Abstract Body (Do not enter title and authors here): Background: The Bruce treadmill protocol is a commonly used protocol for treadmill-based exercise testing in children. However, normative data for endurance time dating from 1978 remains the clinical reference, despite shifting population anthropometrics and fitness levels. This review evaluates trends in endurance times of healthy children since 1978.

Research Question: How have reference values for endurance time using the standard Bruce protocol changed over time for children ages 4-18?

Methods: A systematic search relevant to the research question from the Semantic Scholar corpus was conducted. We retrieved 499 papers and screened in those that met the following criteria: (1) subjects aged 4-18 years old with no identified cardiac or pulmonary pathology, (2) non-metabolic maximal effort exercise tests (achieved ≥85% predicted peak heart rate), (3) utilization of the standard Bruce protocol, (4) endurance times reported by age group, and (5) written in English. Endurance times were grouped into three age (years) categories (4-9, 10-15, 16-18). Bootstrap resampling was performed to estimate the mean and standard deviation for each age group, gender, and time period by repeatedly resampling endurance times with replacement (10,000 iterations). The bootstrapped mean was calculated as the average of these resampled means, while the bootstrapped standard deviation was derived from their standard deviation.

Results: Eight studies (n = 1,947) were included in the final analysis. Endurance times declined over time in both North America (1970s–2000s) and Europe (1990s–2010s). In North America, mean reductions were observed across all combined age groups: 4–9 years (–1.27 min), 10–15 years (–1.57 min), and 16–18 years (–1.00 min). European cohorts showed similar declines in younger groups: 4–9 years (–1.36 min) and 10–15 years (–2.28 min). The 16–18 European group was only assessed at one time period, precluding trend analysis.

Conclusion: Prior studies point to a decline in fitness in pediatric patients over time. This review of the literature affirms this finding and also highlights regional differences in anthropometrics and fitness trends. Given that current reference values are based on outdated data, there is a need to re-evaluate endurance time standards for modern pediatric populations. This data also points to a critical need to address declines in fitness in children in the current era.