Accelerometry and heart rate as a measure of physical fitness: Cross-validation

Purpose: We recently reported on a new method to assess physical fitness, based on the combined use of accelerometry and heart rate (HR) registration. This study tested the validity of the prediction formula in a group of healthy young adults. Methods: Twenty-six healthy subjects performed a maximal incremental test on a bicycle ergometer to determine VO2max. A triaxial accelerometer and a HR monitor were worn for 7 d under free-living conditions. The prediction formula developed in a previous experimental group (EXP) was applied on the cross-validation group (CV). Results: No difference was found in subjects' characteristics between the EXP and CV groups except for accelerometer output (activity counts). Although measured VO2max could be predicted for 80% (P < 0.0001), a paired t-test showed a significant difference between measured and predicted VO2max (178 mL(.)min(-1); P = 0.015). Because of the difference in activity between the EXP and the CV groups, all data were combined and sorted according to activity counts, then two new groups were formed. As a result, EXP and CV groups were created that did not significantly differ in activity or any other parameters. The formula developed in the new experimental group (R-2 = 0.74; P < 0.0001) explained 72% (P < 0.0001) of the variation in VO2max in the cross-validation group, a paired t-test showed no difference between measured and predicted VO2max, and Bland-Altman plotting showed no systematic bias. Conclusion: Although a good correlation was seen between measured and predicted VO2max, in the cross-validation group, care should be taken in applying the prediction formula on groups that differ in physical activity from the current study population.