Exercise training decreases the growth hormone (GH) response to acute constant load exercise

To assess the influence of exercise training on the growth hormone (GH) response to acute exercise, six untrained males completed a 20-min, high- intensity, constant-load exercise test prior to and after 3 and 6 wk of training (the absolute power output (PO) during each test remained constant, X̄ PO = 182.5 ± 29.5 W). Training increased (pre- vs post-training) oxygen uptake (V̇O₂) at lactate threshold (1.57 ± 0.33 L·min^-1 vs 1.97 ± 0.24 L·min^-1, P ≤ 0.05), V̇O₂ at 2.5 mM blood lactate concentration ([HLa]) (1.83 ± 0.38 L·min^-1 vs 2.33 ± 0.38 L·min^-1, P ≤ 0.05), and V̇O(2peak) (3.15 ± 0.54 L·min^-1 vs 3.41 ± 0.47 L·min^-1, P ≤ 0.05). Power output at the lactate threshold (PO-LT) increased with training from 103 ± 28 to 132 ± 23W (P ≤ 0.05). Integrated GH concentration (20 min exercise + 45 min recovery) (μg·L^-1 X min) after 3 wk (138 ± 106) and 6 wk (130 ± 145) were significantly lower (P ≤ 0.05) than pre-training (238 ± 145). Plasma epinephrine and norepinephrine responses to training were similar to the GH response (EPI - pre-training = 2447 ± 1110; week 3 = 1046 ± 144; week 6 = 955 ± 322 pmol·L^-1; P ≤ 0.05; NE - pre-training = 23.0 ± 5.2; week 3 = 13.4 ± 4.8; week 6 = 12.1 ± 6.8 nmol · L^-1; P ≤ 0.05). These data indicate that the GH and catecholamine response to a constant-load exercise stimulus are reduced within the first 3 wk of exercise training and support the hypothesis that a critical threshold of exercise intensity must be reached to stimulate GH release.