The specific force of single intact extensor digitorum longus and soleus mouse muscle fibers declines with aging

In the present study we measured, for the first time, the isometric specific force (SF, force normalized to cross sectional area) generated by single intact fibers from fast- (extensor digitorum longus, EDL) and slow-twitch (soleus) muscles from young adult (2-6), middle-aged (12-14) and old (20-24 month-old) mice. SF has also been measured in single intact flexor digitorum brevis fibers from young mice. Muscle fibers have been classified into fast- or slow-twitch based on the contraction kinetics. Maximum SF recorded in EDL and soleus fibers from young and middle-aged mice did not differ significantly. A significant age-dependent decline in maximum SF was recorded in EDL and soleus fibers from young or middle-aged to old mice. The SF was 377 +/- 18, 417 +/- 20 and 279 +/- 18 kPa for EDL fibers from young, middle-aged and old mice, respectively and 397 +/- 17, 405 +/- 24 and 320 +/- 33 kPa for soleus fibers from age-matched mice, respectively. The frequency needed to elicit maximum force in EDL and soleus fibers from middle-aged to old mice did not differ significantly. In conclusion, the specific force developed by both fast and slow-twitch single intact muscle fibers declines with aging and more significantly in the former.