Effects of cyclic changes in muscle length on force production in in-situ cat soleus

Muscle shortening and stretch are associated with force depression and forc e enhancement. respectively. Previously, we have investigated the effect of combined dynamic contractions (i.e. a single shortening-stretch and stretc h-shortening cycle) on force production (Herzog and Leonard, 2000). In orde r to investigate the relationship between force depression and force enhanc ement systematically, we studied the effects of a single as well as multipl e stretch-shortening and shortening-stretch cycles on the ascending limb of the force-length relationship. Furthermore, by systematically varying the speed and magnitude of stretch preceding shortening and the speed and magni tude of shortening preceding stretch, we investigated the influence of thes e varying contractile conditions on force depression and force enhancement, respectively. Test contractions were performed on cat soleus (n =6) by ele ctrical stimulation using four conceptually different protocols containing a single or repeated stretch-shortening and shortening-stretch cycles. The results of this study showed that: (1) force depression was not influenced by stretch preceding shortening independent of the speed and amount of stre tch; (2) force enhancement was influenced in a dose-dependent manner by the amount of shortening preceding stretch but was not affected by the speed o f shortening; (3) repeated stretch-shortening (shortening-stretch) cycles s howed cumulative effects; (4) the number of shortening steps over a given d istance did not influence the amount of force depression. The findings of t his study support the idea that the mechanism of force depression associate d with muscle shortening is different from that of force enhancement associ ated with muscle stretch. Furthermore. they support and extend our previous findings that stretch-shortening and shortening-stretch cycles are not com mutative. (C) 2001 Elsevier Science Ltd. All rights reserved.