Comparing continuous and intermittent exercise: an "isoeffort" and "isotime" approach

PLoS One. 2014 Apr 15;9(4):e94990. doi: 10.1371/journal.pone.0094990. eCollection 2014.


The present study proposes an alternative way of comparing performance and acute physiological responses to continuous exercise with those of intermittent exercise, ensuring similar between-protocol overall effort (isoeffort) and the same total duration of exercise (isotime). This approach was expected to overcome some drawbacks of traditional methods of comparison. Fourteen competitive cyclists (20±3 yrs) performed a preliminary incremental test and four experimental 30-min self-paced protocols, i.e. one continuous and three passive-recovery intermittent exercise protocols with different work-to-rest ratios (2 = 40∶20s, 1 = 30∶30s and 0.5 = 20∶40s). A "maximal session effort" prescription was adopted for this experimental design. As expected, a robust perceived exertion template was observed irrespective of exercise protocol. Similar between-protocol pacing strategies further support the use of the proposed approach in competitive cyclists. Total work, oxygen uptake and heart rate mean values were significantly higher (P<0.05) in the continuous compared to intermittent protocols, while lactate values were lower. Manipulating the work-to-rest ratio in intermittent exercise, total work, oxygen uptake and heart rate mean values decreased with the decrease in the work-to-rest ratio, while lactate values increased. Despite this complex physiological picture, all protocols showed similar ventilatory responses and a nearly perfect relationship between respiratory frequency and perceived exertion. In conclusion, our data indicate that overall effort and total duration of exercise are two critical parameters that should both be controlled when comparing continuous with intermittent exercise. On an isoeffort and isotime basis, the work-to-rest ratio manipulation affects physiological responses in a different way from what has been reported in literature with traditional methods of comparison. Finally, our data suggest that during intermittent exercise respiratory frequency reflects physiological strain better than oxygen uptake, heart rate and blood lactate.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Athletes
  • Bicycling
  • Blood Gas Analysis
  • Exercise / physiology*
  • Exercise Test
  • Heart Rate
  • Humans
  • Lactic Acid / blood
  • Male
  • Respiratory Rate
  • Rest
  • Time Factors
  • Young Adult


  • Lactic Acid

Grant support

This study received no external funding. The study was supported by the University of Rome “Foro Italico”, Italy. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.