54


PART I FOUNDATION CONCEPTS


are shown slightly overlapping with the thick myosin fi laments—an area called the anisotropic band, or A band. The outer portion of the sarcomere, where only actin is located, is referred to as isotropic bands or I bands. The area in the central portion of the sarcomere, where the thick fi lament does not overlap with the thin fi lament, is called the H zone. The middle of the H zone is the M band.


At rest, cross-bridges project from the myosin but are not coupled with the action. Adenosine triphosphate (ATP) is at the head of the projection, and troponin covers the attachment sites of the actin. The onset of a muscle contraction begins with a nerve impulse arriving via an alpha motor efferent neuron at the motor end plate. This stimulation results in an action potential that travels along the muscle fi ber resulting in the release of Ca2+


,


which interacts with troponin, causing the tropomyosin molecules to change position, exposing the actin recep- tor sites. The head groups of the myosin then bind with the actin, forming a cross-bridge between the actin and myosin (Fig. 3.4B). Hydrolysis of ATP and the release of adenosine diphosphate result in actin sliding toward the myosin. The sliding shortens the muscle fi ber and gen- erates tension in the sarcomere, causing a contraction of the fi bers moving the Z lines closer together. The cross- bridge of the myosin then uncouples from the active site on the actin and retracts. Formation of cross-bridges, relaxation, and then reformation occur repeatedly—as many as 100 times per second. To achieve a strong muscle contraction, a large number of bonding sites repeat this process over and over.


TYPES OF CONTRACTION


Traditionally, the term contraction has been used to describe the tension produced by muscles, as “contract” means to draw together or decrease in length. This drawing together or shortening depicts the actin and myosin pro- teins sliding closer together, as described in the cross- bridge theory. A synonymous term with contraction that may defi ne a more accurate account of muscle tension is muscle activation. Because contractile proteins do not always draw closer together when muscles produce tension, the term contraction can be misleading. This text uses both muscle contraction and muscle activation to describe the active tension produced by muscles when they are stimulated by the nervous system. There are three types of muscle contraction to con- sider: concentric, eccentric, and isometric. The term iso- tonic (equal tension) muscle contraction has been used as a type of muscle contraction encompassing both concen- tric and eccentric contractions. This is an erroneous term to describe these types of contractions, as muscle tension does not stay constant but changes as the angle of the joint changes during movement. (Refer to Chapter 1 to


review how various joint angles have different moment arms resulting in a range of torque forces as the muscle moves the joint through a range of motion.)


Concentric Muscle Contractions


During concentric muscle contractions, there is repeated formation and reformation of cross-bridges until there is shortening of the muscle fi ber. If enough fi bers are activated, and the internal torque produced by the muscle is greater than the external torque produced by the seg- ment’s weight and any other external forces, there will be rotary motion at the joint. One end of the muscle attach- ment to the bone is stable, while the other attachment moves closer to the stable attachment. Lifting a weight during elbow fl exion is an example of a concentric con- traction of the biceps (Fig. 3.5). The concentric contrac- tion is performing positive work on the load and acts as an accelerator of movement against the load.


Eccentric Muscle Contractions


Once the arm begins to lower the weight back down toward its original position, the biceps develops an eccen- tric muscle contraction to perform negative work and to control extension of the elbow. The two attachments of the muscle move away from each other. This action is a lengthening contraction of the muscle where the actin and myosin fi laments form cross-bridges that are repeat- edly formed, broken, and re-formed but become farther apart as the muscle lengthens. The eccentric muscle contraction generates a suffi cient force to control the lowering of the arm against the pulling force of gravity and the weight of the arm. As the muscle lengthens, it acts as a decelerator of the movement. Although the elbow is extending, it is not the result of a contraction of the triceps, but rather from the biceps eccentrically decelerating the weight as it is lowered to the beginning position (Fig. 3.6).


Isometric Muscle Contractions


If the protein cross-bridges produce tension equal to the external forces, the muscle contracts without changing its length. This type of activation is called an isometric muscle contraction. Because the internal torque produced by the muscle equals the external torque, the two attach- ments of the muscle to the bony levers do not change in relationship to each other, and no motion occurs at the joint. During functional movements, the static isometric contraction of the muscle stabilizes one body segment so that other segments can move with control. For example, when a person is typing on a keyboard, the wrist exten- sors can function isometrically to maintain the wrist in a static position while the fi ngers move along the keyboard (Fig. 3.7).


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