Flexibility refers to the range of motion in a joint or group of joints, during a passive movement (passive meaning no active muscle involvement is required to hold the stretch; instead gravity or a partner provides the force for the stretch). Flexibility is a general component of physical fitness. Additionally, good range of motion will allow the body to assume more natural positions to help maintain good posture. This component becomes more important as people age and their joints stiffen up, preventing them from doing everyday tasks. Stretching is therefore an important habit to start and continue as one ages. Flexibility of a joint depends on many factors, particularly the length and looseness of the muscles and ligaments due to normal human variation, and the shape of the bones and cartilage that make up the joint. The primary reasons for increasing flexibility are enhanced performance and reduced risk of injury. The rationale for this is that a limb can move further before an injury occurs.