Background Deficits in neuromuscular control are believed to increase re-injury risk after anterior cruciate ligament reconstruction (ACLR). Previous studies mainly focused on muscles at the operated knee, less on muscles around other joints of the operated and non-operated leg.

Objective To evaluate lower extremity muscle onset times during the transition from double-leg stance to single-leg stance in ACLR and non-injured control subjects.

Design Cross-sectional (retrospective).

Setting Controlled laboratory study.

Participants 20 ACLR subjects (15 females, 5 males), 23.1±13.9 months after ACLR and fully returned to their pre-injury sport, and 20 non-injured control subjects (15 females, 5 males) participated in the study. Both groups were matched for age, gender, weight, height and activity level. All ACL injuries were caused by a non-contact injury mechanism.

Risk factor assessment A transition task from double-leg stance to single-leg stance with eyes closed was performed, using a single force plate and surface electromyography. Movement speed was standardized. Both legs of each subject were tested.

Main outcome measurements Onset times of 10 lower extremity muscles were compared between groups and between legs within the same group. Significance was set at P<.05.

Results The ACLR group showed significant delayed muscle onset times for gastrocnemius (P=0.40), peroneus longus (P=.022), vastus medialis (P=.007), gluteus medius (P=.035) and gluteus maximus (P=.001). No significant different muscle onset times were found between legs in the ACLR group (P>.05).

Conclusions Despite the full return to sport, ACLR subjects showed delayed muscle onset times during the transitional movement, not only around the knee, but also around the hip and ankle of both legs. Whether these deficits are a result or a pre-existing risk factor of the initial ACL injury remains unclear. Secondary prevention strategies should focus on the whole lower kinetic chain of both legs to improve neuromuscular control after ACLR.