Explain muscle firing for voluntary contraction?

• A. Type II before Type I; synchronous firing; constant higher firing
• B. Type II before Type I; asynchronous firing; intermittent lower firing
• C. Type I before Type II; synchronous firing; intermittent higher firing
• D. Type I before Type II; asynchronous firing; intermittent higher firing

Answer: (D)

When you contract a muscle voluntarily, the brain controls force by two linked mechanisms: recruiting motor units and adjusting how fast they fire. The smallest, most fatigue-resistant units—Type I slow-twitch fibers—are activated first because they have the lowest activation threshold. As more force is needed, larger fast-twitch units (Type II) are recruited in sequence, following the size principle. Inside the active pool, motor units do not fire in perfect unison; their impulses are distributed over time, which keeps the movement smooth and helps prevent tremor. As the demand increases, the firing rate of the active units rises, often in bursts rather than at a constant high rate, producing intermittent higher firing. This combination—Type I activated first, asynchronous firing among units, and increasing, burst-like firing as needed—describes how voluntary contraction is achieved.