###### Abstract  
The prevalence of knee osteoarthritis (OA) increases with ageing and this research aimed to identify gait adaptations that could reduce OA by investigating ageing effects on knee joint biomechanics. Participants were 24 healthy young males (18-35 yrs) and 14 healthy older males (60-75 yrs). Three-dimensional motion capture (Optotrak) and walkway-embedded force plates (AMTI) recorded their natural preferred-speed walking and the following parameters were computed: knee adduction moment, knee joint vertical force, foot contact angle, toe-out angle, foot centre of pressure displacement, time to foot flat, step length, step width and double support time. A \(2\times 2\) (age \(\times\) limb) repeated measures mixed model analysis of variance design determined main effects and interactions. [PERSON]'s correlations between knee kinetic parameters and stride phase variables were also calculated. Both knee adduction moment and vertical joint force were larger in the older group. Relative to the young controls, older individuals showed a longer time to foot flat, less toe-out angle and wider steps. Correlation analysis suggested that reduced toe-out angle and increased step width were associated with lower knee adduction moment; furthermore, knee joint vertical force reduced with greater step length. Future research could focus on intervention strategies for managing excessive knee joint stresses to prevent the ageing-related development of knee OA. Keywords: gait biomechanics; knee adduction moment; knee joint force; motion capture; knee joint kinetics +
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contact may, therefore, lead to excessive mechanical energy transfer to the knee joint, stressing the protective articular cartilage and ligaments [6, 7]. While a physical stimulus is important in promoting articular cartilage synthesis, excessive loading in everyday walking for a prolonged period of time can contribute to OA progression [6, 8]. It is, therefore, important to investigate how ageing affects gait biomechanics influence foot-ground impact and associated knee joint kinetics. Our focus in this study was ageing effects on knee joint adduction moments and vertical forces (Figure 1). Knee adduction moment is recognised as the key kinetic marker for Varus deformity and associated OA development, principally in the medial compartment [9]. Reducing adduction moment during early stance has, therefore, been proposed as an OA intervention and gait adaptations; less toe-out angle and increasing step width have been reported to reduce initial peak adduction moment and associated OA risk [10]. In addition, peak vertical knee joint force during early stance reflects the primary load and has been associated with damage to joint structures [11, 12]. Due, in part, to difficulties in modelling knee joint forces their role in OA has been less thoroughly investigated but of the total knee joint force, the vertical component is largest due to gravitational effects on the whole-body centre of mass [13]. [PERSON] et al. [14], for example, reported that increased body mass due to obesity imposed the larger loads on the knee joint and increased OA risks, indicating that vertical force is a risk factor. Most previous experimental protocols designed to modify walking patterns in OA patients instructed participants to walk in certain ways to examine whether the gait adaptations would change knee joint kinetics [15, 16]. In this project, however, as OA is more prevalent in older people, we compared the \"natural\" gait patterns of healthy, physically active older adults and young controls. It was hypothesised that older adults' gait patterns would reflect increased knee loads, indicated by greater knee adduction moments and peak vertical force. We also correlated the knee kinetics and stride phase variables to determine the qualitative aspects of gait adaptations that may be effective in reducing the adverse effects of knee joint kinetics.