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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