Asymmetries in explosive strength following anterior cruciate ligament reconstruction

Background: Despite its apparent functional importance, there is a general lack of data regarding the time-related changes in explosive strength and the corresponding side-to-side asymmetries in individuals recovering from an ACL reconstruction (ACLR). The present study was designed to assess changes in the maximum and explosive strength of the quadriceps and hamstring muscles in athletes recovering from an ACLR. Methods: Twenty male athletes with an ACL injury completed a standard isometric testing protocol pre-ACLR, four and six months post-ACLR. In addition to the maximum strength (F-max), the explosive strength of quadriceps and hamstrings was assessed through four variables derived from the slope of the force-time curves over various time intervals (REDmax, RED50, RFD150 and RED250). Side-to-side asymmetries were calculated relative to post-ACLR measures of the uninvolved leg (standard asymmetries), and relative to pre-ACLR value of the uninvolved leg (real asymmetries). Results: Pre-ACLR asymmetries in quadriceps RFD (average 26%) were already larger than in F-max (14%) (p < 0.05). Six months post-ACLR real asymmetries in RFD variables (33-39%) were larger than the corresponding standard asymmetries (26-28%; p < 0.01). Average asymmetries in hamstrings' RFD and F-max were 10%, 25% and 15% for pre-ACLR and two post-ACLR sessions, respectively (all p > 0.05). Conclusions: In addition to the maximum strength, the indices of explosive strength should also be included in monitoring recovery of muscle function following an ACLR. Furthermore, pre-injury/reconstruction values should be used for the post-ACLR side-to-side comparisons, providing a more valid criterion regarding the muscle recovery and readiness for a return to sports. (C) 2014 Elsevier B.V. All rights reserved.