Master's Two-Step Test was sometimes used, but it was too strenuous for many patients and inadequate for the assessment of respiratory and circulatory function during varying amounts of exercise.

To address these problems, Bruce and Dr. Paul Yu began work on developing a treadmill exercise test.

A Bruce exercise test involved walking on a treadmill while the heart was monitored by an electrocardiograph with various electrodes attached to the body.

Bruce's first paper on treadmill exercise tests, published in 1949, analyzed minute-by-minute changes in the respiratory and circulatory function of normal adults and patients with heart or lung ailments.

In 1950 Bruce joined the University of Washington, where he continued research on the single-stage test, particularly as a predictor of the success of surgery for valvular or congenital heart disease.

In the initial paper, Bruce reported that the test could detect signs of such conditions as angina pectoris, a previous heart attack, or a ventricular aneurysm.

Bruce and his colleagues also demonstrated that exercise testing was useful in screening apparently healthy people for early signs of coronary artery disease.

Signals from the treadmill monitors were sent by dataphone from 15 different test sites to the University Hospital research lab for analysis, and follow-up questionnaires from thousands of patients were accumulated for years.

Bruce published more than 300 scientific articles, a major contribution to the knowledge of cardiovascular physiology in health and disease.