Short communicationA method for concise reporting of joint reaction forces orientation during gait

Estimation of joint reaction forces (JRF) is critical for the understanding of load-related pathologies, such as osteoarthritis (OA). Typically, singular components at discrete time-points are presented without knowledge of their orientation over time. The aim of this study was to develop and demonstrate the utility of a method for the concise and intuitive representation of JRF orientation. A scaled musculoskeletal model of the lower limbs was informed by walking gait data from adults with knee OA (n=10) and healthy controls (C) (n=10). Muscle forces and subsequently JRF were computed. The intersections of the JRF vector and a transverse plane proximal to the joint were computed. The 95% confidence ellipse was computed for these points. This allowed the following metrics to be calculated: the normalised area of the ellipse (A); the ratio of the long and short axes (R); the angle between the long axis of the ellipse and the anterior-posterior axis of the distal segment (α) and the position of the centre of the ellipse relative to the origin of the segment (XC, ZC). At the knee, the OA group displayed a more AP-oriented JRF (OA: α=3.0±28.2 °C: α=−15.3±37.1°, p= 0.03), less overall variation in the JRF orientation (OA: A=0.04±0.03 C: A=0.07±0.03, p<0.001), and less ML fluctuations of the JRF (OA: R=4.17±3.48 C: R=2.41±0.76, p=0.01). Significant effects were also observed at the hip and ankle. The method presented has the potential to allow better visualisation and insight into the behaviour of joint contact forces in clinically relevant pathologies.