Final answer:
Improvements in VO₂ max can occur over several weeks to months with high-intensity interval training, with variability in outcomes attributed to genetic muscle fiber differences and individual exercise responses.
Step-by-step explanation:
The specific timeframe for improvements in VO₂ max with high-intensity interval training (HIIT) compared to training at or below the lactate threshold was not provided in the initial question. However, generally, individuals can see significant improvements in VO₂ max within several weeks to months of consistent high-intensity interval training. By stressing the cardiovascular system more intensively, HIIT can lead to greater adaptations and thus a more pronounced increase in VO₂ max.
Differences in outcomes from an exercise program, such as weight loss variability among individuals, may be attributed to physiological factors such as genetic differences in muscle fiber composition and the individual's unique response to physical activity. Individuals with more slow-twitch fibers can have greater endurance and may improve differently than those with a higher proportion of fast-twitch fibers. Additional factors include the intensity of workouts and the amount of recovery time.