Self-assessment of cardiorespiratory fitness: evaluation of reliability and validity of a step test eliminating a step board.

Purpose: Conventional assessment of cardiorespiratory fitness (CRF) requires specialized equipment and designated spaces, limiting accessibility. To address this, the modified National Institute of Occupational Safety and Health, Japan step test (mJST) was developed for self-assessment without a step board. This study evaluated the reliability and validity of the mJST.

Methods: A total of 131 participants (49% female, aged 30-59) were divided into a derivation group (n = 82) to establish an estimated maximal oxygen consumption ( $\dot{\text{V}}$ O_{2 max}) model using multiple regression, and validation group (n = 49) to test the accuracy of the model. All participants completed the mJST and a  $\dot{\text{V}}$ O_{2 max} treadmill test. The mJST comprised whole-body exercise for 3min and 40s followed by a 2-min recovery period. Heart rates (HRs) during the mJST were self-recorded using a wrist-worn device and measured via electrocardiograph. Test-retest reliability and validity were assessed using intraclass correlation coefficients (ICCs) and Bland-Altman analysis.

Results: Excellent test-retest reliability was observed with electrocardiograph HRs (ICC = 0.92); however, fair to good reliability was observed with the wrist-worn device (ICC = 0.75). Bland-Altman analysis showed that mJST overestimated  $\dot{\text{V}}$ O_{2 max} by 1.24 mL·kg^-1·min^-1, likely owing to discrepancies in HR readings from the wrist-worn device. The correlation between estimated and measured  $\dot{\text{V}}$ O_{2 max} was moderate (r = 0.68).

Conclusions: Eliminating the step board in step tests may aid in regular assessments of CRF to support ongoing health management. Despite limitations in device accuracy, the mJST demonstrated sufficient reliability and validity as a practical tool.