Lateral and forward/backward perturbation while walking will be used to elicit balance recovery responses. The degree of necessary joint force and how this relates to a person's functional capacity at specific joint levels will be determined by the torque demand to capacity ratio (DCR: task demands relative to strength capacity). The DCR provides a joint-specific unified scalar quantity representing the balance recovery demand normalized by the maximum muscle strength capacity of an individual. It would indicate whether a person can recover balance and to what extent of his/her maximum capacity is taxed.