Changes in knee joint angle affect to a negligible extent the distribution of motor points—the motor zones—in the quadriceps muscles

Purpose: Functional electrical stimulation (FES) has been documented to provide invaluable, therapeutic effect. The success of FES applications relies on the positioning of stimulation electrodes closely to the muscle motor points. During joint movement, however, motor points may shift as the muscle length changes. Using an objective procedure to define motor points, we assessed how much and how relevantly knee extensors’ motor point location changes with joint angle. Methods: Current pulses were applied over 121 (11 × 11) equally spaced stimulation sites, determined according to the size and boundaries of the three superficial knee extensors of 17 healthy subjects. Five consecutive monophasic pulses (100 µs, 1pps) were delivered at each site for two knee joint angles (40° and 115°) and two intensities (120 % of motor threshold and maximum tolerable). The average peak torque greater than 60 % of the maximum torque was used to identify motor points across sites. The centroid of motor point clusters—motor zones—was considered to assess motor point displacement and peak torque for different knee joint angles. Results: Significant centroid shifts were observed distally (2.5 % displacement; p < 0.031) and medially (4.4 % shift; p < 0.021), representing ∼ 1 cm shifts in both directions, when going from 40° to 115° of knee flexion. Mean differences in peak torque between joint angles were negligible, however, amounting to less than 2 % of maximal voluntary torque. Conclusion: The displacement of quadriceps motor points with knee position changes is of negligible practical relevance for justifying a variable position of stimulation electrodes in FES protocols.