D.ANANDHI, et al. “Correlation of Functional Capacity Between Six Minute Walk Test and Six Minute Step Test in Chronic Obstructive Pulmonary Disease Patients”. International Journal of Pharma and Bio Sciences, vol. 8, no. 2, June 2017, pp. 234-49, doi:10.22376/ijpbs.2017.8.2.b234-239.