Do isometric workouts actually build muscle?
I believe not. Isometric exercises became popular back in the 60's, particularly with football linemen and wrestling coaches. The idea was that in each of these activities there is a lot of pushing against an opponent's resistance. So, it was thought, if athletes push against immovable objects and hold that push for a certain amount of time (@ 6 seconds), strength would be increased over time. In fact, that was and is true. The problem with it is that strength was only increased in THAT particular position, as only the muscle fibers used during that static push were being recruited. Size, however, was not impacted, due to the previous statement, i.e., limited muscle fiber reruitment. This is why incorporating a "plank" for core strenth has limited functional application: how often do you functionally get into a plank position? I wouild argue, NEVER. To sumarize static, or isometric, exercises do not and cannot increase muscle size, but DO increase muscle strength in the statically held position used, but only to a finite level. If, for example you do a 1/2 range of motion barbell biceps curl, wherein the barbell is held statcally for 10 or more seconds, only half the biceps fibers are being used in any meaningful way. The biceps will get stonger in that 90 degree position but not throughout the normal biceps curl range of motion, and the biceps will certainiy not increase in size by doing it.