Motor unit firing rates of the gastrocnemii during maximal and sub-maximal isometric contractions in young and old men

Abstract

The triceps surae comprises an important group of muscles for human posture and gait. The soleus unlike other limb muscles shows atypical lower firing rates in both old and young adults across various voluntary strength levels, including maximal contractions. The other portion of the triceps surae, the gastrocnemii has not been explored in aging, and despite anatomic, histochemical and age-related morphological differences, they share many common functions with soleus. During multiple visits, 10 active young (23–33years) and 10 active old participants (76–86years) performed a series of plantar flexor isometric contractions at a range of contraction intensities including maximal voluntary contraction (MVC) with tungsten microelectrodes inserted into the lateral (LG) and medial (MG) gastrocnemius. Despite equal and near maximal voluntary activation (VA) (∼98%), MVC torque was ∼46% lower, twitch tension was ∼34% lower, and contractile speed was ∼15% slower in the old men compared with the young. At all isometric torque levels tested (25, 50, 75 and 100% MVC) there were no statistically significant differences in mean motor unit firing rates (MUFRs) between young and old men. In both groups, the range of mean MU firing rates was similar (∼8Hz at 25% MVC to ∼22Hz at 100% MVC). The structural age-related changes in the gastrocnemii are not reflected in neural drive adaptations, indicating that MUFRs may not be a common feature with aging and other factors such as habitual use or anatomical location may be influential.