Sound knowledge of its structure, contractile ability, and biomechanical characteristics is necessary to understand the interrelationships that determine human function. Diverse but interrelated factors affect the capacity of normal skeletal muscle to generate tension to control the body and perform motor tasks, all of which contribute to the magnitude, duration, and speed of force production (Kisner &. Colby, 2002). One of the key concepts in the study of muscle contraction is the length-force relationship.
To understand skeletal muscle biomechanics, it is important to have a good understanding of its anatomy. The skeletal muscle fibers are made up of contractile myofilaments, actin, and myosin as well as the troponin-tropomysin complex. The refractive index differences between these myofilaments are responsible for the striated appearance of skeletal muscle. The light I band, which is composed of the thin filaments actin and the troponin-tropomyosin complex, is divided by the dark Z line and the dark A bands, composed of the thick filament myosin, has the lighter H band in its center. The area between two adjacent Z-lines is called a sarcomere (Ganong, 2003).
One of the biomechanical properties important to muscle physiology is the length-force relationship. The force-length relationship of muscle fibers is converted at the joint to a torque-angle relationship. In humans, the torque-angle relationship often is represented as the force-length relationship of muscle fibers (Kawakami &. Fukunaga, 2006). .
Torque, also known as moment of the arm, is defined as a measurement of the effectiveness of a force in producing rotation about an axis. .It is equal to the product of a force times the perpendicular distance between the site of force application and the axis of rotation (DeLisa, 1998). . Torque can also be expressed as T = Ft sin q, where q is the angle between the direction of force application and the axis of rotation.