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Skeletal Muscle Hypertrophy
Len Kravitz, Ph.D.

Determining the optimal resistance training program for the development of skeletal muscle hypertrophy (size) is of great interest to many personal trainers working with clients who seek increases in muscularity. Morton et al. (2016) summarize current recommendations suggesting loads of 70% to 85% of a person's one-repetition maximum (1RM) are traditionally considered a criterion for maximizing skeletal muscle hypertrophy from resistance training. The authors continue that these loads have been conventionally considered best for recruiting more motor units (the nerve and muscle fibers innervated by the nerve). Two original research studies indicate some meaningful new insights into hypertrophy training and are reviewed in this column.

Study 1. Morton, R.W. et al. (2016). Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men. Journal of Applied Physiology, 121, 129-128.
The purpose of this research was to 1) was to determine the effects of a 12-wk higher-repetition (using lighter weights) vs. a lower-repetition (using heavier weights) weight training intervention on skeletal muscle hypertrophy and strength in resistance-trained men; and 2) to investigate whether the immediate post-workout increase in body hormones was associated with skeletal muscle mass or strength changes.
Participants: Forty-nine resistance-trained (2 yrs of total body weight training) males (ave age =23 yr) volunteered to participate in this study. Prior to the study, all participants were familiarized with the resistance training exercises and completed a 10-repetition maximum (10RM) and1RM on the incline leg press, bench press, knee extension, and shoulder press.

Training Program. The participants were randomly assigned to either a high repetition (HR) (n=24) or low repetition (LR) (n=25) group. The HR group performed 3 sets of 20-25 repetitions per set, with each set being performed to volitional fatigue (i.e., failure). This load was about 30% to 50% of their 1RM. The LR group performed 3 sets of 8-12 repetitions per set (to volitional failure), which corresponded to 75-90% of 1RM. Each participant completed four days/week (Monday, Tuesday, Thursday, Friday) of resistance training. The Monday/Thursday workouts included incline leg press followed immediately by a seated row, referred to as a superset, with a one-minute rest between each superset. Superset two was barbell bench press with cable hamstring curl, and the workout was completed with 3 sets of planks. On Tuesday/Friday, superset one consisted of machine-guided shoulder press with biceps curls, and superset two was triceps extension with wide-grip latissimus dorsi pull downs. The session was completed with 3 sets of machine-guided knee extensions. All training sessions were supervised by a personal trainer, who confirmed each set was performed to failure, and also made sure the correct repetition range was maintained during the 12-week study. Participants consumed 30 g of whey protein twice per day: immediately following RT on training days and 1-2 hours prior to sleep. On non-training days, participants consumed the first dose in the morning and the second dose 1-2 hours prior to sleep.
Results: Unpredictably, the HR and LR were equally effective in stimulating skeletal muscle hypertrophy (as measured with muscle biopsy techniques) in this resistance-trained group of males. As well, when participants were tested periodically for maximal strength, the increases in muscular strength were equally similar, with the exception of the bench press, which increased to a greater extent in the LR group. In addition, post-exercise levels of circulating hormones did not change as a result of the RT intervention and the researchers summarized they were unrelated to changes in muscle mass and strength.
Take-Away Message: The researchers concluded that training to volitional failure is the most important variable for skeletal muscle hypertrophy gains in males. They summarized that trained males exercising to volitional failure between 30-90% 1RM will sufficiently activate skeletal muscle motor units, which is the driving mechanism for skeletal muscle hypertrophy. The researchers propose that if a client's primary goal is to increase 1RM strength, then performing exercise with heavier loads should be periodically (or consistently) required for optimal improvement. Personal trainers always design resistance training programs reflective of a client's goals and preferences. The combined use of heavier and lighter loads or 'rep zones', as in periodized programs, will offer variety in the resistance training stimulus and result in equally effective gains in muscle size and strength, as long as all training sets are completed to failure.

Study 2. Herman-Montemayor, J.R., Hikida, R.S. & Staron, R.S. (2015). Early-phase satellite cell and myonuclear domain adaptations to slow-speed vs traditional resistance training programs. Journal of Strength and Conditioning Research, 29(11), 3105-3114.
Satellite cells (SCs) are cells found between the sarcolemma and basal lamina of muscle. Herman-Montemayor and colleagues summarize that several studies have shown that a resistance training stimulus in young and old males and females directly influences an increase in SC content. The authors continue that an increase in SCs influences the volume of muscle fluid (cytoplasm or sarcoplasm), referred to myonuclear domain (MND) and that SCs are also consequential in long-term changes in skeletal muscle hypertrophy (including the number of muscle fiber nuclei-muscle cells are multi-nucleated). This is the first study to ever investigate the effect of slow-vs normal-speed strength training on varying intensities on SC content, myonuclear (nucleus of muscle cell) number, fiber type and MND.
Participants: Thirty-four untrained (no exercise within the last 6 months) female (ave age = 21 yrs) volunteers were divided into slow speed, traditional strength, traditional muscular endurance, and non-training control groups. Prior to the training, participants completed a 2-week familiarization to learn the correct execution of the resistance training exercises.

Training Program: Exercise group volunteers performed three sets each of leg press, squat, and knee extension on 2 days of the first week and then on 3 non-consecutive days per week for the following 5 weeks. The slow speed group performed 6-10 repetition maximum (6-10RM) for each set with a 10-second concentric and 4-second eccentric contraction for each repetition. The traditional strength group performed all exercises with a 6-10RM, which was an intensity of 80%-85% of their 1RM. The traditional endurance group performed all exercises with a 20-30RM, which was 40%-60% of their 1RM. Both the traditional strength and traditional endurance groups performed 1-2 second concentric contractions with 1-2 second eccentric contractions. Pre-test and post-test muscle biopsies were analyzed for fiber cross-sectional area, fiber type, satellite cell content, myonuclear number, and MND.
Results: The traditional strength group had a significant increase in satellite cell content and MND for the slow twitch (oxidative, long lasting) and fast twitch (glycolytic, short burst) fiber types. The slow speed resistance training group had a significant increase in satellite cells and MND only in the fast twitch fibers. There was no change in satellite cells or MND in the traditional endurance and control groups. The myonuclear number did not change in any group. This is a major research finding (alas, at the molecular level of muscle) which shows that skeletal muscle fiber hypertrophy begins to occur before the addition of new muscle nuclei (see Figure 1).

Take-Away Message: With untrained females, this 6-week study shows that the traditional strength group, which performed all exercises with a 6-10RM, showed the most impressive early molecular changes influencing hypertrophy (e.g. satellite cell content and MND). The slow speed group, which performed 6-10 repetition maximum (6-10RM) for each set with a 10-second concentric and 4-second eccentric contraction had significantly better satellite cell number and MND improvements than the traditional endurance and control groups. A longer training study is needed to identify when molecular changes occur from traditional endurance resistance training. Personal trainers working with female resistance training enthusiasts wishing to gain muscular size and strength should optimally emphasize a loading strategy with a 6-10RM performed with a 1-2 second concentric and eccentric contraction. In addition, the integration of some slow speed concentric and eccentric contraction protocols in the 6-10RM training zones is also a favorable stimuli for molecular changes that accompany skeletal muscle hypertrophy and strength changes in females.

Final Take-Home. Perhaps the most compelling take-home message from the new research is that training to neuromuscular failure, with exercise intensities ranging from 30-90% 1RM, is an essential strategy for personal trainers training clients seeking improvements in muscle size and strength.

Figure 1. Progression of Muscle Cell Adaptations to Reisance Training
With hypertrophy, recent research shows that muscle fibers tend to increase the myonuclear domain (note fluid increase from 'A' to 'B' of the muscle cell) during the early adaptations of skeletal muscle hypertrophy. Long-term changes in hypertrophy show increases in nuclei (see 'C') in the cell and other contractile proteins adaptations.
Source: Herman-Montemayor, J.R., Hikida, R.S. & Staron, R.S. (2015).