Ambulatory tasks and journeys: a framework for free-living behaviour

Background: Standard accelerometer summaries obscure meaningful differences in how people move while upright. We introduce an operational two-class Ambulatory Behaviour Framework that separates Ambulatory Tasks—periods of standing and short continuous stepping bouts (<1 min) that are indicative of activity in a single locus—from Ambulatory Journeys—long continuous stepping bouts (≥1 min) that are indicative of movement between locations. Methods: We analysed thigh-worn activPAL3 data from 3545 participants in the age-46 sweep of the 1970 British Cohort Study (24,815 valid monitor-days). Event-based algorithms grouped upright events and classified them as an Ambulatory Task or Journey; linear models examined associations with sitting time and differences by sex and BMI. Results: Mean upright time averaged 6.50 h day−1; Ambulatory Tasks dominated (5.91 h; 90.6% of upright exposure), whereas Ambulatory Journeys contributed 0.61 h (9.4%). Each additional hour of Ambulatory Tasks corresponded to 0.61 h less sitting (β = −0.61 h; 95% CI: −0.63 to −0.61), while an extra hour of Ambulatory Journeys displaced only 0.04 h of sitting (β = −0.04 h; 95% CI: −0.044 to −0.039). Women accumulated significantly more time in Ambulatory Tasks and less sitting time than men. Both upright behaviours declined with increasing BMI. Conclusions: Ambulatory Tasks substantially replace sitting time, whereas Ambulatory Journeys leave sitting essentially unchanged. Interventions to displace sitting should concentrate on fostering frequent, brief, context-embedded tasks throughout the day. This novel framework yields interpretable, sensor-agnostic metrics to target behaviour change and standardise reporting of free-living mobility.