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Related Experiment Videos

Metabolic and performance responses to constant-load vs. variable-intensity exercise in trained cyclists.

G S Palmer1, L B Borghouts, T D Noakes

  • 1School of Life Science, Kingston University, Kingston upon Thames, Surrey KT1 2EE, United Kingdom.

Journal of Applied Physiology (Bethesda, Md. : 1985)
|September 14, 1999
PubMed
Summary

Steady-state (SS) and variable-intensity (VI) cycling show similar whole-body responses but differ in muscle fuel use. VI cycling spares muscle glycogen and increases glucose oxidation, yet time trial performance remains unchanged.

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Area of Science:

  • Exercise Physiology
  • Sports Science
  • Metabolic Research

Background:

  • Understanding fuel utilization during different exercise intensities is crucial for optimizing athletic performance.
  • Constant-load (steady-state, SS) and variable-intensity (VI) exercise protocols elicit distinct metabolic demands.
  • The impact of these protocols on muscle glycogen utilization and glucose oxidation requires further investigation.

Purpose of the Study:

  • To compare glucose oxidation (Glu(ox)) and glycogen degradation between SS and VI cycling at the same average power output.
  • To assess the subsequent 20-km time trial (TT) performance after these exercise protocols.
  • To investigate differences in muscle fiber recruitment and substrate utilization.

Main Methods:

  • Six trained cyclists completed four trials: two 140-min exercise bouts (SS and VI) with muscle biopsies for glycogen analysis, and two similar trials including a 20-km TT.

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  • During experimental rides, subjects ingested a [U-(14)C]glucose solution to measure Glu(ox) rates.
  • Measurements included O(2) consumption, energy expenditure, heart rate, plasma lactate, and muscle glycogen content (periodic acid-Schiff's staining).

    • Main Results:

    • Whole-body responses (O(2) consumption, energy expenditure, heart rate, perceived exertion) were similar between SS and VI trials.
    • Plasma lactate was significantly higher during VI compared to SS.
    • VI cycling resulted in less total muscle glycogen utilization (49% vs. 65% for SS) and higher total Glu(ox) (99.2g vs. 83.9g for SS).
    • Fewer type I fibers were glycogen-depleted after VI (59%) compared to SS (98%), with greater type II fiber involvement in VI.
    • Despite metabolic differences, subsequent TT performance was not significantly different between SS and VI trials.

    Conclusions:

    • 140 minutes of either SS or VI exercise at the same average intensity elicits similar whole-body metabolic and cardiovascular responses.
    • VI exercise promotes greater glucose oxidation and spares muscle glycogen compared to SS exercise.
    • Differences in skeletal muscle carbohydrate metabolism and fiber recruitment do not translate to performance disparities in a subsequent time trial.