Our muscles actually become weaker during a standard set of high-intensity strength exercise. That is, performing eight to 12 controlled repetitions to the point of muscle fatigue, with both positive and negative muscle contractions, produces some microtrauma to the working tissues. It is believed that a degree of microtrauma is necessary for muscle development.

Apparently, however, there is a direct relationship between the amount of microtrauma and the amount of recovery time needed for tissue repair and building. It is also likely that too much microtrauma leads to tissue injury and a lengthy rebuilding process.

Consider the following hypothetical example of a muscle stimulus-response. Suppose that, as illustrated in Figure 1, your biceps strength is 100 pounds at the beginning of Monday's workout.

Immediately after your exercise session, your fatigued biceps can produce only 94 pounds of strength. The next day, your biceps strength has recovered to 99 pounds, the third day it has climbed to 103 pounds, the fourth day it has dropped to 102 pounds, and the fifth day it has almost returned to its original level of 100 pounds.

In this example, you would experience the best training effect by training your biceps again on Wednesday, two days after your original workout. That is, you should perform your next workout at the high point of your strength building process. Training your biceps again after one day, on Tuesday, would probably be counterproductive, because the muscles have not returned to their previous strength level. Delaying your workout until Thursday should be effective, but the rate of strength development may be a little slower. Waiting until Friday to exercise the biceps again will not interfere with your muscle-building process, but the rate of strength development may be further reduced.

Due to physiological differences, such as the percentage of slow-twitch and fast-twitch muscle fibers, some individuals may attain their peak training response sooner or later than other individuals. Also, advanced trainees typically require more recovery and tissue-building time between successive exercise sessions because their workouts are more demanding. Our research studies with previously untrained participants show excellent results with both two-day and three-day training, but favor the every-other-day exercise protocol.

In the Lab
We conducted two specific studies comparing the muscular effects of two versus three exercise sessions per week during a two-month training period. The first study involved 33 college-age subjects and examined strength gains using the bench press exercise. As shown in Table 1, the trainees who performed two workouts per week increased their bench press strength by 17 percent, and the trainees who performed three workouts per week increased their bench press strength by 24 percent. Although both training groups made significant improvements, the rate of strength-development for the two-workouts-a-week program was about 70 percent of that of the three-workouts-a-week program.

The second study included 48 middle-aged, mostly male subjects and examined lean (muscle) weight gains using a 12-station Nautilus circuit. As presented in Table 1, the trainees who performed two workouts a week increased their lean weight by 3.0 pounds, and the trainees who performed three workouts a week increased their lean weight by 3.9 pounds. Although both training groups made significant improvements, the rate of muscle development for the two-workouts-a-week program was almost 80 percent that of the three-workouts-a-week program.