Effects of rest intervals and training loads on metabolic stress and muscle hypertrophy.

We investigated the effects of volume-matched resistance training (RT) with different training loads and rest intervals on acute responses and long-term muscle and strength gains. Ten subjects trained with short rest (30 s) combined with low load (20 RM) (SL) and ten subjects performed the same protocol with long rest (3 min) and high load (8 RM) (LH). Cross-sectional area (CSA) of the upper arm was measured by magnetic resonance imaging before and after 8 weeks of training. Acute stress markers such as growth hormone (GH) and muscle thickness (MT) changes have been assessed pre and post a single RT session. Only the SL group demonstrated significant increases in GH (7704·20 ± 11833·49%, P<0·05) and MT (35·2 ± 16·9%, P<0·05) immediately after training. After 8 weeks, the arm CSA s in both groups significantly increased [SL: 9·93 ± 4·86% (P<0·001), LH: 4·73 ± 3·01% (P<0·05)]. No significant correlation between acute GH elevations and CSA increases could be observed. We conclude that short rest combined with low-load training might induce a high amount of metabolic stress ultimately leading to improved muscle hypertrophy while long rest with high-load training might lead to superior strength increases. Acute GH increases seem not to be directly correlated with muscle hypertrophy.