Electromechanical stimulation ameliorates inactivity-induced adaptations in the medial gastrocnemius of adult rats.

The efficacy of high-load, short-duration isometric contractions, delivered as one vs. two sessions per day, on blunting inactivity-induced adaptations in the medial gastrocnemius (MG) were compared. Adult rats were assigned to a control (Con) or spinal cord-isolated (SI) group where one limb was stimulated (SI-Stim) while the other served as a SI control (SI-C). One bout of stimulation (BION microstimulator) consisted of a 100-Hz, 1-s stimulus, delivered every 30 s for 5 min with a 5-min rest period. This bout was repeated six times consecutively (SI-Stim1) or with a 9-h rest interval after the third bout (SI-Stim2) for 30 consecutive days. MG weights (relative to body weight) were 63, 72, and 79% of Con in SI-C, SI-Stim1, and SI-Stim2, respectively. Mean fiber size was 56% smaller in SI-C than in Con, and it was 19 and 31% larger in SI-Stim1 and SI-Stim2, respectively, compared with SI-C. Maximum tetanic tension was 42, 60, and 73% of Con in SI-C, SI-Stim1, and SI-Stim2, respectively. Specific tension was 77% of Con in SI-C, and at Con levels in both SI-Stim groups. SI increased the percent IIb myosin heavy chain composition (from 49 to 77%) and IIb+ fibers (from 63 to 79%): these adaptations were prevented by both Stim paradigms. These results demonstrate that 1) brief periods of high-load isometric contractions are effective in reducing inactivity-induced atrophy, functional deficits, and phenotypic adaptations in a fast hindlimb extensor, and 2) the same amount of stimulation distributed in two compared with one session per day is more effective in ameliorating inactivity-related adaptations.