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Electrical muscle stimulation protocols : effects on force production and energy metabolism of the gastrocnemius muscle in humans Dunlop, Robert James


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<.001) reduction in force following 12 stimulations. During protocol A [Pi]/[PCr] increased significantly from resting values (Protocol A=210%, P<0.05) during the first 6 stimulations however, this ratio actually decreased slightly in the last 6 stimulations of the protocol (189%). During protocol B only small changes in [Pi[/[PCr] (48%) occurred in the first 6 stimulations, with no changes occurring in the final 6 simulations. pHi was significantly (P<.001) lower in protocol A following 12 stimulations (Protocol A=6.8±0.16, Protocol B=7.03±0.12). The decline in pHi in protocol A was highly correlated with the decline in force production (R²=0.95). In protocol B pHi recovered slightly between stimulation 9 and 12. ATP concentrations dropped insignificantly during stimulation in both protocols (3±0.1%), with insignificant changes during rest and recovery. These results show that the application of EMS in protocol A produces profound changes in the high energy phosphates and intracellular pH, accompanied by a 30% decline in force production. In contrast, protocol B produced a brief period of pH and force decline followed by a steady state period characterized by insignificant changes in Pi/PCr, pH, and force production. Both protocols exhibited an alteration in the pattern of Pi/PCr and force output changes between 2 and 3 minutes of stimulation, possibly induced by changes in peripheral blood flow. Protocol A induced changes in force output, Pi/PCr and pH similar to those reported during heavy resisted voluntary exercise. For healthy muscle a 10 second stimulation/10 second rest protocol produces mechanical and metabolic changes that closely approximate maximal voluntary resisted exercise.

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