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Muscle Hypertrophy: A Narrative Review on Training Principles for Increasing Muscle Mass

Howe, Louis P. MSc1; Read, Paul CSCS2; Waldron, Mark PhD3

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Strength and Conditioning Journal: October 2017 - Volume 39 - Issue 5 - p 72-81
doi: 10.1519/SSC.0000000000000330
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Abstract

INTRODUCTION

Within a strength and conditioning coach's practice, the development of physical qualities that result in improvements in athletic performance are arguably at the heart of the role. One such physical quality is maximal force development because it relates to the constraints of each specific sport skill (85). Although neurological factors primarily contribute to the development of force, structural adaptations after long-term strength training may also influence force generation capacity (19), though the role of muscle hypertrophy is likely to be exaggerated for increasing maximal strength (49). Nonetheless, increases in muscle mass are typically desirable in instances where athletes are required to produce large forces against their external environment. Indeed, cross-sectional area of the limbs has been associated with both horizontal and vertical power production in elite athletes (93). This concept provides the rationale for developing the musculoskeletal system, before neural factors, as part of a “phase potentiation periodization model” (85). Therefore, increases in muscle mass can, in many circumstances, support athletic development to a point.

Alongside performance considerations, increasing muscle mass may also be required after injury occurrence. During the rehabilitation process, muscle atrophy is a key concern, secondary to unloading (40). In cases where muscle atrophy is heightened because of immobilization, the rehabilitation time has been suggested to be extended (8). Therefore, recovery of lean muscle tissue may be a significant objective that should be achieved to restore the athlete to full function after injury.

Three proposed primary mechanisms are responsible for training adaptations, including: mechanical tension, metabolic stress, and muscle damage (65). Mechanical tension refers to the loading of muscle and is proposed to disrupt skeletal muscle structures, compromising the integrity of individual muscle fibers and leading to cellular responses via stimulation of the mTOR pathway (38). Local metabolic stress involves the accumulation of metabolic by-products, such as blood lactate (B[La]) and hydrogen ions ([H+]), caused by a cumulative demand on fast glycolysis (32,89). It is thought that this metabolic perturbation has the potential to stimulate anabolism through mechanisms associated with increased local release of myokines, greater reactive oxygen species production, cell swelling, and acute hormonal responses (66). Lastly, muscle damage is theorized to lead to hypertrophic responses (37), whereby the inflammatory response and upregulation of muscle protein synthesis to exercise induced muscle damage results in greater muscle size. For the interested reader seeking further understanding of each mechanism and the underlying physiological adaptations, please refer to Schoenfeld (65).

Traditional beliefs of the most effective strategies for the development of lean muscle tissue have recently been challenged (17). This article reviews the current body of literature related to training considerations for increasing muscle hypertrophy in young healthy adults. Where appropriate, this review will indicate the training status of the subjects used for each investigation. As such, this will allow coaches to adequately interpret the discussion in order to suitably apply the findings in their own strength and conditioning practice.

In particular, this article discusses the following foundational factors that should influence a coach's decision-making when designing resistance training programs for increasing muscle size:

  • Training volume
  • Load
  • Training frequency
  • Training to momentary muscular failure
  • Exercise variation
  • Contraction type
  • Exercise order
  • Repetition tempo
  • Interset recovery

Evidence-based recommendations will be provided for the design of effective resistance training programs, with the goal of increasing an athlete's skeletal muscle mass.

This article does not describe, with any detail, the physiological mechanisms for increasing in muscle mass, nor does it examine the evidence relating to specific training techniques aimed at increasing muscle hypertrophy. Likewise, although a variety of nutritional and lifestyle factors can be manipulated to augment the effects of hypertrophy training (57), these will not be discussed herein.

TRAINING VOLUME

Resistance training volume, described as the amount of work completed for any given unit of time, is a key variable for consideration in the pursuit of muscle hypertrophy. Much like increases in maximal strength (44), muscle hypertrophy is enhanced after high-volume longitudinal programs, particularly when multiple sets are used, rather than single set routines (45). This evidence has been used to support the current American College of Sports Medicine (ACSM) recommendations, which suggest prescribing multiple sets for advanced lifters to increase muscle hypertrophy (4). Acutely, significant increases in muscle protein synthesis have been observed after high-volume training in comparison with low volume sessions (14), thus reinforcing this recommendation.

Although acute responses are not linearly accompanied by long-term increases in lean muscle mass (52), numerous longitudinal studies have demonstrated that high-volume training facilitates muscle hypertrophy (20,59,60,63). Using ultrasound to measure elbow flexor and extensor muscle thickness, Radaelli et al. (60) showed that 5 sets of exercises per training session resulted in significantly greater upper arm development when compared to one and 3 set routines, across a 6-month training period. This increase in muscle mass was accompanied by significantly superior gains in 5 repetition maximum (RM) loads for the bench press and lat pull-down after the high-volume intervention (60).

Although findings of higher volumes leading to superior gains in muscle growth are not consistent throughout the literature (11,50), a recent meta-analysis identified a dose–response relationship between training volume and muscle hypertrophy (72). For example, high weekly volumes (>10 total sets per body part, per week) were associated with greater increases in muscle mass than lower volumes (<5 sets, per body part, per week), with an effect size of 0.241 (72). Schoenfeld et al. (72) concluded that higher volume training produces greater gains in muscle mass than lower volume training.

The mechanisms underlying the relationship between high training volume and increased muscle mass are potentially linked to the prolonged metabolic stress (34). A greater number of total sets per body part increases the total duration placed on the relevant energy systems and variety of muscle fibers during training. However, if prolonged metabolic stress is responsible for increases in muscle hypertrophy after high-volume routines, then coaches should carefully manage the types of exercises that comprise their program, particularly if high-load exercises are included. For example, utilization of drop sets has the potential to heighten the accumulation of metabolic by-products by placing greater demand on the fast glycolytic energy systems (33). If a high number of sets of this type of training were to be prescribed across sessions in order to increase volume, then an athlete may be taken beyond their recovery threshold, resulting in a blunting of the hypertrophy response (28).

It is important to emphasize that the relationship between volume and muscle growth is unlikely to be linear. That is; continual increases in training volume would inevitably lead to a plateau in the development of muscle mass. This concept is supported by the recent findings of Amirthalingham et al. (3). In their investigation, no significant difference in muscle hypertrophy was found between subjects who performed either 5 sets of 10 repetitions or 10 sets of 10 repetitions over a 6-week period (3). Coaches should appreciate each individual athlete's recovery capacity when prescribing training programs, with excessive volumes leading to extended overreaching or, perhaps overtraining syndrome (82). The net effect of this would be a reduction in the capacity for anabolic processes because of a heightened catabolic status and increased protein metabolism (46). In this sense, the relationship between volume and hypertrophy training is suggested to follow an inverted U curve (70). As each individual possesses their own capacity to recover from a given amount of work, strength and conditioning coaches should use appropriate testing and monitoring tools to identify thresholds which maximize increases in muscle mass while maintaining the athlete's health.

LOAD

The manipulation of load during resistance training, which is typically presented as the percentage of maximal load that can be used for any given movement, has been proposed as a vital factor in maximizing muscle hypertrophy (65). This is likely to relate directly to the mechanism of “mechanical tension,” as increased load results in an intensification of tension on the musculotendinous unit. However, loads exceeding 85% 1RM, while maximizing mechanical tension, fail to provide adequate stress to the fast glycolytic system because of reduced time under tension (61). Therefore, as a compromise in emphasizing both mechanical tension and metabolic stress simultaneously, moderate loads (70–85% 1RM) are traditionally recommended (4).

It has been suggested that high-load training (>65% 1RM) leads to superior gains in muscle mass because of the recruitment and fatigue of higher threshold motor units (43,56). This outcome might be desirable among athletes because hypertrophy of fast-twitch fibers is known to be much greater than slow-twitch muscle fibers (1,88) and fast-twitch fibers demonstrate higher velocity contractions (96). Fry (29) showed that programs incorporating loads above 50% 1RM led to greater fast-twitch fiber hypertrophy compared with slow-twitch fibers. Furthermore, during short-duration isometric contractions with low load (30–45% 1RM), glycogen depletion is nonexistent in type IIX fibers but rises significantly with increased resistance (90).

Although these findings suggest that type II fibers are stimulated to a greater degree when exposed to heavy loading, it has been suggested that low-load training also recruits fast-twitch muscle fibers, provided the working set is continued close to volitional fatigue (15). When high-load training has been directly compared to low-load training, Mitchell et al. (51) found no significant difference in fiber-type-specific hypertrophy. However, this study has been suggested to be underpowered (56), consequently lacking the sensitivity required to establish a difference in fiber-type hypertrophy between loading strategies. Because this investigation did establish a nonsignificant difference in type I hypertrophy between high-load and low-load training (17 versus 30%, respectively) (51), it may be that the lack of statistical power prevented the identification of differences. Mitchell et al. (51) used a sample size of 12 participants per group, which would achieve a poor statistical power of 0.17, based on our own post hoc analysis, assuming an alpha level of 0.05 and an effect size of 0.3 (d). It is important for coaches to note that this investigation demonstrated very little difference between high-load and low-load training for type II fiber hypertrophy (16 versus 18%, respectively) (51). Further evidence is required to establish whether load determines fiber-type hypertrophy. Of course, coaches shouldn't underestimate the importance of recruiting a wide range of motor units through the prescription of high and low loads. Indeed, in the development of muscle cross-sectional area, which is determined by the increase in myofibrillar proteins and thus muscle fiber diameter, a reliance upon the hypertrophy of type I fibers also exists. Exercises that activate a greater proportion of type I fibers would, therefore, be of equal use in maximizing hypertrophic adaptation.

When considering whole muscle hypertrophy, a meta-analysis by Schoenfeld et al. (75) showed high-load training (>65% 1RM) to be no more effective than low-load training (<60% 1RM) for increasing skeletal muscle mass. However, there was a nonsignificant trend in favor of high-load training (p = 0.076), which might be due to the low number of studies investigating the effects of training load on muscle hypertrophy. The use of low-load training strategies is supported by reports that training at 30% 1RM resulted in greater acute muscle protein synthesis relative to high-load training (90% 1RM), providing low-load training is continued to failure (15). Additionally, Mitchell et al. (51) showed that a low-load training (30% 1RM) regime resulted in similar increases in whole muscle cross-sectional area when compared to high-load training (80% 1RM) over a 10-week period. Furthermore, using a moderate loading scheme (8–12RM) or a variety of training loads (2–4RM, 8–12RM and 20–30RM) across a training week has been shown to result in similar increases in muscle mass after an 8-week intervention (71).

Therefore, when coaches are prescribing a training stimulus for muscle hypertrophy, high loads or low loads may be selected. Traditionally, loads >65% 1RM have been prescribed for hypertrophy programs, which might not be necessary. This information might also be useful for injury rehabilitation, where low-load training can be an effective method to increase muscle mass without the augmented forces associated with high-load training, leading to reduced joint loads.

Although low-load training might be equally as effective for muscle hypertrophy, coaches should not overlook the superior strength adaptations that high-load training provides in comparison with low-load training (10,54,64,67). These differences are explained by the principle of training specificity, whereby the all-out efforts required during high-load training produces maximal force, whereas low-load training requires only low-to-moderate forces in a fatigued state (18). As such, high-load training that results in the accumulation of considerable training volume may allow for increased muscle strength alongside substantial increases in muscle mass. Such adaptations have been defined as functional hypertrophy, where increases in muscle mass and maximal strength occur simultaneously (58).

TRAINING FREQUENCY

Training frequency is defined as the number of training sessions per unit of time. From the perspective of increasing an athlete's muscle mass, training frequency relates directly to training volume. During a single training session, the capacity to recover from the work performed is limited. Therefore, for high levels of training volume to be achieved, multiple sessions are likely to be required. As training volume is a key factor in muscle growth (72), optimizing training frequency will allow for volume to be maximized without excessive fatigue being incurred.

In establishing the optimal training frequency for muscle hypertrophy, Wernborn et al. (94) showed that 2–3 training sessions per week was optimal. This is supported by a recent meta-analysis, identifying that 2 weekly training sessions for the same muscle group led to significantly greater increases in muscle mass compared to one or 3 sessions per week (73). These findings are in contrast to the traditional practices of some bodybuilders, who are reported to train a single muscle group once per week (36). However, it should be noted that both Schoenfeld et al. (73) and Wernborn et al. (94) included both untrained and trained subjects in their analysis, potentially impacting the application for athletic populations.

For the majority of resistance-trained athletes, it is inevitable that the training volume per session and training frequency are inversely related, such that an increase in training frequency leads to a reduction in volume per session. In the case of a high-frequency training program, where a muscle group is trained on multiple occasions, the training volume for each session should be lower to prevent excessive weekly training volumes. High training frequency should be periodized strategically, so that adequate recovery is provided between sessions. Indeed, training the same muscle group before protein synthesis has returned to homeostasis may impair the muscle hypertrophy process (48); thus, 48–72 hours of rest between training sessions for the same muscle group may be required to optimize the training response (73).

Although this general recommendation is likely to be appropriate for many athletes, it has been recently suggested that higher training frequencies may be more beneficial for trained individuals in stimulating greater muscle hypertrophy (23). Because trained individuals adapt to resistance training in the long term by reducing the muscle protein synthesis response (22), distributing training volume across a higher frequency has the potential to increase the total time spent in a positive protein balance by an athlete (23). Such an approach would require a large reduction in training volume per session to avoid the accumulation of excessive fatigue. Dankel et al. (23) suggest strategies of high training frequency may be less than optimal for untrained individuals because the subsequent bouts of resistance training would likely interfere with the increased protein synthesis response for the preceding training session. Although this hypothesis is supported mechanistically, at present little evidence exists to confirm this theoretical model and future investigations are required.

TRAINING TO MOMENTARY MUSCULAR FAILURE

Training to momentary muscular failure results in the inability to produce the necessary force to lift a load through the concentric phase (65). When training to failure, it is hypothesized that maximal motor unit recruitment is achieved resulting in the fatigue of a greater number of muscle fibers (16,97), in turn leading to a greater hypertrophic response. Performing repeated muscle contractions of a fixed load (i.e., a set) to failure is associated with progressive increases in both the perception of effort (5) and muscle activation levels (obtained from surface electromyography (EMG)) (86). These findings infer an increase in the recruitment of high-threshold motor units (86). In support of this, Burd et al. (14) reported no differences in muscle adaptation in low-load or high-load training strategies, providing that each set was completed to failure. However, it is difficult to come to such conclusions where training volume is not equated.

Goto et al. (34) investigated the effects of training to failure on muscle hypertrophy in a 12-week study, where participants were volume-matched and assigned to either a group that trained to failure or a group that incorporated an intraset rest preventing failure from occurring. With volume equated, the “no-rest” group using repetition maximums achieved significantly greater muscle hypertrophy in the quadriceps, along with higher levels of maximal strength (34). This is similar to the findings of Schott (77), who also identified training to failure induced superior hypertrophy adaptations compared to finishing a set before failure.

Although these findings indicate the positive effects of training to failure, caution should be taken with routinely prescribing this approach. Sundstrup et al. (86) reported that complete concentric failure was not required to achieve full muscle activation using EMG analysis, with a plateau occurring during the final 3–5 repetitions with a 15RM load. This is an important consideration because routinely performing resistance training to failure may produce symptoms of overtraining and subsequent threats to the anabolic status of athletes (39). Because many of the studies evaluating the benefits of training to failure are of relatively short duration, the long-term implications are yet to be elucidated. Such findings have been identified in training programs for maximal strength (25). Therefore, it is recommended that practitioners strategically expose their athletes to training that induces failure to prevent the occurrence of overtraining.

EXERCISE VARIATION

Traditionally, bodybuilders tend to subscribe to the notion that broad exercise variation is required to maximize muscle hypertrophy (36). A proposed rationale is that muscles such as the pectoralis major (47) and trapezius (7) perform different movements of the same joint segment via the functional subdivisions of each muscle (6). As such, manipulating exercises has the potential to target large sections of the muscle. For example, in the case of the pectoralis major, using a 15° decline during the bench press results in greater EMG activity of the sternal fibers relative to the clavicular fibers (47). Therefore, to overload specific portions of different muscles, a wider variety of exercises is essential to recruit and fatigue all muscle subdivisions.

The above concept may be extended to muscles that possess numerous fibers orientated at a variety of angles between the origin and insertion. For example, both the long head and short head of the biceps brachii musculature are architecturally classified as fusiform (31); the biceps brachii is not functionally compartmentalized like the pectoralis major muscle. By manipulating shoulder and elbow positioning, the biceps brachii demonstrates a region-specific muscle activation strategy during supination (12). Furthermore, during elbow flexion, the biceps brachii does not uniformly shorten, suggesting that separate muscle fascicles contract concentrically at varying rates thus manipulating the range in the work produced for each muscle fiber (31).

Nonuniform muscle fiber recruitment has also been shown to occur in the hamstrings musculature, with EMG activity varying between the lower and upper fibers, depending on whether the hamstrings were required to flex the knee or extend the hip against resistance (68). This finding was supported by the work of Mendez-Villanueva et al. (53), who used functional magnetic resonance imaging to demonstrate regional differences in muscle activation of each head of the hamstrings during a variety of posterior chain exercises. Likewise, during resisted elbow extensions, multijoint and single-joint exercises have been shown to elicit regional differences in muscle activation. For example, single-joint elbow extension exercise has been shown to increase activation of the distal portion of the triceps brachii (91). Chronic adaptations to these exercises led to greater increases in cross-sectional area in the distal region of the muscle after a 12-week overloaded intervention program (91). Similarly, Wakahara et al. (92) showed that a multijoint elbow extension exercise (dumbbell bench press) increased muscle activation levels of the middle and proximal region of the triceps brachii, leading to greater growth in these areas. This suggests that to maximize hypertrophic adaptation, it is necessary to stress the muscle across its different portions (proximal-distal) using a variety of exercises.

Fonseca et al. (26) showed that changing exercises within a 12-week period was more effective for increasing muscle strength and hypertrophy compared to solely manipulations in training load. Within this investigation, hypertrophy of the vastus medialis and rectus femoris was more for subjects who varied exercises over 3-week cycles, compared to subjects who used the same exercise throughout (26). This evidence supports the concept of using numerous exercises to fully exploit adaptations of muscle hypertrophy.

One potential mechanism for the regional differences in hypertrophy may be the compartmentalization of skeletal muscle (6). Within the neuromuscular system, sections of the muscle are innervated by specific motor units that are responsible for orchestrating the contraction of their respective fibers (6). Indeed, even fusiform muscle fibers terminate intrafascicularly (31,95), meaning that there is potential for various neuromuscular compartments to exist within a given muscle. As fiber-type distribution inside muscles is also region specific (47,83), intramuscular differences are likely to exist relative to function. Therefore, resistance training targeting preferential hypertrophy of fast-twitch fibers with strategies such as eccentric-only training may also lead to nonuniform hypertrophy (discussed in the following section). It is likely that each muscle contains multiple neuromuscular compartments that can be selectively overloaded through varying exercise selection.

CONTRACTION TYPE

Eccentric muscle contractions increase the amount of mechanical stress on the musculotendinous units (7). Although EMG amplitude is lower during eccentric contractions, fast-twitch fibers are preferentially recruited over their slow-twitch counterparts, leading to greater tension per muscle fiber and a bias toward type II fiber damage (78). The greater muscle damage incurred promotes an adaptive response in the fast-twitch fibers, which possess greater potential for growth (1).

Traditional methods to determine loads during a program typically use the concentric strength of the athlete (i.e., percentage of 1RM). However, because eccentric strength can be as much as 45% greater than concentric strength (41), it is likely that eccentric training is rarely exploited to its full potential. Because submaximal eccentric training does not acutely raise muscle protein synthesis when compared to concentric training, the downstream effects on muscle hypertrophy are likely to be limited if traditional approaches to quantify load are used (21). However, when eccentric training is performed with maximal resistance, muscle protein synthesis is significantly greater than load-matched concentric training (55). When eccentric training is applied over a number of weeks, muscle hypertrophy adaptations are shown to be superior to that of concentric training (62). Therefore, supra-maximal eccentric training is likely to induce greater hypertrophic adaptations, assuming that the necessary recovery is provided. However, this is not consistent within the literature, with some studies identifying no difference between modes of contraction (28). This is potentially because of difficulties in matching volume-load between conditions, with eccentric training requiring higher loads.

In a recent meta-analysis, Schoenfeld et al. (76) identified a nonsignificant trend that eccentric-only training induced greater hypertrophic adaptations than concentric-only concentric training for inducing hypertrophic gains (P = 0.076). Mean effect sizes for muscle growth after eccentric-only and concentric-only training were 1.02 and 0.77 respectively, with an effect size difference of 0.27. The authors propose that because of many of the studies included for analysis matching the total repetitions performed and not total work, the higher amount of work completed was likely to be a major influence on these findings (76).

A further consideration with eccentric training is the identification of region-specific hypertrophy. Franchi et al. (27) showed that although muscle hypertrophy of the vastus lateralis was equal between concentric and eccentric training, midportion hypertrophy was higher in the concentric group, whereas the eccentric group experienced greater growth in the distal division. This may be due to the change in muscle architecture, secondary to the activation of altered molecular responses, after a concentric-only or eccentric-only training intervention (27). Eccentric-only resistance training leads to increases in fascicle length, whereas concentric-only training promotes greater pennation angles, indicating a higher number of sarcomeres in parallel (27). This has the potential to alter the force–velocity relationship of any given muscle, with an increased fascicle length (sarcomeres in series) resulting in superior shortening velocities (19). Conversely, a muscle with a larger pennation angles has the capacity to create higher levels of force because of an increased number of sarcomeres in parallel (19).

EXERCISE ORDER

It is generally recommended that multijoint exercises, relying on work being produced by large muscle groups, should be performed in the initial stages of a training session (2). As more repetitions can be completed with any given load earlier in the training session (81), greater long-term accumulation of training volumes will occur in response to exercises that are performed during these periods (80). Although this is dependent upon the design of the training session, it is, therefore, possible that using multijoint exercises at the start of a training session will result in greater hypertrophic adaptations in larger muscle groups.

Although this provides one reason to include multijoint exercises earlier in the training session, little evidence exists to support this hypothesis. This is mostly due to the limited amount of studies investigating the relationship between chronic structural adaptations and exercise order (80). Of the research that does exist, Simão et al. (79) and Spineti et al. (84) both showed that ordering training sessions such that single-joint elbow extension and flexion exercises were performed before the bench press and lat pull-down, resulted in increased triceps muscle volume when compared to the reverse order (effect size = 2.07 and 1.08 versus 0.75 and 0.40). It should be noted that no difference was seen for the biceps musculature between conditions in either investigation (79,84). However, neither study attempted to establish whether structural changes occurred in the pectoralis major and latissimus dorsi muscles, limiting the scope of their conclusions. It is likely that muscles trained and fatigued in the early stages of a training session will accumulate higher training volumes and therefore adapt to a greater extent. As such, practitioners should prioritize exercises for completion in the initial stages of the training session based on the individual needs of the athlete (80).

One issue with performing single-joint exercises before multijoint exercises is that the prefatigued muscle may alter the muscle activation patterns during the multijoint exercise. In both the lower extremity (9) and upper extremity (30), prefatiguing a muscle with a single-joint exercise has been shown to decrease the recruitment of the muscle during a multijoint exercise. This occurs alongside an increased recruitment of the synergistic muscles during the compound movement (30). However, activating a muscle with a single-joint exercise but not to the point of fatigue may increase its activation in the subsequent multijoint exercise (42). Therefore, coaches may order exercises to strategically manipulate the recruitment patterns of prime movers during exercises in order to alter muscle activation patterns.

REPETITION TEMPO

Explosive strength training demonstrates a clear advantage over slow concentric training for strength development (13). This is likely to be due to the higher forces that are required to increase acceleration during the concentric phase of an appropriately loaded lift. However, when attempting to develop muscle mass, this relationship is not evident (69). This potentially occurs because of the reduced lifting speed requiring less force, prolonging the duration of the set leading to increased metabolic stress. When loads are lifted with the intention of achieving a high velocity, the forces will be higher, increasing the tension of the muscle. In such instances where the repetition duration increases, loading must be reduced because of the time component placing increased demands on the involved energy systems (87). Therefore, manipulating repetition tempo is just another example of the inverse-relationship between volume and load.

The available evidence shows equivocal differences between short-repetition and long-repetition tempos for muscle hypertrophy development. Tanimoto and Ishii (87) determined that as long as subjects trained to failure, there was no significant difference in quadriceps hypertrophy after high-load normal tempo training (1-second concentric: 1-second eccentric: 1-second relaxation) when compared to a low-load, slow training group (3-seconds concentric: 3-seconds eccentric: 1-second relaxation). Furthermore, a recent meta-analysis showed no significant difference in muscle growth when comparing regimes consisting of 0.5–8 seconds for completion of the concentric phase of a lift (69). Thus, manipulating repetition tempo between blocks of training provides coaches with another strategy that may provide a novel form of overload through increasing training volume (via long-repetition duration) or load (via short-repetition durations).

INTERSET RECOVERY

Much like repetition duration, coaches may also change interset recovery periods to alter the balance of the training volume–load relationship. With short-duration recovery periods (<30-seconds), training volume may be increased as training density rises. However, if insufficient recovery is provided to fully replenish the anaerobic energy stores (34), the load must be reduced. Likewise, with higher interset recovery periods, greater loads may be used for each set at the expense of maintaining high training density, coupled with additional rest time (24).

Research investigating interset recovery periods suggests that short rest intervals (≤60-seconds) potentially compromise volume load because of drastic reductions in the load used, when compared to longer recoveries (3-minutes) (74). This is supported by Buresh et al. (17), who showed greater increases in quadriceps cross-sectional area when using longer (2.5 minutes), compared to shorter (60 seconds) interset recoveries. However, caution should be applied when interpreting these results, as in each of these studies volume was equated. Such control may remove the benefits of using short recoveries because training density cannot be increased. Whether shorter rest periods permit tolerable increases in training volume has not yet been studied. Therefore, further evidence is required to make clearer recommendations regarding the manipulation of interset rest periods for increasing muscle hypertrophy (35).

CONCLUSION

Certain aspects of traditional hypertrophy training have been recently challenged. A more detailed understanding of key programming variables is therefore required to maximize training effectiveness. Using the training principles outlined in this review, coaches can design and deliver evidence-based hypertrophy training that has the potential to increase athletic performance or expedite recovery from injury.

The current body of evidence suggests that there is no ideal load prescription to maximize muscle hypertrophy. In fact, from a loading perspective, there appears to be very few constraints, providing that the intensity of effort is high. However, an important variable that must be considered is training volume. High training volumes are necessary for maximizing muscle growth. This may be accomplished through a variety of approaches; one being increased training frequency. The current literature indicates that between 2 and 3 training sessions per muscle group per week are most effective, although there may be potential for superior gains in muscle hypertrophy with the prescription of higher frequencies (>3) in trained individuals.

Exercise variation is also important to access all “functional compartments” of individual muscles. This may be accomplished by including variations of basic exercises that place stress on specific muscle divisions. By incorporating a variety of exercises for a single muscle in the training program of an athlete, the hypertrophic response has the potential to be enhanced. Such adaptations may also be obtained with varying the type of contraction because concentric-only and eccentric-only loading strategies have been shown to provide hypertrophic adaptations in different sections of a muscle. Furthermore, it seems that eccentric-only training has the potential to increase muscle mass beyond that of concentric-only training through the increase in total work performed.

Finally, exercise order, repetition tempo, and interset recovery periods may all be manipulated at the program level to present the athlete with a novel stimulus. These variables should be considered in relation to the individual athlete goals and the desired outcome.

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Keywords:

muscle hypertrophy; resistance training

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