Energy cost of isotonic tetanic contractions of varied force and duration in mammalian skeletal muscle

Summary

The enegy cost of muscular exercise was studied in the gastrocnemius muscle of dogs (20 to 25 kg body weight) anesthetized with morphine, chloralose and urethane. The muscle, loaded with 2 to 8 kg, was stimulated supramaximally to rhythmic isotonic contractions, the duration of which was varied from 0.2 to 1.2 sec. The energy expenditure was obtained from the O₂ uptake and the lactic acid output (which was of minor importance), calculated from venous outflow and arteriovenous differences.

With stimulations of 0.2 sec the O₂ uptake was higher with 8 kg than with 2 kg. But when stimulated for 1.2 sec the O₂ uptake was lower with 8 kg than with 2 kg, indicating that less ernegy was spent to maintain a tetanic contraction against 8 kg than against 2 kg. This finding was correlated with the behaviour of blood flow which was lower with 8 kg than with 2 kg, when the stimulation time was 1.2 sec.

Thus in certain limits the O₂ uptake seemed to increase with blood flow, although the performance remained about constant. This was further shown by decreasing mechanical efficiency with increasing flow, and by increasing O₂ consumption for the maintenance of a tetanic contration with increasing flow.