Side Plank - Vashistashana - Pose for Sage Vashistha
Core Stability

Side plank pose is a very integrative, body aligning posture which has been found to be an important part of an exercise approach for reducing scoliosis, deviation in the curvative of the spine. In side plank the obliques, quadratus lomborum, rectus abominis, and transverse abdominis are the primary muscles to support the position. Working consciously to elongate the spine by engaging the transverse abdominis(uddiyna bandha) will create better results for improving posture. In cases of Scoliosis, for the best results, the side plank should be adapted to suit individual cases, best done with a trainer. Otherwise, practice a complete Hatha yoga practice with variety in exercises and balance the time spent on both sides. 

Version 1

Version 2

Beginning variations can be practiced until the necessary strength and balance is developed to stack the feet and raise the top arm. The bottom leg should work especially strong pushing down into the edge of the flexed foot against the floor. Also engage the quadriceps, lifting the kneecap, and push through the heel. The pelvis and side body lift away from the floor by pushing down through leg and the supporting hand. Make your body as straight as possible and fully turn the torso to the side, and away from the floor. Looking upwards will encourage this movement.

Inhale down

Dynamic Side Plank Exercise

Exhale up

As the posture becomes stable you can explore the range of motion in side bending. Create a good extension of the spine first by engaging the transverse abdominis for a few breaths in the side plank. Begin to go down on the inhale, but not resting the full weight of your hip on the floor. Move up on the exhale. The eyes can stay looking down for balance or also going with the movement up and down.

This version of side plank works more balance control and flexibility. Start by grabbing the big toe of the top leg in side plank. Extend the leg to the side. When the leg has full extended begin to lift up and simultaneously push onto the bottom of the foot of the supporting leg. 

More variations and similar poses

Sage Vashistha, for whom this Asana is named after, is one of the saptarishi's, who are said to have learned Yoga directly from Shiva, the first yogi and teacher who appeared in the Himalayas. Vashista is one of only two mortals to have a hymn in the Rigveda dedicated to him. He is accredited with many divine feats as well as narrating Vishnu Purana along with Rishi Pulatsya. He has also contributed to many Vedic hymns and is seen as the arranger of Vedas during Dwapara Yuga. -Wikipedia