Final answer:
Intermediate motor units contain Type IIa (fast-twitch oxidative) fibers, which have properties of both Type I (slow-twitch) and Type IIb (fast-twitch glycolytic) fibers and are suited for both quick and moderately enduring activities.
Step-by-step explanation:
The class of skeletal muscle fibers found in intermediate motor units is Type IIa (fast-twitch oxidative) fibers. These fibers are somewhat of a hybrid of Type I and Type IIb fibers. They have fast contractions like Type IIb fibers but are also rich in mitochondria and capillaries, like Type I fibers, which allows them to use oxygen more efficiently and be more fatigue-resistant. Therefore, they can maintain activity longer than fast-twitch glycolytic fibers but are not as resistant as slow-twitch fibers when it comes to endurance activities.