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A great deal of information about physical adjustment to work can be obtained from quantitative stress testing. Maximal stress limited by symptoms of exertional intolerance is the concept of the 2 min duration work increment test (rectangular-triangular exercise test). Compared to steady-state work tests strict observation of the standardized procedure- and computer assisted evaluation of ergospirometric parameters offer innovatory opportunities: (1) the test is of short duration (8-14 min), (2) the subjects recover rapidly, even from an exhausting test, (3) one is more likely to be able to observe plateauing of VO2, should determination of maximal VO2 be desired, (4) adaptation to increasing work rates and maximal work capacity is assessable, (5) computer technics provide on-line assessment of aerobic and anaerobic power in quantitative terms, (6) measurements proved to be highly reproducible, (7) the relationship between variables such as increments of heart rate and systolic blood pressure, respiratory minute volume, oxygen uptake during the early phase of the non-steady-state condition and the index of anaerobic power, and the influence of factors such as work load and work output, has been studied to derive standard values. Soft-ware programs have been designed to estimate deviation of parameters actually measured from standard values in terms of multiples of the standard deviation of the standard regression line. In particular, evaluating oxygen uptake during short time-intervals (0.5 min) provides information about adequate adaptational forces of the cardio-circulatory system. Energy that is not accounted for by reactions involving the VO2 measured is computed by substracting the caloric equivalent of oxygen uptake during work exceeding the steady-state level during rest from the energy demand to sustain a given work load aerobically. This index of anaerobic power is defined in kcal, cal/kg body wt., and as a percentage of the total amount of energy required (moderately trained athletes 350-500 cal/kg; sedentary people 200-300 cal/kg). A close relationship to parameters of metabolic acidosis (base excess) exists. It is concluded that the physical performance of sedentary people, athletes and patients with impaired cardio-pulmonary function can be more precisely qualified in quantitative terms by means of computer assisted rectangular-triangular ergospirometry. Results obtained in patients with diseased conditions must be carefully interpreted, their condition suggesting the use of more invasive investigations to reveal the pathophysiologic mechanism.