Weight training free weight - Spine extension

 This is example of exercise #2 of  Spine Extension (“Bird Dog”) (Isometric Exercise) with the instructions how doing this exercise:   At the begin on all fours with your knees the below of your hips and your hands below your shoulders.  The unilateral of spine extension:  (a)  Extend your right leg to the rear and reach forward with your right arm. Make sure keep your spine of neutral and your raised arm and leg in line with of your torso. And don’t arch your back or let of your hip or your shoulder sag. And then make sure you hold this position for 10–30  seconds. Then do repeat with your left leg and your left arm. The bilateral of spine extension: (b)  Extend your left leg to the rear and reach forward with your right arm. Keep your spine of neutral and your raised arm and your leg in the line with your torso. And don’t arch your back or let your hip or your shoulder sag. Make sure hold this position for 10–30  seconds. Then do repeat with your right leg and your left arm.  

The Muscles of developed which is the Erector spinae, gluteus maximus, hamstrings and deltoids. You can make this exercise more difficult by making the box of patterns with your arms and your legs.