Of course, muscles are not aware of the number of repetitions they complete. The key to muscle performance and fatigue is the relationship between force and time. Generally speaking, you can produce a high level of muscle force for a relatively short time and a low level of muscle force for a relatively long time. However, to stimulate strength development, you should train your muscles within the anaerobic energy system. This system provides large amounts of energy for up to 90 seconds of high effort exercise.

Training Guidelines
For best strength results, it is recommended that you use enough resistance to fatigue your muscles within 30-90 seconds. As general guidelines, persons with predominantly Type 2 muscles should training about 30-50 seconds per set. At six seconds per repetition (two seconds lifting and four seconds lowering), this represents about five to eight repetitions.

Persons with predominantly Type 1 muscles should train about 70-90 seconds per set. At six seconds per repetition, this represents about 12-15 repetitions. Persons with an even mix of muscle fibers should train about 50-70 seconds per set. At six seconds per repetition, this represents about 8-12 repetitions. These time and repetitions relationships are summarized in Table 2.

Because more people possess a relatively even mix of muscle fibers, a 50 to 70- seconds bout of strength exercise is an excellent training recommendation.

If you choose to train in a slowed manner, completing 14-second repetitions (10 seconds lifting and four seconds lowering), you should use enough resistance to complete four to five repetitions. In this manner, the muscles again reach fatigue within the 50 to 70-second anaerobic range.

Specialized Training
Depending upon the type of activity you undertake, you may want to adjust your training accordingly. To determine the optimum number of repetitions for a particular muscle group, such as the quadriceps, follow this procedure:

1 - Perform 10 leg extensions with relatively light weight.

2 - After a two-minute rest, perform five leg extensions with a moderate weight.

3 - After a two-minute rest, perform one leg extension with a relatively heavy weight.

Continue in this manner until you determine the heaviest weight load you can perform once with proper technique. This is your maximum weight load.

After a five minute rest, perform as many leg extensions as possible with 75 percent of your maximum weight load. This represents the approximate number of repetitions you should perform in this exercise.

If you complete six repetitions, for example, you most likely have a higher percentage of Type 2 muscle fibers and should probably train with about five to eight repetitions per set. At the other extreme, if you complete 13 repetitions, you most likely have a higher percentage of Type 1 muscle fibers and should probably train with about 12-15 repetitions per set.

Overall, it would appear that best strength results are attained when you match your training repetitions to your muscle fiber type. Although muscle fiber type may vary somewhat among muscle groups, we have found this factor to be relatively consistent in most people.

Regardless of your muscle fiber type, the key to strength development is high-effort, anaerobic exercise. The resistance should be sufficient to fatigue the target muscle group within 30-90 seconds of exercise. For most of us, this is best accomplished with a double progressive training system using a protocol of eight to 12 repetitions. First, select a resistance that fatigues the target muscle within 8-10 repetitions. Second, train with this resistance regularly until you can complete 12 repetitions. Next, add 2.5 to five pounds and progress in the same manner until you can perform 12 repetition with the new resistance. By matching the optimum repetition range with your muscle type and by systematically increasing the repetitions and resistance, you will make safe, steady and solid progress toward your ultimate strength potential.