Gait speed, how fast someone walks at their normal, comfortable pace over a measured distance, is one of the oldest and most extensively validated functional biomarkers in geriatric medicine. It's sometimes informally called a "vital sign" for older adults because a single timed walk integrates cardiovascular fitness, muscular strength, balance, and neurological function into one simple, reproducible number.
Like grip strength, gait speed's value comes from what it correlates with rather than from being a single underlying biological mechanism: multiple large studies have linked slower usual walking speed to higher risk of falls, hospitalization, disability, and mortality in older populations, and gait speed is already used clinically as a quick functional screening tool rather than only in research settings.
Gait speed and grip strength are often discussed together as the two simplest, cheapest functional biomarkers available, requiring no lab equipment beyond a stopwatch or hand dynamometer, in contrast to the molecular biomarkers on this list that require blood draws and laboratory processing.