not 100 % sure if it's strain or stress, but basically when they measure your heart rate, or respiration, or anything else, whilst under physical strain, like running on a tradmill.

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Note added at 22 mins (2008-04-11 18:47:33 GMT)
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http://www.everythingfitness.com/heart-rate-monitor.html

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Note added at 29 mins (2008-04-11 18:54:17 GMT)
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found it! under stress (with the help of House MD :-) )

for example "Cardiac stress test"
A cardiac stress test is a medical test performed to evaluate arterial blood flow to (and indirectly the amount of oxygen that will reach) the myocardium (heart muscle) during physical exercise, compared to blood flow while at rest. As an exercise test, the results can also reflect overall physical fitness. These tests do not assess emotional stress or other connotations of the term.
http://en.wikipedia.org/wiki/Cardiac_stress_test

Example from an article:
Several diseases result in difficulties or makes it impossible for the patient to achieve sufficient ventilation for adequate gas exchange of the alveolar space in the lungs. This results in the blood, which leaves the lungs, not having appropriate content of oxygen and carbon dioxide which often is referred to as respiratory insufficiency. Lack of oxygen can mostly be rectified by supplying to the patient breathing gas having a higher oxygen content than that present in air, but when it comes to the carbon dioxide a positive effect cannot be achieved in a corresponding way. At serious respiratory insufficiency, respiratory treatment is applied to ensure the patient a sufficient alveolar ventilation. However, if the function of the lungs is seriously disturbed respiratory treatment can result in the pressure in the airway during injection of the breathing volume being very high, and it can then be a risk that the airway and the lung tissue are further damaged, a phenomenon often called barotrauma. In order to reduce the requirements of ventilation drastic actions are taken such as narcosis and muscle relaxation, reduction of the body temperature and extracorporeal elimination of carbon dioxide, but such measurements are extremely resource demanding and of limited medical utility. (http://www.wipo.int/pctdb/en/wo.jsp?IA=WO1991/19526&DISPLAY=...