Effects of Training with Hybrid Assistive Device on Motor Unit Behavior After Total Knee Arthroplasty: A Randomized Controlled Trial

PURPOSE: Postoperative rehabilitation utilizing the single-joint Hybrid Assistive Limb (HAL-SJ) following total knee arthroplasty (TKA) has been reported to facilitate early functional recovery. This study aimed to elucidate the firing properties of individual motor units (MU) during knee exercise post-TKA with HAL-SJ assistance. METHODS: This single-blinded randomized controlled trial included 24 participants who underwent primary TKA for knee osteoarthritis. Participants were randomly assigned to the HAL-SJ (n = 12) or control (n = 12) groups. All participants followed the same postoperative rehabilitation protocol; however, those in the HAL-SJ group commenced training with the HAL-SJ 7-day post-surgery. The primary outcomes included the discharge rate (DR) obtained from high-density surface electromyography recordings from the vastus medialis (VM) and vastus lateralis (VL) muscles, which were measured preoperatively and 3 weeks postoperatively. The secondary outcomes included knee isometric extension torque and knee pain intensity, assessed using a numerical rating scale (NRS). RESULTS: The DR of the VL muscle at 60% maximal voluntary contraction in the HAL-SJ group was significantly higher postoperatively compared to preoperatively (p = 0.001). In addition, the overall DR of the VL muscle in the HAL-SJ group was significantly higher than that in the control group for higher-threshold MU (HTMU) (p < 0.001). No significant differences were observed in knee extension torque or resting pain NRS scores between the control and HAL-SJ groups. CONCLUSION: Rehabilitation with the HAL-SJ following TKA induced neurophysiological alterations in the VL muscle, characterized by an increased discharge rate of higher-threshold motor units. Although no significant between-group differences were observed in knee extension torque, these findings suggest that HAL-SJ training may facilitate early neural adaptations following TKA. TRIAL REGISTRATION: This study was registered with the University Hospital Medical Information Network (UMIN000053675).