defining isometric and isotonic
Prior to examining the advantages of isometric and isotonic muscle activation methods, it is necessary to examine their differences by the establishing physiological and mechanical characteristics of each.
Isometric muscle activation occurs when there is force generated against a load by which the muscle is attempting to contract, but no movement is generated (Faulkner, 2003). A.V. Hill (1938) found that isometric muscle activation actually does have physiological properties of contraction. There is no displacement achieved in the end-to-end length, however, and thus it is considered to have no net movement.
In isotonic muscle action, displacement of the external force occurs whereby the external force remains constant. Either the muscle is able to overcome the resistance and contract (concentric activation), or be overcome by it and extend (eccentric activation).
As concentric, eccentric, or isometric muscle activation must be involved in every training protocol, their popularity as a topic in literature has been widespread since Adolf Fick first introduced isometric and isotonic as mechanically descriptive terms in 1887 (Faulkner, 2003).
Before making comparing the effectiveness of these two muscle activation techniques in eliciting strength and hypertrophy, the physiology and mechanics that account for muscular adaptations must be understood. This will include fiber type orientation, ability to generate ATP, elastic and passive compliance of the muscle, the length-tension relationship, the force-velocity and torque-velocity relationships, hormonal influence, neural innervations, and the associated amount and rate of activation.