After anterior cruciate ligament (ACL) injury and reconstruction (ACLR), sex differences are prominent during walking, cutting, and jumping tasks.(1,2) Aberrant walking mechanics predominate in males and females, persist up to 2 years after ACLR,(3,4) and are associated with the development of posttraumatic osteoarthritis.(5,6) Kinematic and kinetic variables show continued walking asymmetry in females after ACLR compared to their male counterparts.(2) While these sex differences are important, muscle activity, which may provide further insight to sex-specific adaptations to injury, has not been explored. Muscle timing variables (i.e., onset, peak activity, and offset) may provide insight into neuromuscular changes after ACL injury and ACLR. The purpose of this study was to explore sex-specific changes in muscle activity during gait in non-copers after diagnosis of ACL rupture, and after pre- and post-operative physical therapy. The secondary purpose was to compare muscle activity patterns during gait in individuals 6 months after ACLR to an uninjured control group.
Thirty-nine non-copers (12 females, 27 males) who participated in >=50 hours per year in Level I or II sports and diagnosed with a complete ACL rupture were enrolled. Participants were excluded if they had concomitant grade III knee ligament injuries, meniscus tears with >3 sutures, or chondral lesions >1cm2. Uninjured controls were age, sex, and activity-level matched to participants after ACLR. Gait data were collected at pre- and post- pre-operative physical therapy, and 6 months after ACLR. Surface electromyography (EMG) data were collected at each time point from 7 muscles bilaterally: vastus lateralis and medialis (VL, VM), lateral and medial hamstrings (LH, MH), lateral and medial gastrocnemius (LG, MG), and soleus (SOL). General linear models with generalized estimating equations were used to detect the effects of limb (involved, uninvolved) and time (baseline, after pre-operative physical therapy, 6 months post-ACLR) for muscle activity timing variables of interest, run separately for each sex.
Males presented with more asymmetries than females at pre- and post- pre-operative physical therapy in the VL on (p<0.001) and off (p=0.007), VM on and off (p<0.001), MH off (p<0.001), and SOL off (p=0.021), but resolved most of their asymmetries by 6 months. Conversely, females presented with fewer asymmetries, with only VL off (p=0.027), VM on (p=0.003), and LG (p=0.012) at pre- and post- pre-operative physical therapy. In support of our second hypothesis, females continue to demonstrate asymmetries at 6 months compared to uninjured control participants in the VL on (p=0.001), VM on (p<0.001), VM off (p=0.035), LH off (p=0.030), MH on (p=0.041), LG on (p=0.009), and SOL on (p=0.004).
Sex-specific differences in muscle activity after ACL injury and reconstruction parallel those of kinematic and kinetic changes. While both sexes attempt to normalize inter-limb differences after pre-operative physical therapy, after surgery females have more asymmetries in muscle activity 6 months post-operatively than males when compared to health controls. Future work should consider the sex-specific effects of targeted post-operative rehabilitation to address the aberrant kinematic, kinetic, and muscle activation deficits after ACL injury. These sex-specific biomechanical changes may provide important targets for rehabilitation.
References: 1. Hewett et al., AJSM 2005. 2. Di Stasi et al., JOSPT 2016. 3. Capin et al., JOR 2019. 4. Capin et al., Clin Orthop Relat Res 2017. 5. Wellsandt et al., AJSM 2016. 6. Saxby et al., Knee Surg Sport Traumatol Arthrosc. 2019.