You can perform this exercise standing as well but this puts strain on your back especially if you are using heavy dumbbells like 95 lbs or so. (bodybuilding.com)
This sleeping position tends to place a lot of strain on the back, and this is the main reason why many people wake up with a back pain. (pillowtip.com)
For weight trainers, lifting weights can put much strain on your back. (agymrat.com)