Bent-Over Triceps Extension
This more traditional weight training exercise strengthens the triceps which is the muscle in the posterior upper arm. If you don't have dumbbells at home, try this with a can of food or other household item of appropriate weight.
Start with a weight that you feel you can do for 10 - 12 repetitions and feel a gentle workload. As you become accustomed to the exercise and position you can work with a more challenging level of resistance.
Modifications for Those with Back Issues
If you have a history of low back issues you can modify this exercise to decrease stress to your low back. For both of these modifications draw abdominals in gently and keep your back long, sternum drawn gently up from ribs.
The easier modification for your low back will be to sit toward the front of a armless chair. Rest one forearm across your thighs to provide support for your low back while you perform the exercise with the other arm.
Another common modification for this exercise is to partial kneel on a sturdy bench. You can rest one hand and one knee on the bench (putting your back in a strong table top position) for back support while you perform the exercise with the other hand.
Video Demonstration of Exercise