The dynamic control ratio masks bilateral asymmetries - A gender-specific analysis of 264 healthy and ACL-injured athletes

Isokinetic strength tests are frequently applied to assess anterior cruciate ligament (ACL) rehabilitation processes. However, diverging methodologies cause misleading conclusions. This crosssectional study evaluated the effects of gender (male vs. female), group (healthy vs. ACL-injured) and limb (dominant/healthy vs. non-dominant/ACL-injured) on thigh muscle balance of 138 female and 126 male athletes (50% ACL-injured, averagely 12.8 months after surgery). Balance was analysed between legs (bilateral asymmetry) and between concentric knee extensor (Qcon) and eccentric knee flexor strength (Hecc) (DCR = dynamic control ratio, DCRe = DCR at the equilibrium point). Females were generally 17-27% weaker than males. Independent of gender and time after surgery, ACL-injured athletes demonstrated bilateral asymmetries (7-20%) in peak (PMQcon, PMHecc) and DCRe moments (p <= 0.030; 0.018 <=eta(2)(p)<= 0.215). ACL-injured athletes' affected (24-28%) and unaffected (12-24%) hamstrings and quadriceps peak moments were significantly weaker compared to healthy athletes (p<0.001; 0.061 <=eta(2)(p)<= 0.362). The bilateral asymmetries of PMQcon significantly decreased from early to late self-reported rehabilitation phases (p<0.001; eta(2)(p)=0.158). Peak and DCRe moments detected bilateral asymmetries, whereas DCR revealed similar to 50% false negative attributions. This knowledge provides guidance for future design and interpretation of isokinetic tests.