In skeletal muscle physiology, how is a muscle best described when it is relaxed (not actively contracting)?

• A. It is lengthened to its resting length because no active cross-bridge cycling is occurring.
• B. It is maximally shortened due to continuous motor neuron stimulation at rest.
• C. It is shortened because relaxation increases actin-myosin cross-bridge formation.
• D. It is rigid and fixed at a shortened length due to sustained tetanus.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the state of a relaxed skeletal muscle. When a muscle is relaxed, it is not actively contracting, meaning that the motor neurons are not stimulating the muscle fibers to generate force.
2. Step 2: Recall the role of cross-bridge cycling in muscle contraction. Cross-bridge cycling occurs when myosin heads bind to actin filaments and pull them to shorten the muscle. In relaxation, this cycling is minimal or absent.
3. Step 3: Recognize that without active cross-bridge cycling, the muscle cannot generate tension to shorten. Instead, the muscle returns to or remains at its resting length, which is the length where the muscle is neither stretched nor contracted.
4. Step 4: Eliminate incorrect options by understanding muscle physiology: continuous motor neuron stimulation causes contraction, not relaxation; relaxation does not increase cross-bridge formation; sustained tetanus causes rigidity and shortening, not relaxation.
5. Step 5: Conclude that a relaxed skeletal muscle is best described as being lengthened to its resting length because no active cross-bridge cycling is occurring.