ECCENTRIC CONTRACTION-INDUCED INJURY IN NORMAL AND HINDLIMB-SUSPENDEDMOUSE SOLEUS AND EDL MUSCLES

The primary objective of this study was to compare the magnitude of in jury in mouse extensor digitorum longus (EDL) and soleus muscles induc ed by high-force eccentric contractions. A second objective was to stu dy the effect of altering the daily loading of the muscles through hin dlimb suspension (HS) on the injury. One of two protocols was performe d in vitro: 1) 15 eccentric contractions (n = 20: 10 EDL and 10 soleus muscles) or 2) 15 isometric contractions (n = 20: 10 EDL and 10 soleu s muscles). After the protocol, the decrements in contractile performa nce and lactate dehydrogenase (LDH) release were measured at 15-min in tervals over 1 h. Immediately after the eccentric contraction protocol , markedly greater decrements in maximal isometric tetanic force (P-o) occurred in the normal EDL than in the normal soleus muscles (60.7 +/ - 4.2 vs. 7.6 +/- 2.1%, P less than or equal to 0.0001). LDH release i mmediately after the eccentric contraction protocol was 2.7-fold great er in the normal EDL than in the normal soleus muscles. To investigate the role of recent loading of the muscles in the injury, EDL (n = 9) and soleus (n = 10) muscles from mice subjected to HS for 14 days perf ormed the eccentric contraction protocol. HS resulted in greater decre ments in contractile performance for the soleus muscles (decreases in P-o immediately after the protocol for HS and normal soleus muscles we re 31.0 +/- 1.8 and 7.6 +/- 2.1%, respectively; P less than or equal t o 0.0001) but not for the EDL muscles. HS resulted in a 76% greater LD H release in the soleus muscle but had no effect on LDH release by the EDL muscle. HS did not alter the histochemically determined fiber typ e composition of the soleus muscle. The data demonstrate that EDL musc les are more susceptible than soleus muscles to eccentric contraction- induced injury in normal weight-bearing mice; part of the explanation for the difference appears to be the greater previous loading of the s oleus muscle rather than the fiber type composition per se.