Many people start working out to increase muscle size
or become stronger and capable of lifting heavier weights. Strength
training with external resistance can increase strength and stimulate
muscle growth simultaneously. However, there is a distinct difference
between training for maximum force output (strength) and an increase in
muscle size. Resistance training alone does not induce muscle growth;
the training stimulus has to cause either metabolic or mechanical
fatigue to initiate the physiological mechanisms responsible for
increasing muscle mass.
Momentary muscle fatigue is a signal that an
appropriate amount of either metabolic or mechanical stress has been
applied. Metabolic and mechanical demand on muscle tissue can stimulate
the mechanisms responsible for muscle growth. Therefore, knowing how to
apply the stress in the right way is essential for achieving an optimal
response from a workout program. If you have a client stuck at a plateau
and you’re looking for a way to kick-start his or her workout program,
knowing how to create the proper stress stimulus on muscle tissue can
help you achieve the results your client wants.
Here are eight things to know about how to place the
appropriate demands on muscle tissue to achieve your client’s desired
results:
1. Muscle growth and improvements in strength require
activating high numbers of fast twitch (Type II) muscle fibers. Fast
twitch fibers are capable of producing significant levels of force and
are frequently involved in anaerobic energy production, which makes them
most susceptible to both mechanical and metabolic damage. Type IIb
fibers are completely anaerobic because they used stored adenosine
triphosphate (ATP) to provide the energy to produce a high amount of
force in a short period of time. Type IIa fibers produce energy from
glycogen in a process known as glycolysis, which can take place both
with (aerobically) and without (anaerobically) oxygen. Fast twitch
fibers have a greater diameter (cross-width) than slow twitch fibers and
are responsible for the hypertrophy, or increased fiber size, of a
particular muscle.
2. Mechanical stress refers to the physical forces
applied to muscle fibers. Resistance training causes microscopic damage
to muscle tissue, which in turn signals the biochemical reaction to
produce new satellite cells responsible for repairing the mechanical
structures and building new muscle proteins. This is how strength
training to the point of momentary muscle fatigue initiates the
physiological mechanisms responsible for muscle growth.
3. Metabolic stress is caused by a muscle producing
and using the energy required to fuel contractions. Moderate- to
high-intensity, high-volume strength training programs use the
glycolytic system for producing the energy required for the involved
muscles to work. Anaerobic energy is muscular energy produced without
the presence of oxygen. The result of sustained anaerobic glycolysis is
an accumulation of lactate, inorganic phosphates and hydrogen ions,
which elevate blood acidity leading to an effect known as acidosis.
There is a strong relationship between blood acidosis and the elevated
levels of the hormones that support muscle protein synthesis. When a
muscle works to the point of fatigue or “failure,” it has expended its
supply of available energy. This leads to metabolic stress on the
involved tissue.
4. Mechanical stress is an important and essential
stimulus for creating exercise-induced muscle growth. Metabolic stress
may also be responsible for signaling the body to initiate the
physiological mechanisms responsible for muscle growth. Similar to the
age-old quest of which came first, the chicken or the egg, we’re not
sure which plays a greater role in muscle growth—mechanical or metabolic
stress—because both occur simultaneously, making it difficult to
identify which is more important. However, using a weight heavy enough
to cause momentary fatigue after eight to 15 reps, combined with short
between-set rest intervals, will create both the mechanical and
metabolic stimulus that could lead to the desired adaptations.
5. When fast twitch muscle fibers create energy from
anaerobic glycolysis, it can lead to an important adaptation responsible
for influencing muscle size. As muscle cells continually use glycogen
for fuel, they will adapt by storing more glycogen during the recovery
phase. One gram of glycogen will hold up to 3 grams of water when stored
in muscle cells. Exercising to momentary fatigue not only elevates
mechanical damage to the muscle, it can also deplete stored muscle
glycogen. This results in an increase in muscle size once it is
replenished.
6. One of the long-term adaptations to strength
training is an increase in muscle fiber cross-width, which is also known
as hypertrophy. As the cross-sectional area increases in size, the
fibers have more surface tension and become capable of generating higher
amounts of force. Muscles with a larger cross-sectional area of
individual muscle fibers are capable of producing greater amounts of
force. Myofibrillar hypertrophy refers to the increase in size or
thickness of individual actin and myosin protein filaments, which can
improve the force-production capacity of the myofibrils. Myofibrillar
hypertrophy does not lead to larger muscles; rather, it results in
thicker muscle fibers capable of generating more force. Sarcoplasmic
hypertrophy is an increase in the volume of the fluid-containing
proteins responsible for tissue repair in the intercellular space that
surrounds an individual muscle fiber. Sarcoplasmic hypertrophy can cause
the cross-sectional area of muscle fibers to increase, but most of this
growth is due to the volume of the sarcoplasm and non-contractile
proteins not directly involved with force production. Despite a common
misperception that lifting weights can lead to an immediate increase in
muscle size, it can take eight weeks or longer for significant muscle
growth to occur, even in a well-designed program.
7. Compound, multijoint movements with free weights
such as barbells, dumbbells and kettlebells involve a number of
different muscles and can generate metabolic and metabolic stress during
training. This is especially true when performing eight to 15
repetitions, with momentary failure occurring at the last repetition.
Selectorized machines that focus on muscle-isolation or single-joint
movements allow mechanical stress to be placed directly into localized
tissue. Free weights involve a number of contributing muscles, which can
increase the metabolic demand, while the stabilization provided by
machines allows for heavier loads on specific muscles. Short- to
moderate-duration rest intervals of 30 to 120 seconds help create a
sizable metabolic demand. Completing three to four sets per exercise
ensures an effective amount of stress on the involved tissues. The tempo
of movement should be relatively short (i.e., one to two seconds)
during the concentric phase of muscle action, and should be longer
(i.e., two to six seconds) during the eccentric phase to ensure adequate
mechanical strain. Slower muscle-lengthening (eccentric) actions place a
tremendous amount of both mechanical and metabolic stress on muscle,
which makes it an effective approach when exercising for muscular
growth. Specifically, lengthening under load can create higher levels of
damage, which leads to a rapid rise in protein synthesis to repair the
damage.
8. Drop sets are another technique for creating
muscle growth. A drop set involves using a set weight for as many reps
as possible and then, when fatigue occurs, lowering the amount of weight
used and continuing. Drop sets keep a muscle under tension for an
extended period of time, which means they can induce significant levels
of metabolic and mechanical stress. However, drop sets can also create
extreme discomfort and should only be used by experienced weightlifters
at the end of a workout.
It is not completely clear whether muscle growth is the result of mechanical or metabolic
overload. What is known is that the mechanisms for
muscle growth are best achieved when an exercise is performed to the
point of momentary muscle fatigue. To stimulate muscle growth, a
strength-training program must be applied in a manner that places a
significant amount of mechanical stress on the muscle tissue, while also
creating a sizable metabolic demand.
Identifying whether a metabolic or mechanical
stimulus is more appropriate for a client interested in muscle growth
will take some trial and error. Some clients may be capable of
tolerating the discomfort that comes with training to muscle failure,
while others may prefer a heavier load for fewer reps to induce
mechanical stress. While both mechanical and metabolic stimuli result in
muscle growth, both can also lead to significant muscle soreness. If a
client wants to increase muscle size or get stronger, he or she must
understand what is involved and be willing to put forth a tremendous
effort, thus’ making the phrase “no pain, no gain” completely
appropriate.
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