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 sixmonths post-ACLR. In addition to the maximum strength (Fmax), 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 (RFDmax, RFD50, RFD150 and RFD250). 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 Fmax (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 Fmax 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.