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Abstract

Electrical Muscle Stimulation (EMS) is commonly used in rehabilitation medicine to promote strength gains in skeletal muscle. However, despite its widespread use, the physiological effects of alterations in many EMS application variables has yet to be investigated. The stimulation duty cycle (work:rest ratio) is commonly manipulated by clinicians, without any physiological rationale for the use of a particular protocol. Understanding the mechanical and metabolic response of stimulated muscle to manipulation of this application variable is necessary in order optimize the efficacy of EMS as a training stimulus. To evaluate the metabolic and mechanical effects of two EMS protocols commonly utilized in rehabilitation, the gastrocnemius muscle of healthy male subjects (n=8) was stimulated via surface electrodes (Medtronic Respond II) using a 1:1 work/rest ratio (Protocol A-10 sec stim./10 sec off) or a 1:5 work/rest ratio (Protocol B-10 sec stim./50 sec off) for 12 repetitions. Each subject was placed supine on a specially fabricated foot pedal ergometer situated in the bore of a Phillips 1.5 tesla NMR unit. Muscular force production was measured during stimulation at 0.5 second intervals via a load cell connected to the foot pedal by a glass fibre rod, and interfaced with a microcomputer for continuous data acquisition. Relative changes in [PCr], [Pi], [ATP] and intracellular pH (pHi) were obtained during stimulation and recovery, by ³¹P NMR spectroscopy using a 1.5 cm RF surface antenna. The RF coil interrogated a 15cc hemispherical volume of tissue to a maximal depth of 1.5 cm. Results showed that protocol A produced a 30.4±1.3% decline in muscular force production while protocol B yielded a significantly smaller (13±0.8% - P