Robert is training for a track race. He slightly tears his calcaneal tendon. The doctors prescribe P.R.I.C.E. and give him medicine for inflammation and pain. Robert most likely has a

Correct Answer: B

Rationale: The calcaneal (Achilles) tendon connects the gastrocnemius and soleus to the calcaneus. A slight tear from training is a muscle strain, an overstretching or tearing of muscle or tendon, treated with P.R.I.C.E. (protection, rest, ice, compression, elevation) and anti-inflammatories. A sprain involves ligaments, not tendons. Repetitive strain injuries result from overuse, not a single tear. Tendon strain isn't a standard term strains encompass tendon injuries. Muscle strain is the correct answer, as it fits the Achilles injury and treatment, common in runners.

Correct Answer: D

Rationale: The muscular system accounts for about 50% of body weight, primarily from skeletal muscles, with smooth and cardiac adding less mass. This reflects their extensive presence over 600 muscles supporting movement and structure. Twenty percent is too low, ignoring skeletal bulk. Forty percent is close but underestimates, per anatomical data. Seventy percent overstates, leaving little for bones, organs, and fat. Fifty percent aligns with standard physiology, emphasizing muscles' role in force generation and mass, balancing the body's composition, distinct from lower or higher guesses misaligned with muscle distribution.

Correct Answer: C

Rationale: Calcium ions, essential for muscle contraction, are stored in the sarcoplasmic reticulum, a specialized endoplasmic reticulum surrounding myofibrils. Upon a nerve signal, it releases calcium into the sarcoplasm to bind with troponin, initiating actin-myosin interaction. T-tubules conduct the signal inward but don't store calcium they facilitate its release. The sarcolemma is the muscle fiber's membrane, encasing the cell, not a storage site. Mitochondria produce ATP, powering contraction, but lack calcium storage capacity. The sarcoplasmic reticulum's role as a calcium reservoir is unique, ensuring precise regulation of contraction and relaxation, distinguishing it from conduction, structural, or energy-producing components, critical for the sliding filament mechanism in muscle physiology.

Correct Answer: C

Rationale: After ATP depletion in muscle contraction, creatine phosphate rapidly donates a phosphate to ADP, regenerating ATP in seconds via creatine kinase. This immediate source sustains short bursts of activity. Cellular respiration, in mitochondria, follows later, using oxygen for sustained ATP but takes time. Lactic acid breakdown isn't a process lactic acid forms in anaerobic conditions, not regenerating ATP. Fermentation (anaerobic glycolysis) produces ATP slowly, secondary to creatine phosphate. Its quick, oxygen-independent action distinguishes creatine phosphate, bridging energy gaps, unlike slower respiration or fermentation, vital for initial muscle energy demands.

Correct Answer: D

Rationale: The gluteus maximus is named for size (maximus, largest) and location (gluteus, buttock region), the biggest muscle there, extending and rotating the thigh. Shape (e.g., trapezius) or action (e.g., flexor) could apply elsewhere, but here, size and gluteal position define it. Combining both aspects highlights its prominence and placement, distinct from single-trait names, reflecting its anatomical and functional significance in the hip.