This one will simultaneouslywork all the muscles of the upper body: your pecs, back, biceps, triceps and abs. Your legs will even benefit from their role of stabilizing the body.
Place two kettlebells or dumbbells about shoulder-width apart on the floor. Get in the top position of a push-up with your hands on the handles. Tighten up your entire body and squeeze the handles as if you were going to crush them. With your right hand, push hard into the ground while pulling the weight to the waist with your left. Lower the weight slowly and with control. Repeat on the other side.