The most common source of vibration is machinery–a car engine, for instance. The last major source of image shake is vibration, a resonance that causes repeated bumps at rapid and regular intervals.
You don’t notice this because your brain compensates for the bumping image, but your brainless camcorder dutifully records a picture that jumps with every jarring step. Every time you take a step you send a small shock wave through your body that causes what you see to bounce slightly. The second biggest cause of camera shake is movement from place to place.
