The purpose of this study was to compare knee extension torque variability in patients with ACL reconstructed knees before and after exercise. Thirty two patients with an ACL reconstructed knee (ACL‐R group) and 32 healthy controls (control group) completed measures of maximal isometric knee extension torque (90° flexion) at baseline and following a 30‐min exercise protocol (post‐exercise). Exercise included 30‐min of repeated cycles of inclined treadmill walking and hopping tasks. Dependent variables were the coefficient of variation (CV) and raw‐change in CV (ΔCV): CV = (torque standard deviation/torque mean x 100), ΔCV = (post‐exercise – baseline). There was a group‐by‐time interaction (p = 0.03) on CV. The ACL‐R group demonstrated greater CV than the control group at baseline (ACL‐R = 1.07 ± 0.55, control = 0.79 ± 0.42, p = 0.03) and post‐exercise (ACL‐R = 1.60 ± 0.91, control = 0.94 ± 0.41, p = 0.001). ΔCV was greater (p = 0.03) in the ACL‐R group (0.52 ± 0.82) than control group (0.15 ± 0.46). CV significantly increased from baseline to post‐exercise (p = 0.001) in the ACL‐R group, while the control group did not (p = 0.06). The ACL‐R group demonstrated greater knee extension torque variability than the control group. Exercise increased torque variability more in the ACL‐R group than control group. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1165–1170, 2015.