Skeletal muscle are thought to have both direct and

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skeletal muscle are thought to have both direct and indirect actions, and some of the effects are thought to be mediated through the actions of IGF-1 in an autocrine and paracrine fashion to promote hypertrophic adaptations [68]. There is a significant amount of research supporting this belief, as it has been shown that circulating IGF-1 levels are increased following the administration of GH [40, 71, 72]. Furthermore, there is wealth of research suggesting that the administration of exogenous GH in GH deficient populations have been shown to reduce body fat and more importantly increase muscle mass [73]. As a result of this observation, it has led to the belief that GH may play a significant anabolic role in skeletal muscle growth. Acute elevations in GH concentrations post-exercise are extremely sensitive to resistance training. Specifically, it has been demonstrated that an exercise-induced increase in GH levels have been highly correlated with the magnitude of skeletal muscle hypertrophy (both in type I and type II muscle fibers) [74]. The magnitude of the GH response to resistance training appears dependant on the intensity, volume, rest periods, exercise selection and sex [35]. Similar to testosterone, hypertrophy type schemes (moderate-intensity, high- volume, short rest) have shown to produce the greatest GH response compared to conventional strength type schemes (high-intensity, low-
volume, long rest) [20]. A summary of the studies that have directly compared the acute GH response to hypertrophy vs. strength type resistance exercise protocols are displayed in Figure 4. For example, Hakkinen and Pakarinen [64] compared the acute hormonal response to two different heavy resistance protocols in male athletes (1) 10 sets x 10 reps at 70% 1RM of the squat exercise (hypertrophy scheme) and (ii) 20 x 1RM at 100% 1RM (strength scheme). It was reported that the strength scheme produced a slight increase in GH (not significant), while a substantial increase in GH was observed in the hypertrophy scheme. It is suggested that the acute GH response to resistance exercise is highly influenced by the total work completed and its metabolic properties [20]. Thus, resistance exercise protocols that elicit higher blood lactate levels tend to produce the greatest augmented response in GH (protocols that typically incorporate hypertrophy schemes) [17, 18, 26, 64]. Furthermore, high correlations have been reported between blood lactate and GH concentrations [64], and it has been proposed that the accumulation of hydrogen ions produced by the build-up of lactate may be the most important factor influencing the release of GH [75]. Figure 4. Acute growth hormone responses from studies that have directly compared hypertrophy and strength resistance type exercise in men. * denotes significant difference (P <0.05) from corresponding resting or pre-exercise value.