Skeletal Muscles

1. Muscles are connective tissues and they are three types. Skeletal, smooth and cardiac (refer chapter1). Smooth muscles (unstriped & involuntary) line the internal organs and cardiac muscles form the heart. Skeletal muscle (striped & voluntary) is attached to at least two bones via tendons in order to cause and control the movement of one bone with respect to other bone. When muscle fibers contract under the stimulation of a nerve, the muscle exerts a pull on the bones to which it is attached. The development of tension in the muscle has been possibly only due to contraction of muscle fibers. The muscle contraction can take place as a result of muscle shortening (concentric contraction), or muscle lengthening (eccentric contraction) or without any apparent change in length of the muscle (static or isometric interaction).

2. The contractile element (motor unit) consists of many sarcomere elements connected in a series arrangement as shown in the figure.

The muscle force is generated within these sarcomeres by lengthening or shortening of the muscle. The force and torque developed by a muscle depend upon number of sarcomeres (motor units) within muscle, number of sarcomeres utilized, the manner of change of length of muscle, the velocity of muscle contraction and length of the lever arm of the muscle force. Two different forces are generated in a muscle. The contractible elements of the muscle produce active tension due to the voluntary muscle contraction. The passive tension is developed within the connective muscle tissues when the muscle length surpasses its resting length. The net force is the resultant of these two forces. A typical tension versus muscle length diagram is given in the figure.

At resting, length to the number of cross bridges between filament is maximum. Hence active tension (Ta) is maximum & passive tension (Tp) is Zero. On lengthening of muscle, the filaments are pulled apart resulting in reduction of number of bridges. Hence active tension (Ta) reduces. At full extended position, active tension (Ta) becomes zero. 3. The outcome of muscle contraction is always tension. Hence a muscle can only exert a pull and it cannot exert a push. A muscle is also named according to the function it performs. A muscle is called ‘agonist’ if it causes movement through concentric contraction. An ‘antagonist’ muscle controls the movement by eccentric contraction. Hence the biceps during flexion of the fore arm is ‘agonist’ as the length of muscle decreases and the bicep during extension of the forearm is ‘antagonist’ as the length of the muscle increases.