Which of these muscles has fascicles parallel to the midline?

Correct Answer: C

Rationale: The rectus femoris has fascicles running parallel to the midline of the body, as part of the quadriceps group, extending the knee with fibers aligned longitudinally along the thigh. External obliques have diagonal fascicles, aiding trunk rotation. Transverse abdominis runs horizontally, compressing the abdomen. Biceps femoris has longitudinal but slightly angled fascicles in the posterior thigh, not strictly midline-parallel. Rectus femoris' straight, midline-aligned fascicles (rectus means straight ) make it the correct answer, reflecting its anatomical orientation and role in linear force production distinct from the angled or transverse patterns of other options.

Correct Answer: A

Rationale: The origin is the tendon's attachment to a stationary bone, providing a fixed point for muscle contraction, like the scapula for the biceps brachii. The insertion attaches to the movable bone (e.g., radius), belly is the muscle's fleshy part, and aponeurosis is a broad tendon sheet. Origin is the correct answer, as it defines the stable anchor in muscle anatomy, essential for understanding how contraction produces motion by pulling the insertion toward it.

Correct Answer: A

Rationale: Voluntary muscles, synonymous with skeletal muscles, allow conscious control over actions like lifting an arm or turning the head. Linked to bones via tendons, they respond to brain signals via the somatic nervous system, enabling deliberate movement. No control describes involuntary muscles smooth and cardiac working automatically, like digestion or heartbeats. Automatic function fits those, not voluntary ones. Muscles fused together misrepresents anatomy muscles are distinct, not merged. Voluntary muscle's defining trait is this control, essential for daily tasks, contrasting with involuntary types' autonomic roles, highlighting its role in purposeful, cognitive-directed motion across limbs, neck, and torso.

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: D

Rationale: Thin filaments in a sarcomere consist of actin, troponin, and tropomyosin, forming a complex that interacts with myosin for contraction. Actin provides the structural backbone, tropomyosin covers binding sites in rest, and troponin binds calcium to shift tropomyosin, exposing sites for myosin. Only actin oversimplifies, ignoring regulatory proteins. Only myosin misplaces it myosin forms thick filaments. Actin and myosin together suggest a mix, but they're separate filaments. The trio of actin, troponin, and tropomyosin defines thin filaments, enabling controlled contraction, distinct from incomplete or incorrect combinations, critical for the sliding filament theory and muscle movement precision.