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 P-31 NMR in the gastrocnemius and soleus muscles were evaluated in order to compar e the anaerobic ATP costs of voluntary and electrically induced exerci se. Continuous isometric contraction at 40 % of maximum force and repe ated isometric contractions at approximately 75% of maximum force (con traction plus relaxation period of 0.5 s plus 2 s) were studied. 2. An aerobic ATP turnover in soleus and gastrocnemius muscles was slower du ring continuous voluntary contraction than during continuous electrica lly induced contraction (0.36 +/- 0.04 versus 0.63 +/- 0.05 mmol (kg w et wt)(-1) s(-1), P<0.05, in soleus; 0.19 +/- 0.03 versus 1.04 +/- 0.0 4 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 we re 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 m uscle). 4. During continuous voluntary contraction, in contrast to con tinuous 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 th e 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 i n the soleus muscle during brief voluntary and brief electrically indu ced contractions. 6. The results show that the anaerobic ATP costs wer e higher for electrically induced exercise than for voluntary exercise when continuous submaximal contraction was performed but not when bri ef high-intensity contractions were performed. The gastrocnemius muscl e contributes to total force production relatively less than tile sole us muscle during continuous voluntary plantar flexion at 40% of the ma ximum voluntary contraction.