The motor unit consists of a single motor neuron and its branches and all the muscle fibers innervated by that motor neuron. Muscular tension is increased by recruitment and summation.
1. Concentric Actions
Concentric action occurs when a muscle acts forcibly enough to actually shorten. This shortening pulls the bones of attachment close to each other, causing movement of the joint. For example: The muscles responsible for the flexion of the elbow joint act with sufficient force to shorten, which pulls the lower arm toward the upper arm. Although the pull is on all the bones of attachment, usually only the bone farthest from the trunk moves during a concentric action. To stand up from a squat position, the body must extend at the hip joints and knee joints, but gravity resists that extension. The muscles must develop sufficient force to overcome the gravitational force; if sufficient force is developed, the muscle shortens in a concentric action, pulling the bones to cause extension. Resistance training with free weights uses gravitational pull as the resistance; the use of pulleys changes the direction of the gravitation pull, offering resistance to movement in other directions. Water resists movement of submerged body parts in all directions.
To exercise muscles by using gravity as the resisting force, the movements must be done in the direction opposite the pull of gravity (away from earth). Arm abduction (moving arms laterrally away from body) from a standing position occurs opposite the pull of gravity, so a concentric action is required by the muscles that will pull the humerus into the abducted position. Movements such as shoulder horizontal flexion and horizontal extension executed from a standing position over parallel to the ground and therefore are not resisted by gravity. Internal tissue friction is the only resistance, but concentric action by the muscles responsible for these movements is still necessary. During these movements, gravity is still trying to draw the arm toward the earth, but it is not interfering with the horizontal movement. To perform horizontal flexion and extension against the resistance of gravity, the performer must get into a position in which these movements are away from the pull of gravity. To horizontally extend the shoulder joints against gravity, the performer can lie prone on a bench or floor or stand with the truck flexed at the hip. Horizontal flexion against gravity can be done from a supine (lying face up position) position on the floor.
A concentric action is also necessary for rapid movement, regardless of the direction of the other forces. When an external force could cause the desired movement with any muscular action, but too slowly, concentric actions produce the desired sped. An example of this is seen in the arm movements during the seconds’ count of a jumping jack, when the arms adduct from their abducted position: Gravity would adduct the arms, but concentrically acting muscles speed up the adduction.
A muscle that is very effective in causing a certain joint movement is a prime mover, or agonist. Assistant movers are muscles that are not as effective for the same movement. For example, the peroneus longus and brevis are prime movers for eversion of the intertarsal joints, but they assist plantar flexion of the ankle joint only a little. During concentric action, muscles that act opposite to the muscles causing the concentric action, the antagonist muscles, are basically passive and lengthen as the agonists shorten. For example, for the elbow to flex against gravity, the muscles responsible for elbow flexion act concentrically, while the antagonists, or the muscle responsible for elbow, relax and lengthen passively. In some fitness activities, such as aerobic dance, however, the antagonist muscles can offer more resistance to the concentrically acting muscles. This resistance occurs when all the muscles are activated (as in a bodybuilder pose) during movements.
2. Eccentric Action
An eccentric contraction occurs when a muscle generates tension that is not great enough to cause movement but instead breaks the speed of movement caused by another force. The muscle exerts force, but its length increases. Arm abduction requires a concentric action of muscles; gravity will adduct the arm back down to the side. To adduct the arm more slowly than gravity does, the same muscles that acted concentrically to abduct the arm act eccentrically to control the speed of the lowering arm. Eccentric actions also may occur when a muscles maximum effort is not great enough to overcome the opposing force; movement will be caused by that force despite the maximally activated muscle, which is still lengthening. An example of this may occur when a person with the elbow joint flexed 90 degrees is handed a heavy weight. The exerciser tries to flex the elbow joint or even maintain the 90 degree position but lacks the strength to do so. The elbow joint extends despite the efforts to flex it. The elbow joint extends despite the efforts to flex it. Muscle antagonist to the eccentrically acting muscles passively shortens during the movement.
3. Isometric Action
During an isometric or static action the muscle exerts a force that counteracts an opposing force. The muscle length does not change, so no movement occurs, and the joint position is maintained. The contractile part of the muscle shortens, but the elastic connective tissue lengthens proportionally, so there is no overall change in the entire muscle length. Holding the arm in an abducted position or maintaining a semi-squat position requires isometric action, producing just enough muscle force to counteract the pull of gravity and resulting in no movement. The effort involved in trying to move and immovable object (pushing against a wall) is another example of isometric actions; although the amount of muscular force can be maximal, the joint does not move.
A backward pelvic tilt desired during some exercises is maintained by isometric action of the abdominal muscles after they have acted concentrically to tilt the pelvis backward. During all resistance exercises that involve the arms or legs, the trunk muscles should act isometrically to stabilize the trunk and help prevent injury.
In Summary
The roles of muscles can cause movement (concentric), decelerate movement caused by another force (eccentric action), or prevent movement (isometric action). Muscles may also act isometrically to stabilize or prevent undesirable movement. For example, during a push up exercise, gravity tends to cause the vertebral column and hip joint to hyperextend. Isometric action of the abdominal muscles prevents this sagging; activating the abdominal muscles stabilizes the trunk in its proper position.
Another function of the muscle is to counteract an undesirable action caused by the concentric action of another muscle. The concentric action of most muscles causes more than one movement at the same joint or causes movement at more than on joint. If only one of those movements is intended, another muscle must act to prevent the undesirable movement. For example, concentric action of the upper trapezius fibers, which cause the depression and adduction, neutralize the unwanted action and help the desired action. The biceps brachii muscle causes both elbow flexion and radioulnar supination; for only flexion to occur, the pronator teres counteracts the supination.
Key Point:
A concentric action, which shortens the muscle and therefore pulls the bones of attachment, is necessary to effect joint movement in a direction opposite another force, such as gravity, as well as rapid joint movement regardless of the direction of any other forces. An eccentric action, which lengthens the muscle, controls the velocity of movement caused by another force. An isometric action, which does not change the length of a muscle, prevents movement.