Type of measurement used influences central and peripheral contributions to quadriceps weakness after anterior cruciate ligament (ACL) reconstruction

ObjectiveThe relative contribution of muscle size and voluntary activation (VA) on quadriceps strength after anterior cruciate ligament (ACL) reconstruction remains inconclusive. Here, we aimed to determine the contributions of muscle size and VA on quadriceps strength in ACL-reconstructed patients and determine if contributions were similar if unilateral outcomes (i.e. ACL-reconstructed limb) or the LSI was used. Design: A cross-sectional study.SettingA university research laboratory.ParticipantsSixteen individuals 6–12 months after ACL reconstruction (Age: 22.3 ± 6.0yr, Height: 1.7 ± 0.1 m, Mass: 68.7 ± 11.5 kg) were recruited.Main outcome measuresQuadriceps isometric strength and VA, via the interpolated triplet technique, were assessed bilaterally. Ultrasound images were acquired of the vastus lateralis to calculate cross-sectional area (CSA) in both legs. LSI’s were computed for all variables by expressing values of the reconstructed leg as a percent of the non-reconstructed leg. Separate stepwise linear regressions were performed to examine the contribution of VA and CSA on quadriceps strength. Model 1 used LSI for all outcomes and model 2 used outcomes from the reconstructed leg.ResultsWe observed between limb deficits of 27.78% in quadriceps strength, 13.61% in vastus lateralis CSA, and 13.18% in VA (P < 0.05). Strength LSI was significantly predicted by VA LSI (R² = 0.45, P < 0.01), but not by CSA LSI (R² = 0.01, P =0.87). Reconstructed leg strength was significantly predicted by VL CSA (R² = 0.50, P < 0.01) but not quadriceps VA (R² = 0.08, P =0.11).ConclusionsThe contributions of VA and CSA on quadriceps PT differed greatly if LSI or reconstructed leg outcomes were used. Evaluation of VA and CSA in unison may be provide a more holistic understanding of the sources of muscle weakness after ACL reconstruction.