Auditory-cued terminal knee extension enhances vastus medialis motor unit function in the early phase following ACL reconstruction: a randomized controlled trial

BACKGROUND: Following anterior cruciate ligament reconstruction (ACLR), the vastus medialis (VM) commonly exhibits functional deficits largely attributed to neuromuscular alterations. This study aimed to investigate the immediate effects of terminal knee extension (TKE) exercise synchronized with auditory cues on VM motor unit behavior in individuals 2 weeks postACLR. METHODS: The study recruited 38 individuals who underwent primary ACLR 2 weeks postsurgery and they were randomly assigned to experimental or control groups. The experimental group performed three sets of TKE exercises synchronized with auditory cues; the control group performed identical exercises without auditory cues. Before and after the intervention, VM muscle activity, motor unit action potential (MUAP), and motor unit firing rate (MUFR) were assessed during knee extension using a Trigno-Galileo electromyography system. Within and between-group differences were analyzed using parametric or nonparametric methods, as appropriate, on the basis of data distribution. Statistical significance was set at p < 0.05. RESULTS: Data from 20 participants (10 experimental, 10 control) were included in the final analysis. The auditory-cued group demonstrated significantly greater increases in peak [12.59 (IQR 17.89) µV, p = 0.043] and average [11.31 (IQR 3.04) µV, p = 0.029] MUAP compared with the noncued group [6.26 (IQR 11.02) µV and 2.24 (IQR 9.94) µV, respectively]. Moreover, only the auditory-cued group showed significant postintervention increases in average electromyography (EMG) activity [pre: 14.23 (IQR 11.37) µV; post: 17.60 (IQR 20.94) µV; p = 0.007], average MUAP [pre: 43.07 (IQR 34.30) µV; post: 50.33 (IQR 36.12) µV; p = 0.005], peak MUAP [pre: 55.54 (IQR 50.35) µV; post: 61.69 (IQR 52.92) µV; p = 0.005], and peak MUFR (pre: 10.50 ± 4.29 pps; post: 12.66 ± 3.36 pps; p = 0.016) after the intervention. CONCLUSIONS: A single session of TKE exercise synchronized with auditory cues significantly increased MUAP in the VM muscle in individuals 2 weeks post-ACLR. These findings likely reflect enhanced acute neural drive and improved VM motor unit recruitment. Auditory-cued TKE may represent a potential strategy to enhance neuromuscular control during the early postoperative phase. However, these effects reflect only immediate responses, and the persistence of these adaptations with longer-term training remains to be determined. TRIAL REGISTRATION: NCT06662955 (25 October 2024, on ClinicalTrials.gov).