This is a major energy source in a hurdle race to the leg muscles

Correct Answer: D

Rationale: In a hurdle race, a high-intensity, endurance-based activity lasting over a minute, leg muscles primarily rely on oxidative metabolism for energy. This process uses oxygen to break down glucose, fats, and proteins in mitochondria, producing ATP efficiently via the aerobic pathway, sustaining prolonged effort. Glycolysis provides quick ATP anaerobically but fatigues muscles due to lactate buildup, insufficient for a race's duration. Lactate and pyruvate are intermediates, not primary sources. Preformed ATP is limited, depleting in seconds. Oxidative metabolism dominates in events requiring sustained power, like hurdling, where oxygen delivery supports muscle contraction over time, distinguishing it from short-burst energy systems and aligning with the aerobic demands of such races.

Correct Answer: A

Rationale: A muscle twitch, a single contraction-relaxation cycle from one stimulus, has three phases: latent (signal delay to contraction start), contraction (cross-bridge activity shortens muscle), and relaxation (calcium removal, filament separation). A myogram records this, not the event itself. Summation is multiple twitches overlapping, not a single cycle. A motor unit is a neuron and fibers, not a phase sequence. The twitch's distinct stages define its mechanics, distinguishing it from recordings, cumulative effects, or anatomical units, fundamental to muscle response analysis.

Correct Answer: D

Rationale: Zygomaticus (major and minor) lifts the mouth's corners upward and laterally, forming a smile. Buccinator compresses cheeks, not smiling. Frontalis raises the brow, unrelated to lips. Orbicularis oris purses lips, as in kissing, not smiling. Zygomaticus' specific pull on the mouth defines smiling, distinct from cheek, brow, or lip-closing actions, central to happy expressions.

Correct Answer: A

Rationale: Latissimus dorsi, spanning the lower back, adducts and extends the arm, lowering it from a raised position, as in swimming strokes. Pectoralis major pulls forward. Serratus anterior protracts scapula. Trapezius elevates shoulders. Latissimus dorsi's posterior action sets it apart, crucial for arm return, distinct from chest, scapular, or shoulder roles.

Correct Answer: A

Rationale: Gastrocnemius, on the posterior leg, forms the calf's bulk, plantarflexing the foot, as in walking. Semitendinosus, a hamstring, is thigh-based. Tibialis anterior, anterior leg, dorsiflexes. Vastus lateralis, a quadriceps, is thigh-front. Gastrocnemius' calf prominence and action set it apart, key for locomotion.