Fit folks live for benchmark numbers—1RM, PRs, body fat percentage . Perhaps the most braggable stat available, though, is your VO2 max. Technically speaking, this is the maximum amount of oxygen you can utilize during exercise, but for all intents and purposes, it’s a marker of just how fit you actually are. “You want the biggest VO2 max as humanly possible, since this number defines how intense a workout your body can handle. The higher the number, the harder and longer you can go in a workout,” says San Diego-based trainer Matt Pippin, CSCS, founder of Pippin Performance . Generally, fit guys will be between 42.5 (good) and 52.5 (superior) in their 20s, 41 to 49.5 in their 30s, and 39 to 48.1 in their 40s. Now, traditionally, your VO2 max is determined by a treadmill test where you’re hooked up to a ventilator, run at different paces, and a machine measures how much oxygen you consume with each breath, then how much of that oxygen your body uses to fuel your exercise. 3 Tips f