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 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.

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 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.

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.

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