Observation of high and low frequency muscle fatigue by means of 31P nuclear magnetic resonance.

Abstract

The changes in energy metabolites and corresponding force curves were studied during muscle fatigue with respect to excitation frequency (20 and 50Hz) for contracting muscle in situ and by means of 31P nuclear magnetic resonance (NMR) technique in vitro. The rat hind limb muscle group was stimulated via the tibial nerve by 0.5ms duration square pulses at different frequencies for a period of three minutes. The force signals recorded during repeated experiments were fed through an A/D converter and averaged by the use of a desk-top computer. 31P-NMR spectra were recorded from gastrochemius and soleus muscle of resting and after stimulation. Results indicated that 1) Iarge and significant decreases in high energy phosphate compounds (e.g., creatine phosphate and adenosine triphosphate (ATP)) were observed during prolonged electric stimulation at 50Hz in both the gastrocnemius and soleus muscles, 2) during 20Hz stimulation, muscle fiber type dependent changes in energy metabolites were observed: i. e., the gastrocnemius manifested a significant decline in creatine phosphate and ATP contents (p<0.01) while the soleus hardly showed any such changes, and 3) these metabolite changes occurring during different frequency stimulations were well reflected upon the rate of force decline in such that the calculated time constant for force fatigue during 50Hz stimulation was significantly shorter than obtained during 20Hz stimulation (p<0.01). The observed difference in the force fatigue time constants could be, at least in part, explained by the difference in the rate of ATP hydrolysis (e.g., the energy cost for contraction, Ca++ diffusion and uptake, ATP-linked sodium-potassium pump). It was suggested that force fatigue may ultimately be due to a failure of the rate of energy supply to meet demand, but the precise expression of this defect might vary, such that failure of membrane excitation and/or activation may predominate over failure of the energy supply.