Strength is a matter of the made-up mind.
Remember to keep your movements slow and controlled so your muscles are doing the work, not your momentum. Start with your feet out in front. Keep your back close to the chair as you bend your elbows and lower your buttock towards the floor. To increase resistance, either balance something heavy such as a dumbbell in your lap or raise your legs up onto a bench as shown. Keep your elbows close to your ears as you straighten your arms and extend the dumbbell directly above your head.