Motor unit firing rate control of agonist skeletal muscle during voluntary isometric and shortening contractions with limb movement
Abstract
Voluntary movements throughout mammalian lifespan require skeletal muscle contractions that are controlled by time- and recruitment-dependent firing rate patterns. Single motor unit (MU) activity reflects the final efferent neural drive to the muscle, yet the underlying neural control of movement at the MU level is not well understood. Using intramuscular electromyography single MU recordings, relationships between voluntary contraction kinematics and MU firing rates were evaluated in vivo, in groups of young and older adult participants.
The purpose of study one was to characterize how MU firing rates are differently scaled among muscles relative to voluntary contraction intensity. Across 12 different muscles, MU firing rates were compared between voluntary intensities and two adult age groups. Findings supported that firing rates are muscle and voluntary intensity dependent. But with adult ageing, firing rates were more similar across muscles, although more dissimilar in response to voluntary contraction intensity.
The purpose of study two was to compare whether MU firing rates recorded from the anconeus muscle change similarly with an age-related effect in both isometric and limb movement contractions. Results indicated that firing rates were lower in older adults during isometric contraction but not during limb movements, supporting observations of greater descending drive during movements in older adults, and reflected at the single MU level.
The purpose of study three was to investigate how MU firing rates recorded from the anconeus and triceps brachii muscles are related with elbow extension movement kinematics. Effects of phase lag between comparisons were minimized by relating each firing-time to separated kinematic parameters. Through correlation analysis, results showed that firing rates of both muscles were directly related to limb torque, but not velocity or position, supporting that MU activity controls movements indirectly through torque.
In study four, MU firing rate trajectories of the anconeus and triceps brachii muscles were compared between isometric and limb movement contractions. During self-initiated contractions, results indicated that characteristics of the trajectory were dependent on the task. Whereas during reaction-cued contractions, MU firings occurred sooner relative to the go-cue, suggesting that descending inputs determine the trajectory, but the state of limb-movement further modifies trajectory timing.