This is a popular glute activation exercise, and Dr Stuart McGill uses it for glute activation when working with patients who have lower back pain.
It is safe on the back, if you do it properly, and yet still allows you to extend both hips.
It provides you with an opportunity to connect with your glute contraction, a chance to master your deliberate hip extension, with the right muscles.
Lie on your back, with both your legs bent to square 90˙ angles.
Position your arms along your sides, and put your hands on your glutes to feel their level of activation. Do not put your hands underneath you, in between your glutes and the ground. Position them to feel your glutes from the side.
First, contract your glute muscles as though you are squeezing a card or coin in between your bum cheeks. With your hands, you should feel the muscles stiffen, or harden, if you get it right.
Once you are sure you have activated your glutes, then, lift your hips up. Squeeze your glutes even harder while you lift, making sure that they are involved in your lifting.
At the top position, use your hands to feel your glute activation again and make sure that they are fully contracted.
Lower your hips back to the floor before repeating.
Your priority is to make sure that you activate your glutes, and that you lift your hips with your glutes driving the movement. If your glutes are not doing this, then you will likely be lifting your hips with your lower back and hamstrings instead. If you feel fatigue in your lower back, you are probably not activating your glutes enough, or at all.
Slightly bracing your core throughout the movement will ensure that your spine is stabilized and supported.
Complete 2 to 3 rounds of 10 reps, plus hold the last rep up for 10 seconds if you are comfortable to do so.