Grip Strength Percentile.

Grip strength is a remarkably honest proxy for total-body muscle strength and quality, and muscle is the organ of longevity — it disposes of glucose, protects against falls, and tracks the loss of function that ends independence. Because it is cheap, fast, and reproducible, epidemiologists have measured it in hundreds of thousands of people.

Leong et al. (Lancet 2015, ~140,000 adults across 17 countries) found each 5 kg drop in grip strength was associated with a 16% higher all-cause mortality risk — a stronger signal than systolic blood pressure. The good news is that grip and the posterior chain respond fast to heavy carries, rows, deadlifts, and dead hangs, so percentile is a number you can deliberately climb.