The Force-Velocity Relationship of the Soleus Muscles of Mice During After-Loaded Isotonic Contractions and After-Loaded Isotonic Releases

Abstract

Hypotheses were tested that a muscle stimulated at a frequency that elicited a maximal rate of force development (dP/dto) was fully activated during after-loaded isotonic contractions by the time the muscle developed the force necessary to displace the after-load and consequently the shortening velocity was maximum. whereas a muscle stimulated at a rate that elicited maximum tetanic force (Po) was not fully activated and under similar circumstances could not displace after-loads at maximum velocities. As controls for fully activated muscle, the same muscles were stimulated to develop Po and were then released to each after-load. Soleus muscles from 14 mice were stimulated directly in vitro at 25º C. Shortening velocities of each muscle were measured at 13 different after-loads using 4 different protocols: stimulation at a frequency that produced Po (approximately 90 Hz) with an after-loaded isotonic contraction: stimulation at 90 Hz to Po with a quick-release to each after-load; stimulation at a frequency that produced dP/dto (approximately 250 Hz) with an after-loaded isotonic contraction; or stimulation at 250 Hz to Po with a quick-release to each after-load. The maximum velocity of unloaded shortening (V max) was extrapolated from the force-velocity relationship. A significant difference was observed between the V max of isotonic contractions at 90 Hz (V max=4.17±0.l4 Lf/s) and that at 250 Hz (V max=4.82±O.l4 Lf/s). Based on a repeated measures analysis of variance of the velocities at 13 after-loads, I concluded that soleus muscles were not fully activated when stimulated at a frequency that produced Po unless stimulated to Po and subsequently quick-released to the after-loads. whereas muscles stimulated at a frequency that produced dP/dto were fully activated in all cases except during isotonic contractions at after-loads above 0.40. Furthermore, the extrapolation of measured velocities of a whole muscle to V max is not valid and the practice should be discontinued.