Effect of ageing on the regulation of motor unit force in rat medial gastrocnemius muscle

The influence of ageing on the regulation of force through the firing rate (force–frequency relationship) and motor unit contractile output were investigated in three types of motor unit (MU): FF, FR and S, in the medial gastrocnemius muscle. A control group of young (5–10 months) Wistar rats was compared to three groups of older (20–21, 24–25 and 28–30 months) animals. The optimal tetanus characterized by the maximum contractile output (force–time area – FTA – per single pulse) was determined. During ageing, the steep part of the force–frequency relationship of medial gastrocnemius MUs shifted towards lower stimulation rates. However, in all MU types of the oldest rats, the opposite shift (towards higher rates) was observed. Ageing induced a substantial increase in the maximal FTA per pulse, particularly in S and FF units, but only subtly altered the fusion index of the optimal tetanus of MUs. Moreover, a transient increase in the mean forces of FF MUs was revealed in the groups of 20–21 and 24–25 months rats, and a significant decrease in the fatigue resistance of FR MUs accompanied ageing. These findings increase our understanding of the functional mechanisms responsible for changes in rate coding and alterations in muscle fatigability during ageing.