Sex differences in knee extension rate of torque development and patient-reported function among individuals with anterior cruciate ligament reconstruction

Abstract

Purpose: Men and women with anterior cruciate ligament reconstruction (ACLR) continue to receive similar rehabilitative care despite evidence that women are more likely to experience poor clinical outcomes, a more than 4 times greater risk for second ACL injury, and a 45% greater risk of developing post-traumatic osteoarthritis (PTOA) when compared to men of similar age and activity level. While the mechanism of PTOA development following ACLR remains unclear, altered knee joint loading characteristics during activities of daily living have been hypothesized as potential contributors to the early stages of this process. Recently, knee extension strength, knee extension rate of torque development (RTD), and patient-reported psychological readiness have been shown to influence lower extremity loading characteristics, but it remains unclear if sex differences are present in these outcome measures. If present, this would help to explain the disparate outcomes experienced by men and women with ACLR. Identification of modifiable outcomes such as knee extension strength characteristics and psychological readiness early after ACLR may provide clinicians with feasible targets for patient-centered intervention within this population. Therefore, the primary purpose of this study was to determine the effect of participant sex on measures of knee extension strength characteristics and psychological readiness among men and women within the first year after ACLR. Methods: Thirty-three women (age= 19.0±4.3 years, time since surgery= 6.8±1.6 months) and 29 men (age= 21.4±6.8 years, time since surgery= 7.4±2.3 months) with a history of ACLR were enrolled in an on-going cohort study from which these data were obtained. Participants were included if they were between the ages of 13 and 40, had a history of ACLR, were within 2 weeks of their final clinical visit to their orthopaedic surgeon, and had not yet returned to sport participation. Participants completed a detailed health history, the ACL Return to Sport After Injury (ACL-RSI) scale, and the International Knee Documentation Committee (IKDC) subjective knee evaluation form after which they performed a bilateral assessment of knee extension maximal voluntary isometric contraction (MVIC) strength using a multimodal dynamometer. After a standardized familiarization protocol, participants completed 3 knee extension MVIC trials during which they were instructed to “kick out as hard and fast as possible” while being provided visual feedback on a nearby monitor. Peak knee extension MVIC torque (Nm) was documented from each trial and normalized to body mass (Nm/kg). Additionally, RTD100 was calculated as the slope of the torque x time curve from contraction initiation to 100ms following contraction initiation (Nm*kg-1*s-1) and RTD200 was calculated as the slope from of the torque x time curve from 100ms to 200ms after contraction initiation (Nm*kg-1*s-1). Participant demographics were compared between the sexes using independent samples t-tests while patient-reported outcome measures and knee extension strength characteristics were compared using ANCOVA with months since surgery as a covariate. Results: There were no significant sex differences in age (p= 0.10), time since surgery (p=0.65), patient-reported knee function (Female= 80.9±11.2, Male= 85.1±9.2; p= 0.11), or psychological readiness for sport (Female= 69.3±23.8, Male= 62.8±24.5; p= 0.25) among participants with ACLR. Female participants displayed lesser involved limb (Female= 1.99±0.53 Nm/kg, Male= 2.71±0.71 Nm/kg; p < 0.001) and contralateral limb (Female= 2.62±0.59 Nm/kg, 3.17±0.52 Nm/kg; p < 0.001) knee extension MVIC torque as well as lesser involved limb knee extension RTD100 (Female= 6.41±2.39 Nm*kg-1*s-1, Male= 8.24±3.17 Nm*kg-1*s-1; p= 0.01) and involved limb RTD200 (Female= 6.62±2.21 Nm*kg-1*s-1, Male= 9.43±3.40 Nm*kg-1*s-1; p < 0.001; Figure 1) when compared to male participants. Conclusions: Our findings indicate that women experience larger magnitude deficits in involved limb knee extension strength and RTD when compared to men during the terminal phase of clinical care following ACLR. This is despite no significant differences in patient-reported knee function or psychological readiness between the sexes. While not perceived as a functional limitation among women with ACLR, persistent quadriceps dysfunction and the associated alterations in lower extremity loading characteristics may set the stage for early degenerative changes in knee joint articular cartilage structure. Early identification of sex-based disparities in modifiable functional outcomes (knee extension strength and RTD) following ACLR may provide clinicians with goals for patient-centered impairment-based rehabilitation strategies to improve lower extremity loading characteristics which are implicated in the early progression of PTOA.