Biomechanical Alterations in the Unweight Phase of the Single-Leg Countermovement Jump After ACL Reconstruction.

Anterior cruciate ligament reconstruction (ACLr) often leads to asymmetries between limbs, with variable return-to-performance rates in athletes. The single-leg countermovement jump (SLCMJ) is commonly used to assess postoperative knee function. However, limited research has explored deficits specifically during the unweighting phase of the jump. This study assessed 53 recreational athletes (11 females, 42 males) between 6 and 9 months post-ACLr using a dual force plate system (1000 Hz). Each participant performed three maximal-effort SLCMJs per limb. Outcome measures included jump height, negative peak velocity, minimum force, and center of mass (COM) displacement. Paired -tests and Wilcoxon tests were used to compare the ACLr limb with the contralateral limb. Compared to the healthy limb, the ACLr limb showed significantly lower negative peak velocity (-0.80 ± 0.40 m/s vs. -0.94 ± 0.40 m/s, < 0.001), higher minimum force (36.75 ± 17.88 kg vs. 32.05 ± 17.25 kg, < 0.001), and reduced COM displacement (-17.62 ± 6.25 cm vs. -19.73 ± 5.34 cm, = 0.014). Eccentric phase duration did not differ significantly. Athletes post-ACLr demonstrate altered neuromuscular control during the early SLCMJ phase. These findings highlight the importance of rehabilitation strategies targeting eccentric strength and symmetry restoration.