Contributions of Neural and Mechanical Input to Spinal Motoneuron Output in Human Force Control

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Production of functional movement is malleable; it can be trained to improve output and over time it deteriorates with age. This age-related functional deterioration in movement control is more pronounced in women than in men. Over the next 5 years, studies will be conducted to ascertain the contributions of spinal motoneuron output in the production of steady movement to understand greater age-related decline in force control in women compared to men. Spinal inputs that modulate motoneuron output in the biceps brachii will be quantified in the evaluation of force control in isometric and dynamic contractions as well as functional movement in the elbow flexors of young (18-30 years) and old (>75 years) men and women. The biceps brachii muscle, through distinct anatomical and physiological compartments of the short and long heads are ideal for studying how small changes in motoneuron output impacts force production, especially forearm control that is central to functional movement. Initial projects will focus on the contribution of reflex feedback in modulating motoneuron output differentially between heads, and the impact on force production. Functional movement will be modeled through the task of lifting and lowering a cup for drinking to quantify motoneuron output with control tasks of dynamic contractions that are matched for position and acceleration with the functional task. Experiments will use surface and indwelling electromyography (EMG) to measure the activity of specific muscles and motor units that regulate the production of steady contractions. Ultrasonography is included to quantify the mechanical contributions of muscle and tendon. Additional to neural output, force transfer through muscle and tendon is integral to steady force control. Through a series of investigations we will be the first to quantify single motor unit activity during functional movement and ascertain how reflex feedback is integrated to alter spinal motoneuron output between synergist muscle compartments in the evolution of producing steady contractions as men and women age. This research will generate fundamental knowledge about spinal networks in humans that collectively contribute to understanding motoneuron output in the regulation of force transfer that creates functional upper limb movement. This research is significant as integration between neural and mechanics is integral to understanding the plasticity of the neuromuscular system that contributes to sex-specific age-related decline of functional movement. Consistent with prior investigations, this research program will result in advanced training for approximately 20 highly qualified personnel enabling technical skill development in neurophysiology applicable to scientific discovery in basic human movement science.