## 4 Discussion

Table 1: Correlations, knee joint kinetics (adduction moment and vertical force) and gait parameters; SD = standard deviation indicating intrasubject variability over multiple gait cycles. * = \(p\) < 0.05, ** = \(p\) < 0.01.  
moment. [PERSON] et al. [27] also reported the possibility that toe-in gait may stimulate certain lower limb muscles and potentially help relieve knee joint loading. Negative correlations between toe-out angle and step width identified in the current research implied that wider steps would be closely associated with less toe-out angle (i.e., toeing-in), also found by [PERSON] et al. [15]. Increased step width, the medio-lateral displacement between the two heels, tends to result in pointing the toes inward relative to the heels. Age-related increase in step width and reduced toe-out angle may, therefore, be adaptive in achieving lower knee adduction moment among older individuals. Contrary to expectation, our investigation of foot control mechanics, including foot contact angle, COP control and time to foot flat showed few direct associations with knee joint kinetics. Preventing flat-foot contact and the associated increase in time to foot flat were hypothesised to dissipate foot contact impact but the obtained results suggest that knee joint kinetics may be independent of these foot control mechanics. It is, however, possible that such foot impact dissipation strategies could reduce stress on the ankle or other lower limb joints. Toe-out gait was, however, found to be associated with larger COP displacement, possibly disadvantageous in dynamic balance control [28]. A further interesting finding, common to both age groups, was negative correlations between step length and knee joint force. Taking longer steps is possibly related to reduced knee joint force. This is a reasonable hypothesis because rather than stressing the knee joint, force may be utilised to increase step length. Efforts to take longer steps may reduce knee joint vertical compression. Interestingly, larger foot contact angle was associated with increased step length. Dorsiflexion at heel contact is responsible for increasing foot contact angle and ageing often impairs dorsiflexor strength [29]. Strength training or footwear interventions to compensate reduced dorsiflexion can possibly contribute to longer strides, which may reduce _vertical_ knee joint forces by directing ground reaction force _anterior_. Caution should, however, be exercised with older adults because increased foot contact angle was accompanied by larger knee adduction moment. Shorter double support time is also considered to promote longer step length. As prolonged double support time has been identified as a typical gait adaptation to secure balance, particularly among frail populations including seniors [20], gait training to reduce excessively long double support time may help increase step length and potentially contribute to a reduction in knee joint vertical force. Balance support should be carefully considered when walking with longer steps or shorter double support time than usual for training purpose [9]. While the current research aimed to investigate whether ageing was a factor affecting knee joint kinetics and associated OA risks [7], the primary aim was to identify effective gait modifications to reduce OA risks by focusing on knee adduction moment and knee joint vertical force. Based on the current findings, practical intervention strategies to reduce OA risk should be devised with the following considerations. First, some reports have not identified direct links between these parameters and OA risk. From the perspective of gait biomechanics, knee joint kinetics should indicate stress of the knee joint, whereby damage beyond a threshold intensity, frequency and duration could lead to OA. Most well-known is adduction moment that compresses the medial part of the knee but controversy remains as to whether knee adduction moment is related to OA [7].