Lactate levels quantify just how deep into anaerobic hell an athlete is. As exertion progresses, oxygen levels in the blood drop, starving tissues of the O2 they crave. At this point, pyruvic acid starts to be converted into lactic acid, thereby increasing the blood's lactate level. Once the accumulation begins, the athlete's performance is likely to slip. Understanding where and when an athlete reaches this threshold would be a valuable data point, allowing workouts to be optimized and focused, to increase endurance or sharpen a skill. But getting such data currently requires a costly instrument and a sample of blood from a pin prick. So only elite athletes undergo such analysis, and only do so in a lab setting.