HLTH 4340 Week 11 Cardiorespiratory Final Paper Example

Cardiorespiratory Endurance From Heart to Field Test: Why Oxygen Delivery Sets VO2max, What Training Changes First and How a 12-Minute Run Stands In for the Laboratory

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Walden University

HLTH 4340: Fit and Well: Core Concepts in Exercise Science

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Month Day, Year

Cardiorespiratory Endurance From Heart to Field Test: Why Oxygen Delivery Sets VO2max, What Training Changes First and How a 12-Minute Run Stands In for the Laboratory

Component and Line

This paper examines cardiorespiratory endurance, the capacity to sustain large-muscle work by supplying oxygen to the muscles and using it there. Its line is that, in healthy people exercising with large muscle groups, the ceiling on this capacity, maximal oxygen uptake (VO2max), is set mainly by how much oxygen the heart and blood can deliver, not by how much the muscles can extract. Each section tests that line against a different kind of evidence: the physiology that produces the capacity, what training changes, how the capacity is measured and how the measured number relates back to the tissue.

The Physiology: From Lung to Mitochondrion

Oxygen uptake is described by the Fick principle: oxygen uptake equals cardiac output multiplied by the difference in oxygen content between arterial and mixed venous blood. Cardiac output is heart rate times stroke volume. The arteriovenous oxygen difference reflects how much oxygen the muscles remove from each liter of blood. Every structure along the path, from the lungs through the heart, the blood and the capillaries to the mitochondria in muscle fibers, could in principle limit the maximum.

Bassett and Howley (2000) reviewed the evidence on which of these sets the limit and concluded that VO2max is limited by the ability of the cardiorespiratory system to deliver oxygen to exercising muscle. They gave three lines of support. When oxygen delivery is changed, by blood doping, low-oxygen air or beta-blockade, VO2max changes with it. The rise in VO2max with training comes mainly from a higher maximal cardiac output, not from a wider arteriovenous difference. And when a small muscle mass is given more blood than it would normally receive, it can consume oxygen at a far higher rate per kilogram than during whole-body exercise, which shows that muscle has spare capacity to use oxygen.

Adaptation: What Training Changes First

If delivery is the limit, training should raise VO2max mainly through the heart and blood. Montero et al. (2015) tested this in 16 untrained healthy young men who completed six weeks of supervised endurance training, three to four sessions a week. Peak oxygen uptake rose by about 9% and peak cardiac output by about 7%. Red blood cell volume, plasma volume and hemoglobin mass all increased. In the muscle, capillary density and mitochondrial volume rose, but mitochondrial respiratory capacity did not change. The researchers then removed blood to return red cell volume to its pretraining level, and peak oxygen uptake fell by about 7%, largely erasing the gain. In their analysis, peak cardiac output, red cell volume and hemoglobin mass independently predicted peak oxygen uptake. They concluded that the early training gain was mainly due to cardiac output and the blood's capacity to carry oxygen.

Training responses also vary widely between people. In the HERITAGE Family Study, Bouchard et al. (1999) trained 481 sedentary adults from 98 families for 20 weeks. The average gain in VO2max was about 400 milliliters per minute, but some people gained little or nothing while others gained more than 1 liter per minute. Response varied about two and a half times more between families than within them, and the authors estimated a maximal heritability of 47% for the response.

Two cautions apply. Montero's sample was small, young and male, and HERITAGE enrolled only white adults in the analysis reported, so neither describes everyone. Muscle adaptations are not unimportant: Bassett and Howley (2000) note that mitochondrial changes improve submaximal endurance by increasing fat oxidation and reducing lactate accumulation, even if they do little to raise the maximum.

Measurement: Laboratory Criterion and Field Estimate

The laboratory criterion for this capacity is VO2max measured by expired gas analysis during a test to exhaustion on a treadmill or cycle ergometer. It requires equipment, trained staff and a maximal effort. Field tests trade accuracy for convenience. The best known is the 12-minute run published by Cooper (1968), in which a person covers as much distance as possible in 12 minutes. In 115 men in the U.S. Air Force, distance covered correlated with treadmill-measured VO2max at r = 0.897, which allowed VO2max to be predicted from distance.

A correlation near 0.9 is strong for a group but leaves room for error in an individual. The test also depends on pacing skill and motivation, and the original sample was young men, so the equation may be less accurate in women, older adults or people with little running experience. A field value is a predicted VO2max and should be reported as one.

Integration: From the Number Back to the Tissue

The field number connects to the tissue through the Fick principle. Distance in a 12-minute run depends on the speed a person can sustain, which depends on the oxygen they can take up, which depends on how much blood the heart can pump with each beat and how much oxygen that blood carries. A person who trains for six weeks and runs farther in the test has, on the evidence of Montero et al. (2015), most likely increased stroke volume and red cell volume, the delivery side of the equation. The same improvement in distance could also reflect better pacing or running economy, which a field test cannot separate from VO2max; this is part of its error.

The integration also explains why responses differ. If delivery sets the limit, people whose hearts and blood volume respond strongly to training will gain the most, and the HERITAGE finding that response clusters in families fits a capacity whose main limits are partly inherited.

Conclusion

Cardiorespiratory endurance is limited at its maximum by oxygen delivery. Bassett and Howley (2000) set out the evidence for that limit, Montero et al. (2015) showed that early training gains in young men came mainly from cardiac output and blood volume, and Bouchard et al. (1999) showed that the size of those gains varies widely and runs in families. The 12-minute run from Cooper (1968) offers a practical estimate of the same capacity, strongly related to the laboratory value in the group it was built on but carrying error for any one person. Read together, the evidence traces one line from the heart and blood to a distance on a track.

References

Bassett, D. R., Jr., & Howley, E. T. (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine & Science in Sports & Exercise, 32(1), 70-84. https://doi.org/10.1097/00005768-200001000-00012

Bouchard, C., An, P., Rice, T., Skinner, J. S., Wilmore, J. H., Gagnon, J., Pérusse, L., Leon, A. S., & Rao, D. C. (1999). Familial aggregation of VO2max response to exercise training: Results from the HERITAGE Family Study. Journal of Applied Physiology, 87(3), 1003-1008. https://doi.org/10.1152/jappl.1999.87.3.1003

Cooper, K. H. (1968). A means of assessing maximal oxygen intake: Correlation between field and treadmill testing. JAMA, 203(3), 201-204. https://doi.org/10.1001/jama.1968.03140030033008

Montero, D., Cathomen, A., Jacobs, R. A., Flück, D., de Leur, J., Keiser, S., Bonne, T., Kirk, N., Lundby, A.-K., & Lundby, C. (2015). Haematological rather than skeletal muscle adaptations contribute to the increase in peak oxygen uptake induced by moderate endurance training. The Journal of Physiology, 593(20), 4677-4688. https://doi.org/10.1113/JP270250