Background Hashemi et al. (2011) put forward a new theory of ACL injury mechanism, named the ‘hip extension knee flexion paradox’ that attempted to integrate itself with the most convincing findings in ACL research. This theory of mechanism stated that non-contact ACL injury would occur when the following four criteria are simultaneously met: 1) the tibial plateau has a posterior slope 2) the knee is near full extension upon application of a dynamic ground reaction force 3) activation of musculature about the knee is delayed and 4) hip extension and knee flexion occur concurrently. In their paper, Hashemi et al. (2011) also suggest that fatigue may play an important role in increasing the risk of non-contact ACL injury by delaying activation of musculature about the knee joint. Hip-knee coordination and the effect of fatigue on it, have however not previously been investigated. Purpose The purpose of this study was to quantify hip – knee coordination in the sagittal plane during a 45° cutting movement. Methods Nine female team sports players with experience at international or national level and no history of ACL injury were recruited. Written consent was obtained at the time of testing. Cutting was assessed under pre and post fatigue conditions, whilst motion capture data were collected for the lower limbs using QTM (Qualysis AB, Göteburg, Sweden). In each condition, ten successful 45° cuts were captured for the dominant leg. The fatiguing protocol comprised drop maximal vertical jumps and a multi-directional short sprint exercise. Hip and knee sagittal joint angles were extracted between the time of foot contact to 40 ms, the window in which ACL injury is believed to occur. A coordination pattern of 1) hip flexion – knee flexion, 2) hip extension – knee flexion, 3) hip extension – knee extension, 4) hip extension knee flexion was determined for each time point. The average time that each participant spent in each coordination pattern was calculated and the median coordination pattern was determined for each participant at each time point. Wilcoxon tests were executed in SPSS (IBM SPSS Version 22, Armonk, NY) to determine the effect of fatigue on the percentage time spent in hip extension – knee flexion and hip extension – knee extension. Results The average modal coordination pattern of the group, both pre- and post-fatigue, was to extend the hip and knee for the first 10 ms after foot contact. By 25 ms, all participants had changed coordination pattern to flex at the knee whilst continuing to extend at the hip. Both pre- and post-fatigue, a greater percentage of time (p<0.05) was spent in hip extension – knee flexion than in hip extension – knee extension. No change was seen in the percentage time spent in hip extension – knee flexion after fatigue. Practical Applications Most players adopted a hip extension – knee extension pattern at foot contact before switching to hip extension – knee flexion. The initial observation of hip extension – knee extension was interesting as it represents stiffness at both the hip and knee joint, which has been suggested as high risk for non-contact ACL injury (Pollard et al., 2010). The latter coordination pattern has also been hypothesised as high risk for non-contact ACL injury and was dominant both pre- and post-fatigue. The absence of change in coordination patterns after fatigue suggests this risk factor was not accentuated as a result of a short duration fatiguing protocol in an elite, healthy population
