Previous studies have demonstrated that resistance training (RT) is beneficial to increase muscle strength, improve functional ability and the ability to rapidly produce force, known as the contractile rate of force development (RFD) in older adults. However, much less research has focused on the effect of RT on the lower extremity muscle strength, contractile RFD and impulse in middle-aged and older people with musculoskeletal conditions, especially for osteoporosis (OP) (or osteopenia) or knee osteoarthritis (KOA). Therefore, the purpose of this study was to investigate the effect of RT on the lower extremity muscle strength, RFD and impulse in middle-aged and older people with musculoskeletal conditions, especially for OP and knee OA (KOA). The investigators hypothesized that such a training program would lead to induce not only specific muscle strength enhancement but also an increment in contractile RFD and impulse.