CHQ: SUBJECT: uncontrollable jerking movements
  MESSAGE: When I am exercising hard, the left side of my body will start to do strange things. When lifting weights overhead, my left upper lift will jerk up into sort of a sneer.  When doing core work, my left arm will start jerking so hard that it's impossible to hold onto anything. The jerking stops after the exercise ends; but my left arm feels worn out.  No pain. A few months ago, my left leg completely lost any strength for weeks. It first appeared when I was just trying to do a squat with a plastic pipe in my hand. Eventually was able to work strength back. Again - no pain.  Should I be alarmed?
Summary: What causes jerking movements during exercise?