What is hypertrophy?
Hypertrophy is an increase in the cross-sectional area of the muscle, mainly due to an increase in saromeres and filaments in parallel (Schoenfeld, 2010). There are many highly detailed molecular underpinnings to how this occurs, but in essence they are up-regulating protein synthesis and down-regulating protein catabolism (Sandri, 2008).
Why increase it?
An increase in muscle mass is associated with a host of positive benefits for performance, whether it is increasing the capacity for maximal strength (Ikai et al., 1968; Maughan et al., 1983) or improving performance in collision sports like Rugby and American Football. Particularly in old age, muscle mass is of particular importance in order to combat the onset of sarcopenia (Hameed et al., 2002) . Additionally, many average trainees simply want to add muscle to their frame for aesthetic purposes.
Mechanisms of hypertophy
There are three main mechanisms by which hypertrophy can occur from training: mechanical tension, metabolic stress, and muscle damage (Schoenfeld, 2010).
Mechanical tension is directly related to the mass lifted x duration. In other words, how heavy the weight you lift is and how many sets and reps are completed. It is likely a mistake to simplify this mechanism into the common fitness media term of ‘time under tension’. Though there is a dearth in research on this topic in trained individuals, there is a quite a lot on untrained that suggests that significantly slowing the tempo of a repetition does not result in more hypertrophy (Keeler et al., 2001; Neils et al., 2005; Tanimoto and Ishii, 2006; Tanimoto et al., 2008).
Metabolic stress is a product of a build up metabolites due to high intensity activity that requires anaerobic energy. This creates hypertrophy through many potential mechanisms, such as cell swelling and upregulation of growth-promoting hormones. Practical examples would be doing 20 rep squats.
The last mechanism is muscle damage, which can occur when (1) a new stimulus is presented to the muscle (e.g. a new exercise) or through eccentric muscle contraction (e.g. a slow negative on a bicep curl). The high forces created by eccentric muscle action creates significant tears in the muscle, which are repaired in the form of hypertrophy Farthing et al., 2003). However, eccentrics should be carefully planned in a training programme because they take longer to recover from than conventional training. (Schoenfeld, 2010)
Volume for the purposes of this discussion can be defined as the sets x reps. Perhaps the best correlated variable with muscular hypertrophy is training volume. Recently, Schoenfeld et al. (2017) found a dose-response relationship between increasing training volume per week and the degree of hypertrophy attained. These findings validated previous studies which had found higher volume programmes to be superior for hypertrophy (Schoenfeld, 2010)
Training for hypertrophy is different to training for strength, power or endurance, in that it seems to be much less intensity specific. Muscle hypertrophy has been shown to occur using loads as light as 30% and as heavy as 90% (Kumar et al., 2009; Mitchell et al., 2012, Morton et al., 2016; Ogasawara, 2013). However, these findings have not been established in well-trained subjects (Schoenfeld, 2013). Furthermore, a more recent analysis from Schoenfeld et al. (2016b) implies that for the majority of well-controlled studies, 50% is the minimum intensity at which hypertrophy occurs. The reason for this threshold is not exactly clear, however Schoenfeld (2013) speculates that there exists a minimum and maximum threshold outside of which either volume or load is not high enough to stimulate growth. This may be why the ‘hypertrophy range’ is often stated as a weight that is challenging for approximately 6-12 reps per set, as this allows high volumes to be amassed with relatively heavy loads.
Due to early research that found growth hormone levels were elevated in shorter rest periods, there arose a belief that shorter rest periods are always superior for hypertrophy. However the training literature does not reflect this view. For one thing, growth hormone is not responsible for hypertrophy (Henselmans & Schoenfeld, 2014). Research on trained individuals reveals have not shown a correlation between shorter rest periods and greater hypertrophy (Ahtianen et al., 2005; De Souza-Junior et al., 2010). Rest periods that are of too short duration may actual be harmful for hypertrophic gains, as they do not allow full recovery between sets, which will negatively impact the amount of work that can be done in the next set (Henselmans, and Schoenfeld, 2014). Additionally, researc from Buresh et al. (2009) found that the muscles of the upper body hypertrophied greater when longer rest periods were utilised.
In volume-matched training studies, greater training frequencies are generally shown to produce greater hypertrophy, with 2 days per week per muscle group being superior to one (Schoenfeld et al., 2016a). For example, Häkkinen and Kallinen (1994) found that female tennis players had increased hypertrophy gains when comparing 2 days vs. 1 day per week.
In order to optimise hypertrophy, training likely requires the athlete to use loads of ≥50%, high training volumes, and a minimum training frequency of 2 times per week per muscle group. Training should seek to exploit all 3 mechanisms of hypertrophy. This means that a well-structured hypertrophy programme should incrporate heavy weights at lower reps for mechanical tension, high reps with little rest to induce ischemia and metabolic stress, and slow eccentrics to induce muscle damage. Shorter rest intervals can be used for inducing metabolic stress, but most sets should probably allow sufficient rest in between so that strength recovers as much as possible.
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