These below three exercises are isometric...the joints are static. This means the muscles are "stationary" during the contraction phase. This holding of a position can be more difficlut than dynamic (or isotonic) movements, obviously depending on the time duration. What is especially important is ensuring controlled breathing throughout the static muscle contraction.
We will be using these core isometrics as "active rest" periods between isotonic movements. These are generally done to either failure (cannot hold the position any longer) or for time (ex: 30 seconds).
Above is the traditional PLANK.
The above is the SIDE PLANK. Ignore the segments with adding weight.
The above is the SUPERMAN.