Final answer:
For events lasting 21 to 60 minutes, the athlete would work at less than 90% VO2 max, utilizing the aerobic energy system to sustain endurance over the duration.
Step-by-step explanation:
In events lasting 21 to 60 minutes, the athlete would typically work at < 90% VO2 max. During prolonged periods of exercise, such as events that last between 21 to 60 minutes, athletes tend to operate at a submaximal level that is sustainable over the duration of the event. While short-term, high-intensity activities, such as hurdling, might require an athlete to tap into anaerobic respiration, the aerobic energy system is predominantly used in longer events, and the athlete's exertion tends to align with a VO2 max that is less than their absolute maximum to maintain endurance.