Meeting Abstract
During animal movement, muscle length oscillations are common. We used a stretch-shortening cycle (SSC) protocol that stimulated muscles over the first third of lengthening to determine eccentric contraction properties during cyclical length changes. Mouse extensor digitorum longus (EDL, fast twitch) and soleus (slow twitch) muscles were used in the experiments. Additionally, due to the attenuation of eccentric contraction properties in mdm mice, these muscles were evaluated under the SSC protocol and results compared to wild-type muscles. We calculated the rate of force development (RFD) during eccentric contractions and total work. The results indicate that the RFD is larger in the EDL than the soleus, while both have larger RFDs than isometric contractions. Both EDL and soleus display large negative work, showing their absorptive properties during eccentric contractions. It is apparent that the EDL and soleus differ in muscle deactivation rates; the soleus maintains force well after stimulation ends while the EDL drops in force immediately after deactivation. We suggest that this is caused by the differences in calcium uptake rates. These results suggest that the soleus functions as a spring, while the EDL functions as a shock absorber. Mdm muscles showed reduced RFD and negative work compared to wild-type muscles, suggesting that a key component of the eccentric contraction mechanism is lost by the mdm mutation. The mdm mutation involves a deletion in part of the N2A region of titin, which could be involved in titin activation – a prediction of the winding filament hypothesis. Taken together, the SSC approach used here appears to be a good strategy to study and compare mechanics of different muscles with varying fiber types and functions.
