Human movements are controlled by muscles and the forces of gravity. Muscles are used to hold a position, to raise or lower a body part, to slow down a fast moving segment (arm ,leg) and to generate speed in the human body or to propel an object into the air (football, baseball, javelin) . Muscles are capable contracting rapidly and vigorously, but they tire easily and require rest after only brief periods of activity.
Tension developed by the muscles applies compression to the joints enhancing joint stability. However, in some joint positions, the tension generated by the muscles can act to pull the segments apart and create instability.
Muscles are used asymmetrically in most activities in which one side of the body will use a specific set of muscles, and the other side of the body will be using opposite or different muscles. This muscle relationship is true in activities such as golf, bowling, baseball, walking and racquet sports that have nonsymmetrical use of the arms, legs, and trunk. Muscles are used symmetrically in activities such as weight lifting and jumping where both sides of the body are performing the same movement
using the same muscles.
Exercise programs for a young healthy population will most likely involve exercises that push the muscular system to high levels of performance. Muscles can exert force to and develop power. For this reason many athletic training programs with the young and healthy are designed to achieve maximum performance. The same principles used for young, active individuals can be scaled down to accommodate persons of limited ability. If we use the elderly as an example, it is apparent that the reduction of physical strength is one of the major factors affecting daily activity.
Continue Article 
Visual How To Measure