Whole-body and skeletal muscle responses to divergent modes of exercise training and detraining in middle-aged men

While short-term (~six weeks) HIIT induces rapid increases in skeletal muscle oxidative capacity, the anabolic potential of HIIT for promoting concurrent gains in skeletal muscle mass has received less scientific inquiry. The experiments undertaken for this thesis investigated skeletal muscle adaptive responses following HIIT, resistance exercise training (RET) and endurance exercise training (ENT), and after a subsequent period of detraining, in sedentary, middle-aged men. Thirty-five sedentary, males (39±3 y) performed six weeks of either ENT (n=12), HIIT (n=12) or RET (n=11) followed by 2.5 weeks of detraining. Skeletal muscle gene and protein expression, muscle fiber characteristics, body composition, muscle thickness, muscle strength, aerobic capacity, resting metabolic rate and glucose control were assessed at baseline, and after exercise training and detraining.

Lean mass increased after RET and HIIT (+3.2±1.6% and +1.6±2.1%, P<0.05). Muscle strength (sum of leg press, leg extension and bench press 1RMs) increased after all exercise training interventions (RET: +25±5%; HIIT: +10±5%; ENT: +7±7%, P<0.05). Aerobic capacity increased only after HIIT and ENT (+14±7% and +11±11%, P<0.05). Type I and II muscle fiber size increased for all groups after exercise training and remained elevated after detraining (main effect of time, P<0.05). Following detraining, the gains in lean mass and muscle strength were maintained in RET and HIIT groups, but maximal aerobic capacity declined below post-exercise training levels in HIIT and ENT (P<0.05). Androgen receptor, Akt and mTOR total protein increased after exercise training in all groups. Vitamin D receptor and apelin receptor mRNA increased after exercise training in all groups (main effect of time, P<0.05).

Six weeks of HIIT resulted in the most pronounced skeletal muscle adaptation prior to detraining in middle-aged men. While only HIIT and RET resulted in increases in lean mass and muscle thickness, gene and protein expression of markers implicated in muscle growth responses were largely similar across all exercise training modes. Short-term detraining did not negatively impact gains in muscle size, mass or strength irrespective of exercise modality.