Keeping a muscle under contraction for long periods of time leads to residual tension in the muscle after the workout. The sign of a healthy muscle is one that can contract strongly and then relax as completely and as quickly as possible.
Note that the normal state of the musculature should be relaxed with sufficient tone to maintain body posture. When a muscle is not in use, it should be relaxed. If the muscle does not relax you have residual tension which leads to body tension that can cause headaches or even muscle soreness. A classic example of this is the residual tension in the upper trapezius muscles on both sides of the junction of the neck and shoulder.
In addition, in order to maintain a muscle contracted at all times, you generally have to hold your breath or concentrate only on the static contraction. This, in turn, can inhibit the range of motion possible. For example, when doing abdominal exercises, as for example the crunch or sit up, you should work the muscle full range — this means rising up approximately 45 degrees.
When you reach approximately a 45° angle, the abdominals will then be fully shortened and tensed. When you return back to the initial position they should relax. How long the muscle relaxes depends upon your rate of doing the repetitions. This pattern holds true for all exercises. First you tense and then you relax.
Breathing helps in this regard. When you inhale and hold your breath, you create greater tension which assists you in rising up when the muscle contracts. When you exhale as you return to the initial position, — or even after you returned to the initial position — you relax.
The ability to tense the muscles and then relax the muscles is a learned activity. It is something that you should practice and master to make all your weight training exercises more effective. In addition, you will find yourself more relaxed and capable of working more effectively at the task you are working on.
However, if you strive to keep the muscles under contraction all the time, as is often recommended by many trainers, you’ll find that you are still tense after the workout and any work that follows will not be at your best. Our culture appears to create much stress and tension; do not add to it with your strength workouts which in the long run are supposed to help make you more productive.
See Kinesiology of Exercise for more information on exercise execution.