Physical Therapy exercises are designed to increase core strength and endurance. Goals of treatment are to increase back extensor, oblique, and flexor muscle strength and coordination. With greater strength and endurance you will be able to hold the position with less shaking. Also bring the two feet on top of each other. Finally work toward marinating a straight lateral plank position.
The exercises should always be performed in a pain free manner. Concentrate on holding the position for as long as possible. Try for 3-5 sets per day.
Performing the exercise on an unstable surface increases the difficulty and challenges the core muscles. The plank exercise increase strength and endurance of the rectus abdominus and oblique muscles.