Grip strength, measured with a simple handheld dynamometer, is one of the more surprising entries in longevity biomarker research: a 10-second test of hand-squeeze force turns out to be a strong, independently replicated predictor of all-cause mortality and age-related functional decline, including in large multi-country cohort studies. It's now used clinically as a proxy for overall muscular strength and frailty risk in older adults, not because grip strength itself is what matters, but because it correlates closely with total-body muscle mass and neuromuscular function, both of which decline with age and both of which are harder to measure directly.
Part of what makes grip strength valuable as a research tool is exactly what makes it unglamorous: it's fast, cheap, requires no lab, and produces a single reproducible number, which is why it shows up consistently across large population studies where more expensive functional tests would be impractical at scale.
Like VO2 max, grip strength is a functional rather than molecular biomarker, and it responds to resistance training, giving it the same practical advantage of being both predictive and directly actionable, unlike some of the harder-to-influence molecular markers on this list.