Effects of contraction mode and stimulation frequency on electrical stimulation-induced skeletal muscle hypertrophy

We compared the skeletal muscle hypertrophy resulting from isometric (Iso) or eccentric (Ecc) electrical stimulation (ES) training with different stimulation frequencies. Male Wistar rats were assigned to the Iso and Ecc groups. These were divided into three further sub-groups that were stimulated at 10 Hz (Iso-10 and Ecc-10), 30 Hz (Iso-30 and Ecc-30), or 100 Hz (Iso-100 and Ecc-100). In experiment 1, the left plantarflexor muscles were stimulated every other day for 3 weeks. In experiment 2, mammalian target of rapamycin complex 1 (mTORC1) signaling was investigated 6 hours after one bout of ES. The contralateral right muscle served as control (non-ES). Ecc contractions comprised forced dorsiflexion combined with ES. The peak torque and torque-time integral during ES was higher in Ecc group than that in Iso group in all stimulation frequencies examined. The gastrocnemius muscle weight normalized to body weight (MW/BW) in ES side was increased compared to the non-ES side by 6%, 7%, and 17% in Ecc-30, Iso-100, and Ecc-100 groups, respectively, with a greater gain in Ecc-100 than Ecc-30 and Iso-100 groups. The p70S6K (Thr389) phosphorylation level was higher in Ecc-30 and -100 than Iso-30 and -100 group, respectively. The peak torque and torque-time integral were highly correlated with the magnitude of increase in muscle mass and the phosphorylation of p70S6K. These data suggest that ES-induced muscle hypertrophy and mTORC1 activity are determined by loading intensity and volume during muscle contraction independent of the contraction mode.

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