Recruitment results in

Correct Answer: B

Rationale: Recruitment increases contraction strength by activating more motor units neurons and their muscle fibers as effort rises. This graded response builds force, like lifting heavier weights, without changing individual fiber contraction strength. Atrophy is muscle wasting from disuse, opposite to recruitment's effect. All-or-none applies to single fiber response, not the cumulative effect of recruitment. Fatigue follows prolonged effort, not recruitment itself. Stronger contractions via recruitment reflect motor unit summation, distinguishing it from wasting, binary responses, or exhaustion, key to muscle power modulation.

Correct Answer: A

Rationale: The rotator cuff comprises infraspinatus, supraspinatus, teres minor, and subscapularis, stabilizing the shoulder joint and enabling rotation. 'Intraspinatus' is a typo correct is infraspinatus. Teres major and deltoid assist shoulder movement but aren't cuff muscles. These four anchor the humerus, distinct from larger or adjacent muscles, vital for shoulder integrity and motion.

Correct Answer: C

Rationale: Fibromyalgia features chronic muscle pain, tenderness, stiffness, and elevated substance P, a pain neurotransmitter, with no clear cause. Cancer may cause pain but isn't muscle-specific or tied to substance P. Diabetes affects nerves, not primarily muscles. Tetanus causes spasms from infection, not chronic ache. Fibromyalgia's diffuse, persistent symptoms set it apart, a musculoskeletal disorder.

Correct Answer: D

Rationale: Muscle contraction and relaxation drive movement through a coordinated process. Contraction occurs when nerve impulses trigger calcium release, enabling myosin heads to bind actin and pull filaments together, shortening the muscle and moving attached structures, like bones in skeletal muscle. Relaxation follows when calcium is pumped back into the sarcoplasmic reticulum, halting myosin-actin interaction, allowing the muscle to lengthen and rest. The incomplete option likely misrepresents a molecular detail (e.g., 'Xn' for ions or proteins), but lacks clarity. Cell layer thickness affects force, not the process itself. Vessel linings support circulation, not contraction mechanics. The contraction-relaxation cycle, powered by ATP and regulated by calcium, is the correct mechanism, universally applicable to skeletal, cardiac, and smooth muscle functions, making it the accurate statement despite the truncated option.

Correct Answer: B

Rationale: Type IIb (or IIx in humans) muscle fibers are fast-twitch, with high motor unit strength and fast contractile speed, suited for explosive efforts like sprinting, but they have low oxidative capacity, relying on anaerobic glycolysis, leading to rapid fatigue. High fatigue resistance, typical of slow-twitch (Type I) fibers with rich mitochondria and myoglobin, doesn't fit Type IIb's profile they tire quickly due to limited aerobic endurance. Low oxidative capacity aligns with their anaerobic nature, and strength and speed reflect their power. Fatigue resistance misaligns with their design, distinguishing them from endurance-focused fibers, key to understanding muscle fiber specialization.