The maximum oxygen uptake (V̇O₂ max), determined from graded maximal or submaximal exercise tests, is used to classify the cardiorespiratory fitness level of individuals. The purpose of this study was to examine the validity and reliability of the YMCA submaximal exercise test protocol performed on a newly-designed rectilinear stepping ergometer (RSE) that used up and down reciprocating vertical motion in place of conventional circular motion and giving precise measurement of workload, to determine V̇O₂ max in young healthy male adults. Thirty-two young healthy male adults (32 males; age range: 20-35 years; height: 1.75 ± 0.05 m; weight: 67.5 ± 8.6 kg) firstly participated in a maximal-effort graded exercise test using a cycle ergometer (CE) to directly obtain measured V̇O₂ max. Subjects then completed the progressive multistage test on the RSE beginning at 50W and including additional stages of 70, 90, 110, 130, and 150W, and the RSE YMCA submaximal test consisting of a workload increase every 3 minutes until the termination criterion was reached. A metabolic equation was derived from the RSE multistage exercise test to predict oxygen consumption (V̇O₂) from power output (W) during the submaximal exercise test (V̇O₂ (mL·min^-1 )=12.4 ×W(watts)+3.5 mL·kg^-1·min^-1×M+160mL·min^-1, R²= 0.91, standard error of the estimate (SEE) = 134.8mL·min^-1). A high correlation was observed between the RSE YMCA estimated V̇O₂ max and the CE measured V̇O₂ max (r=0.87). The mean difference between estimated and measured V̇O₂ max was 2.5 mL·kg^-1·min^-1, with an SEE of 3.55 mL·kg^-1·min^-1. The data suggest that the RSE YMCA submaximal exercise test is valid for predicting V̇O₂ max in young healthy male adults. The findings show that the rectilinear stepping exercise is an effective submaximal exercise for predicting V̇O₂ max. The newly-designed RSE may be potentially further developed as an alternative ergometer for assessing cardiorespiratory fitness and the promotion of personalized health interventions for health care professionals.

• The rectilinear stepping exercise is a simple modality of exercise, which requires only up and down movements of the legs. It overcomes the mechanical dead centers of circular motion and is mechanically efficient. It is potentially applicable to a large group of populations.

• The RSE gives an accurate measurement of power output and ensures a constant power output independent of stepping cadence.

• The RSE submaximal exercise test is valid and feasible for estimating V̇O₂max in young healthy male adults compared with the CE maximal exercise test.

• The rectilinear stepping exercise is an effective submaximal exercise mode for predicting V̇O₂max. The RSE designed for this study may be potentially developed as a new and alternative ergometer to assess cardiorespiratory fitness and could be used in the future by healthcare professionals to promote personalized health interventions.