In electrically induced muscle contractions, which muscle fibers are recruited first?

• A. Small diameter, slow twitch fibers
• B. Large diameter, fast twitch fibers
• C. Both fiber types equally
• D. Intermediate fibers

Answer: (B)

In the context of electrically induced muscle contractions, the correct answer is that small diameter, slow twitch fibers are recruited first. This aligns with the principles of muscle recruitment that dictate how muscle fibers are activated based on their characteristics.

When an electrical stimulus is applied, the first fibers to be recruited are typically those with smaller diameters and slower contraction speeds. These slow twitch fibers are more fatigue resistant and are primarily utilized for endurance activities. They require a lower threshold for activation, allowing them to respond efficiently to electrical stimulation.

On the other hand, large diameter, fast twitch fibers are recruited later in the contraction process. These fibers are designed for rapid and powerful contractions but have a higher activation threshold and are more prone to fatigue. While they play a crucial role in high-intensity activities, their recruitment occurs after the slower fibers.

The recruitment strategy follows the principle known as Henneman's size principle, which dictates that motor units are recruited from smaller to larger sizes based on the intensity of the stimulus. This principle is fundamental in understanding muscle physiology and training regimens.

In summary, the correct response highlights the physiological mechanisms governing muscle fiber recruitment in response to electrical stimulation, emphasizing that small diameter, slow twitch fibers are activated first due to their lower activation thresholds.