This is a classic exercise for triceps, but you should be careful and I don´t recommend it if you have lower back issues. As you imagine, holding a weight behind your head creates tension in your lower back.
Stand-up holding a barbell or e-z bar with palms facing forward and hands closer than shoulder width apart. Feet about shoulder width apart. Lift the barbell above your head until your arms are fully extended. Keep your elbows in.
Inhale. Keeping your upper arms close to your head and elbows in, lower the barbell behind your head until your forearms touch your biceps. The upper arms should remain stationary and only the forearms should move.
Exhale as you contract your triceps, extending your arms to the starting position.