Association Between High-Intensity Knee Extensor Torque Steadiness and Center-of-Pressure Path Length During a...

Quadriceps function contributes to lower-extremity support during single-limb tasks. It can be characterized by maximal voluntary contraction (MVC) and submaximal torque steadiness. MVC reflects maximal knee extensor torque-generating capacity, whereas torque steadiness reflects the ability to regulate submaximal torque output. These aspects of quadriceps function may contribute to postural regulation during single-limb balance tasks, including lower-extremity reach tasks. However, their relationships with center of pressure (COP) behavior during reach-hold tasks remain unclear.

A total of 32 healthy young adults performed quiet single-leg stance, forward reach-hold, and backward reach-hold tasks. Anteroposterior (AP) and mediolateral (ML) COP path lengths were recorded. Knee extensor MVC and torque steadiness at 10%, 40%, and 70% MVC were assessed. Torque steadiness was quantified using the coefficient of variation from the most stable five-second window within each 15-second trial. Associations between knee extensor variables and task-specific COP measures were examined using Pearson's or Spearman's correlation coefficients, as appropriate. The Holm procedure was applied to adjust for multiple correlation tests. A supplementary simple linear regression analysis was performed for the association that remained significant after Holm correction.

Compared with single-leg stance, AP COP path length was greater during forward and backward reach-hold, whereas ML COP path length was greater during backward reach-hold. In the unadjusted analyses, normalized MVC torque and torque CV at 40% MVC were moderately associated with AP COP path length during forward reach-hold. Torque CV at 70% MVC was also moderately associated with AP and ML COP path lengths during backward reach-hold. After Holm correction, only the positive association between torque CV at 70% MVC and AP COP path length during backward reach-hold remained statistically significant (ρ = 0.48, 95% CI (0.14, 0.71), unadjusted p = 0.006, Holm-adjusted p = 0.035). Greater high-intensity torque variability was therefore associated with a longer AP COP path. In the supplementary regression analysis, torque CV at 70% MVC explained 18.9% of the variance in AP COP path length (R² = 0.189, p = 0.013).

Reduced knee extensor torque steadiness at high intensity was associated with greater AP COP path length during backward reach-hold in healthy young adults. Associations involving maximal torque capacity, moderate-intensity torque steadiness, and ML COP behavior were observed only in the unadjusted analyses and should be regarded as exploratory. These findings suggest that high-intensity torque regulation may be relevant to AP postural behavior during a demanding backward reach-hold task; however, confirmation in larger and clinical populations is required.