Females recruit faster alpha motor neurons than males during the patellar stretch reflex: evidence for neurophysiological sexual dimorphism

Objective Human neurophysiology is sexually dimorphic where the pre-motor latency of the patella stretch reflex (PR) in females occurs earlier than males; an effect unaccounted for by stature difference. Using surface electromyography we investigated whether this difference is due to dimorphism in the conduction velocity (MNCV) of the associated alpha-motor neurons. Methods Extracellular linear electrode arrays measured vastus lateralis (VL) motor unit action potential activity (MUAP). MNCV was determined from the relationship between the latency of MUAP initiation and VL distance during a PR. Results In females, the PR premotor latency occurred 2.1±0.3 ms earlier than in males and recruited alpha-motor neurons with a MNCV 5 ms −1 faster (p<0.02) than males: 52.4±1.5 5 m s −1 (n=40) compared to 47.6±1.3 5 m s −1 (n=41) respectively. Conclusions In males the longer PR premotor latency was accounted for by their slower MNCV. Our finding is contrary to the observations of larger diameter alpha-motor axons and somas seen in males. Since the PR yields MNCVs values at the lower end of published values supports the “size-principle” of motor unit recruitment with a physiological stimulus. Significance Our findings evidence gender dimorphism in a routine neurophysiologic test which suggest androgen modulation of skeletal muscle innervation.