Motor Neuron Discharge Characteristics of the Knee Extensors in Single and Multijoint Isometric Motor Tasks and the Effects of Foot Positioning: A Preliminary Analysis

This study examined the influence of task type, knee angle, contraction intensity, and foot positioning on the motor neuron discharge characteristics of knee extensors during single-joint knee extensions and multijoint leg extensions. Ten male participants performed isometric contractions at 10% and 30% of maximum voluntary contraction at 90° and 135° knee angles with identical hip angles. For the 90° leg press, the influence of foot positioning was explored additionally. Motor unit activation was assessed through decomposition of high-density surface electromyography signals to analyze discharge rate, recruitment threshold, and derecruitment threshold. The results showed no significant differences between the vastus medialis and vastus lateralis muscles across all conditions. Knee angle did not affect motor unit characteristics, and foot positioning (internal or external rotation) in the leg press did not alter discharge characteristics, except for neutral foot positioning at 30% maximum voluntary contraction. Significant differences were found between single- and multijoint tasks in all motor unit activation characteristics. Additionally, significant differences in discharge rate were observed between 10% and 30% maximum voluntary contraction. These findings suggest that task type and contraction intensity influence motor unit activation, but knee angle and foot rotation do not have a significant impact on the motor neuron discharge characteristics of knee extensors.