Effect of tubocurarine on human soleus and gastrocnemius muscles.

The effect of a bolus injection of tubocurarine (0.1-0.13 mg X kg-1 i.v.) was followed in six young male subjects by registration of the rectified smoothed electromyogram (rsEMG) from the m. soleus (71 +/- 5.1 (s.e. mean) % slow twitch muscle fibers) and from the m. gastrocnemius (54 +/- 3.1% slow twitch muscle fibers). Volitional muscle strength was recorded in isometric plantar flexions with the knee fully (0 degrees) extended where m. soleus and m. gastrocnemius both are active, and with the knee bent 90 degrees where m. soleus is dominating force development. During maximal action of the drug, the rsEMG from the soleus muscle was reduced to 30 +/- 4.0% of the control value, while the rsEMG from the gastrocnemius muscle was reduced to 53 +/- 5.2% (P less than 0.01). Muscle strength with the knee extended showed 53 +/- 7.5% force left, while 44 +/- 6.4% of the muscle strength remained when the knee was bent (P less than 0.01). The results suggest that tubocurarine affects human muscles in proportion to their slow twitch muscle fiber content.