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1 Brain Research Institute, University of California-Los Angeles, Los Angeles, California, United States
2 Department of Health and Physical Education, Aichi University of Education, Kariya, Japan
3 Physiological Science, University of California-Los Angeles, Los Angeles, California, United States
4 Department of Physiology and Biophysics, University of California-Irvine, Irvine, California, United States
5 Brain Research Institute, University of California-Los Angeles, Los Angeles, California, United States; Physiological Science, University of California-Los Angeles, Los Angeles, California, United States
* To whom correspondence should be addressed. E-mail: rrr{at}ucla.edu.
The efficacy of high-load, short duration isometric contractions, delivered as one vs. two sessions/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 sec stimulus, delivered every 30 sec for 5 min with a 5 min rest period. This bout was repeated 6 times consecutively (SI-Stim1) or with a 9-hr 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. Mean fiber size was 56% smaller in SI-C than in Con, and 19 and 31% larger in SI-Stim1 and SI-Stim2 compared to SI-C. Maximum tetanic tension was 42, 60, and 73% of Con in SI-C, SI-Stim1 and SI-Stim2. Specific tension was 77% of Con in SI-C, and at Con levels in both SI-Stim groups. SI increased the percent IIb MHC 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 to one session/day is more effective in ameliorating inactivity-related adaptations.
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