DETERMINANTS OF WORK PRODUCED BY SKELETAL-MUSCLE - POTENTIAL LIMITATIONS OF ACTIVATION AND RELAXATION

The objective of this study was to estimate the limitations imposed by the kinetics of activation and relaxation on the ability of slow skel etal muscle to produce mechanical work. These estimates were made by t he following methods: 1) using the work loop technique and measuring t he actual mechanical work (W-A) produced by rat soleus muscles (n = 6) at four different frequencies (0.5, 1, 2, and 4 Hz) and seven differe nt amplitudes of length change (1, 2, 3, 4, 5, 6, and 7 mm); 2) determ ining the force-velocity relationships of the soleus muscles and using this data to quantify the theoretical mechanical work (W-T) that coul d be produced under the work loop conditions described above; and 3) s ubtracting W-A from W-T The difference between W-T and W-A was interpr eted to represent limitations imposed by activation and relaxation. Un der certain conditions (high frequency, small strain), factors control ling the kinetics of activation and relaxation reduced the mechanical work of the soleus muscle by similar to 60%. Hence, activation and rel axation collectively represent important factors limiting the producti on of mechanical work by slow skeletal muscle.