Orishimo, KF, Fukunaga, T, Kremenic, IJ, Rao, S, Magill, R, Ling, WK, McHugh, MP, and Nicholas, SJ. Countermovement jump inefficiency is mostly independent of quadriceps weakness in athletes returning to sport after anterior cruciate ligament reconstruction. J Strength Cond Res XX(X): 000-000, 2024-After anterior cruciate ligament reconstruction (ACLR), comparing ground reaction force (GRF) metrics during countermovement jumps (CMJs) and isokinetic quadriceps strength testing may aid in return-to-sport decision-making. The purpose of this study was to compare asymmetries in GRF metrics during bilateral and unilateral CMJs and asymmetries in quadriceps strength between patients after ACLR and healthy athletes. Twenty-two patients who had undergone ACLR and 12 healthy athletes performed isokinetic tests of quadriceps strength and maximal-effort bilateral and unilateral CMJs on force plates. Countermovement jumps force and isokinetic measurements were compared using repeated-measures analysis of variance. Associations between asymmetries in CMJ metrics and isokinetics were assessed using correlation analysis. In the patients, significant asymmetries in knee extension strength were found (24.5% at 60 °·s-1, 13% at 180 °·s-1). In addition, asymmetries were found in 4 of 6 GRF metrics during bilateral CMJs and in 10 of 15 metrics during unilateral CMJs. The control group showed no significant asymmetries in quadriceps strength or CMJ GRF metrics. Asymmetry in knee extension strength was not correlated with any bilateral CMJ asymmetries and only 2 unilateral CMJ asymmetries. Asymmetry in knee extension power was correlated with 3 bilateral CMJ asymmetries and 3 unilateral CMJ asymmetries. A comparison of GRF profiles revealed functional deficits on the involved side of the patients during both CMJs, indicating difficulty with stretch-shortening cycle function. Asymmetries were partially explained by deficits in quadriceps power but mostly independent of quadriceps weakness. Return-to-sport assessments after ACLR should include the assessment of the biomechanical efficiency of lower extremity stretch-shortening cycle function.