moves off of the myosin binding sites on actin.

Correct Answer: C

Rationale: Tropomyosin moves off actin's myosin-binding sites when Ca²⁺ binds troponin, which shifts tropomyosin, allowing myosin to bind actin for contraction. Ca²⁺ and troponin don't move off they enable the shift. Troponin doesn't block sites or move myosin it's regulatory. Troponin moving tropomyosin reverses roles tropomyosin shifts due to troponin. Tropomyosin's displacement by troponin-Ca²⁺ distinguishes it, key to exposing sites, unlike static or reversed pairings, central to sliding filament theory.

Correct Answer: C

Rationale: Muscle roles define movement dynamics. An antagonist opposes or reverses an agonist's action, relaxing during the agonist's contraction e.g., triceps antagonize biceps in elbow flexion. Agonists drive the primary motion, synergists assist, and fixators stabilize. The antagonist's counteraction ensures controlled, reversible movements, a key biomechanical principle distinguishing it from supportive or driving roles, essential for coordinated skeletal motion.

Correct Answer: D

Rationale: Sustained activity beyond 30-40 minutes relies on aerobic respiration in mitochondria, oxidizing pyruvate (from glycolysis) with oxygen to produce 36-38 ATP per glucose far more than stored ATP (seconds), glycolysis (minutes), or creatine phosphate (15 seconds). This efficient, oxygen-dependent process supports endurance, distinguishing it from short-burst energy sources, critical for prolonged muscle function in activities like running.

Correct Answer: B

Rationale: Adductor longus names its action (adduction) and size (long), not location, unlike latissimus dorsi (back), rectus femoris (thigh), and biceps brachii (arm). Located on the thigh, its name prioritizes function, distinguishing it from location-centric terms, useful for understanding muscle roles beyond positional cues.

Correct Answer: D

Rationale: The flexor carpi ulnaris flexes the wrist, bending it toward the forearm's palmar side, mirroring the flexor carpi radialis' action, differing only in position (ulnar vs. radial). It doesn't flex the arm or fingers, nor extend like its extensor counterpart. This wrist-specific flexion distinguishes it, key to hand positioning movements.