Muscle strain can be caused by overworked muscles that attempt to protect your joints from painful movement. As arthritis causes swelling and inflammation of your joints, connecting muscles feel the stress. The swollen joints can alter the muscle attachments to your bones, setting them at odd angles and causing stress and strains. Muscle weakness also plays a role in determining how severe osteoarthritis in the knee will be. It has been assumed that muscle weakness develops because pain prevents the arthritis sufferer from fully using the joint. However, a study of elderly patients suggests that, at least for women, muscle weakness may come first. The weakness may then contribute to progressive joint damage. Exercising to strengthen muscles has been shown to reduce pain and improve mobility. In fact, many of the problems common to arthritis sufferers-loss of flexibility, muscle atrophy, muscle weakness, pain, depression, and fatigue-can be alleviated through exercise.