Quadriceps force fluctuation during maximal isometric contraction is altered in acl injury and is associated with lower limb functional performance: a cross-sectional study with athletes

Abstract

Objective: This study aims to investigate the effects of ACL injury on force fluctuation measures (stability and complexity) of the quadriceps muscle, and how these measures relate to functional performance.

Methods: Forty-two athletes (14 females) with unilateral ACL injury were assessed. Quadriceps maximal isometric contraction was performed for both limbs before any surgical intervention. A 1.5s window from maximal torque signal was used to compute average torque (AT), force stability (coefficient of variation, CV), and complexity (sample entropy, SE), from which the limb symmetry index (LSI, % of contralateral limb) was estimated. The single hop test (SHT) was also performed and expressed as LSI. A nonparametric approach was used for between-limb comparison (Wilcoxon test) and to assess the relationship between force variables and SHT (Spearman correlation).

Results: Significant limb asymmetry was found, with the injured limb exhibiting smaller AT (p < 0.001) and higher SE (p = 0.044) than the contralateral limb. A significant, moderate association was found only for LSI-SE and LSI-SHT (rho = -0.535, p = 0.002).

Conclusions: In summary, along with the average torque, force complexity was altered in ACL-injured limbs of professional athletes, and was associated with larger asymmetries in functional performance, corroborating its relevance as a marker of ACL injury-related neuromuscular deficits.