Second, the researchers observed that ratings of perceived exertion steadily increased over the course of the workout even as sprint times held steady. On average, RPE was 10 on a 20-point scale after the 3rd sprint and increased to 14 after the 12th sprint. This finding suggested that the subjects got a sense for how fatigued they should feel at any given point in the workout to avoid becoming totally exhausted before the workout was completed. Finally, the researchers observed a negative correlation between the individual students’ rest times and their VO2max measurements. In other words, the more aerobically fit an individual student was, the less time he tended to rest between sprints. Since a higher VO2max does enable a person to recover faster after high-intensity exercise efforts, this observation suggested that individuals of various aerobic fitness levels were able to accurately sense their capabilities and make appropriate training decisions based on these feelings. While the subjects of this study were able to feel their way toward discovering the optimal work/rest ratio for speed intervals in less than two weeks, this was a rather small thing to discover when every other parameter of the workout was given to them. In the real world, the modern sport of distance running had existed for more than half a century before anyone even got the idea to run intervals. So while you could reinvent the most effective workouts on your own by feel, you really should not try. Recovery runs, base runs, long runs, progression runs, fartlek runs, hill repetitions, interval runs, tempo runs, and a few other types of workouts are proven to work well for every runner, and learning them is step one of becoming a runner. An overview of the basic run workout types is presented in the Appendix.