The Influence of Accelerometer Epoch Length on Associations of Physical Activity Intensity and Volume with Bone Outcomes

Abstract

Two accelerometer metrics (intensity-gradient and average-acceleration) can be used to determine the relative contributions of physical activity (PA) volume and intensity for health, but it is unknown whether epoch length influences the associations detected. This is important when considering bone health, as bone is particularly responsive to high intensity PA, which may be underestimated by longer epochs. This study aimed to assess the associations between average-acceleration, a proxy measure of PA volume, and intensity-gradient, reflective of PA intensity distribution, from PA data from 1-s to 60-s epochs at age 17 to 23 years with bone outcomes at age 23 years. This is a secondary analysis of 220 participants (124 females) from the Iowa Bone Development Study, a longitudinal study of bone health from childhood to early adulthood. Accelerometer-assessed PA data, captured at age 17 to 23 years, were summarised over 1-s, 5-s, 15-s, 30-s, and 60-s epochs, to generate average-acceleration and intensity-gradient from each epoch length, averaged across ages. Regression analysed associations between mutually adjusted average-acceleration and intensity-gradient with dual-energy X-ray absorptiometry assessed total-body-less-head (TBLH) bone mineral content (BMC), spine areal bone mineral density (aBMD), hip aBMD, and femoral neck cross-sectional area and section modulus at age 23 years. Intensity-gradient was positively associated with TBLH BMC in females, with spine aBMD in males, and with hip aBMD and geometry in both sexes, when a 1 to 5-s epoch was used. Average-acceleration was positively associated with TBLH BMC, spine aBMD and hip aBMD in males, generally when the adjustment for intensity-gradient was from > 1-s epochs. Intensity and volume were important for bone outcomes in both sexes and males, respectively. A 1 to 5-s epoch length was most appropriate to assess the mutually adjusted associations of intensity-gradient and average-acceleration with bone outcomes in young adults.

Accelerometry Measured Movement Behaviors in Middle-Aged and Older Adults: Cross-Sectional Analysis of the ELSA-Brasil Study

“…62,63 Moreover, associations of MVPA with sociodemographic factors and health outcomes do not vary with epoch length. [64][65][66] Therefore, the variation in prevalence of physical inactivity we found would likely be similar, even with different epoch lengths. Lastly, a systematic review suggested that the protective effect of device measured PA on mortality plateaus around 130 to 140 minutes per week.…”

Adapting the Intensity Gradient for Use with Count-Based Accelerometry Data in Children and Adolescents

“…We also used 15 s epochs rather than the 5 s epochs that were used when the IG was initially designed [4]. This may have contributed to the lower IG seen with the count IG, as previous work has shown that as the epochs' size increases, the IG becomes shallower [26]. Furthermore, the lowest intensity bins in the adjIG likely differed in the nature and range of activity compared with the accIG, as the adjIG bins included more values above the sedentary threshold than the first bins in the accIG.…”

The influence of accelerometer epoch length on physical activity output in adolescent athletes

“…In many studies of adolescents, PA is usually investigated in relation to health, and the most meaningful PA indicators are not only vigorous activities, but also energy expenditure, moderate-to-vigorous activity, and sedentary hours ( Joensuu et al, 2018 ; Papadopoulou et al, 2021 ; Skinner et al, 2023 ; Wang et al, 2019 ). This study also analyzes PA as part of a project to investigate the impact of youth athletes’ lifestyle on their growth and development, so in future research, it might be better to consider both of a short epoch that can better capture physical movements and a long epoch that can better reflect physiological energy expenditure levels.…”