The side plank hold is a variation on the active Clam Shell. In both the hold and active clam shell keep your down leg pressed into the floor at the knee. Keep your elbow directly below your shoulder as you lift your body up from the floor. In the active version wear a band above the knees and open and close by lifting the free knee and balancing your foot on the shoe of the down leg.
In the hold version demonstrated here, lift your arm up which will open up your abdomen. Hold this for 30 seconds.
When you are ready, you can add the runner version shown here to this basic move. In this version bring your knees up to your chest and then extend straight down again. You can do 10 to 15 repetitions. This version builds hip and core strength as well as the upper gluteal muscles strengthened in the active clam shell or clam shell hold.