Selectively reshaping a muscle phenotype: Functional overload of cat plantaris

Abstract

Functional overload (FO) of the fast plantaris muscle was studied in treadmill-exercised (FO-Ex) or sedentary (FO-Sed) adult cats. Mechanical, phenotype, and kinematics analyses were performed. 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. 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.