(48) QUADRICEPS STRENGTH SYMMETRY AND SINGLE-LEG JUMP HEIGHT SYMMETRY AFTER ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION

Purpose: Quadriceps strength and single leg vertical jump height of the operative (OP) limb can be negatively impacted after anterior cruciate ligament reconstruction (ACLR). Therefore, it is possible that the relationship between quadriceps strength and jump height may differ between the non-operative (NOP) and OP legs after ACLR. This relationship can be modeled with limb symmetry indices (LSI) of quadriceps strength and jump height in ordinary least squares regression or through linear mixed models, which allow for the within-subjects repeated measures design of both OP and NOP legs to be included in the model. Therefore, the purpose of this investigation was to determine if the relationship between quadriceps strength and single leg jump height differs between legs in individuals after ACLR. Methods: Twenty-four (21 female; mean±SD=28.3±8.6 years; 48.5±31.8 months post-ACLR) individuals volunteered to participate. Participants completed three single-leg vertical jumps on a force plate to determine jump height (JH) and three quadriceps maximal voluntary isometric contractions (MVIC) in an isokinetic dynamometer on each leg. The NOP leg was tested prior to the OP leg on both assessments. The LSI was calculated as [100 x (OP÷NOP)] for MVIC and JH. Paired sample t-tests assessed between-leg differences in MVIC (Nm) and JH (cm) at p≤0.05 with Cohen’s d effect sizes. Ordinary least squares regression was conducted to determine if there was a relationship between MVIC LSI and JH LSI. Next, a linear mixed model was fit with JH as the outcome variable with the independent variables of MVIC, leg (OP vs. NOP) and the MVIC x leg interaction and a random intercept for participant. From the linear mixed model, estimated marginal means of the linear trends for the OP and NOP MVIC-JH relationship were calculated to determine if the relationship differed between legs. For both models, participant age, body mass, and time since ACLR were entered as covariates. Results: Both MVIC (mean diff.±SE; 22.0±5.8 Nm, p< 0.001, d=0.784) and JH (2.12±0.31 cm, p< 0.001, d=1.39) were greater in NOP leg than OP. The LSI model was not significant (p=0.761, R²=0.089), indicating no relationship between quadriceps MVIC and JH LSI. The linear mixed model indicated that the trends for MVIC and JH did not differ between legs (interaction p=0.008; OP slope=0.0334±0.01; NOP slope=0.018±0.010). However, there was a positive trend for MVIC (β=0.026; 95% CI=0.006 to 0.048). The leg coefficient was also significant (β=-1.87; 95% CI= -3.14 to -0.61), supporting the JH t-test results. Conclusions: The NOP leg outperformed the OP leg in both MVIC and JH. However, from the LSI model, it can be concluded that greater strength symmetry (LSI) does not positively relate to jump height symmetry. While from the linear mixed model, although MVIC had a significant positive relationship with jump height (0.026 cm increase in jump height for every 1 Nm increase in MVIC), this relationship did not differ between legs. Practical Applications: Practically, due to the positive effect of MVIC in the mixed model, strength may influence single leg jump height after ACLR, but this does not differ between surgical leg status. Therefore, to attempt to attenuate jump height differences between legs, plyometric exercises may need to be continually emphasized long after ACLR instead of a sole continued focus on strength symmetry. Acknowledgements: None.