popular application and associated validity
In regards to the use of each protocol, isotonic exercises were initially used for athletic enhancement whereas static exercises were particularly popular among individuals with joint complications or a lack of the necessary motor control to perform dynamic exercises appropriately. Recently, training regimens for a variety of strength and hypertrophy goals has incorporated both protocols (SOURCE).
The common belief of these protocols is that isotonic exercise elicits greater hypertrophy, particularly through the eccentric phase, as well as dynamic strength, whereas isometric exercise can produce more significant strength gains at specific joint agnles. Though this is largely supported in literature, there is also an exceedingly strong body of evidence in contrast.
Adams, Cheng, Haddad & Baldwin (2004) found the relation of greater hypertrophy with isotonic exercise, particularly the eccentric phase compared to that of isometric to be completely contrary. In their testing protocol, isometric exercise accounted for a 14% hypertrophic gain compared to 12% from concentric-only contractions and 11% from eccentric-only muscle activation. Though this is not conclusively the case from all related literature, it is certainly not an uncommon result. This raises questions regarding the validity of the effectiveness of each muscle activation protocol.
To sufficiently address this issue, the underlying causes for the body's hypertrophic adaptation and strength gain must be understood and compared to the mechanical and physiological demands each protocol places on the body.