Contraction-induced injury: Interaction of initial length and work input.

Abstract

For single pliometric (lengthening) contractions of whole skeletal muscles initiated solely from optimal fiber length (L$\rm\sb{f}),$ the most important factor determining the subsequent deficit in isometric force production is the work input during the stretch. The purpose of this dissertation was to clarify the association of the force deficit following pliometric contractions with specific mechanical events, using a range of initial fiber lengths to provide a more comprehensive simulation of in vivo conditions. The hypothesis was that for whole muscles when fully activated, regardless of the initial length, the force deficit following the stretch is primarily a function of the work input. Extensor digitorum longus muscles of mice were maximally activated in situ and lengthened at 2 L$\rm\sb{f}$/sec from one of three initial fiber lengths (90%, 100%, or 120% of L$\rm\sb{f})$ to one of three final fiber lengths (150%, 160%, or 170% of L$\rm\sb{f}).$ No single mechanical factor, including the work input (r$\sp2$ = 0.34), was sufficient to explain the differences in force deficits observed among groups. Therefore, the force deficit arises from a complex interaction of mechanical events. With the data grouped by initial fiber length, the correlation between the average work and the average force deficit was high (r$\sp2$ = 0.97 to 0.99). Consequently, differences in force deficits among groups were explained best on the basis of the initial fiber length and the work input during the stretch. The specific hypotheses were subsequently tested that for single permeabilized fiber segments: (i) as with whole muscles, an increased force deficit occurs following pliometric contractions initiated from longer compared with shorter fiber lengths, even when the work input is the same, and (ii) the increased force deficit at longer compared with shorter lengths is associated with an increase in the sarcomere length heterogeneity. Single permeabilized fiber segments from soleus muscles of rats were maximally activated in vitro and lengthened at 0.5 L$\rm\sb{f}$/sec through one of seven protocols of varying initial and final fiber lengths. The force deficits observed following single stretches of the same relative displacements increased with increasing initial length, even though the work input was not different. For average regional sarcomere lengths, the mean and maximum increased with increasing initial length, as did the standard deviation and range. These results support the working hypothesis that following contraction-induced injury, the force deficit is a function of the sarcomere length heterogeneity, and the proportion of sarcomeres on the descending limb of the length-force curve.