Side Plank or Vasisthasana increases not only flexibility, but strength, too. Once you begin adding this pose to your practice, you will quickly see results in your arms, legs, and core strength. It is a powerful pose that makes people feel powerful! To make the pose accessible for everyone, there are several subtle variations.
Begin in Downward Facing Dog, then shift forward into a high plank position. Then, rotate to the right side, so the outer edge of your right foot and your right hand are supporting your body weight. Keep the right elbow locked and strong. Your left foot can be staggered in front of the right foot or stacked on top of it. You can also bend the right knee and use it like a kickstand for the body. The left leg should be long and strong with the outer edge of the foot on the floor. Your left arm should be lifted toward the sky, in line with the right arm. Then, return to Downward Facing Dog and repeat on the left side.
Lowering the bottom knee to the floor provides one variation. You can also use the top leg for support, too. To do this, you keep the bottom leg straight with the outer edge of the foot on the floor. Then, bend the top knee and put the foot on the floor. Lift the hips up to the sky as you do this.
If you want to add difficult to the pose, there are several other variations. They all require keeping the bottom foot on the floor and lifting the top leg. In one variation, you simply lift the top leg. In another, you lift the top leg, but place the foot on the inner thigh of the lower leg, like tree pose. In another, more difficult variation, you lift the top leg as high as possible while holding on to the big toe with the top hand.