Proper form for the bird-dog was described. Here are a couple variations that can be done to target the anti-rotation core musculature, such as the multifidus and the obliques!
There were three variations performed in this video:
Clip 1: is a regression of the typical bird dog in quadruped. Having your torso stabilize on the swiss ball takes a lot of the core stabilization out of the exercise. Many people feel awkward when first performing a bird dog, this variation can be used for learning the movement. Once the motor control has been attained the next step is to progress to quadruped.
Clip 2: is a progression that will require rotation of the thoracic spine as well as good hip mobility in order to touch the elbow to the opposite knee.
Clip 3: is a progression that requires much more BALANCE & CORE STRENGTH, make sure you can perform 15 bird dogs in quadruped without fatigue before progressing to this push-up position variation. *It is important to note that you should never extend at your lumbar spine when performing this, focus on engaging the entire core and extend only at the hips. Improving core stabilization has been shown to prevent and treat non-specific lower Back pain.