The plank is an excellent full-body exercise, particularly for the core and upper body. It is weight bearing through the hands and wrists (or, if modified, through the forearms), which is good for strengthening the bones of the wrists, arms and shoulders.
The knee plank is a plank that reduces the load on the hands and wrists as well as the demand on the core. If you have wrist pain or if a standard plank is too difficult, this is a good way to modify.
Begin on the ground with your hands placed on the floor shoulder-width apart, your neck and spine long, and your knees and feet on the floor.
Press the hips up so that your weight is supported through the hands and knees. Make sure your spine is long, your neck is straight and your belly is drawn in. You may need to adjust where your hand placement to find this position.