The Age Associated Alteration in Eccentric Contraction Properties during a Stretch Shortening Cycle in Fast and Slow Twitch Mouse Muscles

Abstract

During animal movement, cyclical changes in muscle length are common. However, age-associated muscle mechanics are not well understood. We used a stretch-shortening cycle (SSC) protocol that stimulated muscles over the first third of lengthening to determine eccentric contraction properties during length oscillations in mice that ranged in age from 30 - 400 days. Mouse extensor digitorum longus (EDL, fast twitch) and soleus (slow twitch) muscles were used in the experiments. We calculated the rate of force development (RFD) during eccentric contractions and total work. A passive SSC was also conducted to measure passive properties, because an age-associated increase in muscle stiffness was expected. The results indicate that the RFD and total negative work increased with increasing age in a linear fashion. The contribution of passive muscle components remained consistent or increased only slightly with age. Further, our results suggest that an age associated increase in muscle stiffness is associated with components of active muscle. These could include age-associated changes in cross bridge cycling or an increase in stiffness of the titin filament. Titin has been shown to contribute to passive and active stiffness in eccentrically contracting muscles. Because titin stiffness is modified by a variety of pathways, we argue future work should focus on identifying age-associated changes in titin stiffness.