[digdugdirk]

Follow up details - isometric exercises don't have to be just bodyweight, and can be trained for strength by performing the exercise against something to constrain the motion. Imagine crawling under your bed before performing the plank (now more of a pushup, since you're pressing with your chest muscles) mentioned above. You can push as hard as possible, but you won't budge the bed.

They're incredible for returning from injury, since you can specifically train ranges of motion, and the person performing the exercise has full control of the "load" - aka how hard to exert themselves.