Although various strength tests and their outcome measures have been proposed for anterior cruciate ligament (ACL) reconstruction (ACLR), their measurement properties still remain relatively underexplored. The aim of this study was to investigate the longitudinal construct validity of the standard isokinetic (IKT) and isometric test (IMT), and of the IMT of alternating consecutive maximal contractions (ACMC). In addition, the concurrent validity of ACMC was assessed and compared with the validity of IMT. The strength of quadriceps and hamstrings in 20 male athletes with an anterior cruciate ligament (ACL) injury were assessed before ACLR, 4 and 6 months after ACLR, by means of IMT, ACMC, and IKT performed at 60 and 180° · s(-1). Significant between-session differences in muscle strength variables were found in the involved quadriceps (F > 6.5; p ≤ 0.05), but not in the uninvolved leg (F < 2.5; p > 0.05). Coefficients of variations in the uninvolved leg (all below 13.5%) were lower than the involved leg (11.7-22.1%). Intraclass correlation coefficients were moderate-to-high for the uninvolved leg and low-to-high for quadriceps of the involved leg. The concurrent validity of ACMC with respect to the IKT (r = 0.57-0.92; p ≤ 0.05) was comparable with the validity of IMT (r = 0.52-0.87; p ≤ 0.05). We conclude that the explored longitudinal construct validity of most of the evaluated variables could be sufficiently sensitive to detect the effects of the applied rehabilitation procedures. In addition, the obtained sensitivity and concurrent validity and the potential advantages of ACMC over IMT, all suggest that ACMC could be a particularly promising method for routine testing of neuromuscular function after ACLR.