Study instruments and outcomes: Primary outcome measures: reactive balance control, as indicated by lower extremity muscle activation onset latency; hip and ankle strategies and centre of pressure movements will be measured by electromyography, electrogoniometry and a force platform, respectively. Secondary outcome measures: knee muscle strength, balance confidence and fall history will be assessed by an isokinetic dynamometer, Activities-specific Balance Confidence Scale (Chinese) and interviews, respectively (pre-, post- and follow-up measurements).