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The study aimed to investigate the impairments in quadriceps neuromuscular function, including strength, rate of torque development (RTD) and activation failure (QAF) early after an ACL injury. A cross-sectional study was conducted. Thirty patients with a primary ACL injury within three months, aged 18 to 40 years old, with a pre-injury Tegner Activity Scale of not less than 6, and who were scheduled for ACL reconstruction were included. Thirty healthy controls with matched age, physical activity level and body mass index were also recruited. All quadriceps neuromuscular functions were measured on an isokinetic dynamometer with knee flexion at 45°. Quadriceps strength was measured by maximal voluntary isometric contractions (MVIC). Early (RTD0-50) and late (RTD100-200) phases of RTD were calculated as the average slope of the torque-time curve of the MVIC from 0 to 50 milliseconds and 100-200 milliseconds, respectively. QAF was quantified by the central activation ratio (CAR) measured by superimposed burst technique. The results of Mann-Whitney U test showed that when compared with the healthy limbs, the injured limbs of the ACL group showed lower quadriceps strength (P<0.001), RTD0-50 (P<0.001) and RTD100-200 (P<0.001); the uninjured limbs showed lower quadriceps strength (P=0.009), RTD0-50 (P=0.006) as well as greater QAF (P=0.010). To conclude, bilateral quadriceps suffered from neuromuscular impairments early after an ACL injury.

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