How effective is isometric exercise?

R U ready: Any exercize result depends on what you put into it strength gains only occur at the angle you use in the exercise. During an exercise, blood flow to the muscle stops, blood pressure rises, and less blood flows back to the heart. This could be dangerous if you have heart problems.

At what: It's not clear what your goal is. They are good for muscle stabilization and in the rehabilitation setting. But to be honest, these types of static contractions, don't allow for a muscle to work through it's range of motion. It won't likely help with building size and strength. It may help with decreasing blood pressure and maintaining some degree of strength.