Across 16 years of follow-up in 324,284 UK Biobank adults aged 40-65, 2,674 hip fractures occurred. Middle quintiles of leisure-time activity carried the lowest risk: roughly one hour daily cut hip fracture hazard by 22% in women (HR 0.78) and 26% in men (HR 0.74). Vigorous activity showed the strongest signal, with 60-90 minutes weekly linked to hazards about 35% lower (HR 0.65 women, 0.64 men). Benefits flattened at the highest activity volumes.
The plateau is the interesting part. It fits a model in which weight-bearing, higher-impact loading builds bone density and muscle strength, while very high volumes may add injury or fall exposure, or reflect energy deficiency in a minority of extreme exercisers. The vigorous-activity result echoes trial evidence that impact and resistance training protect bone, and it suggests mid-life exercise intensity may matter more than raw time. Several caveats apply. Activity was self-reported at a single baseline visit, so misclassification is likely. UK Biobank volunteers are healthier and more affluent than the general population, and residual confounding is plausible: active people differ in diet, smoking, alcohol use, and baseline bone health. Healthy-survivor effects and reverse causation cannot be excluded in an observational design. This is a preprint and has not been peer reviewed, so the estimates could shift. The findings are confirmatory rather than paradigm-shifting, but they offer a concrete, attainable target: a few vigorous sessions weekly, not marathon-level volume.