Association of Smartwatch-Based Heart Rate and Physical Activity With Cardiorespiratory Fitness Measures in the Community: Cohort Study (Preprint)

<sec> <title>BACKGROUND</title> Resting heart rate (HR) and routine physical activity are associated with cardiorespiratory fitness levels. Commercial smartwatches permit remote HR monitoring and step count recording in real-world settings over long periods of time, but the relationship between smartwatch-measured HR and daily steps to cardiorespiratory fitness remains incompletely characterized in the community. </sec> <sec> <title>OBJECTIVE</title> This study aimed to examine the association of nonactive HR and daily steps measured by a smartwatch with a multidimensional fitness assessment via cardiopulmonary exercise testing (CPET) among participants in the electronic Framingham Heart Study. </sec> <sec> <title>METHODS</title> Electronic Framingham Heart Study participants were enrolled in a research examination (2016-2019) and provided with a study smartwatch that collected longitudinal HR and physical activity data for up to 3 years. At the same examination, the participants underwent CPET on a cycle ergometer. Multivariable linear models were used to test the association of CPET indices with nonactive HR and daily steps from the smartwatch. </sec> <sec> <title>RESULTS</title> We included 662 participants (mean age 53, SD 9 years; n=391, 59% women, n=599, 91% White; mean nonactive HR 73, SD 6 beats per minute) with a median of 1836 (IQR 889-3559) HR records and a median of 128 (IQR 65-227) watch-wearing days for each individual. In multivariable-adjusted models, lower nonactive HR and higher daily steps were associated with higher peak oxygen uptake (VO&lt;sub&gt;2&lt;/sub&gt;), % predicted peak VO&lt;sub&gt;2&lt;/sub&gt;, and VO&lt;sub&gt;2&lt;/sub&gt; at the ventilatory anaerobic threshold, with false discovery rate (FDR)–adjusted &lt;i&gt;P&lt;/i&gt; values &amp;lt;.001 for all. Reductions of 2.4 beats per minute in nonactive HR, or increases of nearly 1000 daily steps, corresponded to a 1.3 mL/kg/min higher peak VO&lt;sub&gt;2&lt;/sub&gt;. In addition, ventilatory efficiency (V&lt;sub&gt;E&lt;/sub&gt;/VCO&lt;sub&gt;2&lt;/sub&gt;; FDR-adjusted &lt;i&gt;P&lt;/i&gt;=.009), % predicted maximum HR (FDR-adjusted &lt;i&gt;P&lt;/i&gt;&amp;lt;.001), and systolic blood pressure-to-workload slope (FDR-adjusted &lt;i&gt;P&lt;/i&gt;=.01) were associated with nonactive HR but not associated with daily steps. </sec> <sec> <title>CONCLUSIONS</title> Our findings suggest that smartwatch-based assessments are associated with a broad array of cardiorespiratory fitness responses in the community, including measures of global fitness (peak VO&lt;sub&gt;2&lt;/sub&gt;), ventilatory efficiency, and blood pressure response to exercise. Metrics captured by wearable devices offer a valuable opportunity to use extensive data on health factors and behaviors to provide a window into individual cardiovascular fitness levels. </sec>