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Abduction (Chapter 1, 11). Position or frontal plane movement of the body segment away from the midline. In the foot, abduction occurs in a transverse plane around a vertical axis.

Acceleration (Chapter 2). A rate of increase in velocity.

Accessory motion (Chapter 1). Also known as component motions. Motions that take place at a joint as a natural smooth accompaniment to active range of motion.

Acetabular fossa (Chapter 9). Central area of acetabulum, devoid of hyaline cartilage, which houses fibroelastic fat pad and ligamentum teres.

Acetabulum (Chapter 9). The cup of the pelvis into which the head of the femur fits to form the hip joint, structurally comprised of portions from all three pelvic bones.

Actin (Chapter 3, 4). The thinner protein filament that contains troponin and tropomyosin, which control the binding between actin and myosin that occurs during muscle contraction.

Action potential (Chapter 3). Electrochemical signal propagated within nervous system whereby an excitable nerve or muscle cell is sufficiently depolarized then repolarized.

Active insufficiency (Chapter 4). When a muscle that crosses more than one joint is at its shortest position but the joint still has more motion available. The muscle’s actin and myosin overlapping sites are used up, but the joint has not reached the end of its motion. This occurs in muscles that cross more than one joint; e.g., the hamstrings are used to fully extend the hip but are unable to fully flex the knee simultaneously.

Active tension (Chapter 4). Force produced by the muscle itself through crossbridge activation between the actin and myosin fibers of muscle tissue. Of all factors contributing to overall muscle force, active tension is the largest. See passive tension.

Adduction (Chapter 1, 11). Position or frontal plane movement of the body segment toward the midline. In the foot, adduction occurs in transverse plane around a vertical axis.

Adductor tubercle (Chapter 9). Projection proximal to the medial epicondyle at the distal femur, so named because the adductor magnus muscle attaches to it.

Aerobic metabolism (Chapter 3). Oxidative metabolism of fats, carbohydrates, and proteins for the production of energy within the muscle.

Afferent nerves (Chapter 3). Sensory nerves that send impulses into the central nervous system.

Agonist (Chapter 4). Muscle or muscle group that is primarily responsible for producing a motion.

Akinesia (Chapter 3, 12). Difficulty initiating movement. It is a condition that is seen typically in Parkinson’s disease.

All-or-none law (Chapter 3). Principle in which all of the muscle fibers ...

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