Introduction
It has been well established that resistance exercise evokes an acute anabolic response promoting skeletal muscle adaptation. Manipulation of program variables, including exercise intensity, volume, and rest intervals, alter the imposed mechanical and metabolic stresses which may influence muscle growth and strength development (2). Resistance exercise paradigms are often divided into protocols designed to promote an increase in either hypertrophy or strength. Hypertrophy-style protocols (HYPs) typically involve greater volume (3–6 sets; 8–12 repetitions), moderate intensities (<85% 1 repetition maximum [1RM]), and short rest intervals (30–90 seconds), whereas strength-style protocols (STRs) typically involve higher intensities (≥85% 1RM), low volumes (2–6 sets; ≤6 repetitions), and longer rest intervals (3–5 minutes) (1). Historically, the classification of HYPs and STRs resulted from empirical observation of the training styles of bodybuilders and powerlifters, respectively. Additionally, these protocol designs are typical of specific mesocycles (e.g., hypertrophy and strength, respectively) used by strength/power athletes in their periodized training program (24). However, the literature supporting such classifications is surprisingly sparse in trained individuals, and the distinct classifications of such protocols may be an oversimplification.
Given that greater muscle size, and therein more force-generating contractile proteins, is positively correlated with muscle strength (40), it is understandable that skeletal muscle mass is desired by many types of athletes to enhance athletic performance, increase body size, and improve esthetic appearance. Human skeletal muscle hypertrophy is a function of the balance between muscle protein synthesis and muscle protein breakdown, where a positive net protein balance (i.e., synthesis exceeds degradation) facilitates gains in muscle mass over time (42). Furthermore, skeletal muscle protein synthesis appears to be regulated by intramuscular signaling proteins specific to the multiprotein phosphorylation cascade, mammalian target of rapamycin complex 1 (mTORC1) (20). Resistance exercise stimulates an increase in muscle protein synthesis and is well established to promote remodeling of skeletal muscle (8,36,50). A complete discussion of the regulation of skeletal muscle hypertrophy is beyond the scope of this article and interested readers are referred to the following recent reviews (17,42).
Long-term studies comparing the effects of HYP and STR resistance exercise protocols on muscular adaptation in untrained men have yielded somewhat mixed results (7,9,39). However, the findings from studies conducted on previously untrained individuals cannot be generalized to a well-trained population. Trained individuals possess a lower adaptive ability to resistance training (21). Specifically, individuals with greater training experience appear to have an attenuation in intramuscular anabolic signaling and protein synthesis rates after resistance exercise (10,19,27,49). Additionally, initial strength gains in previously untrained individuals appear to be associated with neurological adaptations involving a more efficient activation pattern of skeletal muscle (45). Thus, the purpose of this review was to examine our current understanding of the acute anabolic responses and training-induced muscular adaptations after HYP and STR resistance exercise in trained individuals.
Comparisons of Hypertrophy-Style and Strength-Style Resistance Training Protocols
Muscle Activation
Resistance exercise is characterized by repeated bouts of loaded contractions that result in rapid recruitment of type II muscle fibers stimulating muscle protein synthesis (13). The specific skeletal muscle adaptations elicited by resistance training appear to be related to the patterns of motor unit activation occurring during exercise (28). Muscle activation is influenced by the number of motor units activated, the firing rate, and motor unit synchronization (16), which appears to be recruited in accordance with the size principle during voluntary muscle contraction (23). That is, as higher intensity loads are achieved, greater activation of high threshold type II motor units occurs. However, high threshold type II motor units also appear to be recruited during exercise at submaximal intensities (52,62), and several studies have suggested a progressive recruitment of type II fibers into the contraction process when using submaximal intensities to momentary muscular failure (4,6,44). The rise in muscle activation has been attributed to increased motor unit recruitment as a compensatory mechanism for sustaining contractile force as fatigue accumulates and glycogen is depleted (58). In trained men, muscle activation assessed via surface electromyography (EMG) has shown to be greater when performing resistance exercise to failure at moderate- (75–80% 1RM) vs. low-intensity (30% 1RM) resistance exercise (26,55). Hence, greater intensity loads appear to be a more effective means of increasing EMG amplitude than increasing the number of repetitions performed with lighter loads even when the end point is muscular failure (35). However, these results do not reflect those of typical HYPs and STRs given that both protocols use relatively high-intensity loads during training. When comparing lower body HYP and STR training protocols, similar EMG muscle activation patterns have been reported in trained men (18,46). EMG analysis of the vastus lateralis during a lower body HYP (70% 1RM; 4–6 × 10–12 repetitions; 1-minute rest intervals) and STR resistance exercise protocol (90% 1RM; 4–6 × 3–6 repetitions; 3-minute rest intervals) revealed no statistical difference in mean muscle activation in trained men during the barbell back squat and leg press exercises (18). Similarly, Nicholson et al. (46) reported similar decrements in EMG muscle activation patterns after lower body HYP and STR. Nevertheless, it is important to acknowledge that surface EMG assessment is not a direct measure of motor unit recruitment. Although motor unit recruitment influences EMG amplitude, the potential for muscular adaptation may not be inferred from surface EMG assessment (65). The research in a trained population is limited and further investigation into muscle activation during HYP and STR resistance exercise protocols is warranted.
Acute Anabolic Responses
The acute anabolic response to resistance exercise plays an important role in skeletal muscle adaptation, as long-term changes result from the cumulative effects from transient alterations in protein synthesis after acute bouts of exercise (11). Manipulation of program variables (e.g., exercise intensity, volume, and rest interval length) has been suggested to promote differing anabolic responses, yet the optimal parameters of a resistance training program for regulating muscle protein synthesis remain unclear (2). The acute systemic response after HYP and STR resistance exercise protocols has been extensively examined demonstrating several distinctions in circulating measures, whereas the anabolic response within the exercised musculature after each protocol warrants further investigation.
Regardless of training status, the stimulus of muscle contraction that occurs during differing intensities of resistance exercise influences the endocrine response (32). Compared with a typical STR, HYP routines have consistently shown to produce significantly greater elevations in anabolic hormones (12,22,30,34,41,59,64). Additionally, HYP seems to reduce plasma volume to a greater degree than STR, which augments the molar exposure of hormones at the tissue receptor level (18). It has been suggested that a greater endocrine response increases the likelihood of hormone-receptor interaction thereby enhancing the probability of an anabolic effect, leading to the supposition that HYP are superior for eliciting an anabolic response (3,30,32). However, recent research has challenged this hypothesis by demonstrating that physiological fluctuations in ostensibly anabolic hormones do not enhance muscle protein synthesis (66), intramuscular anabolic signaling (60,66), or resistance training–induced muscle hypertrophy (43). Although HYPs are associated with greater elevations of growth hormone (30,59), testosterone (12,41), and cortisol (12,41,59,64) when compared with STRs, systemic hormonal elevations do not appear to enhance the anabolic activity within skeletal muscle. Transient hormonal elevations appear to play a permissive, rather than stimulatory, role in the regulation of muscle protein synthesis (42). Testosterone, growth hormone, and insulin-like growth factor 1 have been suggested to be far more important for developmental growth rather than exercise-induced muscle growth (38). Although it is premature to draw definitive conclusions as to whether or not the postexercise anabolic hormonal response is associated with muscle hypertrophy, the majority of evidence to date seems to suggest that transient elevations in circulating hormones do not enhance anabolic activity within the muscle (17). Thus, the differing hormonal response after HYP and STR resistance exercise protocols may not have consequential implications on muscular adaptation, and implementation of a resistance training program based on a hormonal response may be futile.
The accumulation of metabolic byproducts (e.g., markers of muscle damage and metabolic stress) is also understood to occur in relation to changes in program variables. Resistance exercise induces significant microtrauma to muscle fibers promoting elevations in markers of muscle damage such as myoglobin and lactate dehydrogenase (47). Metabolic stress manifests as a result of the accumulation of metabolites (i.e., lactate, H+, Pi) from contractile-induced hypoxia. Both HYP and STR resistance exercise protocols augment markers of muscle damage, although STR appears to induce greater elevations (18). However, indirect markers of muscle damage have not shown to be a consistent indicator of exercise-mediated adaptation. In fact, muscle hypertrophy has even been observed in the relative absence of muscle damage (5,14). The association between exercise-induced elevations of markers of muscle damage in promoting anabolism and muscular adaptation remains to be fully elucidated (53). Markers of muscle damage appear to be a consequence of resistance exercise, rather than a primary stimulus of muscular adaptation. Alternatively, exercise-induced metabolic stress may promote anabolism, as lactate has shown to augment anabolic signaling proteins (48). Similarly, elevations in blood lactate have been demonstrated to be weakly associated (r = 0.38) with intramuscular anabolic signaling after resistance exercise in trained men (51). Acute muscle hypoxia associated with resistance training may serve to further augment metabolic buildup and, hence, stimulate hypertrophic adaptations (54). Several studies have observed significantly greater metabolic stress after HYP resistance exercise protocols compared with STR resistance exercise protocols (18,41,59). Oxygen delivery to the muscle appears to be compromised to a greater degree during an HYP as a result of arterial compression over a longer period of time (61). Additionally, energy provision is primarily derived from the phosphagen system during an STR yielding minimal metabolic buildup. In contrast, an HYP is associated with greater reliance on anaerobic glycolysis promoting metabolic buildup. However, the mechanisms by which metabolic stress influences the anabolic response to resistance exercise remain unclear.
Several research studies have observed distinctions in the systemic response after HYP and STR (12,22,34,41,59,64), whereas few studies have investigated intramuscular activity after different resistance exercise protocols. Hulmi et al. (25) reported greater intramuscular mTORC1 activation after an HYP (80% 1RM, 5 × 10 repetitions, 2-minute rest interval) compared with very low-volume STR (100% 1RM, 15 × 1 repetition, 3-minute rest interval) using the bilateral leg press. However, the implemented protocol does not depict a typical STR regimen, and the subjects did not have any experience in regular resistance training. We recently investigated intramuscular mTORC1 signaling in conjunction with circulating concentrations of hormones, markers of muscle damage, and lactate after a typical lower body HYP (70% 1RM; 4–6 × 10–12 repetitions; 1-minute rest intervals) and STR resistance exercise protocol (90% 1RM; 4–6 × 3–6 repetitions; 3-minute rest intervals) in trained men (18). As anticipated, the HYP stimulated a greater hormone and lactate response, whereas the STR workout elevated markers of muscle damage to a greater degree. However, the regulation of anabolic signaling proteins within mTORC1, including Akt, mTOR, p70S6k, and RPS6, were not significantly different after each protocol. Despite significant differences in the endocrine response and byproduct accumulation, both protocols appeared to elicit similar intramuscular anabolic signaling in resistance-trained men. It is plausible that the relationship between intensity load and muscle protein synthesis may reach a plateau between intensities of ∼60 to 90% of 1RM (33).
Muscular Adaptation
The optimal training paradigm for maximizing the adaptive response to resistance exercise remains to be clearly defined. Although a recent meta-analysis noted a strong trend for superiority of heavy loading (≥65% 1RM) with respect to hypertrophy and strength outcomes compared with lower loads (≤60% 1RM) (57), both HYP and STR routines typically use an intensity load above 65% 1RM. It has been suggested that the 6 to 12RM loading range may provide the best combination of intensity load and volume for hypertrophy, whereas high-intensity, low-repetition resistance exercise has been suggested to be most appropriate for increasing strength (31).
Limited research has compared the long-term training adaptations after traditional HYP and STR resistance exercise protocols in trained individuals. Schoenfeld et al. (56) investigated muscular adaptations after an 8-week, volume-equated, HYP training program (split routine; 3 × 10RM; 1.5-minute rest intervals) vs. an STR training program (total body routine; 7 × 3RM; 3-minute rest intervals) in resistance-trained men. Interestingly, no significant difference between protocols was noted for muscle thickness of the biceps brachii assessed via ultrasonography. However, the STR group produced significantly greater 1RM bench press strength and displayed a tendency to produced significantly greater 1RM back squat strength. Both HYP and STR training appeared to promote similar increases in muscular size, yet STR was superior for enhancing maximal strength. In this study, total volume load (i.e., repetitions × sets × load) was purposefully equated between groups to ensure that the results were not confounded by the amount of work performed. However, workout volume is characteristically different between HYP and STR program designs when implemented by athletes. Furthermore, the single ultrasonographic assessment of the biceps brachii limits the understanding of hypertrophic adaptations to a full-body resistance exercise routine.
Recently, Mangine et al. (37) compared an HYP training program (70% 1RM; 4 × 10–12 repetitions; 1-minute rest intervals) and an STR training program (90% 1RM; 4 × 3–5 repetitions; 3-minute rest intervals) in resistance-trained men without equating for volume (37). Given that an additional concern when examining divergent resistance exercise protocols in trained individuals is the novelty of the stimulus, Mangine et al. (37) employed a 2-week preparatory training program immediately preceding the training intervention to ensure that participants began the training program with a comparable training base. Pre- and posttraining assessments included lean tissue mass via dual energy X-ray absorptiometry; muscle cross-sectional area and thickness of the vastus lateralis, rectus femoris, pectoralis major, and triceps brachii muscles via ultrasonography; and 1RM strength in the back squat and bench press exercises. After 8 weeks of training, the STR routine stimulated significantly greater gains in lean arm mass and 1RM bench press strength compared with the HYP routine. However, no other differences between protocols were noted for measures of lean mass, muscle size, or strength. The greater gains in some measures of muscle size and maximal strength observed after the STR routine indicate that using a greater intensity load might provide a superior stimulus for muscle hypertrophy and strength in trained men. Thus, these results provide further evidence that strength development among well-trained individuals may only be realized with greater intensity loads (21).
Limitations and Future Research
Muscular adaptation after regimented resistance training is highly variable between individuals (43,63) and is purportedly influenced by nutritional support, muscle fiber type distribution, and genetic predisposition (2,29). Furthermore, analysis of competitive lifters have suggested that those who tend to adhere to STR programs (i.e., powerlifters) exhibit a preferential hypertrophy of type II muscle fibers, whereas those who adhere to HYP programs (i.e., bodybuilders) may experience greater hypertrophy of type I muscle fibers (15). An additional limitation when researching divergent program variables is that muscle adaptations may be enhanced when unaccustomed program variables are used. Therefore, researchers should be mindful of subjects' training history, including training style, when implementing a seemingly familiar training stimulus. For example, if participants were following a similar type of program to the one used in the study, then fewer improvements may be expected. However, if participants were following a very different type of program compared with the one used in the study, then the change in stimulus may drive improvements and hyperinflate the efficacy of the program. Thus, it is imperative for future research studies to accurately depict prior training of the participants. Moreover, training studies that evaluate muscular adaptations after training interventions in trained individuals require longer training durations to observe meaningful differences and are, therefore, scant. Research studies have not investigated the time period necessary to observe the development of muscle mass that is optimal for elite strength/power athletes, bodybuilders, or powerlifters (i.e., years). Nevertheless, maximizing hypertrophy over the long term may be more complex than what can be extrapolated from current research, and may involve training across the intensity-volume continuum (1).
Practical Applications
The current evidence suggests that the classification of HYP and STR is an oversimplification, and practitioners are advised to look beyond the classification of resistance exercise protocols when aiming to elicit specific physiological responses. Despite the classification of training paradigms, HYP and STR resistance training routines appear to elicit similar magnitudes of muscle growth, whereas STR routines appear to be more conducive to increasing strength in resistance-trained individuals. As hypertrophy and strength are typically trained during separate stages of a periodized training program and may be of greater or lesser importance to an athlete with respect to his or her sport, an understanding of the acute responses and muscular adaptations associated with each resistance exercise protocol may allow athletes and coaches to make appropriate recommendations based on the goals of the athlete.
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