O2 uptake and work by in situ muscle performing contractions with constant shortening.

The oxygen uptake by the gastrocnemius-plantaris muscle group of the dog was measured during brief submaximal isotonic-tetanic contractions. Shortening in the contractions was kept constant while the load was altered by adjusting the stimulation voltage applied to the motor nerve. Incompletely fused tetanic contractions were produced by 200 msec trains of impulses at a frequency of 20/sec. The trains were delivered once a second. The rate of oxygen uptake (VO2) was linearly related to the external work rate (W). The relationship between VO2, microliter/g min, vs W, gM/g min, is described by the equation VO2 = 18.0 + 3.36 W. The average maximal gross efficiency was 12.5%. The major determinant of VO2 during contractions appeared to be the number of muscle fibers which were activated during the contractions.