SIMULTANEOUS STIFFNESS AND FORCE MEASUREMENTS REVEAL SUBTLE INJURY TORABBIT SOLEUS MUSCLES

The time course of force generation and the time course of muscle stif fness were measured in rabbit soleus muscles during eccentric contract ion to understand the underlying basis for the force loss in these mus cles. Muscles were activated for 600 msec every 10 sec for 30 min. Sol eus muscles contracting isometrically maintained constant tension thro ughout the treatment period, while muscles subjected to eccentric cont raction rapidly dropped tension generation by 75% within the first few minutes and then an additional 10% by the end of 30 min. This indicat ed a dramatic loss in force-generating ability throughout the 30 min t reatment period. To estimate the relative number of cross-bridges atta ched during the isometric force generation phase immediately preceding each eccentric contraction, stiffness was measured during a small str etch of a magnitude equal to 1.5% of the fiber length. Initially, musc le stiffness exceeded 1300 g/mm and, as eccentric treatment progressed , stiffness decreased to about 900 g/mm. Thus, while muscle stiffness decreased by only 30% over the 30 min treatment period, isometric forc e decreased by 85%. In isometrically activated muscles, stiffness rema ined constant throughout the treatment period. These data indicate tha t, while soleus muscles decreased their force generating capability si gnificantly, there were a number of cross-bridges still attached that were not generating force. In summary, the loss of force generating ca pacity in the rabbit soleus muscle appears to be related to a fundamen tal change in myosin cross-bridge properties without the more dramatic morphological changes observed in other eccentric contraction models. These results are compared and contrasted with the observations made on muscles composed primarily of fast fibers.