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Skeletal Muscle Hypertrophy

Clients who want more muscularity need to face one of the most fundamental facts of skeletal muscle hypertrophy: Training to failure is crucial to building bigger muscles. Two original research studies reinforce this reality while offering meaningful new insights into hypertrophy training.

The authors of one of the studies noted that the current recommended standard for resistance training to maximize skeletal muscle hypertrophy suggests loads of 70%–85% of a person's one-repetition maximum (1-RM). Conventional wisdom is that these loads do the best job of recruiting motor units, each of which is a nerve plus the muscle fibers innervated by that nerve (Morton et al. 2016). Here's a closer look at these two studies:

High-Rep vs. Low-Rep Training

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 (1), 129–38.

Goals. This research had two objectives:

  1. Compare how resistance-trained young men responded to a training regimen with high reps and lighter weight vs. one with low reps and heavier weights.
  2. Investigate whether the immediate postworkout increase in body hormones was associated with changes in skeletal muscle mass or strength.

Volunteers. Forty-nine resistance-trained men (average age 23) with 2 years of body-weight training joined the 12-week study. Before the study, they were familiarized with the resistance training exercises and completed a 10-repetition maximum (10-RM) and a 1-RM on the incline leg press, bench press, knee extension and shoulder press.

Resistance training program. The men were randomly divided into high-rep and low-rep groups. The high-rep group performed 3 sets of 20–25 reps to failure; the load was about 30%–50% of 1-RM. The low-rep group performed 3 sets of 8–12 repetitions to failure, corresponding to 75%–90% of 1-RM.

Each volunteer lifted weights 4 days a week (Monday, Tuesday, Thursday and Friday). Each training session included two different supersets (with a 1-minute rest between them), plus an additional exercise. On Mondays and Thursdays, the first superset consisted of incline leg press followed immediately by seated row; the second consisted of barbell bench press followed by cable hamstring curl. The workout wrapped up with 3 sets of planks. On Tuesdays and Fridays, the first superset began with machine-guided shoulder press and ended with biceps curl, and the second superset began with triceps extension and ended with wide-grip latissimus dorsi pull-down. The session finished with 3 sets of machine-guided knee extensions.

A personal trainer supervised all training sessions, confirming that each set was performed to failure and within the correct repetition range throughout the study. The men consumed 30 grams of whey protein twice a day—immediately after weightlifting on training days and 1–2 hours before sleep. On nontraining days, they consumed the first dose in the morning and the second dose 1–2 hours prior to sleep.

Results. Unpredictably, the high-rep and low-rep resistance training regimens were equally effective in stimulating skeletal muscle hypertrophy (measured with muscle biopsies). Furthermore, when the men were tested periodically for maximal strength, the increases in muscular strength were equal, except with bench press, which produced bigger increases in the low-rep group. Postexercise levels of circulating hormones did not change as a result of the resistance training intervention, and the researchers said they were unrelated to changes in muscle mass and strength.

Take-away message. The researchers concluded that training to failure is the most important variable for skeletal muscle hypertrophy gains in trained men. They said that trained men exercising to failure with loads between 30% and 90% of 1-RM will sufficiently activate skeletal muscle motor units, the driving mechanism for skeletal muscle hypertrophy.

The researchers proposed that if a client's primary goal is to increase 1-RM strength, then exercising with heavier loads should be periodically (or consistently) required for optimal improvement. Personal trainers always design resistance training programs to reflect a client's goals and preferences. Combining heavier and lighter loads or "rep zones," as in periodized programs, creates variety in the resistance training stimulus and produces equally effective gains in muscle size and strength—as long as all training sets are completed to failure.

Understanding Satellite Cells

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), 3105–14.

Goals. Several studies have shown that resistance training in men and women triggers an increase in satellite cells, which lie between the sarcolemma and basal lamina of muscle. Additionally, research has shown that an increase in satellite cells influences the volume of muscle fluid (cytoplasm or sarcoplasm), also called the myonuclear domain. Satellite cells play a key role in long-term changes in skeletal muscle hypertrophy, including in the number of muscle fiber nuclei (muscle cells are multinucleated).

The goal of this study was to investigate—for the first time—how slow- and normal-speed strength training protocols in varying intensities influence satellite cell content, myonuclear (muscle cell nucleus) number, fiber type and myonuclear domain.

Volunteers. Thirty-four untrained (no exercise in 6 months) women (average age 21) were divided into slow-speed, traditional strength, traditional muscular endurance and nontraining control groups. Before the training, the women completed a 2-week course that taught them how to execute the resistance training exercises correctly.

Training program. Exercise group volunteers performed 3 sets each of leg press, squat and knee extension on 2 days of the first week and then on 3 nonconsecutive days per week for the following 5 weeks. The slow-speed group performed all exercises at 6-RM to 10-RM, with a 10-second concentric contraction and a 4-second eccentric contraction for each repetition; intensity was 40%–60% of 1-RM. The traditional strength group also performed all exercises at 6-RM to 10-RM, but with 1- to 2-second concentric and eccentric contractions; intensity was 80%–85% of 1-RM. The traditional endurance group performed all exercises at 20-RM to 30-RM, with 1- to 2-second concentric and eccentric contractions; intensity was 40%–60% of their 1-RM. Pretest and posttest muscle biopsies were analyzed for fiber cross-sectional area, fiber type, satellite cell content, myonuclear number and myonuclear domain.

Results. The traditional strength group showed significant increases in satellite cell content and myonuclear domain for slow-twitch (oxidative, long-lasting) and fast-twitch (glycolytic, short-burst) fiber types. The slow-speed resistance training group showed significant increases in satellite cell content and myonuclear domain only in fast-twitch fibers. There were no changes in satellite cell content or myonuclear domain 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. This 6-week study of untrained women demonstrated that the traditional strength group—which performed all exercises at 6-RM to 10-RM—showed the most impressive early molecular changes influencing hypertrophy (satellite cell content and myonuclear domain). The slow-speed group, which performed all exercises at 6-RM to 10-RM with a 10-second concentric contraction and a 4-second eccentric contraction, had significantly better satellite cell content and myonuclear domain improvements than the traditional endurance and control groups. A longer training study will be needed to identify when molecular changes occur from traditional endurance resistance training.

Personal trainers working with female resistance training enthusiasts wishing to gain muscle size and strength should optimally emphasize a loading strategy of 6-RM to 10-RM performed with 1- to 2-second concentric and eccentric contractions. In addition, integration of some slow-speed concentric- and eccentric-contraction protocols in training zones of 6-RM to 10-RM will provide another favorable stimulus for molecular changes that accompany skeletal muscle hypertrophy and strength changes in women.

Final take-home message. Perhaps the most compelling take-home message from the new research is that training to neuromuscular failure, with exercise intensities ranging from 30%–90% of 1-RM, is an essential strategy for personal trainers with clients seeking bigger muscles and more strength.

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