FORCE OUTPUT AND ENERGY-METABOLISM DURING NEUROMUSCULAR ELECTRICAL-STIMULATION - A P-31-NMR STUDY

The purpose of this study was to determine the acute physiologic effec ts of two electrical stimulation protocols commonly used for muscle re habilitation, Surface electrodes were used to provide 12 stimulations of the calf musculature, In protocol A the duty cycle was fixed at 1:1 (10-second stimulation: 10-second rest); far protocol B it was 1:5 (I O-second stimulation :50-second rest), We continuously recorded isomet ric plantarflexor force in six healthy male subjects during stimulatio n using a load cell connected to a foot pedal ergometer. Metabolic cha nges in the stimulated gastrocnemius muscle mere monitored in the supi ne position using P-31-NMR spectroscopy (Phillips 1.5 tesla NMR machin e), Relative changes in phosphocreatine (PCr), inorganic phosphate (Pi ), and intracellular pH (pH(i)) were obtained during stimulation and r ecovery, using a 1.5 cm RF surface antenna, Over the 12 stimulations, protocol A produced a significantly (p < 0.001), greater force decline (protocol A: 30.4 +/- 1.3%, protocol B: 13 +/- 0.8%); a significantly (p < 0.005), greater increase in Pi/PCr (protocol A: 210%, protocol B : 50%); and a significantly (p < 0.001), lower pH(i) (protocol A: 6.8 +/- 0.16, protocol B: 7.03 +/- 0.12). We conclude that the shorter dut y cycle produces more fatigue throughout the stimulation period, possi bly as a result of greater intracellular acidosis and reduced availabi lity of the high energy phosphate PCr. The clinical application of thi s finding relates to the selection of a stimulation protocol that maxi mizes strength gains in atrophic vs healthy muscle.