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)
- Depending on the force requirements, a given number of muscle fibers are recruited.
- The recruited muscle fibers fatigue from performing metabolic work.
- 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 - 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 its 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).
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 (GH), 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).
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, its 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 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