Muscle Synergy Differences During A Single-limb Loading Task Following Anterior Cruciate Ligament Reconstruction

Neuromuscular deficits in the surgical limb after anterior cruciate ligament reconstruction (ACLR) persist for years, posing a risk factor for secondary injury and long-term-sequelae. Muscle synergy analysis characterizes muscle activation patterns related to a patient’s motor deficits and/or compensations during a specific movement. PURPOSE: To investigate muscle coordination patterns via muscle synergy analysis during a single-limb drop-land task in ACLR and healthy controls. METHODS: Five adults with unilateral ACLr (time since surgery: 16.5 ± 1.5 months) and 5 matched controls with no history of knee injury, performed a single-leg drop-land task. For the current study, we were interested in the deceleration phase of the drop-land task which was defined as the time from initial contact (vertical GRF>20 N) to maximum knee flexion. Electromyography (EMG) was recorded from the vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), semitendinosus (ST), biceps femoris (BF), medial (mGas) and lateral gastrocnemius (lGas) and normalized to the peak EMG value recorded from the drop-land task. To characterize the muscle coordination patterns for motor control strategies, we performed muscle synergy analysis using non-negative matrix factorization (NNMF). To categorize muscle synergies, k-means clustering was used. Intra-class correlation coefficient (ICC) was then used to examine the internal consistency of all muscle synergies in each cluster. To determine group-specific synergies, the ICC value of muscle synergies over 0.9, indicating excellent reliability, were categorized in the same cluster. RESULTS: Muscle synergies of all paricipants were grouped into 7 clusters. Within these clusters we identified group-specific muscle synergies between ACLR and Control groups. Compared to control group, the ACLR group showed relatively greater BF and ST activation at the initial phase of landing while relatively less VL and VM activations during the mid-phase of landing. Additionally, the ACLR group had greater mGas activation than the Control group. CONCLUSIONS: The ACL group exhibited increased activation in the hamstrings and mGas, potentially aiding in preventing anterior tibial translation and enhancing postural stability during single limb support.