C57BL/6 life span study: Age-related declines in muscle power production and contractile velocity

Quantification of key outcome measures in animal models of aging is an important step preceding intervention testing. One such measurement, skeletal muscle power generation (force * velocity), is critical for dynamic movement. Prior research focused on maximum power (P_max), which occurs around 30–40 % of maximum load. However, movement occurs over the entire load range. Thus, the primary purpose of this study was to determine the effect of age on power generation during concentric contractions in the extensor digitorum longus (EDL) and soleus muscles over the load range from 10 to 90 % of peak isometric tetanic force (P₀). Adult, old, and elderly male C57BL/6 mice were examined for contractile function (6–7 months old, 100 % survival; ~24 months, 75 %; and ~28 months, <50 %, respectively). Mice at other ages (5–32 months) were also tested for regression modeling. We hypothesized and found that power decreased with age not only at P_max but also over the load range. Importantly, we found greater age-associated deficits in both power and velocity when the muscles were contracting concentrically against heavy loads (>50 % P₀). The shape of the force-velocity curve also changed with age (a/P₀ increased). In addition, there were prolonged contraction times to maximum force and shifts in the distribution of the myosin light and heavy chain isoforms in the EDL. The results demonstrate that age-associated difficulty in movement during challenging tasks is likely due, in addition to overall reduced force output, to an accelerated deterioration of power production and contractile velocity under heavily loaded conditions.