Hypertrophy Energy Balance

Abstract

The present chapter delves into the topic of muscle hypertrophy in detail, focusing on defining what muscle hypertrophy is, the types of hypertrophy, the mechanisms, and the relationship with resistance training, as well as the variables affecting hypertrophy such as nutrition, rest, exercise selection, training volume, and training frequency, among others. The importance of mechanical tension, metabolic stress, and muscle damage as triggers for muscle hypertrophy is emphasized. Various types of muscle hypertrophy are explored, including connective tissue hypertrophy and sarcoplasmic and myofibrillar hypertrophy. The text also delves into how hypertrophy mechanisms relate to resistance training, highlighting the significance of mechanical tension and metabolic stress as stimuli for muscle hypertrophy. In a practical point of view, the text also discusses factors like nutrition and recovery, highlighting the importance of maintaining a positive energy balance and adequate protein intake to promote muscle growth optimally. Training variables such as exercise selection, exercise order, intensity, volume, frequency, and tempo of execution are discussed in detail, outlining their impact on muscle hypertrophy. The text provides a comprehensive overview of muscle hypertrophy, analyzing various factors that influence the ability to increase muscle mass. It offers detailed information on the biological mechanisms, types of hypertrophy, training strategies, and nutritional and recovery considerations necessary to achieve optimal results in terms of muscle hypertrophy.

1. Introduction

1.1 What is muscle hypertrophy?

Muscular hypertrophy refers to the excessive increase of muscle, more specifically, skeletal muscle. With this definition, it becomes complicated to delimit the term and make muscular hypertrophy measurable, in addition to acquiring a negative connotation on a certain average. We will be able to give a more adequate definition if we look at the concept referred to in the specialized scientific literature when the term muscular hypertrophy is mentioned. Muscle hypertrophy according to Haun and Vann [1] is defined as an increase in the size (volume) of skeletal muscle, accompanied by an increase in minerals, proteins and quantity of energy substrates (e.g., glycogen or intramuscular triglycerides).

The scientific variables most related to the increase in the volume of muscle (kg) tissue are [2] as follows: Fat-free mass (FFM) was calculated as “all that is not fat”, subtracting fat weight from body weight, or when the measurements were obtained by dual X-ray absorptiometry was calculated as lean tissue plus bone mineral content [3]. Lean muscle mass (LMM), lean mass, lean body mass, bone-free lean body mass or mineral-free lean mass was calculated as the fat-free mass minus the bone mineral content (DXA) or as fat-free mass minus the estimated weight [4] of the live bone by the equation of Heymsfield, Smith [5]. Skeletal muscle mass (SMM) or skeletal muscle was defined as lean muscle and was calculated by anthropometric equations, by proprietary algorithms when using bioimpedance or by estimates based on dual X-ray absorptiometry data [6].

1.2 Types of muscle hypertrophy

If muscle hypertrophy is defined as an increase in the size of skeletal muscle, we must identify which components of muscle tissue produce this increase. A logical and fairly accepted hypothesis is that muscle hypertrophy is primarily due to an increase in the contractile elements of muscle tissue [7]. However, studies have failed to demonstrate that increases in muscle contractile proteins and units (sarcomeres and myofibrils) are proportional to increases in muscle cell area [1]. Two things can be concluded from this: the first is that hypertrophy of contractile elements is not the only one and, moreover, it is not known if it is the one that most influences the hypertrophy of the whole muscle.

We can differentiate three types of hypertrophy or forms of muscle tissue growth [1, 8]:

• Connective tissue hypertrophy (extracellular matrix): increase in the volume of the extracellular matrix accompanied by an increase in minerals and protein quantity.

• Sarcoplasmic hypertrophy: chronic increase in sarcolemmal and/or sarcoplasmic volume accompanied by an increase in mitochondrial volume, sarcoplasmic reticulum, t-tubules, and/or enzymes or the content of energy substrates such as glycogen and intramuscular triglycerides.

• Myofibrillar hypertrophy: increase in size and/or number of myofibrils, accompanied by an increase in the number of sarcomeres or the amount of sarcomeric proteins directly related to the structure and contractile force generation of the sarcomere. Hypertrophy of the contractile elements can be achieved by an increase in the number of myofibrils or the addition of sarcomeres to existing myofibrils. The addition of sarcomeres can occur in series or in parallel, the latter being the most important contributor to exercise-induced muscle hypertrophy [7].

1.3 Muscle hypertrophy mechanisms and their relationship with resistance training

Resistance training (RT) is a popular physical activity recommended for the enhancement or maintenance of musculoskeletal health [9]. It is typically performed with free weights, resistance machines and isokinetic equipment.

In addition, we know that RT is the best training method for increasing muscle mass, superior to aerobic or cardiovascular type training [10]. In turn, RT is the best non-pharmacological method of activating variables linked to hypertrophy and, therefore, the best method for maintaining or gaining muscle mass [11].

Hypertrophy-inducing stimuli are defined as those signals (internally) induced by strength training, which are of sufficient magnitude and duration to trigger hypertrophy of skeletal muscle mass [12]. These signals are recognized by sensors that subsequently trigger muscle protein synthesis through the activation of protein complexes such as mTORC1.

In addition to mTORC1 there are other chemical signaling pathways that induce hypertrophy, such as, among others, insulin and growth factor IGF1, this pathway is also activated with RT, specifically through muscle stretching [13], which is related to the mechanical stress exerted on the muscle during exercise.

Other RT-response pathways that trigger hypertrophy include androgenic markers, myostatin and activin, among others [14].

The main stimuli or mechanisms that trigger the hypertrophic response associated with RT were enunciated by Schoenfeld [7]:

1.3.1 Mechanical tension

When a muscle contraction occurs, whatever the type, it causes an increase in intramuscular pressure captured by mechanical sensors and translated into the corresponding chemical signal at the physiological level. The simple act of walking or running already involves an intramuscular pressure of 200–300 mmHg in the soleus muscle [15]. Therefore, sedentary people who do not practice any type of exercise, simply because they are subjected to the force of gravity on a daily basis, receive sufficient mechanical stimulus to maintain and not decrease muscle mass.

Muscle contraction is not the only way to produce a mechanical stimulus that can result in muscle hypertrophy. Passive muscle stretching can also produce hypertrophy as long as it is very intense or ballasted with resistance [16, 17].

Nowadays, it is considered the main mechanism of hypertrophy since, equalizing mechanical tension, measured as time under tension (TUT), hypertrophy is similar between different types of training protocols [18]. Mechanical tension is load-dependent; the greater the load, the greater the mechanical tension for the muscle, but when in two sets with different percentages of one-repetition maximum (1RM), the same character of effort is reached, which must be high or very high, and the mechanical tension generated in the set is equalized. This is because in the series with higher external load, it will have been possible to perform a small number of repetitions and the TUT will be lower, while in the series with lower load, it will last longer and will reach a similar mechanical stress based on accumulating repetitions of lower mechanical stress [19]. This is why, the mechanical tension and therefore the hypertrophy that we are able to generate through this is dependent on the character of the effort and not so much on the percentage of 1RM used for the series, although obviously both variables are directly related.

1.3.2 Metabolic stress

Metabolic stress induced by weight training could be defined as the accumulation of metabolites, particularly inorganic phosphorus, lactate, H+ protons, and associated muscle hypoxia, which facilitate the increase of these metabolic byproducts and stimulate muscle hypertrophy [20].

One of the most studied metabolites as a signal of metabolic stress has been lactate, a metabolite produced in intense efforts of 30 seconds to 2 minutes duration, times in which the classic RT sets of 6–15 repetitions where anaerobic energy pathways predominate are usually found [21]. It has also been seen how lactate administration in cells in vitro promotes anabolism and cell growth [22], and metabolic stress may be a mechanism of muscle hypertrophy in humans.

The two factors that most affect lactate production and the consequent metabolic stress are as follows: the relative load or number of repetitions per set, together with the character of the effort or proximity to muscle failure of these sets [23]. Therefore, the greater the range of repetitions and character of effort, the greater the metabolic stress.

The character of the effort or proximity to failure is also related to the mechanical stress, as this increases notably in the repetitions closer to failure. Therefore, it could be that the increase in muscle mass associated with metabolic stress is a chemical translation of stimuli of a mechanical nature. When attempts have been made to test whether a greater accumulation of metabolic products increases hypertrophy in the absence of mechanical stress, it has been found that this does not occur and that hypertrophy is dependent on mechanical stress [24, 25].

1.3.3 Muscle damage

Exercise Induced Muscle Damage (EIMD) can be defined as a set of alterations or changes at the morphological level that occur in muscle and connective tissue in response to exercise. This muscle damage manifests itself in alterations at different levels in the muscle fiber such as sarcolemma, sarcoplasmic reticulum, loss of structural proteins, necrosis, microtraumas in the contractile elements (discontinuity in the Z bands), as well as in the connective tissue and in the extracellular matrix [26].

As it is sometimes complicated to measure direct markers of muscle damage, muscle damage can also be measured with indirect markers as follows:

• Reduced ability to produce force [27]

• Edema or swelling produced in muscle tissue.

• Delayed Onset Muscle Soreness (DOMS)

• Reduced joint range of motion due to muscle limitations.

• Increased concentration of certain proteins in the blood such as creatine kinase (CK) or myoglobin.

• Increased levels of inflammatory cytokines in blood such as tumor necrosis factor-alpha (TNF-α) or interleukins such as 1β or 6.

Each of these markers has a specific recovery time [28], but in all of them, the residual effect is affected by the magnitude of the initial damage, and the more the muscle damage, the longer the recovery time.

The association of muscle damage with muscle hypertrophy is due to a series of events and signaling that occurs in the body when muscle damage occurs, such as the inflammatory response, the activation of satellite cells, the cellular swelling, and the production of IGF-1 [29].

Eccentric training has been shown to have greater effects on hypertrophy than concentric training [30]. This could find its explanation in that it is a type of training that generates greater muscle damage or else to that it is a type of training that generates greater mechanical stress. As with metabolic stress, it is difficult to separate the effect of muscle damage on hypertrophy from the effect of mechanical stress on hypertrophy.

Muscle damage decreases as we repeat the type of training that has caused this damage, and this is the effect known in the literature as repeated bout effect (RBE). A few weeks after repeating the training stimulus, the markers of muscle damage diminish to a great extent. If muscle damage was necessary, we would have to find a way to provoke it again once it diminishes with repeated exposure to the same type of training.

This clashes with hypertrophic adaptations that usually occur after 8–10 weeks of exposure to the appropriate stimulus, when muscle damage is already almost nil due to adaptations to training [31, 32]. If muscle adaptations occur when muscle damage is almost nonexistent, it is worth asking whether muscle damage is a direct cause of hypertrophy or whether it is just another means with a small effect on hypertrophy and associated with the main mechanism which is mechanical tension. Without going any further, workouts that involve great muscle damage, as could be eccentric work on slopes, do not generate significant hypertrophy [26]. However, Blood Flow Restriction (BFR) workouts, which barely generate muscle damage, could generate the same hypertrophy as conventional RT [33].

1.4 How much hypertrophy can we expect?

The possible amount of muscle mass (MM) that can be increased depends on the method of measurement and the period of evaluation. Our research in this regard has shown that resistance training can obtain improvements ranged from 1.6 kg in FFM and 1.7 kg in LMM, to 1.1 kg in MM [2]. Regarding the characteristics of the participants, there are no variables (neither the age nor the training status of the participants) that moderate the gains in hypertrophy. In addition, with respect to the characteristics of the training, the only single variable that moderates inversely the gains in hypertrophy is the number of sets per workout, showing that an excess of sets per workouts a effects negatively the amount of muscle growth. These muscle mass gains do not include studies that have used anabolic steroids to achieve these increases, and these changes are much greater when pharmacological aids are used [34].

2. Variables affecting hypertrophy

Now that we have to determine which mechanism induces muscle hypertrophy from RT. We can look at the variables that affect how much muscle we can build. When it comes to RT, most people would think first thought in the weight they need to lift, the number of repetitions and series they need to do, or which exercise to execute. All these will have a place in the variables that affect muscle hypertrophy, but in order to study all the edges that have an influence on the hypertrophy process, we need to give it a wider look.

With this purpose, we have developed a visual map of the variables, backed by science, that a personal trainer should take into consideration when preparing a muscle hypertrophy plan for a client (Figure 1).

Figure 1.

Hypertrophy map factors.

Within the hypertrophy map, we can gather the factors into different groups by similarity of the factors; by doing so, we obtain three categories of variables:

• Subject-related or non-modifiable variables

• Nutritional and recovery variables

• Training variables

2.1 Subject related or non-modifiable variables

Personal differences between subjects directly affect the ability to generate hypertrophy; some of these variables such as gender, age, or previous training experience are observable to the naked eye or can be known with just one question. On the other hand, the genetics of each individual is difficult to know unless a complex study is performed, which is usually not available when training someone. The main limitation of this group of variables is that although we can know them and know their effects on hypertrophy, we cannot modify them; despite this, it will be essential to take them into account within the hypertrophy program, in order to establish realistic goals for muscle mass gain.

2.1.1 Biological sex

Men and women, due to genetic factors, have different body composition. In the distribution of muscle mass, women have a greater percentage of their muscle mass in the lower part of the body, while men present greater similarity between the lower and upper part. Likewise, the differences in total muscle mass between men and women are less marked in the lower than in the upper body [35].

Although sex differences exist, there is much evidence available indicating that the relative muscle hypertrophy that can be generated is the same regardless of sex and muscle group. This hypertrophy is relative; in other words, it depends on the initial muscle mass present in the muscle. Therefore, as men generally have greater muscle mass, they can increase their muscle mass to a greater extent than women [36]. But in the case of presenting two subjects of different sexes, with a similar muscle mass in a given muscle, the capacity to hypertrophy said muscle would hardly be affected by gender differences; other variables could mainly influence the differences between subjects.

2.1.2 Age

The peak of a person’s muscle mass is reached during youth between 20 and 40 years of age, at which time the maximum values of other health parameters such as bone mineral density are also reached [37]. From that peak point muscle mass decreases at a rate of 0.3% annually if resistance training is not performed [26].

As one advances in age, it becomes more difficult to increase and maintain muscle mass, with age more and more subjects become non-responders to the same training program or hypertrophic stimulus [38]. However, it is always possible to increase muscle mass even in older subjects, especially if they have no previous training experience [39], with the associated health benefits. For trained subjects with a great muscular development during their youth, it will be very difficult to increase muscle mass, reaching a point where it will be impossible for them to increase hypertrophy and they will have to focus on losing muscle mass as gradually as possible [40].

2.1.3 Genetics

There is a limit to the amount of muscle mass that a person can increase over a lifetime, but each person has a different genetic limit or potential. When the same training load is applied to people who apparently have a similar training status, we see that they respond very differently in terms of muscle mass gains [41].

These genetic differences allow us to divide subjects into cluster groups of high responders, low responders, non-responders, etc. Depending on how much hypertrophy, they can build from a same stimulus.

Following this methodology, it has been identified that there are certain genetic polymorphisms and variants that are associated with greater gains in muscle mass, strength, power, or VO_2max [42].

2.1.4 Previous training experience

The training level of the subject is also a parameter to take into account, not so much because of its direct involvement but because it is a reference for the use of other variables. In the early stages of training, when one goes from not controlling any of the variables that affect hypertrophy to starting to perform activities and behaviors that enhance it, the change is so great that it means an increase in muscle mass even when the training and nutrition parameters are not optimized, as a little is better than nothing. Given this premise, one would expect that, after this initial stage of improvement, the adaptations would be reduced by the appearance of RBE, but nothing could be further from the truth. Benito, Cupeiro [2] determined that the hypertrophic response depending on the training experience could be equally large in novice and experienced subjects, with less response from those subjects between 2 and 4 years of RT experience (Figure 2).

Figure 2.

Differences in hypertrophy (kilograms of fat free mass) as a function of level in years of training experience [2].

This increase in the capacity to generate hypertrophy as the subject becomes more experienced could be explained not so much by physiological factors but also by the optimization of known and controllable variables. As one gains experience in training, one also learns to train and feed better or more in accordance with the physical objective set. This, added to other factors such as the time invested in achieving the objective and the importance of training in the lives of those who practice it, makes the very experienced optimize their possible gains in muscle mass as opposed to those who, despite having a greater margin for improvement, are not able to take full advantage of the variables that affect the increase in muscle mass. In addition to these implications of experience on the hypertrophy variables, we also found that previous experience with the same type of hypertrophic stimulus affects the stimulus response, which is why it must be taken into account when conducting and analyzing studies if the subjects are subjects without previous experience in RT or if on the contrary they are subjects with experience in RT. This nuance affects not only the magnitude of the effect of the intervention but also the implications that the intervention may have on the subject. The epigenetic response to RT stimuli such as, among others, muscle stretching is dependent on the subject’s level of training [43].

2.2 Nutritional and recovery variables

Within the factors that can influence the hypertrophy process are the group of nutritional factors which, in turn, are related to the recovery from the training sessions. The protein balance, where there is a sufficient excess of proteins, necessary for the construction of muscle mass, which is directly related to the need or not of protein supplementation, and on the other hand, those processes that accelerate the recovery processes or enhance the reconstruction of muscle mass, such as the use of anabolic steroids. The effect of these nutritional factors, on the processes of recovery and energy availability, has a direct connection with the metabolic response that any training stimulus may produce.

It would appear that a slightly hyperproteic, incorporating at least 1.6 g of protein per kilogram of body mass, is appropriate in hypertrophy interventions [44], although it is true that this value may change depending on other variables such as sex, training status, previous training experience, and the use of anabolic steroids.

What is clear and beyond doubt is that a negative energy balance can reduce or even eliminate the ability to hypertrophy in humans, even when weight training with high loads. A slightly positive energy balance and a diet rich in amino acids (2 to 3 times the RDA recommendations) is needed to promote maximal muscle growth [45, 46].

When it comes to nutritional intake, a slightly hypercaloric diet, made of whole foods or high-calorie supplements, combined with RT increases FFM [47]. The calorie surplus just needs to be slight, as if it is too big nor only will there be a MM increase if not also a body fat increase [48], especially in advanced lifters who have small chance of taking advantage of aggressive hypercaloric conditions [49].

There is a wide variety of nutritional intakes, low carbohydrates, high fats, intermediate fasting, etc. All of them with subtypes within it, while most of them claim to be the most effective to improve body composition, it has been proved that all of them have advantages and drawbacks [50]. Concerning FFM, any kind of nutritional intake could possibly increase hypertrophy when combined with RT, as long as the amount of protein intake is high enough [50]. Regarding the other nutritional parameters that could affect the increase in MM, it should be noted that not all of them affect in the same way; it depends on the individuals following the diet program. This should be taken into consideration when designing nutritional plans, as the response is inevitably individual.

The use or not of nutritional supplements will depend on the nutrient intake of each person, so before providing any supplement, it is necessary to analyze whether or not they are necessary to maximize the process of muscle hypertrophy.

Referring to recovery process, in the past few years, there has been a huge increase in recovering methods, cryotherapy, foam rolling, pressotherapy, ice baths, etc. As always, we tend to forget the most simple but effective methods, in this case, sleep. To optimize muscle hypertrophy, sleeping time, and quality needs to be considered, as it is the main time when the muscle will recover from training and grow as a response to the given stimulus [51]. The lack of sleeping time and or quality, could negatively affect hypertrophy, not only the new MM increases but also the already existing MM, what would lead to muscle atrophy and, in worst case, scenarios, sarcopenia, and cachexia. This is due to the effect of sleep deprivation, which causes a decrease in the activity of muscle protein synthesis (MPS) and other anabolic pathways, while also activates catabolic signaling and degradation pathways [51].

Some of the above-mentioned recovery methods have been related to hypertrophy increases, for instance, ice baths or cold water immersions. This method is used right after finishing the training session, with the claim of accelerating the recovery process, but the hypertrophic response clearly does not benefit from it, cold water immersions attenuate the activity and changes in satellite cells, which regulate muscle hypertrophy, therefore reducing the dehypertrophic response of the muscles [52, 53].

In contrast with cold water immersions, we have heat therapy, but despite the change in temperature, there is no change on its effectiveness to increase muscle hypertrophy, same as before, and there was not a difference in MM increments by using or not localized heat pads [54].

Moreover, other recovery methods have not yet been studied along with hypertrophy, and this is the case of pressotherapy. This new method has recently gained prominence among athletes, but its effects on muscle recovery and thus on its possible impact on hypertrophy are not yet clear [55].

That is why, all in all, the main focus of the recovery process should not be on the use of new methods and devices, but to rest properly and according to the personal demands.

2.3 Training variables

2.3.1 Type of activity

Not all types of training are equally efficient in producing muscle hypertrophy, as not all of them involve the principles of mechanical tension, metabolic stress, and muscle damage. In some training methods, even when these mechanisms are present, hypertrophy is not produced, as it is dependent on very specific conditions. The training method that has proven to be most effective and efficient in generating hypertrophy is weight training or RT [10].

2.3.2 Exercise selection

Exercises are usually chosen to work a target muscle, but within the variety of exercises that exist for the same muscle group, not all generate the same hypertrophy, as, depending on muscle involvement, regional hypertrophy of muscle zones can be produced [56]. Small changes in the placement of a joint during the execution of an exercise can lead to small adjustments and changes in the distribution of hypertrophy generated [57]. This can be especially interesting for athletes such as bodybuilders [58], who depend on their physical appearance for their sport performance and where every muscle detail can make a difference.

When we think of different exercises for the same muscle group, one of the first things that comes to mind is the material we can use to perform the exercise, dumbbells, barbells, bars, pulleys, machines, etc. For years, it has been hypothesized whether free weights, being more demanding or harder, generated greater hypertrophy, or whether the use of machines, created for the isolated work of a muscle group, enhanced it to a greater extent. Well, both hypotheses were at least partially wrong. A recent research by Hernández-Belmonte et al. [59] showed that, matching the rest of the training variables such as volume, intensity, etc. There are no differences in hypertrophy generated by the use of free weights or machines. This is due to the fact that the organism does not understand the exercises performed; it simply receives the stimulus signal and triggers the response to this mechanical stimulus in the form of chemical signaling that increases protein synthesis and hypertrophy.

Another point that generates debate within the selection of exercises is whether to use multi-joint or mono-joint exercises, as happened with free weights and machines there are advocates and detractors of both, even so, and as happened with the previous point, no significant differences have been found between using multi-joint or mono-joint exercises [60].

With this we could think that, by selecting the exercise that we like the most for a muscle group, and performing all the corresponding series in this exercise, we could reach the optimal hypertrophy of that muscle group. Despite this logical reasoning, we must remember that there is the RBE or adaptation of the response to the same stimulus, which is why it is advisable to vary the exercises for each muscle group to maximize hypertrophy [61, 62], this variation of exercises should be more related to biomechanical aspects (resistance profile, muscle activation, etc.) of the exercise than to the material used. Moreover, in general, varying the exercises for the same muscle group helps to increase motivation and avoid monotony [63], which also helps to optimize results.

2.3.3 Order of exercises

Now that we have determined that we must perform several exercises to maximize the hypertrophy of a muscle group, we must order them within the session. Although, as we mentioned before, there are no differences between performing global or analytical exercises, there are differences in how one affects the other, so it is important to know how the order in which they are programmed affects them [64], as the fatigue they generate is not the same. Placing the exercises of the target muscles at the beginning of the session could have slight advantages when it comes to optimizing the hypertrophy of these muscles due to the production capacity and increase in strength available at the beginning of the sessions [4].

2.3.4 Effort character

The character of effort refers to the number of repetitions performed in a set versus those that could potentially have been performed in that same set [65]. Therefore, maximum effort character would be to perform all possible repetitions with the given load, until reaching muscular failure. Muscle failure is the point at which, despite applying all possible force, the concentric phase of the exercise in question cannot be completed [66]. It remains to be determined whether this muscle failure should be considered as the complete achievement of concentric ROM in all exercises, as in very demanding exercises toward the end of the concentric run one might not reach muscle exhaustion but not be able to complete the entire run.

Of all the hypertrophy variables associated with RT, the nature of the effort is one of those with the greatest implication on the muscular hypertrophy that can be generated by RT, and it could be argued that it is the most important, with the exception of training volume. This is due, in part, to mechanical tension, as we saw in the section corresponding to this concept; mechanical tension is the main mechanism of hypertrophy and also increases as the series progresses, when fatigue and the nature of the effort increase.

Indeed, having a high effort character in the series performed is one of the main keys for this series to increase our muscle mass, as demonstrated by the scientific evidence to date [67, 68]. This could lead us to think that performing all sets until muscle failure would be the best way to optimize hypertrophy, but this is not the case, in studies where a high or very high effort character has been compared with muscle failure, it has been observed that there are no significant differences in terms of the hypertrophy produced [69, 70], although muscle failure does lead to greater fatigue, greater accumulation of metabolites and potentially greater muscle damage [71]. Therefore, the use of muscle failure in training should be used as another tool and taken into account when planning.

To measure or estimate the character of effort, there are several tools, such as scales of perceived effort, such as the Borg scale; the repetitions in reserve (RIR), which is the number of repetitions that we leave undone before reaching muscle failure, if in a series we can perform 12 repetitions and 10 are performed, it would be a series with RIR 2; another option is also the velocity-based training, where the proximity to failure is estimated with the percentage of speed loss of the concentric phase compared to the first repetition. This method is very dependent on the attentional focus on performing the concentric phase at the highest possible speed. All of these are valid ways of estimating the proximity to failure and making a set an effective set for hypertrophy.

In summary, the character of the effort is the parameter that marks whether a series is effective for hypertrophy or not, in general, it is considered that a series of high effort character, which is performed with an RIR 4 or lower, can be quantified as a training series [72]. The degree of effort required in each series will influence the tolerable training volume and the perceived exertion of the training [73].

2.3.5 Relative intensity - load type

Relative intensity refers to the percentage of the 1RM selected in the series to be performed. With each percentage of the 1RM, a certain number of repetitions can be achieved before reaching muscle failure [65]. This number will be different between subjects for the same percentage of the 1RM of each subject [74]. These percentages of 1RM are divided into high loads (1-5RM), medium loads (6-15RM), and low loads (15RM or more) [75].

Current evidence tells us that muscle mass gains can be achieved with either high, medium, or low loads [76, 77, 78], as long as some considerations are taken into account for each of them. In all cases, one should have a high or very high effort character, close to muscle failure. Excessively low loads, below 20% of the 1RM, which would be equivalent to 60-70RM, do not produce as much hypotrophy as other load ranges [79]. In the series with low loads is, if possible, even more important, the proximity to muscle failure, as it is the only way to try to match the mechanical stress that occurs in series with higher loads [80]. For its part, the series of high loads, especially if they occur in multi-joint or global exercises, do not require reaching muscle failure if not only to RIR 3–4 [72].

Although, as we have seen, you can conduct hypertrophy with almost any type of load, the use of medium loads is still the recommended for most people and for most of the time, for its efficiency and convenience [81, 82]. High and low loads should be used as a complementary tool within training with which to give variety of stimulus to the muscle.

2.3.6 Volume

The most adequate method used, also in research, for the quantification of volume in RT is the number of effective series per muscle group [83]. Now, not all the series can be considered effective, for this, we return to the character of the effort, as we determined previously the series must have a high stove character, RIR < 4, so that this is sufficiently stimulating to generate hypertrophy. Also, this character of the effort should be taken into account depending on the type of load of the series, it is necessary to apply a greater character of the effort when using lower loads so that the series can be effective.

If the character of the effort is the variable that marks if a series is effective for hypertrophy or not, the volume of training is the variable that has more relation with how much can be hypertrophied, but is always more volume more hypertrophy?

In the first studies analyzing training volume in the form of effective sets, such as that of Wernbom, Augustsson [84], it was observed that, for the biceps brachii, the recommended volume was 4–6 sets per session and 2–3 sessions per week, which is equivalent to 8–18 sets per week, while for the quadriceps, it was 8–30 sets to optimize hypertrophy. This review was not misguided, as the latest reviews that study the optimal volume per muscle group, speak of 10 to 20 weekly sets per muscle group [85]. Below 6 weekly sets per muscle group, it is really difficult to produce hypertrophy [86], especially in subjects already trained in RT with hypertrophy objective; possibly for untrained subjects, it is a sufficient stimulus to start but that should quickly approach the 8–10 sets indicated by the reviews.

We already have the limit of the minimum effective volume more or less clear, not to go below 6–8 weekly series and, if possible, to be closer to 10–12, at least. But what about the upper limit?

In some studies such as that of [87, 88], where different groups were compared with different training volumes, it was observed that those who were around 20 weekly sets per muscle group generated greater hypertrophy than those who were closer to 30 weekly sets. On the other hand, the authors of [85] establishes 20–25 sets as the weekly recoverable volume, at which point more fatigue accumulates than hypertrophy is generated, which could be counterproductive for hypertrophy due to a possible subsequent poor recovery. On the other hand, a recent study carried out in men with 5 years of experience in RT, i.e., fairly trained subjects, measured the effect of adding weekly series on the quadriceps, starting from 22 weekly series, an already high volume, until reaching in the case of one of the groups to 52 weekly series. In this study, the group performed a higher volume obtained greater hypertrophy [89]. The results of this study should be taken with caution, due to the nature of the highly trained subjects, the progressive overload approach, and the fact that only the muscle group analyzed was trained during the study. Taking all this into consideration, we cannot say that always more volume is going to be better, there is a point, different for each person and muscle group, where the maximum tolerable or recoverable volume is reached [90]. When this upper limit of the volume is reached not only may it not increase more if not even be a smaller increase than when using a lower volume. This point should be adjusted to each subject and muscle group based on their level of training and ability to adapt and will change as training progresses and based on the approach taken at any given time.

Now that we have the volume in weekly series, it is necessary to distribute it in the workouts, to perform all the series for the same muscle group in the same training session is not optimal, especially if this volume is higher than 8–10 series [91]. The recommended would be around 6–8 sets per muscle group per session, not going below 4 and not exceeding 10 sets per muscle group per session. Therefore, an intermediate or advanced athlete who usually performs 16 weekly sets of a muscle group should divide it into at least two weekly sessions. This is mainly due to the high effort character that must be maintained in the series for hypertrophy.

In the exercise selection section, we already established that it was important to perform more than one exercise per muscle group to provide stimulus variety [61, 62]. Therefore, it will be important to divide the weekly series that are programed in different exercises for the same muscle group. Just as it is not good to perform all sets of the same exercise, it is not good to perform one set in each exercise. Krieger [92] determined that performing 2–3 sets per exercise provided 40% more hypertrophy than performing only one, while performing 4 to 6 sets already diluted this effect and that although greater hypertrophy was generated by greater volume, this did not have as much increase as in the first sets. Therefore, it is important to perform 2 to 4 sets per exercise.

2.3.7 Training frequency

The training frequency is the number of sessions or days in which training is performed. Taken into RT it is usually referred as the number of days in which a muscle group is trained during a microcycle or week of training; thus, if each muscle group is trained once a week, it will correspond to a frequency of 1 while 2 or 3 days of the same muscle group would be frequency 2 and 3, respectively. Often, the frequency of training does not depend only on physiological factors, as we will explain below, but also on the availability of the subject, which is why the design of efficient training plans is paramount [93].

The frequency is a variable directly dependent on the volume and intensity, I understand this as the character of the effort, the higher the volume and the more intense the training, the higher the frequency must be to be able to tolerate that work. At equal volume and intensity and as long as both are tolerable, the frequency of training does not make a difference on the hypertrophy that is generated [94, 95]. Therefore, we should understand frequency as a tool to regulate training volume.

Frequency has a very high relationship with volume and therefore also with the parameters of optimal volume and tolerable volume, as explained above. As we have seen, the weekly sets that some subjects may require to generate hypertrophy can range from 12 to 25–30 weekly sets [85]; these exceed the 10–12 sets that are tolerable in a single training session [91]. Therefore, for those weekly volumes that exceed the tolerable daily volume, it will be necessary to increase the frequency [96, 97], not so much because this will have an effect on hypertrophy but to tolerate and distribute the volume required to hypertrophy.

As we have also mentioned above, the volume increases as one progresses in training and in the development of muscle mass, therefore, in novice subjects, a low volume may be assumable with a frequency 1, while in more advanced subjects, a type 2 frequency will be required, being able to reach frequencies 3 and 4 at specific times or specialization mesocycles for a specific muscle group in advanced subjects.

The frequency of training is also closely related to the type of training routine, those fullbody routines, where the whole body is worked in each session, tend to have a higher frequency than Split or Weider routines, where the frequency is usually 2 or 1. This distribution of muscle groups in different training days has not been shown to have an influence on hypertrophy [98, 99] provided that the volume is the same. In cases where volume is not well controlled, a higher frequency usually leads to greater hypertrophy, but this is linked to the fact that a higher training volume is usually produced.

In conclusion, the variable that determines the amount of hypertrophy is the volume, the frequency is dependent on it and must be used to maximize the gains generated by the optimal training volume.

2.3.8 Execution time (tempo) and time under tension

The Time Under Tension (TUT) is the time it takes to perform a series, formed by the time of each of its repetitions; adding all the series of the same muscle group, we obtain the TUT of each muscle group in the session.

In RT, each repetition is composed of four phases: concentric, eccentric, and two transition phases, from concentric to eccentric and from eccentric to concentric. The duration of each of these phases is what is known as movement tempo or cadence of execution. The time spent in each of these phases directly affects the number of repetitions that can be performed in the series and therefore the character of the effort of the series, since, against the same external load, performing a given number of repetitions will not involve the same effort, this is due to the tempo in which these repetitions are performed. Therefore, it is essential to set the tempo of execution of each phase in the RT.

Regarding the effects of movement tempo on muscle hypertrophy, as long as the total time of the repetition is between 0.5 “and 8”, hypertrophy is not negatively affected [100], so the margin of time we have to perform a repetition is quite wide. Seen that the total time of the repetition does not have excessive influence on hypertrophy, one might ask whether the time in each phase of the repetition does.

Hypertrophy benefits from increased TUT in the eccentric phase, due to the increased mechanical tension and muscle damage that this generates, so it is interesting that, within stretch-shortening cycles, the eccentric phase is slowed down to maximize hypertrophy [30]. This is due, among other things, to the fact that the load used during the series is always the same regardless of the phase, while muscle strength is not the same in concentric as in eccentric [101]. Therefore, increasing the intensity in the eccentric phase with increasing the time in that phase has effects on hypertrophy, because it intensifies the series by increasing the character of effort.

On the other hand, increasing the time in the concentric phase does not seem to have clear benefits on muscle hypertrophy [102]. Therefore, it is not advisable to deliberately slow down this phase but to allow it to slow down when the character of the effort at the end of the series requires it [103].

The movement tempo that is paused must be in accordance with the level of demand of the repetitions. It must take into account the character of effort and the type of load. When working with high loads or when already close to failure, it is not always possible to follow the stipulated cadence, as the acyclic muscular resistance will have been exceeded. A point from which the speed of execution cannot be modulated at pleasure, especially that of the concentric phase. Despite the fact that the established cadence is not met, the series must continue to ensure the proximity to muscular failure.

Movement tempo is an important variable to be taken into account when RT is patterned. As we will see later, where the attentional focus is placed during a set has implications on the resulting hypertrophy. Although in the first repetitions of a set a stipulated cadence can be met, maintaining a specific tempo and focusing the attention on it can affect the quality of the set when proximity to failure is approaching [104].

2.3.9 Rest time

It is the time that elapses between sets or exercises of a workout. Despite the classic recommendation to use short rest times (60″) to elevate hormonal values and thus maximize hypertrophy, today it is known that there is no relationship between these variables [105]. Rather the other way around, too short rests can negatively affect the hypertrophic response such as muscle protein synthesis (MPS) [106].

This is why new trends, guided by scientific evidence, recommend longer rest times, seeing that hypertrophy generated with 3-minute rests is superior to 1 minute [107] and that, in general, 2-minute rests can be optimal for hypertrophy [108]. There is no scientific support for shortening rests when hypertrophy is the goal. The feeling of higher intensity from training more often does not trigger greater muscle mass gains [109].

2.3.10 Range of movement

The range of motion (ROM) refers to the distance in degrees that are covered in a particular joint during the execution of an exercise. If all possible degrees of freedom of that joint are traversed for a given exercise, it is called full ROM, whereas if only part of the possible degrees of movement is performed, it is called partial ROM [110].

Current scientific evidence tells us that, the main key to the selection of a ROM for an exercise is that this ROM goes through muscle stretch [111] either through the use of a full ROM or through a partial ROM in the area of muscle stretching or elongation [112]. The latter option of partial repetitions in stretching has taken quite a bit of traction in the scientific literature recently, sometimes surpassing full ROM in terms of the hypertrophy produced [112, 113].

This effect, known as stretch-mediated hypertrophy, whereby hypertrophy is produced by stretching a muscle, either with very demanding passive stretches or RT where the muscle is brought to the stretch, has been shown to have effects on increasing muscle mass [16, 114]. This is because muscle stretching within RT increases mechanical tension to a greater extent than RT in muscle shortening, thereby also increasing TUT intensity [17].

The effects of muscle stretching on hypertrophy can be seen not only in ROM comparisons but also between exercises with the same ROM. Those exercises in which at the beginning or the end of the ROM have a greater muscular stretch produce greater hypertrophy than those with a more reduced ROM in which the muscle is not taken to maximum possible stretch [115, 116]. By this same principle, the use of ballasted stretching between sets of RT could have positive effects on hypertrophy [117]. In this case, it would be mainly due to the fact that, by maintaining tension on the muscle at the end of the series with load, the TUT and the training volume are increased, and it would be necessary to study whether at the same volume and TUT these ballasted stretches between sets would have superior effects on muscle hypertrophy.

2.3.11 Resistance profile

The resistance profile is the result of putting together the torque that a joint undergoes throughout the course of an exercise, with the capacity of the muscle to produce force during that same course. This capacity of the muscle to produce force as a function of its length reduces as muscle shortening occurs [118]. This shows us that, depending on the position of the body segment which is being trained and how the external forces affect it, an intensity curve will be produced during the ROM reflecting how demanding the exercise is along its ROM [119], this curve is known as resistance profile.

This training variable has, to date, little scientific support behind it, which is why it has not yet been possible to determine whether there is an optimal endurance profile when seeking to maximize hypertrophy [120]. However, we do know that once an exercise has been selected, it is important that the ROM passes through the zone of greatest mechanical demand of the exercise [121], or what is the same, to pass through the highest zone of the resistance profile curve, as that is where the mechanical stress produced by the exercise is greatest.

2.3.12 Muscle action type

As mentioned above, the most common way to generate hypertrophy is through muscle contraction, although as we have seen, muscle stretching can also induce hypertrophy. Within muscle contraction, the use of concentric or eccentric contraction can also have an effect on the hypertrophic outcome.

When comparing the use of only concentric versus eccentric contractions, it has been found that strength gains are specific to the type of contraction while in hypertrophic gains, while both contractions can generate an increase in muscle mass, eccentric contractions generate a greater increase in hypertrophy [122], with the increase in muscle mass in eccentric being very similar to when concentric and eccentric phase are used at the same time [123]. Eccentric overload training generates great muscle damage, accumulates a lot of fatigue, and usually requires supervision and specialized equipment [124], so although it has a place in RT, especially in cases of sports performance, its use when the objective is hypertrophy is not indispensable and, if used, a greater control of the character of the effort should be taken as in this case the eccentric muscle failure is not the same as the concentric and can lead to much more fatigue [125] in addition to possible muscle injuries.

As purely eccentric work has similar results to using concentric and eccentric work in the same set, it will be more interesting in general to use a classical RT work, where the concentric phase is performed in the time required by the load demand and the eccentric phase is slowed down and controlled slightly in order to take advantage of the eccentric work [30]. By doing this, we will increase the TUT and mechanical tension produced by patterning a movement tempo that spends more time in the eccentric than concentric phase [104].

2.3.13 Attentional focus

In the world of bodybuilding and strength training in general, it has long been believed that focusing attention on the muscle being worked during the set increased muscle mass gains, and this attentional focus is referred to as the mind-muscle connection.

Indeed, focusing attention on a muscle can lead to slight increases in the activation of that muscle [126]. Given that, when approaching near failure, it is difficult to maintain attentional focus on anything other than generating force as best one can. We have also investigated how external assistance with verbal instructions affects maintaining that muscle focus and mind-muscle connection. The result was the same as above, and verbal instructions have a slight effect on muscle activation of the target muscle [127, 128].

Despite the fact that there is the ability to improve muscle activation by focusing attention on the muscle, we know that, if the muscular disposition in front of the load is correct, the nervous system itself is responsible for recruiting the muscles with greater mechanical advantages to carry out the demanded movement [129]. This is why, even more important than focusing on the mind-muscle connection, it is essential to select exercises that put the target muscle in an ideal biomechanical position to exert force.

In summary, it has been evidenced that, focusing attention during the series on an internal focus, such as the mind-muscle connection, generates more hypertrophy than focusing on external factors [130]. This does not mean that one should slow down the concentric phase of an exercise looking to maximally notice the muscle, as we saw earlier. This has no positive effect on hypertrophy [102].

During a set, the focus should be on moving the load in the plane where the target muscle is most efficient and moving the load at a controlled speed that allows us to ensure we work the target muscle, without slowing down voluntarily. At the end of the series, when it is difficult to maintain this focus, we should seek to be close to failure, keeping the repetitions as homogeneous as possible to avoid the intervention of other synergistic muscles that would take work away from the target muscle.

3. The effects of energy balance on muscle hypertrophy

The process of building muscle mass is a process that costs energy (endergonic), which means that if we provoke an increase in this type of tissue, it will be at the cost of obtaining the energy necessary for it.

As mentioned above, what is clear and beyond doubt is that a negative energy balance can reduce or even eliminate the ability to hypertrophy in humans, even when weight training with high loads. A slightly positive energy balance and a diet rich in amino acids (two to three times the RDA recommendations) is needed to promote maximal muscle growth [45].

In the mathematical models proposed so far that attempt to predict muscle growth, the energy balance is one of the most essential [131, 132, 133]. In the model proposed by Torres, it is clear that the path that muscle growth can develop is very different depending on whether the intake is hypocaloric, normocaloric, or hypercaloric. For this reason, it is worth asking how much energy will be necessary for this process. An increase of approximately 200 kcal over the normocaloric diet is a good start, and in any case, any desired increase that does not produce an increase in the percentage of body fat, which would make it clear that there is an excess of unnecessary Kcal in that energy balance. It is enough to measure every 2 weeks the percentage of fat, to know that the increase of kcal is being adequate. Recent studies have found that larger energy surplus, 15% above maintenance calories, leads to a bigger body mass increase while also increasing fat mass while smaller surplus, 5% above maintenance, will lead to a smaller body mass gain while nearly reaching the same MM gains [134]; therefore, just a slight surplus above the normocaloric diet should be enough to optimize muscle hypertrophy while limiting the increase in fat mass (Figure 3).

Figure 3.

Effects of three types of diets on the growth of muscle mass (left side) and fat mass (right side) in mathematical models proposed by Torres et al. (figure modified with permission of the author).

3.1 Body recomposition

One of the questions that we, personal trainers and physical trainers, are often asked is whether if it is possible to produce an increase in muscle mass while producing a reduction in the fat mass, which has been called body recomposition. In the light of science, we must say that this process has been moderately studied and that it possibly exists [135].

Body recomposition is especially useful in individuals with high fat percentage, low MM, and low RT experience, as in these kind of individuals, any change for the better will have a positive impact on body composition. As a result, the research done in postmenopausal women has proven that, in this kind of population body, recompositing is possible [136, 137]. Same for the elderly, starting RT and dieting at any age has an impact of increase of MM and decrease on fat mass [138, 139].

Although it might seem that this is a process that only occurs in people with a low level of fitness, it has been shown that it can also occur in people with previous experience in RT, even when not following a nutritional plan [140, 141]. The problem in these cases is that although changes occur, they are pretty small, the amount of time used during a body recomposition to lose 1 kg of fat mass and gain 1 kg of MM might be enough time for a bigger decrease in fat mass and a bigger increase in MM if focused only on one goal at a time. When a nutritional protocol is carried out, the changes can be substantially greater than when only training is taken into account [142]. The possibility of producing a body recomposition is given regardless of the level of fitness [143], for both men [144] and women [145]. Body recomposition exists for all populations, only that some of them, can take greater advantage of it while, in others, it would be more interesting to perform classic processes of muscle mass gain and fat loss.

One population that could benefit from body recomposition is elite competitors, not so much because in their case, the effect of recomposition is very large and because they are athletes who must maintain an optimal state of fitness throughout the year, with a low percentage of body fat. Body recomposition may be sufficient to generate the small changes in body composition required by these athletes while maintaining performance [146, 147]. Among these athletes, we could also find bodybuilders; in their case, this body recomposition occurs when they go from a stage of muscle mass gain to one of fat loss to prepare for a competition. In the first weeks of transition between stages, a body recomposition can occur, and the loss of fat is due to the reduction of calories in the diet while there is still an effect on the increase in muscle mass [148, 149].

It is necessary to say that the process of increasing muscle mass is an anabolic process, and that the process of fat reduction is a catabolic process that can conflict, in fact, we have seen in the previous section as a deficiency of energy in the energy balance can compromise muscle growth. Body recomposition should be taken into consideration as a protocol for some individuals as it can occur in any population [150]; however, sometimes, depending on how much of an increase in MM or decrease in fat mass is required, we believe that an adequate periodization of training by focusing on one goal at a time, could produce more relevant effects than trying to achieve mixed training targets.

3.2 Practical applications

In a practical point of view, in order to be effective in the process of increasing muscle mass, it is necessary to analyze the hypertrophy map individually. Establishing those factors that are not modifiable (with the focus on analyzing the potential hypertrophy for each individual); and adapting those factors that are modifiable within the parameters described in this chapter.

Resistance training has proven to be the most effective training for improving muscle hypertrophy. This is due to the presence of the hypertrophy mechanisms in this type of training. Within the hypertrophy mechanisms, mechanical tension has proven to be the most important one when studying muscle gains, muscle damage and metabolic stress are variables derived from and influenced by mechanical tension, and muscle damage and metabolic stress alone do not induce muscle hypertrophy. Therefore, trainers should seek to optimize mechanical tension within resistance training.

Taken into account the training factors, the variables which have been shown to have the greatest importance on muscle hypertrophy are the character of effort (RIR 4 or lower) and the volume of training (between 12 and 25 weekly series per muscle group), depending on the other factors.

To optimize hypertrophy, it is crucial to sustain a slight positive energy balance, typically around 200 kcal above maintenance levels. This slight surplus supports muscle growth by providing the additional energy needed for effective recovery and adaptation following resistance training.

When it comes to nutritional factors, scientific evidence supports that at least 1.6 g of protein/kg of body weight is necessary to achieve a significant effect on hypertrophy.

4. Conclusions

To effectively enhance muscle mass, it is essential to evaluate the hypertrophy map on an individual basis. This involves identifying non-modifiable factors to assess each person’s hypertrophic potential and adjusting the modifiable factors according to the guidelines outlined in this chapter.

Nutritional factors are intrinsically related to recovery. Assuming that hypertrophy always occurs in the recovery phase, poor recovery could be a limiting factor in the muscle growth generated by resistance training.

Although in the field of sports science almost all studies focus on training variables, the present chapter demonstrates that this is only a part of the factors involved in the process of increasing muscle mass.

Maintaining slight positive energy balance is essential to support muscle growth and optimize hypertrophy outcomes during resistance training. Controlling the degree of positivity in the energy balance, by monitoring changes in body fat, and maintaining an adequate protein balance, with or without artificial supplementation, is crucial for optimizing muscle hypertrophy outcomes.

Although body recomposition is scientifically feasible, it might not be the best option for everyone. While obese and overweight people may go through a body recomposition process when initiating resistance training for other subjects such as healthy trained population, it may be more efficient for a periodization process where the goal is to increase muscle mass and then decrease fat mass. Body recomposition may be also interesting in high performance sports, as these athletes need to maintain a certain body weight during the season.

Acknowledgments

We would like to thank the Universidad Politécnica de Madrid and the LFE Research Group for the opportunity to continue working in a center of national and international reference.

Conflict of interest

The authors declare no conflict of interest.