Your heart rate remains elevated throughout the entire workout, which increases your endurance. In fact, data from an American Council on Exercise study showed that participants’ heart rates were elevated to 90 percent of maximum heart rate, which was sustained throughout two CrossFit workouts. Fitness industry guidelines suggest a HRmax between 64 to 94 percent in order to improve cardio endurance. Scientists also found that individuals averaged 80 percent of VO2max, which is the maximum amount of oxygen a person can utilize during intense exercise. Industry guidelines suggest 40 to 85 percent of VO2max for improving cardio fitness and body composition.