Muscle fatigue occurs

Correct Answer: A

Rationale: Muscle fatigue sets in when ATP depletes, halting cross-bridge cycling as myosin can't detach or re-cock without energy, despite calcium presence. This follows prolonged activity outpacing ATP regeneration. The latent period, pre-contraction, involves signal delay, not fatigue. Relaxation begins as calcium returns to the SR, not ATP exhaustion. Lactic acid breakdown isn't a fatigue marker it accumulates, not depletes, during anaerobic effort. ATP shortage directly impairs contraction, distinguishing fatigue from timing phases or metabolic byproducts, reflecting energy failure's impact on muscle performance.

Correct Answer: B

Rationale: Deltoid abducts the arm horizontally, lifting it sideways from the body, due to its lateral fibers' orientation. Biceps brachii flexes the elbow. Trapezius elevates or adducts scapula. Triceps brachii extends the elbow. Deltoid's abduction role, centered at the shoulder, sets it apart, vital for arm positioning, distinct from elbow or scapular functions.

Correct Answer: C

Rationale: Quadriceps extend the knee, straightening the leg, while hamstrings flex it, bending it, acting as antagonists at the knee joint. Abduction-adduction involves lateral-medial thigh motion, not their primary knee focus. Extension-flexion defines their opposing actions, distinct from side movements, crucial for leg dynamics.

Correct Answer: B

Rationale: Sarcomere shortening, the basis of muscle contraction, occurs via the sliding filament mechanism. Myosin heads, protruding from thick filaments, bind to actin on thin filaments, forming cross-bridges. Using ATP-derived energy, these heads execute a power stroke, pulling actin toward the sarcomere's center specifically the A band's midpoint reducing the distance between Z lines and shortening the sarcomere. Acetylcholine initiates this by triggering an action potential, but it's not the direct shortening mechanism. Calcium release from the sarcoplasmic reticulum enables myosin-actin binding by shifting tropomyosin, a crucial step but not the act of shortening. ATP breakdown powers the process, yet the sarcoplasmic reticulum stores calcium, not hydrolyzes ATP for shortening. The myosin-actin interaction is the mechanical driver, making it the precise answer, central to muscle movement and the sliding filament theory.

Correct Answer: C

Rationale: The external occipital protuberance, a midline bump on the occipital bone, is termed the inion, a landmark for neck muscle attachment (e.g., trapezius). The nasion is the forehead-nose junction, a facial point. The acromion, on the scapula, is a shoulder feature, not cranial. The mental protuberance is the chin's prominence on the mandible. Inion's specific occipital location and role in muscle anchorage distinguish it, aligning with skull anatomy, unlike facial, shoulder, or mandibular misnomers.