Association between neural-drive variability and motor function in middle-aged and older adults

The purpose of this review was to examine the extent to which variability in the cumulative set of action potentials sent from the spinal cord (neural drive) to limb muscles can explain the variance in performance of middle-aged and older adults on the NIH Toolbox measures of motor function. Neural-drive variability is attributable to two spinal mechanisms: low-frequency oscillations in the shared synaptic input onto alpha motor neurons and synaptic noise in the generation and transmission of action potentials by individual motor neurons. Evidence indicates that the coefficient of variation for force during submaximal contractions (a behavioral proxy for neural-drive variability) explains moderate amounts of the variance in performance scores on the tests for balance, dexterity, and endurance, whereas locomotion is associated with motor-unit discharge characteristics in calf muscles and handgrip strength seems to be related to the timing of motor-unit activity across forearm-hand muscles. These findings indicate that some of the variance in performance on a range of dynamic motor tasks can be explained by neural-drive variability during submaximal isometric contractions. Future work needs to expand the repertoire of tasks from which measures of neural-drive variability are derived, which may increase its explanatory power for the observed variance in performance scores of middle-aged and older adults on Toolbox measures of motor function.