Do Motoneurons Slow With Aging? A Systematic Review And Meta-Analysis With Meta-Regression

Nervous system maladaptation is linked to the loss of muscle force production and motor control with aging. However, the mechanisms specifically underpinning these muscle functional limitations remain unclear. Motoneuron discharge rates are a critical determinant of force production and strongly impact motor control; thus, lower discharge rates could underpin force and physical function losses during aging. The present meta-analysis with meta-regression summarises the findings of studies comparing motoneuron discharge rates between young and older adults and examines whether a muscles functional role and anatomical location influences discharge rates across contraction intensities during aging. Meta-analysis revealed lower discharge rates in older compared to young adults in lower body flexor (standardised mean difference, SMD=-0.86; 95%CI: -1.20, -0.51; p<0.001) but not extensor (SMD=-0.18; 95%CI: -0.50, 0.15; p=0.29) muscles or upper body muscles (SMD=-0.79; 95%CI: -1.75, 0.18; p=0.11). Meta-regression revealed that the differences in discharge rate between young and older adults increase with contraction intensity in upper body muscles ({beta}=-0.033, p=0.004). These findings suggest that motor function loss with aging might be partly explained by reduced motoneuron discharge rates; however, this decrease varies according to a muscles anatomical location, functional role, and contraction intensity.