I have never come close to finishing an endurance race anywhere near the time of the winners. However, I take some solace in the notion that the ability to sustain a high level of aerobic activity probably has a strong genetic basis, and therefore regardless of how hard or long I train, there will always be others that simply have a higher intrinsic capacity for exercise. Wouldn't it be great to understand the functional basis for such differences in aerobic capacity? Rather than blaming one's genes, wouldn't it be more satisfying to pinpoint where in the oxygen transport system (from the lungs through the blood to the tissue) the differences between“natural” endurance athletes and the rest of us are manifested?Although physiologists have been exploring what limits aerobic capacity for decades, recent experiments published by Henderson and others approach these questions in a novel and intriguing way.

Henderson and colleagues have used artificial selection to develop lines of rats with different inherent exercise capacities. Their selective regimen worked as follows. Animals from a large founder population were trained for one week to run on an inclined treadmill. The following week these rats underwent five days of testing to see how far they would run during a progressive test to exhaustion. The 13 rats with the longest runs were considered high-capacity runners (HCR); the 13 with the shortest runs were low-capacity runners (LCR). HCR and LCR rats were randomly bred, and their offspring underwent similar testing and categorization based on treadmill endurance. Again, the 13 high/low extremes were selected and bred randomly. Following seven generations of this selective breeding, HCR and LCR animals differed substantially in their running capacity: an average HCR rat ran close to 1600 meters before exhaustion, while average LCR rats ran just over 220 meters before quitting.

When the team measured the rats' maximal oxygen consumption rate, they found it was significantly greater in HCR than LCR animals. Various physiological tests were then performed on these animals to explore potential differences in oxygen transport at various points in the oxygen transport system such as lung ventilation, lung oxygen diffusion capacity, cardiac output and oxygen diffusion from the blood into tissues. However, no differences in lung ventilation or lung-blood diffusing capacity were found. The team found higher levels of cardiac output in HCR rats, but these were offset by relative decreases in blood hemoglobin and arterial oxygen concentration in these same animals. Hence, differences in maximal oxygen consumption rate were not mediated by differences in convection in the lungs or circulatory system, or by differences in diffusion between the alveoli and lung capillaries. Rather, differences in oxygen diffusion/extraction at the level of the tissues seem to underlie differences in maximal oxygen consumption rate between HCR and LCR individuals.

Thus, divergent selection on endurance capacity led to rats that differ greatly in their maximal rates of oxygen uptake. Moreover, these differences in oxygen consumption are determined largely by differences in oxygen transport at the tissues rather than at other sites in the oxygen transport system. An obvious next step is to use these evolved lines to explore in more detail the structural and/or physiological mechanisms underlying these differences in tissue oxygen transport.

Henderson, K. K., Wagner, H., Favret, F., Britton, S. L., Koch,L. G., Wagner, P. D. and Gonzalez, N. C. (
). Determinants of maximal O2 uptake in rats selectively bred for endurance running capacity.
J. Appl. Physiol.