We aimed to investigate the role of loading intensity and volume in neuromuscular electrical stimulation (ES)-induced skeletal muscle hypertrophy in rats. Male Wistar rats (8－9 wk, n=35) were assigned randomly to the following groups. Experimental (EX) 1: 60%FAIL (n=6), 60% of maximum isometric torque (MIT) to task failure at supramaximal voltage (45 V); 30%VM (n=6), 30% of MIT with volume-matched to 60%FAIL; 15%VM (n=6), 15% of MIT with volume-matched to 60%FAIL. EX2: 30%FAIL (n=6), 30% of MIT to task failure at supramaximal voltage; 30%1/4V (n=6), 30% of MIT with 1/4 volume of 30%FAIL; 30%1/8V (n=5), 30% of MIT with 1/8 volume of 30%FAIL. For ES training, left plantar fl exor muscles were stimulated via a surface electrode (0.5 ms pulse, 50 Hz, 2 s on/4 s off ) every other day for 3 weeks. The contralateral right muscles served as control (non-ES). In EX1, the gastrocnemius muscle weight normalized to body weight (MW/BW) in ES side was increased above non-ES side by 10.9±1.3%, 6.6±1.1%, and 3.5±0.5% in 60%FAIL, 30%VM, and 15%VM groups, respectively, with a greater gain in 60%FAIL than 30%VM and 15%VM. In EX2, MW/BW was higher in ES side than in non-ES side both in 30%FAIL (6.1±1.3%) and 30%1/4V (6.8±1.2%) but not in 30%1/8V groups, with no difference between groups. Moreover, the extent of muscle hypertrophy was higher in 60%FAIL group than in 30%FAIL group, despite the loading volume of 60%FAIL group was about half of that in 30%FAIL group. There was no change in myofi brillar protein concentration between ES and non-ES side in all groups examined. These data suggest that the higher loading intensity, rather than the loading volume, is recommended to maximize muscle hypertrophy with ES training.
