Cardiorespiratory fitness is an established predictor of metabolic disease and mortality. Fitness is directly measured as maximal oxygen consumption (VO$_{2}max$), or indirectly assessed using heart rate responses to standard exercise tests. However, such testing is costly and burdensome because it requires specialized equipment such as treadmills and oxygen masks, limiting its utility. Modern wearables capture dynamic real-world data which could improve fitness prediction. In this work, we design algorithms and models that convert raw wearable sensor data into cardiorespiratory fitness estimates. We validate these estimates' ability to capture fitness profiles in free-living conditions using the Fenland Study (N=11,059), along with its longitudinal cohort (N=2,675), and a third external cohort using the UK Biobank Validation Study (N=181) who underwent maximal VO$_{2}max$ testing, the gold standard measurement of fitness. Our results show that the combination of wearables and other biomarkers as inputs to neural networks yields a strong correlation to ground truth in a holdout sample (r = 0.82, 95CI 0.80-0.83), outperforming other approaches and models and detects fitness change over time (e.g., after 7 years). We also show how the model's latent space can be used for fitness-aware patient subtyping paving the way to scalable interventions and personalized trial recruitment. These results demonstrate the value of wearables for fitness estimation that today can be measured only with laboratory tests.

Having regular exercise while studying can help boost pupils' exam grades in both French maths, according to researchers, who say it develops their cognitive skills. To understand the influence fitness has on learning, experts from the University of Geneva, Switzerland, tested education and activity levels of 193 pupils aged 8 to 12. By combining data on fitness, and exam results, they found a link between better cardiorespiratory fitness and higher marks in mathematics and French grammar. However, the team say the link was indirect, with physical fitness improving executive functions and cognitive flexibility, which in turn helps with subjects that rely on specific and structured answers, such as mathematics. The researchers say schools and administrators should consider the importance of exercise and movement when planning timetables and allocating budgets.