During maximal exercise (>90% intensity), which type of muscle fibers are primarily recruited?

During maximal exercise at intensities greater than 90%, the body primarily relies on Type IIb muscle fibers. These fibers, also known as fast-twitch fibers, are specifically designed for generating high levels of force and power over short durations. They are characterized by their ability to rapidly contract and produce significant amounts of energy through anaerobic respiration, which is critical during heavy exertion.

While Type I fibers, which are slow-twitch, are more suited for endurance activities and Type IIa fibers can contribute to both aerobic and anaerobic energy production, the demand for explosive strength and speed at maximal intensities necessitates the overwhelming recruitment of Type IIb fibers. During such high-intensity efforts, the nervous system prioritizes the recruitment of muscle fibers that can generate the most force in the shortest amount of time, which are the Type IIb fibers.

Thus, the selection of all three types of fibers, as suggested in the answer choice, reflects a broader context of muscular recruitment but emphasizes that Type IIb fibers are the dominant ones engaged during maximal efforts. However, since max intensity workouts predominantly utilize Type IIb fibers, focusing solely on them would be more accurate for understanding maximal exercise physiology.