Final answer:
Middle-distance running events such as the 400m and 800m predominantly involve Type IIa (Fast Oxidative) muscle fibers, which are capable of both aerobic and higher tension contractions. Type IIx fibers may also play a role due to their quick contraction ability, but they fatigue more rapidly.2) Type IIa muscle fibers
Step-by-step explanation:
The type of muscle fibers used for 400m / 800m activities, such as middle-distance running events, typically involve both aerobic and anaerobic energy systems. Therefore, these events rely predominantly on Type IIa muscle fibers (Fast Oxidative) and to some extent on Type IIx muscle fibers (Fast Glycolytic).
Type IIa fibers, also known as fast oxidative fibers, are rich in mitochondria and capillaries and appear red. They can sustain aerobic activity but can also generate higher tension contractions than slow-twitch fibers, making them suitable for activities like the 400m and 800m where endurance and speed are both required.
On the other hand, Type IIx fibers are the fastest muscle type in humans and can contract more quickly and with a greater amount of force. However, they are less dense in mitochondria and myoglobin, rely on anaerobic metabolism, and can only sustain short bursts of activity before fatigue sets in.