Isometric training is considered the least effective. This is due to the fact that strength gains are specific to the angle of muscle contraction. This means that if a joint is in a 90 degree angle during the isometric contraction, the muscle is strengthened primarily at that angle. Angles greater or less than 90o, strength gains are not as great. It goes back to working the joint through its full range of motion.
Isometric training is most beneficial in sports where static contractions are performed regularly.