As with most forms of training, and with things that sound too good to be true, isometric training has its definite disadvantages. These include the fact that applying force at one specific angle will only serve to strengthen the muscle at that particualr angle. Therefore, the isometric contraction would have to be applied to many different angles on the muscle group being worked in order to strengthen it over a full range of motion. This would virtually eliminate the time advantage of isometric exercise. In addition, isometric exercise causes a signifigant increase in both systolic and diastolic blood pressures, which leads to an increased workload on the heart and its oxygen demands. The static contraction may also cause a respiratory effort to be made against a closed glottis, thereby potentially creating the Vasalva maneuver. Obviously, these physiological changes associated with isometric exercise are not very favorable, especially in older individuals, unfit individuals, and those who already have high blood pressure or risk factors for heart disease.
Isometric contractions may still be used to some degree of effectiveness either alone, or perhaps better in conjunction with concentric and eccentric contractions. For example, one could isometrically contract at the top of a movement, such as in a bicep curl, by holding the weightmomentarily before lowering the weight. Also, at the end of a set, isometric contractions could be used to try and overcome the resistance brought about by concentric, and even eccentric, muscular failure.