Day-to-day variables can also affect results. Sleep, nutrition, hydration, recovery, and even equipment can influence how well someone performs on test day. "The thing about endurance sports is that what you put in is what you get out," says McQuality. In lab testing, his team found that carbon-plated running shoes slightly improve VO2-related performance by increasing efficiency, allowing runners to sustain higher workloads before fatigue sets in. Taken together, these factors help explain why VO2 max is best viewed as a context-dependent snapshot, not a fixed measure of physical fitness. It's most useful when tracked over time, under similar conditions, and alongside other markers of performance and health.

Garmin has some of the best proprietary fitness software around. Here's how to interpret all that meticulously gathered data. All products featured on WIRED are independently selected by our editors. However, we may receive compensation from retailers and/or from purchases of products through these links. So, you've got a shiny new Garmin watch .