Reduced knee extensor torque steadiness and increased motor unit discharge rate variability in young people with patellofemoral pain: a pilot study

Purpose: The aim of this pilot study was to evaluate differences in vasti muscle motor unit (MU) firing between individuals with current patellofemoral pain (PFP) and asymptomatic controls during submaximal isometric knee extension contractions in both single-joint (knee extension) and multi-joint (leg press) exercises. Methods: Ten individuals with PFP and ten age- and gender-matched asymptomatic controls performed submaximal isometric contractions (10–70% maximum voluntary isometric contraction; MVIC) during single-joint and multi-joint exercises. High-density surface electromyography assessed MU discharge properties of the vastus medialis (VM) and vastus lateralis (VL), while torque steadiness was quantified using the coefficient of variation of torque. Results: Neural drive to the vasti muscles was comparable between groups across both exercises however, individuals with PFP exhibited reduced torque steadiness during single-joint compared to multi-joint exercises. This reduction in torque steadiness was accompanied by increased MU discharge rate variability at higher torque levels (50–70% MVIC), particularly for the VL muscle at 70% MVIC. For those with PFP, their pain intensity was also higher during single-joint exercises, which may have further contributed to the aforementioned neuromuscular impairments. Additionally, MU firing-torque relationships revealed neuromuscular adjustments in people with PFP, indicated by significantly lower cross-correlation values during multi-joint exercises compared to the asymptomatic controls. Conclusion: Physically active people with PFP exhibit reduced torque steadiness, increased discharge rate variability, and potentially altered MU firing-torque relationships during single-joint knee extension exercise. These MU adaptations likely reflect neuromuscular adjustments to ongoing PFP, helping to sustain force production despite impaired motor control and potentially mitigating pain during multi-joint exercises.