The study, led by Stuart J. Fairclough of Edge Hill University and colleagues, followed 249 children aged eight to nine years, just over half of them girls, drawn from seven primary schools in Pennine Lancashire in northwest England. The schools spanned a range of neighborhood deprivation levels, and the participation rate was remarkably high: 81.6 percent of all Year 4 children in the recruited schools took part. Each child wore either an ActiGraph GT9X or an Axivity AX3 triaxial accelerometer on the non-dominant wrist for twenty-four hours a day over seven consecutive days, at three points during the 2023–24 school year: November to December 2023, February to March 2024, and June to July 2024.

What makes this study technically distinctive is its choice of outcome measures. Rather than relying on traditional cut-point methods, which classify accelerometer epochs into categories like moderate-to-vigorous physical activity, the researchers used two directly measured metrics that describe the entire 24-hour movement profile. The first is average acceleration, or AvAcc, which represents the mean magnitude of dynamic acceleration across the day and serves as a proxy for total physical activity volume, expressed in milligravitational units, or mg. The second is the intensity gradient, or IG, which captures the curvilinear relationship between how intensely a person moves and how much time they spend at each intensity level. IG values are always negative, and higher, less negative values indicate that movement is more evenly spread across the full intensity spectrum, including more time at mid-range and vigorous intensities.

Used together, these two metrics tell a richer story than either alone. AvAcc correlates strongly with conventional moderate-to-vigorous activity, but the intensity gradient adds information about the shape of a child’s activity distribution that cut-point methods simply cannot see. Both metrics have been independently associated with health and wellbeing outcomes in children and adults, which means that a change in one without the other can carry real clinical meaning. In this study, that distinction proved decisive, because the two dimensions of movement behaved in strikingly different ways over the school year.

The headline result concerns weekdays. Weekday AvAcc was stable between autumn and the winter-spring measurement window, then rose significantly by summer, with an adjusted increase of 3.94 mg relative to winter-spring and 4.43 mg relative to autumn. The intensity gradient, however, barely moved. In other words, children accumulated more total movement on school days as summer arrived, but that extra volume was spread fairly evenly across the intensity spectrum rather than concentrated in harder bursts. The researchers suggest that longer daylight hours and warmer temperatures — average daily temperature climbed from around 4 to 6.7 degrees Celsius in the colder windows to 18.3 degrees in summer, with daylight extending from roughly 8.5–10.6 hours to 16.7 hours — unlocked more outdoor breaks, outdoor physical education, active commuting, and neighborhood play, all of which add volume without necessarily transforming the intensity profile.

Weekends told a different story. Both weekend AvAcc and weekend IG remained essentially flat across all three time points, and both were lower than their weekday counterparts throughout the year. This pattern fits the structured days hypothesis: school days impose a predictable architecture of breaks, lessons, and commutes that reliably generates movement, while weekends hand children large blocks of discretionary time in which sedentary and low-intensity choices often win. The MX metrics, which describe the acceleration above which a child’s most active minutes are accumulated, reinforced the point. On weekdays, the most active single minute of the day reached between 2,203 and 2,434 mg in summer terms, while weekend equivalents were consistently lower. Still, this was a highly active sample: at every time point, the children’s most active sixty minutes exceeded the intensity of brisk walking, and they accrued ten to fifteen minutes daily at intensities equivalent to running.

The second aim of the study took the analysis a step further by asking whether all children follow the same trajectory. Using k-means clustering on standardized autumn AvAcc and IG values, the researchers sorted the children into three weekday groups — Most Active, Somewhat Active, and Least Active — and three analogous weekend groups. The weekday clusters were not socially neutral: the Most Active group was 61.6 percent boys, while the Least Active group was 60.5 percent girls, echoing the well-documented gender gap in children’s activity. Weekend cluster memberships, by contrast, were nearly balanced by sex, suggesting that the looser structure of weekends may open different activity opportunities, such as family walks or community sports, that appeal to girls who are less active during the school week.

Cluster trajectories revealed a subtle but important pattern. All three weekday groups increased their AvAcc from winter-spring to summer, by roughly 3.6 to 4.6 mg, but only the two less active groups showed significant gains from autumn to summer, at 5.2 and 5.8 mg respectively. The Least Active group was also the only one whose weekday intensity gradient improved significantly, rising by 0.05 from autumn to summer. In other words, the children who needed it most responded most strongly to the seasonal shift, and their gains were more consistent across the intensity spectrum than those of their already-active peers. Weekend trajectories showed no significant within-cluster changes, though the Least Active children again displayed the most positive, if statistically uncertain, trends.

Are such small changes meaningful? The authors argue they may well be. Emerging evidence in adults proposes that a daily AvAcc increase of roughly 1 mg constitutes a minimum clinically important difference for health benefits, and related work by the same team suggests that modest increases — as little as three minutes of vigorous activity per day — are associated with meaningful reductions in children’s body mass index z-scores. The practical implication is that incremental, incidental movement accumulated throughout the day, rather than dramatic intervention-driven spikes, may be the realistic and effective target for health promotion in this age group.

The study is not without limitations. The sample came from a single English region, all schools belonged to an active schools network, and the children were unusually active to begin with, which may have created a ceiling effect that blunted observable gains. Missing accelerometer data was substantial, particularly for weekend wear, and the researchers addressed this with multiple imputation by chained equations, generating separate weekday and weekend datasets that precluded direct statistical comparison between day types. Accelerometer data alone also cannot reveal why trajectories changed; no contextual data on weather-specific behavior, school programming, or family routines were collected. Even so, the findings carry a clear message for intervention design: seasonal influences on activity volume are real and exploitable, weekday and weekend behavior follow different rules, and a one-size-fits-all program is unlikely to serve the Most Active and Least Active children equally well. Future work, the authors suggest, should follow children longer and pair the accelerometry with contextual data so the mechanisms behind these trajectories can finally be pinned down.

Subject of Research: Longitudinal trajectories of accelerometer-measured physical activity volume and intensity in primary school children across the school year

Article Title: Trajectories of children’s physical activity volume and intensity across the school year: the Ready, Set, Move project

Article References: Fairclough, S. J., Clifford, L., Banks, J., Edwards, P., Gilmour, A.-M., Tyler, R., Brown, D. M. Y., Rowlands, A. V., & MacDonald, M. (2025). Trajectories of children’s physical activity volume and intensity across the school year: the Ready, Set, Move project. Journal of Activity, Sedentary and Sleep Behaviors, 5(1), Article 1. https://doi.org/10.1186/s44167-025-00091-x

Image Credits: AI Generated

DOI: 10.1186/s44167-025-00091-x

Keywords: physical activity, children, accelerometer, average acceleration, intensity gradient, school year, seasonal variation, weekdays, weekends, public health, longitudinal study, pediatrics