Is there a leisure time PA measurement bias?
Within the scientific literature, there is a distinction to be made between ‘physical activity’ and ‘exercise’; the latter being a type of physical activity, alongside sport, active travel, physical recreation etc. The United States Department of Health and Human Services defines exercise as “Physical activity that is planned, structured, repetitive, and designed to improve or maintain physical fitness, physical performance, or health. Exercise encompasses all intensities.” 
Whilst the typical social gradient holds true for exercise and other types of leisure time physical activity – the same may not be true for total physical activity (PA) levels. A review published last year concluded that the assumed relationship between (PA) and socioeconomic status (SES) is mostly a relationship between leisure time PA and high SES. No such relationships for all other PA types (active travel, occupational etc.) exists and may actually indicate that individuals from low SES groups are more active. 
 Department of Health and Human Services. 2018 Physical Activity Guidelines Advisory Committee Scientific Report. Washington: Department of Health and Human Services; 2018.
 Stalsberg R and Pedersn A. Are Diiference in Physical Activity across Socioeconomic Groups Associated with Choice of Physical Activity Variables to Report? International Journal of Environmental Research and Public Health. 2018; 15: 1-23.
Competing interests: No competing interests