OPTIMAL SHORTENING VELOCITIES FOR IN-SITU POWER PRODUCTION OF RAT SOLEUS AND PLANTARIS MUSCLES

Force-velocity (FV) relationships have been used previously to calcula te maximal power production and to identify an optimal velocity of sho rtening (Vopt-iv) to produce such power in skeletal muscle. The cyclic al nature of muscle position during locomotion for muscles such as the soleus and plantaris is such that either constant force or velocity i s rarely attained. In the present study, the work loop technique, a te chnique developed to measure maximal attainable power output from musc les undergoing cyclic length changes, was undertaken to determine whet her simulating in vivo function alters the power-velocity relationship of the soleus and plantaris and, in particular, the velocity of short ening that produces maximal power (Vopt-w1). FV relationships were det ermined for both soleus (n = 4) and plantaris (n = 4) muscles in situ from adult female Sprague-Dawley rats by measuring shortening velociti es during afterloaded isotonic contractions. The velocity that produce d maximal power using FV relationships, Vopt-fv, was 54.6 +/- 0.7 mm/s for the plantaris vs. 20.2 +/- 1.2 mm/s for the soleus. Then, the wor k loop technique was employed to measure net power from these same mus cles at multiple cycling frequencies (1.5 to 4.0 Hz for the soleus; 4. 0 to 8.0 Hz for the plantaris). Multiple power-velocity curves were ge nerated (one at each cycle frequency) by varying the strain (1-8 mm). Thus, at each cycle frequency, Vopt-w1 could be identified. For both t he plantaris and soleus, Vopt-w1 at each cycle frequency was not diffe rent from their respective Vopt-fv value. Thus both fast and slow skel etal muscles have inherent optimal shortening velocities, identifiable with FV relationships, that dictate their respective maximal attainab le mechanical power production using the work loop technique.