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Background  Maximal oxygen uptake (VO 2max ), a vital physiological indicator, has been widely used in many fields. In recent years, the measurement method of VO 2max  has been widely explored in various populations, but few studies have been conducted for women drug abusers. For the importance of VO 2max  in the formulation of aerobic exercise intensity for drug users, the present study estimated VO 2max  using the step test index combined with heart rate variability in women with drug use disorder.    Methods  Forty women methamphetamine (MA) users without cardiovascular disease and dyskinesia participated in a cardiopulmonary exercise test (CPX) and a 3-minute step test. Each of them performed a heart rate variability (HRV) monitoring test after the step test, and VO 2max  was estimated by step test index and HRV.    Results  (1) The step test index had a significant positive correlation with VO 2max . The standard deviation of normal-to-normal interval (SDNN) had a significant positive correlation with VO 2max  and a significant positive correlation with the step test index; (2) the R-square values of the estimated VO 2max  by step test index and post-SDNN for overall MA users were 0.29 and 0.22, with an accuracy of 93.19 and 92.85%, respectively; (3) the R-square values of the estimated VO 2max  by step test index and post-SDNN in group I were 0.27 and 0.36, respectively, with an accuracy of 94.04 and 93.99%. The R-square value of the estimated VO 2max  by step test index in group II was 0.44, with an accuracy of 92.65%, however, post-SDNN cannot adequately estimate the VO 2max  in group II; and (4) there was no significant difference in VO 2max  obtained by CPX, step test index, or post-SDNN, regardless of overall or grouping variable analysis.    Conclusion  The 3-minute step test combined with HRV can estimate the VO 2max  of women MA users to a certain extent, but the size and the coverage of the sample size should be further considered. In the future, more methods, such as machine learning or artificial neural networks, should be used.

An Experimental Study of Step Test Index Combined With Heart Rate Variability in Estimating Maximum Oxygen Uptake in Women With Drug Use Disorder