Which muscle fibers are likely engaged when electrical stimulation is applied at a motor level?

• A. Type I fibers
• B. Type II fibers
• C. Both Type I and Type II fibers
• D. Type III fibers

Answer: (C)

When electrical stimulation is applied at a motor level, it typically stimulates both Type I and Type II muscle fibers. Type I fibers, also known as slow-twitch fibers, are more resistant to fatigue and are primarily engaged in endurance activities. They are activated during low-intensity, longer-duration exercises. On the other hand, Type II fibers, which include both Type IIa (fast oxidative) and Type IIb (fast glycolytic), are recruited for high-intensity, short-duration activities and fatigue more quickly than Type I fibers.

The nature of electrical stimulation allows for the recruitment of a wide range of muscle fibers, taking into account various thresholds of activation depending on the intensity and duration of the stimulus. Therefore, under motor-level stimulation, both fiber types can be activated to respond to different demands. This characteristic is crucial in rehabilitation and training settings, where neuromuscular stimulation techniques aim to enhance strength and endurance by effectively targeting multiple fiber types simultaneously.

Type III fibers, also known as sensory fibers, are not involved in muscle contraction and therefore do not play a role in this context.