Not being active enough can cause the Quadriceps, Hamstrings and Calf muscles to get weak. When those muscles get weak, the knee joint absorbs more force. These additional forces will cause negative stresses on the knee. As the joint stresses, it breaks down. This can be a cause of early Arthritis. Also, there is more stress on the ligaments, which can cause tears.
More often than not, pain in your hips can easily be misdiagnosed. The pain may actually be coming from your lower back as radiating pain. Since the nerves that control the legs start from the lower back, compromised nerves can cause pain in the hip. Just like if you were having heart problems, you’ll feel the pain in your arms.