The aim of this study was to determine if maximal effort, evidenced by peak HR was attained during the 20m shuttle-run test in a naturalistic setting. Shuttle-run test performance and peak HR were measured in 208 volunteers (11-16 years). Peak HR was 196 (95% confidence interval (C.I.) 194-198 bpm). The relationship between test performance and peak HR was assessed by regression. There was a weak, but statistically significant relationship between test performance and peak HR (R2 = .029, p = .029) but with such a low coefficient of determination (less than 5% criterion), poor performances were not associated with low peak HR values or underestimation of maximal performance. Peak HR values (196 bpm) were higher than cited criterion values (185 bpm) for maximal effort in laboratory studies. In a naturalistic setting, the 20m shuttle-run test elicits a maximal effort in most children.