Author(s): CHU, N., CHEN, Y.J., TZENG, Y.T., LEE, H.J. , Institution: NATIONAL TAIWAN NORMAL UNIVERSITY, Country: TAIWAN, Abstract-ID: 379

INTRODUCTION:
Secondary ACL injuries are common, with twice as many occurring on the non-injured side. Lower extremity asymmetry after ACL reconstruction (ACLR) may increase reinjury risk. The single-leg drop jump (SLDJ) assesses functional recovery after ACLR. Muscle activation asymmetry during SLDJ landing could indicate injury risk. This study evaluated lower limb muscle activation asymmetry during SLDJ in athletes after ACLR return to sports.
METHODS:
Participants: Ten ACLR and eleven healthy athletes were recruited to this study. The conditions of ACLR acceptance were: unilateral and one ACL reconstructed surgery; all athletes have completed the rehabilitation and returned to sports. All participants were without lower limbs musculoskeletal injury within six months.
Procedures: Subjects performed SLDJ from 30cm while electromyography (EMG) recorded muscle activation of the vastus lateralis (VL), vastus medialis (VM), and biceps femoris (BF) on both lower limbs. The EMG data were collected at 2000 Hz. Vertical ground reaction force(vGRF) data during landing were simultaneously collected using a Kistler force plate at a sampling frequency of 2000 Hz. The raw EMG data underwent band-pass filtering (10-500 Hz), and Root Mean Square calculations were performed with a time window of 20 ms. Maximum voluntary isometric contractions were used to normalize muscle activation. Muscle activation levels were calculated using a 100 ms window before and after the landing instant, defined as when vertical ground reaction force exceeded 20 N.
Statistics: Muscle activation variables were compared between ACLR and healthy groups using independent t-tests. Symmetry indices were calculated as injury side/non-injury side x 100% for the ACLR group and as non-dominant side/dominant side x 100% for the healthy group. Significance was set at α<.05.
RESULTS:
The ideal symmetry value is 100%. Before the first landing, we found that the ACLR group had significantly higher asymmetry in BF activation compared to the healthy group. The ACLR group had higher BF activation on the injured side than the non-injured side. The healthy group had larger vGRF asymmetry than the ACLR group. There was significant asymmetry in VM and BF activation between groups. In the ACLR group, VM activation was lower on the injured side while BF activation was higher. In the healthy group, the opposite pattern was seen.
Before the second landing, we found both groups had asymmetric VM activation, but the ACLR group had greater asymmetry. On the second landing, a similar pattern of asymmetric VM and BF activation was seen as on the first landing for both groups. The ACLR group had lower VM and higher BF activation on the injured side compared to the non-injured side. The opposite was seen for the healthy group when comparing dominant and non-dominant sides.
CONCLUSION:
The asymmetrical muscle activation patterns seen in the ACLR group when landing could shift load to the non-injured leg and increase injury risk. The healthy group also showed asymmetry but in the opposite direction, which may elevate injury risk in the non-dominant leg. Addressing these asymmetries through targeted training may help reduce second injury risk when returning to sports after ACLR.