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Selectively reshaping a muscle phenotype: functional overload of cat plantaris
Roland R Roy1, Hui Zhong, Ryan J Monti
1Brain Research Institute, University of California, Los Angeles, Box 951761, 1320 Gonda Neuroscience and Genetics Building, Los Angeles, California 90095-1761, USA. rrr@ucla.edu
Introduction:
Functional overload (FO) of the fast plantaris muscle was studied in treadmill-exercised (FO-Ex) or sedentary (FO-Sed) adult cats.
Methods:
Mechanical, phenotype, and kinematics analyses were performed.
Results:
Plantigrade vs. normal digitigrade posture was observed early post-FO. Relative plantaris mass was greater in FO-Sed (10%) and FO-Ex (60%) cats than in controls 12 weeks post-FO. Specific tension was similar across groups, indicating functional hypertrophy. Fiber size was greater, percent slow fibers higher, percent IIa myosin heavy chain (MHC) higher, and IIx MHC lower in FO-Ex than controls. Twitch and half-relaxation times were longer, and the frequency-tension curve shifted toward that observed in slow muscles. Electromyography (EMG) and tendon force amplitudes during stepping were larger, and the yield (lengthening) phase occurred at a longer muscle length before compared with after FO.
Discussion:
Reshaping the plantaris phenotype was highly dependent on the overload stimulus, indicating that electrical stimulation paradigms used during rehabilitation should be performed with the muscles under "loaded" conditions.
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