Gross efficiency, on the other hand, improves as a function of power output but falls as a function of speed of movement. At very low speeds, an athlete's gross efficiency is very high because there is not enough time to decelerate or accelerate certain muscles. As speed increases, so does gross efficiency until it reaches a maximum at about 180 degrees per second. All-out efficiency, on the other hand, remains constant regardless of speed. It is the rate at which an athlete produces energy in the form of heat and mechanical work during exercise that determines how efficient they are. As we will see, however, there is a limit to how much an athlete can improve their all-out efficiency. Maximum oxygen uptake (VO2max) is a measure of how much oxygen our bodies can absorb from the air during exercise. It is one of the most important indicators of athletic performance because everyone has limits on how much oxygen they can consume before they start to slow down or fail outright. Scientists have found that for athletes to remain competitive, they should aim to improve their VO2max by about 2% every year. So overall, efficiency increases as a function of speed but reaches a maximum where more efficient athletes are no more productive than less efficient ones.