Plank exercises are used on many sports teams for development of the core muscles, i.e. the abdominal, lower back and sometimes hip muscles. In reality however, the plank is a very poor exercise for strengthening the core for athletes because it does not improve dynamic movements or develop strength through a full range of motion.
If you do the plank as part of a back rehabilitation program, the exercise can be of value for stability of the lower spine. Understand that for back stability static muscular contractions are good as are dynamic exercises. But even here, dynamic exercises in the long run are much more effective.
Dynamic strength is more functional as it can be used not only in your sport’s actions but in your daily movements. Static or isometric strength is very specific to a particular point in the range of motion and is effective mainly for holding (stabilizing) purposes. This means it enables you to hold a position for an extended period of time.
This is why the plank exercise is effective for developing static strength to hold the body in a straight line posture. It is not effective for enhancing any movements that use the core muscles, especially dynamic movements with the axis of rotation in the waist or hips. Thus any value gained from doing the plank exercise is specific only to holding the position seeing during the exercise.
It should also be noted that the plank exercise involves mainly the abdominal muscles when holding the push-up position. It does not involve the lower back muscles and only partially the hip flexors. The side plank position however, does involve all the core muscles but not equally. Only the face-up plank position involves mainly the lower back muscles in a static contraction.
If you are an athlete it should be obvious that the plank is not for you. Athletes need dynamic movements and dynamic exercises to develop the muscles as they are used in execution of the sport skills. Core exercises should be strengthened in a manner that enables you to execute any and all movements, especially as they are needed for high-level play.
Some of the best exercises for strengthening the core abdominal and lower back muscles include 45° sit-ups, reverse sit-ups, reverse trunk twist, back raise and back raise with a twist. If you also desire to strengthen the hip muscles then you should also do specific exercises for the hip abductors, adductors, flexors and extensors.
For more information on execution of these and other exercises and the muscles that they develop, see Biomechanics and Kinesiology of Exercise. It is a very detailed book that is well illustrated with muscle and exercise pictures. It contains complete descriptions of not only how the exercise should be done, but also what happens when you change grips, stance, execution,etc.