Explosive Hamstrings Strength Asymmetry Persists Despite Maximal Hamstring Strength Recovery Following Anterior Cruciate Ligament Reconstruction Using Hamstring Tendon Autografts

Purpose: To investigate the differences in maximal (isometric and concentric peak torque) and explosive (rate of torque development (RTD)) hamstring and quadriceps strength symmetry between males and females during early and late phase rehabilitation after anterior cruciate ligament reconstruction (ACLR) using hamstring tendon (HT) autografts and to determine the interaction of time and sex on maximal and explosive strength symmetry. 
Methods A total of 40 female and 53 male participants were assessed during early (3-6 months post-53 operative) and late (7-12 months) phases of rehabilitation following ACLR. Maximal (concentric and isometric 54 peak torque) and explosive (isometric RTD) hamstring and quadriceps strength were assessed at each timepoint and presented as limb symmetry index (LSI).
Result Maximal concentric hamstrings asymmetry (Early: 86 ± 14; Late 92 ± 13; p = 0.005) as well as maximal concentric (Early, 73 ± 15; Late 91 ± 12; p <0.001) and explosive (Early: 82 ± 30; Late: 92 ± 25; p = 58 0.03) quadriceps asymmetry decreased from early to late rehabilitation. However, there were no significant changes in maximal isometric quadriceps strength and explosive isometric hamstring strength in the same time period. Females had a larger asymmetry in maximal concentric (Females: 75 ± 17; Males: 81 ± 15; p = 0.001) and explosive (Females: 81 ± 32; Males: 89 ± 25; p = 0.01) quadriceps strength than males throughout rehabilitation. There were no sex differences in maximal and explosive hamstring strength. There were no sex by time interactions for any variables. 
Conclusion Explosive hamstring strength asymmetry did not improve despite recovery of maximal hamstring strength during rehabilitation following ACLR with HT autografts. While sex did not influence strength recovery, females had larger maximal and explosive quadriceps strength asymmetry compared to males throughout rehabilitation following ACLR.