A stretching or tearing of a muscle is a

Correct Answer: C

Rationale: A strain is a muscle stretch or tear from overexertion, like lifting too heavy. Cramps are painful, involuntary contractions. Spasms are sudden contractions, not tears. Sprains affect ligaments. Strain's injury focus distinguishes it, common in sports, unlike contraction-based or ligament issues.

Correct Answer: B

Rationale: The sarcomere is the basic functional unit of skeletal muscle tissue, defined as the segment between two Z-lines where actin and myosin filaments overlap to produce contraction via the sliding filament mechanism. This repeating unit shortens during muscle action, driving movement. Muscle fibers, single cells containing many sarcomeres, are structural units, not the smallest functional ones. Myofibrils, bundles of sarcomeres within fibers, organize contraction but aren't the base unit sarcomeres are their components. The sarcoplasmic reticulum stores calcium, enabling contraction, but doesn't contract itself. The sarcomere's role as the contractile core distinguishes it, fundamental to muscle function, unlike larger structural or supportive elements, aligning with its physiological definition.

Correct Answer: A

Rationale: A synergist assists an agonist, the prime mover, by enhancing its action or stabilizing joints, like brachioradialis aiding biceps in elbow flexion. A fixator stabilizes the origin bone, indirectly supporting but not directly helping the agonist's motion. An insertion is the muscle's moving attachment, not a helper muscle. An antagonist opposes the agonist (e.g., triceps vs. biceps), hindering, not aiding. Synergists' cooperative role boosts efficiency and precision, distinguishing them from stabilizing, structural, or opposing functions, essential for coordinated movement.

Correct Answer: C

Rationale: Sternocleidomastoid, from sternum/clavicle to mastoid, flexes the head forward when both sides contract, a prime mover in neck flexion. Occipitofrontalis raises eyebrows and wrinkles forehead, not flexing. Corrugator supercilii furrows brows, a facial expression muscle. Masseter elevates the mandible for chewing, not head flexion. Sternocleidomastoid's bilateral action drives head bending, distinguishing it from facial or jaw muscles, critical for neck movement.

Correct Answer: A

Rationale: Gluteal muscles (gluteus maximus, medius, minimus) attach the leg to the pelvic girdle, with maximus extending the hip (e.g., standing up). Obturators rotate the thigh. Adductors pull it inward. Abductors (e.g., gluteus medius) move it outward, not extending. Gluteals' size and extension role distinguish them, key for hip motion.