Athletes train by "stressing and recovering". On one day, they take a hard workout which damages their muscles, on the next day, they feel sore and take easy workouts, and when the soreness goes away, take a hard workout again. They also break down individual workouts into intervals of stress and recovery. After warming up, they increase the intensity of the workout until they feel burning in their muscles, become short of breath, or exceed a certain heart rate. Then they slow down and when they have recovered partially, they increase their intensity again. They repeat these stress and recovery intervals until their muscles start to stiffen and they are then stop the workout. A report from The National Institute for Occupational Safety and Health in Morgantown, West Virginia shows that the shorter the rest during an interval, the longer it takes to recover (Medicine & Science in Sports & Exercise, August 2005).
If you are a regular exerciser, you probably have already noticed this in your own body. Runners may take an interval workout of running ten quarter miles averaging 65 seconds each, with a 110 yard jog lasting three minutes between each hard run. If they shorten their recoveries to two minutes, they tire earlier, their muscles feel more sore afterwards, and it takes them longer to recover. The same applies to weightlifters. A weightlifter may do four sets of ten repetitions of lifting a 150 pound weight 10 times and resting for three minutes between each set. If he shortens his interval rest to one minute, he may not be able to finish his workout, feels far more soreness during the workout and still be sore for many days after that workout.
Athletes learn their ideal interval rest durations through trial and error. They may want to rest until their pulses drops enough for them to feel comfortable, or for them to be able to slow breathing rate down towards normal, or wait until their muscles lose soreness and they feel fresh. They do not wait for complete recovery of resting heart or breathing rate, or complete recovery from muscle soreness. Runners and cyclists often use heart rate monitors or just a clock to determine when they will do their next interval. Weight lifters usually wait for their bodies to "feel" recovered. You can use whatever yardstick for recovery you like, but if it takes you longer than two days to recover from an interval workout, you are probably exercising too intensely, doing too many repetitions, or not taking a long enough interval rest.
