In addition to genetic factors, both the intensity and duration of endurance training have an important impact on the magnitude in training induced changes in VO2 max. For example, prolonged periods of endurance training (i.e., two to three years) can produce large improvements (e.g., 40%-50% increases) in VO2 max in genetically gifted individuals (29). Also, high intensity endurance training can increase VO2 max by as much as 50% in individuals with the genetic potential to respond favorably to endurance training (14, 41). For example one high-intensity study revealed a large (e.g. 30%-S0% improvement) and linear increase in VO2 max over ten weeks of training (41). These results are in contrast with many training studies that show a leveling off of the VO2 max values after only a few weeks of training. The much larger increase in VO2 max with this ten-week training program was due to a higher intensity, frequency, and duration of training than are commonly employed in en durance exercise programs. We next discuss the physiological adaptations that are responsible for endurance t raining induced improvements in VO2 max.