Big Fitness Project: Hypertrophy Mechanisms [1/3] - Mechanical tension, Training Strategies and Techniques

Three factors are responsible for initiating the hypertrophic response to resistance exercise: mechanical tension, muscle damage, and metabolic stress (1-5).

What makes muscle grow is known since 1975 (6). It is suggested that increased tension development (either passive or active) is the critical event in initiating compensatory growth (6).

Mechanical Tension

Mechanical tension seems to be the primary drive for the hypertrophic response (6), mechanical tension alone can produce muscle hypertrophy (7). Increased force development is the critical event in initiating compensatory muscular growth (6,8,9,10).

Mechanical forces are converted into chemical signals in a process called mechanotransduction. This causes molecular and cellular responses in myofibers and satellite cells (11), and mechanical stress alone can directly stimulate mTOR (initiation of protein synthesis) (12,13). Mechanical stress plays a critical role in muscle hypertrophy processes. There’s higher and significant increase in muscle activity during eccentric actions with flywheel training (ECC overload) (14).

In one study the increased in muscle activity was associated to an 11% increase in strength and 6% in muscle mass (15), suggesting that mechanical stress plays a critical role in muscle hypertrophy processes.

Strength and adaptations

Mechanical loading is a critical stimulus to increase strength and size of skeletal muscle (16).

Strength gains are specific to the movement that is trained (37). Moreover, strength gains are due to a combination of muscle hypertrophy and neural adaptations (17,38). In turn, neural adaptations are largely specific to the movement and load used in training (38).

However, changes in muscle size are smaller and slower than changes in strength (18). Interestingly, certain resistance training routines employing high degrees of muscle tension have been shown to largely induce neural adaptations without resultant hypertrophy (19,20).

Strength is increased to a greater extent with high intensity training (21,22,39), even when whole muscle hypertrophy is comparable (23,39).

Training strategies

1. Intensity

Intensity (i.e., load) have a significant impact on hypertrophy. This is usually referred to as a percentage of 1RM in relation to the number of repetitions that can be performed with that percentage.

Repetitions can be classified into 3 basic ranges: low (1–5), moderate (6–12), and high (15+). Different energy systems are used in each range. For example, the phosphocreatine system is used for low repetition sets, and anaerobic glycolysis is used in moderate repetition sets (24). Both low reps and moderate reps elicit a hypertrophic response. However a moderate range (6–12 reps) optimizes the hypertrophic response (1,25,26,27).

Evidence suggests that there is a maximum threshold for tension-induced hypertrophy, above which metabolic factors become more important than additional increases in load (1).

2. Rest Interval

It follows that mechanical tension is maximized by long rest periods, however at the expense of metabolic stress (1,28,29), which attenuate the maximal hypertrophic response.

As with repetitions ranges, rest intervals can be classified into 3 categories: short (30 seconds or less), moderate (60–90 seconds), and long (3 minutes or more).

Short rests does not allow for sufficient time to regain muscular strength, and impairs muscular performance in subsequent sets (30,31). Conversely, long rest intervals afford full recovery of strength between sets, promoting full force capacity for subsequent sets (32).

3. Time under tension

Time under tension is another strategy for mechanical stress. However to extent the time under tension within a set the intensity of load has do drop. Increasing and maintaining continuous tension throughout a set may enhance the potential for microtrauma and fatigueability. Slow-twitch fibers benefit by increased time under. Time under tension has been shown to stimulate optimal growth (33).

4. Repetition Speed

Evidence suggests that faster repetitions are more beneficial for hypertrophy. Performing concentric actions at 1-second cadence vs. three seconds has greater impact on both muscle thickness in elderly men (34). Training at very slow velocities (i.e., superslow training) has been shown to be suboptimal for the development of strength and hypertrophy (1,35,36).

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