What makes the muscles to be strong?

Correct Answer: D

Rationale: Exercise strengthens muscles by stressing fibers, prompting growth and repair. Resistance training or cardio increases muscle mass, endurance, and power through repeated contraction and recovery. Diet and food provide nutrients protein, carbs but without exercise, they don't build strength directly, only support it. Sleeping aids recovery, allowing muscle repair, but doesn't actively strengthen. Exercise's mechanical stress triggers hypertrophy, enhancing fiber size and efficiency, unlike diet's passive fuel role or sleep's restorative one. Regular activity, like lifting or running, directly fortifies muscles, making it the primary driver, supported but not replaced by nutrition and rest, critical for physical capability.

Correct Answer: C

Rationale: Myosin, the thick filament in sarcomeres, features cross-bridges protruding heads that bind actin during contraction. These bridges, powered by ATP, pull actin inward, driving the power stroke. Troponin, on thin filaments, binds calcium to regulate contraction, lacking bridges. Actin forms thin filaments, receiving cross-bridges, not bearing them. Tropomyosin shields actin's sites, also without bridges. Myosin's cross-bridges are unique, enabling force generation, distinguishing it from actin's structural role or troponin and tropomyosin's regulatory functions, essential for the sliding filament mechanism and muscle movement.

Correct Answer: C

Rationale: Hypertrophy is muscle size increase from training, as resistance stress thickens fibers via protein synthesis, enhancing strength and mass, like in weightlifting. Atrophy is size loss from inactivity, opposite to training's goal. Fatigue is temporary exhaustion, not size change. Hyperplasia, fiber number increase, is rare in humans, unlike hypertrophy's fiber growth. This adaptation reflects muscle's response to mechanical overload, distinct from shrinkage, energy depletion, or theoretical cell addition, central to exercise-induced development.

Correct Answer: D

Rationale: The sternocleidomastoid, from sternum and clavicle to mastoid, rotates and tilts the head side to side, as in looking over the shoulder. Arm abduction is deltoid's role. Breathing involves diaphragm and intercostals, not this neck muscle. Shoulder shrugging is trapezius. Its head-turning action distinguishes it, vital for neck mobility, unlike arm, respiratory, or shoulder functions.

Correct Answer: D

Rationale: Biceps brachii flexes the elbow, bending the forearm toward the arm, while triceps brachii extends it, straightening it. Abduction and adduction involve lateral movement, not their primary elbow focus. Flexion-extension defines their antagonistic pair, distinct from side motions, key for arm bending and straightening.