You don't tell me when in the course of the exercise routine the shaking occurs, but I suspect it occurs after you have been exercising for a while, at least, because what you describe usually indicates muscle fatigue occurring with tensions approaching maximal force. It sounds also as if your symptoms occur during "eccentric" muscle contraction (that is, contraction while the muscle is lengthening, as opposed to "concentric" contraction which is contraction in which the muscle shortens), which generally involves more force in the muscle. The combination of increasing fatigue plus excessive force can trigger discoordinate impulses from muscle and tendon stretch receptors, and chemoreceptors, thus providing competing "contract" and "relax" signals to the muscle, resulting in the sort of sensation you describe. The sensation should only last for a brief period, and should quickly subside with rest. You can probably avoid the sensation entirely by moderating your exercise program just a little, and progressing the intensity of it just a little more slowly. Also, be sure that you maintain good nutrition and hydration.