Reduced motoneuron excitability and common synaptic inputs of the aged human vastus lateralis

The age-related loss of neuromuscular function has been comprehensively documented with a disproportionate loss in the quadriceps compared to other muscle groups. Although muscle atrophy may partially account for strength decline, it is further influenced by alterations of the nervous system. The outputs of motoneurons are heavily dependent on their intrinsic excitability and the level of common synaptic inputs they receive. However, these have not yet been described in aged human quadriceps. High density surface electromyography (HDsEMG) signals were collected from the vastus lateralis of ten young [mean(SD), 24(6.2)y] and ten older [66(7.7)y] men during a 20-s ramp contraction peaking at 20% of maximal voluntary isometric contraction. EMG signals were decomposed to identify individual motor unit discharges, and motoneuron persistent inward current (PIC) amplitude and intramuscular coherence were estimated. Older participants produced significantly lower peak knee extensor torque (p<0.001) and poorer force control ability (p=0.002) than young. Older participants also had lower peak discharge rate (p=0.016), PIC amplitudes (p=0.001) and coherence estimates in the beta frequency band in both ascending (p=0.026) and descending phases (p=0.015) when compared to young. These data provide new evidence that intrinsic motoneuron excitability and common synaptic input is lower in the vastus lateralis of older males when compared to young, and highlights the feasibility of this method in the clinically and functionally important quadriceps, further underpinning age-related loss of force control occurring independent of the loss of muscle mass. ### Competing Interest Statement The authors have declared no competing interest.