Energy metabolism of the gastrocnemius and soleus muscles during isometric voluntary and electrically induced contractions in man.

1. Phosphocreatine (PCr) and intracellular pH detected by 31P NMR in the gastrocnemius and soleus muscles were evaluated in order to compare the anaerobic ATP costs of voluntary and electrically induced exercise. Continuous isometric contraction at 40% of maximum force and repeated isometric contractions at approximately 75% of maximum force (contraction plus relaxation period of 0.5 s plus 2 s) were studied. 2. Anaerobic ATP turnover in soleus and gastrocnemius muscles was slower during continuous voluntary contraction than during continuous electrically induced contraction (0.36 +/- 0.04 versus 0.63 +/- 0.05 mmol (kg wet wt)-1 s-1, P < 0.05, in soleus; 0.19 +/- 0.03 versus 1.04 +/- 0.04 mmol (kg wet wt)-1 s-1, P < 0.001, in gastrocnemius). 3. There was no significant difference in anaerobic ATP turnover between voluntary and electrically induced exercise when repeated brief contractions were performed (0.22 +/- 0.05 and 0.30 +/- 0.04 mmol (kg wet wt)-1 s-1, respectively, for the soleus muscle and 0.57 +/- 0.03 and 0.66 +/- 0.07 mmol (kg wet wt)-1 s-1, respectively, for the gastrocnemius muscle). 4. During continuous voluntary contraction, in contrast to continuous stimulated contraction, anaerobic ATP turnover was slower (P < 0.05) in the gastrocnemius than in the soleus muscle, which also showed a higher electromyogram amplitude (41.1 +/- 1.1% of maximum) than the medial gastrocnemius muscle (21.4 +/- 3.6% of maximum, P < 0.001). 5. Anaerobic ATP turnover was faster (P < 0.05) in the gastrocnemius than in the soleus muscle during brief voluntary and brief electrically induced contractions. 6. The results show that the anaerobic ATP costs were higher for electrically induced exercise than for voluntary exercise when continuous submaximal contraction was performed but not when brief high-intensity contractions were performed. The gastrocnemius muscle contributes to total force production relatively less than the soleus muscle during continuous voluntary plantar flexion at 40% of the maximum voluntary contraction.