1. The physiological responses to repeated continuous (progressive) exercise together with the relationship between continuous and discontinuous work tests on an upright bicycle ergometer have been studied in healthy male subjects.

2. The results show a marked decline in mean cardiac frequency (fH) from 145 beats/min to 124 beats/min during the first four experiments at a given submaximal oxygen intake of 1·5 1/min and thereafter a smaller decline to reach 118 beats/min on the final (sixteenth) day of the investigation. This latter decrease was associated with a small but significant rise in maximum oxygen intake (V̇O2 max) which occurred from Day 8 onwards. Pulmonary minute ventilation at an oxygen intake of 1·5 1/min (V̇E 1·5) decreased by about 5 1/min immediately following the first occasion of submaximal work but thereafter remained unchanged. There was however no decrease in O2 cost of exercise, as V̇O2 at a fixed rate of external work remained unchanged throughout the investigation.

3. At maximal effort there was no significant decrease in either V̇E max or fHmax, but the fH at which the V̇O2 max was reached declined significantly from Day 8. Thus the asymptotic nature of the fH/V̇O2 curve which was very pronounced on Day 1 virtually disappeared following the third visit to the laboratory. The accuracy of predicting V̇O2 max from fH and V̇O2 increased noticeably from − 15·4 ± 8·9% on Day 1 to −6·5 ± 10·5% on Day 7 and −0·9 ± 3·4% on Day 15. The possible physiological basis and implications of the results are discussed.

4. No significant differences were found between continuous and discontinuous work. Thus in large scale population studies of work capacity or in the evaluation of training programmes in rehabilitation studies, a continuous test may be used but attention must be paid to familiarization procedures. At least three preliminary test periods are necessary before the results become reproducible and reliable.

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