Excitability of Anterior Horn Cells Following Repetitive Knee Joint Movements at Different Speeds

Introduction F-waves are compound muscle action potentials generated by the antidromic activation of alpha motor neuron axons and the backfiring of anterior horn cells in response to electrical stimulation. Several F-wave parameters are used to assess the state of alpha motor neurons. Previous studies have demonstrated that repetitive thumb movements can decrease the F/M amplitude ratio in the abductor pollicis brevis muscle of healthy individuals. This suggests that repetitive movements may be a practical and accessible self-training method for alleviating muscle hypertonia. However, it remains unclear whether similar effects occur in the quadriceps femoris muscle following repetitive knee joint movement. Furthermore, the speed of movement may influence F-wave responses. This study aimed to investigate the effects of repetitive knee joint movements at different speeds on the excitability of anterior horn cells, as assessed by F-waves recorded from the vastus lateralis (VL) muscle before and after the movements. Methods Eighteen healthy adults participated in the study. F-waves were recorded from the VL muscle before, immediately after, and four and eight minutes after each task. In the 1 Hz trial, participants performed repetitive knee joint flexion and extension in synchrony with a 1 Hz auditory cue; in the 2 Hz trial, they moved in time with a 2 Hz cue. In the control trial, participants remained at rest. For F-wave recordings, the cathode of the stimulation electrode was placed at the distal one-fifth point along the line connecting the greater trochanter and the superolateral margin of the patella, with the anode placed at the distal one-fifth point along the line connecting the greater trochanter and the lateral joint space of the knee. The cathode of the recording electrode was positioned at the midpoint between the cathode of the stimulating electrode and the superior lateral border of the patella, and the anode was placed at the patella. The ground electrode was positioned on the anterior surface of the lower leg. Electrical stimulation was delivered at 1.2 times the intensity required to evoke the maximal M-wave amplitude, with a duration of 0.2 ms, a frequency of 0.3 Hz, and a number of stimuli of 60. The bandwidth filter was set between 20 and 3,000 Hz. Outcome measures included M-wave amplitude, F-wave persistence, and the F/M amplitude ratio. Since the Shapiro-Wilk test indicated non-normality, an aligned rank transformation was applied, followed by two-way repeated-measures ANOVA. Results There were no significant changes in the outcomes before or after repetitive knee movements. Additionally, movement speed did not affect the outcomes. Conclusions Unlike a previous report showing decreased excitability of anterior horn cells that innervate the abductor pollicis brevis following repetitive thumb movements, our study found no changes in VL excitability after repetitive knee movements performed at different speeds. These results imply that the modulation of anterior horn cell excitability by repetitive movements depends on the body part involved.