The seated back extension is an exercise for your lower back, especially the Erector spinae. Strengthening the lower back will improve your posture and might help you to reduce back problems.
However, the back extension machine is controversial among many fitness professionals, because this is a completely unnatural movement that puts a lot of stress on your lower back. Repeatedly flexing your lower back under a load can cause injury to the lumbar discs, and the rigid position the machine holds you in doesn’t allow your core, glutes, and hamstrings to contract as they should to protect you.
If you choose to do the machine back extension, it is important to start with a low weight and carefully progress from there.
It is very important to do this exercise in a slow and controller manner. Keep all movements slow, jerking the back will increase the risk of pulling a muscle or even spinal injury.
Avoid hip movement. Push your hips to the back of the seat by pushing the feet into the platform throughout the whole movement.
Only lean back as far as you can with a good form. The back should be straight and abs engaged the whole time.
If you experience lower back pain, decrease your range of motion or avoid this exercise altogether.