Section 2: What Causes Muscle growth?
Weight training results in an alteration in the rate of protein
synthesis and degradation (34,35). Following resistance training,
levels of 3-methylhistidine (a marker of protein breakdown) increase
(34) and protein synthesis increases over the next 24-36 hours (36-38).
Although the exact stimulus for growth is not known, research supports
one or more of the following factors as critical to stimulate growth:
high tension, metabolic work, eccentric muscle actions and the hormonal
response to training (33,39-41). Additionally, sufficient nutrients and
protein must be available to support the synthesis of new muscle
proteins. Each factor is discussed in further detail.
1. Tension
For a fiber to adapt, it must be used during an activity (24). Recall
from chapter 17 that muscle fiber recruitment is primarily determined by
the load which must be lifted. The minimum tension considered to
stimulate growth and strength gains is roughly 60% of 1RM. Recall also
that muscle fibers continue to be recruited up to about 80-85% of 1RM at
which time further force production occurs through rate coding.
Therefore optimal Type II muscle fiber involvement will occur with loads
between 60-85% of 1RM (approximately 6-20 reps). However, simply
recruiting a fiber is not sufficient to make it adapt.
2. Metabolic work
Once a muscle fiber is recruited, it must do more work than normal for
adaptations to occur (40). Recent research has found that muscle growth
is greater with longer sets and that the metabolic changes (increased
blood metabolites such as lactic acid, phosphate, etc) seen with longer
set times may be part of the growth stimulus (42-44). It has also been
suggested that increased levels of lactic acid may play a role in the
growth stimulus possibly explaining why sets of 20-60 seconds (in the
anaerobic glycolysis range) seem to give better growth than shorter sets (45).
Tension plus metabolic work: the time under tension hypothesis
Factors 1 and 2 combined make up the time under tension (TUT)
hypothesis of growth (39,46). TUT simply says that fibers must develop
sufficient tension for a sufficient time period to adapt. While the
exact amount of time necessary to stimulate growth is not known and will
most likely vary from muscle fiber to muscle fiber, it has been
suggested that set times between 20-60 seconds (corresponding with
anaerobic glycolysis) be used for one or more sets (23,24,46). Even
within the context of high tension for sufficient time, growth is not
guaranteed. We also need to consider how the time under tension is
spent. Although muscles only contract, depending on the relationship
between the force generated, and the load which must be lifted, one of
three types of muscle actions can occur.
The first is referred to as a concentric muscle action, where the
muscle shortens while contracting, lifting the weight. The second is
isometric muscle action, where the muscle does not change length while
contracting, and the weight does not move. The third is eccentric
muscle action, where the muscle lengthens while contracting, and the
weight is lowered.
Performing 40 seconds of pure concentric work is not the same as
performing 40 seconds of isometric work is not the same as performing 40
seconds of pure eccentric work. The third part of the growth stimulus
is thought to be the eccentric muscle action, which has different
characteristics than concentric or isometric actions.
3. An eccentric muscle action
Numerous studies have have compared concentric only training to
eccentric only training. Most find that the eccentric training groups
experiences more growth even when the total number of repetitions (time
under tension) performed by both groups is identical (2,47-51) .
There are a number of physiological differences between the performance
of concentric and eccentric muscle actions, summarized below. In
general, force capacity during an eccentric muscle action is
approximately 30-40% greater than that during a concentric muscle action
(8,9). That is, if 100 pounds can be lifted by a muscle, 130 to 140
pounds can be lowered.
Additionally, Type II muscle fibers (which show the greatest amount of
growth) are preferentially recruited during eccentric actions (8,9). As
Type II fibers have a greater force production capacity than Type I,
this may partly explain the greater strength seen during eccentric training.
During eccentric muscle actions, less muscle fibers are recruited
(8,9). This means that the fibers recruited receive more overload per
fiber (54) which may explain the preferential growth seen. Finally,
eccentric but not concentric lifting stimulates protein synthesis (9).
If eccentric actions are the primary stimulus for growth, the question
arises of why perform concentric (lifting) muscle actions at all? First
and foremost, concentric actions are responsible for most of the
metabolic work during training contributing 84% of the total metabolic
work (49). Additionally, concentric strength limits eccentric strength
(55). That is, you can only lower as much weight as you can lift unless
you have partners lift the weight for you, so that it can be lowered.
This implies that periods of concentric only training (to improve
concentric strength capacity) may be useful so that more weight may be
used during the eccentric portion of the lift.
A final observation about eccentric training is that heavy eccentric
loading is associated with most of the muscle soreness from training
(56). Twenty-four to thirty-six hours after training, soreness occurs
and is called delayed onset muscle soreness (DOMS). DOMS is thought to
reflect direct mechanical damage (small tears) in the muscle fibers
(57). Following eccentric induced trauma, the muscle undergoes an
adaptation to prevent further damage and DOMS from the same overload (58).
It has also been suggested that tears to the cell membrane allow
calcium to flow into the cell, activating enzymes which break down
protein (59,60). Full recovery from this type of eccentric trauma is
completed with 4-7 days suggesting that the same muscle should not be
worked any more frequently than that, at least not with heavy eccentric contractions.
Another possible mechanism by which eccentric muscle actions may be
involved in muscle growth is through satellite cell proliferation (61).
Satellite cells are a type of cell located on the surface of muscle
fibers involved in muscle cell regrowth.
In response to both hormonal and mechanical stimuli (such as muscle
damage), satellite cells become active to help with tissue repair. In
animal models, satellite cell activity is involved in muscle hyperplasia
(generation of new muscle fibers). Although hyperplasia does not appear
to play a role in human growth (14), heavy eccentric muscle actions may
have the capacity to stimulate satellite cell proliferation by damaging
the cell and causing a local release of insulin-like growth factor 1
(IGF-1) (61-63).
All of the above information (tension, metabolic work, and eccentric
induced damage) has led to the development of the following schema,
which requires further validation, for muscle damage and growth. (59)
1. Depending on the force requirements, a given number of muscle fibers
will be recruited.
2. The recruited muscle fibers fatigue from performing metabolic work.
3. Upon reaching fatigue, individual fibers reach a point, termed
ischemic rigor, where they physically ‘lock up’ due to insufficient ATP.
This ‘locking up’ occurs during the concentric part of the movement
4. The subsequent eccentric muscle action causes small tears to occur in
the muscle, stimulating remodeling and growth.
The schema presented above fits well with the TUT hypothesis. To
stimulate the maximum number of fibers requires performing a high set
time with a high tension (within a range of 20-60 seconds). As each
fiber has a different fatigue time (based on it’s physiological
characteristics), each will require a relatively shorter or longer set
time to lock up and be damaged. As only the fibers which are fatigued
and damaged will adapt by the subsequent eccentric contraction, varying
set times may be necessary for optimal growth (24).
4. The hormonal response to weight training
Weight training affects levels of many hormones in the human body
depending on factors such as order of exercise, loads, number of sets,
number of repetitions, etc. The primary hormones which are affected by
weight training are growth hormone, testosterone, the catecholamines,
and cortisol.
The hormonal response to exercise is thought to be of secondary
importance to the factors listed above in terms of muscle growth. With
the exception of testosterone, the hormonal response to weight training
primarily affects fuel availability and utilization (64).
Growth hormone (GH)
GH is a peptide hormone released from the hypothalamus in response to
many different stimuli including sleep and breath-holding (65). At the
levels seen in humans, it’s main role is to mobilize fat and decrease
carbohydrate and protein utilization (66). The primary role of GH on
muscle growth is most likely indirect by increasing release of IGF-1
from the liver (66).
GH release during weight training appears to be related to lactic acid
levels and the highest GH response is seen with moderate weights (~75%
of 1RM), multiple long sets (3-4 sets of 10-12 repetitions, about 40-60
seconds per set) with short rest periods (60-90 seconds). Studies using
this type of protocol (generally 3X10 RM with a 1’ rest period) have
repeatedly shown increases in GH levels in men (67,68) and women (69,70)
and may be useful for fat loss due to the lipolytic (fat mobilizing)
actions of GH. Multiple sets of the same exercise are required for GH
release (70).
Testosterone
Testosterone is frequently described as the ‘male’ hormone although
women possess testosterone as well (at about 1/10th the level of men or
less) (1). Testosterone’s main role in muscle growth is by directly
stimulating protein synthesis (65,71). Increases in testosterone occur
in response to the use of basic exercises (squats, deadlifts, bench
presses), heavy weights (85% of 1RM and higher), multiple short sets (3
sets of 5 repetitions, about 20-30 seconds per set) and long rest
periods (3-5 minutes). Studies have found a regimen of 3X5RM with 3’
rest to increase testosterone significantly in men (67,68,72) but not in
women (69). It is unknown whether the transient increase in
testosterone following training has an impact on muscle growth.
Insulin-like growth factor 1 (IGF-1)
IGF-1 is a hormone released from the liver, most likely in response to
increases in GH levels (62). However, the small increases in GH seen
with training do not appear to affect IGF-1 levels (73). More likely,
IGF-1 is released from damaged muscle cells (due to eccentric muscle
actions) and acts locally to stimulate growth (42,63).
Cortisol
Cortisol is a catabolic hormone meaning that it breaks down larger
substances to smaller (i.e. triglycerides to fatty acids and glycerol,
and proteins to amino acids). It is released from the adrenal cortex in
response to stress such as exercise or starvation. Cortisol may have a
role in the tissue remodeling seen with heavy resistance training as it
increases protein breakdown at high levels (1). Increases in cortisol
tend to mirror the increases seen in growth hormone (74) and it has been
suggested that the increase in cortisol is a necessary part of the
muscle remodeling stimulus. (65,70) The basis for this is that the
breakdown of tissue is necessary to stimulate a rebuilding of that same tissue.
Catecholamines
The major role of catecholamines (adrenaline and noradrenaline) is fuel
utilization. As described previously, increases in levels of adrenaline
and noradrenaline increase liver output of glucose, mobilize fat from
adipose tissue, and stimulate glycogen breakdown in muscles. High
intensity weight training with multiple exercises increases
catecholamine levels similar to that seen in sprint training (65,75).
The overall effect of the rise in catecholamine levels is an increase in
blood glucose and stimulation of fat breakdown.
Adequate nutrients and energy
Once muscle growth is stimulated, the final requirement for growth to
actually occur is an excess of nutrients and energy (63). Reduced
calorie diets put the body in a systemically catabolic (tissue
breakdown) condition due to changes in hormone levels. Low-calorie
diets cause a decrease in growth promoting hormones such as insulin and
thyroid while increasing growth inhibiting hormones such as adrenaline,
glucagon, and cortisol (63). Similarly, overfeeding causes and
increase in those same hormones and an increase in lean body mass as
well as fat (76). It is generally impossible, except for beginners or
those returning from a layoff, for most individuals to gain muscle while
losing fat at the same time.
Simply put, the body must either be systemically catabolic (for fat
loss) or systemically anabolic (for muscle gain). Attempting to gain
significant amounts of muscle while losing fat at the same time or vice
versa tends to minimize the results of either goal. Most individuals
find that focusing on either fat loss or muscle growth yields the best
results. The CKD is somewhat unique among diets in that it couples a
catabolic phase (at below maintenance calories) with an anabolic phase
(at above maintenance calories), meaning that the potential to gain
muscle and lost fat simultaneously exists. This topic is discussed in
greater detail in chapter 12.
Progressive overload: the ultimate determinant of growth
Irrespective of the above factors, the ultimate key to larger and
stronger muscles is progressive overload. Individuals have achieved
growth using from 1 rep to 50 reps with a number of different protocols,
so it is impossible to say unequivocally that there is a ‘best’ program
for stimulating growth. As long as stress continues to be applied to
the body and muscles are forced to work against progressively greater
loads, assuming adequate recovery and nutrients are provided, growth
should occur in the long run. The above discussion is an attempt to
optimize the nature of the growth stimulus.
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