This study is part of a continuing collaboration with the University of California at San Diego, first established in 1993. The purpose of this study is determine the conditions under which oxygen delivery limits or impairs muscle performance. Previous studies demonstrated a sensitive balance between oxygen delivery and uptake in athletes, but concluded that this relationship was absent in untrained sedentary normals. These findings suggested that the oxygen delivery capacity of the capillary system far exceeded the ability of the muscle mitochondrial system to utilize oxygen in normal, undeveloped muscle, while in athletic muscle, the maximal performance of the muscle was limited by this same capillary system. However, it was not known whether this was due to natural variations in humans, or whether this was a developmental adaptation in trained muscle. To test this hypothesis, the untrained normals were put through a ten-week training session and retested. The results after training demonstrated a trend in sensitivity to blood oxygenation similar to the response of the athletes, suggesting that muscle normally has an abundant reserve of oxygen supply, but that muscle development results in a better match between oxygen delivery and utilization. This also suggests that all muscle possesses a natural limit to maximal performance which is ultimately determined by the capillary network.