Lactate, also known as lactic acid, is a substance that builds up in the muscles during strenuous exercise. Technically a byproduct of incomplete breakdown of carbohydrate, lactic acid is metabolized fairly easily during a low-intensity workout. As your workouts become more challenging, your body starts to produce more lactic acid in the active muscles, eventually reaching a state where you’re producing it too quickly for your body to metabolize. The lactic acid then builds up, causing your muscles to fatigue and stopping them from working properly. Yeah, that horrible burny feeling? That's it. This muscle exhaustion is your lactate threshold and an incredibly important part of your fitness level. Why? Most people reach their lactate threshold at 45 to 65 percent of their VO2 Max, it improves with your workout dedication and training.