A skeletal muscle fibre (cell) consists of many sections (units) which contract. What is the name given to one of the units that contract?

Correct Answer: A

Rationale: A skeletal muscle fibre contains myofibrils, each a chain of sarcomeres repeating units between Z-lines that shorten during contraction via actin-myosin sliding. Sarcolemma is the membrane, sarcoplasm the cytoplasm, and fasciculus a fibre bundle not contractile units. Sarcomeres' role as the smallest contractile segment distinguishes them, summing to fibre-wide contraction, fundamental to muscle mechanics and force production.

Correct Answer: D

Rationale: Sternocleidomastoid names origins (sternum, clavicle) and insertion (mastoid process), guiding neck motion. Tibialis anterior, extensor digitorum longus, and rectus femoris denote location, action, or fibre direction not attachments. This origin-insertion naming distinguishes it, aiding precise anatomical mapping.

Correct Answer: C

Rationale: A convergent fascicle pattern features fibers converging from a broad origin to a narrow insertion, like a fan, maximizing force direction versatility, as in the pectoralis major, which spans the chest to insert on the humerus. Biceps brachii has parallel fascicles along its length. Gluteus maximus is multipennate, with angled fibers for power. Rectus femoris has a bipennate structure, not convergent. Pectoralis major's triangular, convergent design, enabling shoulder adduction and flexion, matches this pattern, making 'c' correct.

Correct Answer: C

Rationale: The erector spinae, a muscle group along the spine (e.g., iliocostalis, longissimus), provides postural support, extending the back to maintain upright stance against gravity. Arm movement involves deltoid or latissimus dorsi. Pelvic stabilization is more gluteal or abdominal. Rotation involves obliques or rotatores, not primarily erector spinae, which focuses on extension. Its role in posture, critical for standing/sitting, makes 'c' the correct function.

Correct Answer: B

Rationale: The gastrocnemius, a two-headed muscle overlying soleus, is the largest in the lower leg, spanning from femoral condyles to the Achilles tendon, driving plantarflexion (e.g., calf raises). Soleus , beneath it, is smaller but assists. Tibialis anterior dorsiflexes, and tibialis posterior inverts both are slimmer. Gastrocnemius's bulk and superficial prominence, critical for locomotion, make 'b' the correct answer.