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

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.