"Let's start by examining what happens when you exercise. In skeletal muscle, the cells never contract individually. Rather they contract as groups of muscle cells that are collectively connected to a motor nerve originating in the spinal cord; the combination of the motor nerve cell (neuron) and the muscle cells it innervates is known as the motor unit.
"The size of the motor units determines the precision of movement that a particular muscle can produce. For example, in the muscles of the larynx, or voice box, each motor nerve usually connects to only two or three individual muscle cells, allowing very fine gradations of strength. On the other hand, large muscles that serve gross movement--such as the gastrocnemius muscle in the lower leg--have motor units made up of motor nerves that each control 2,000 or more muscle cells.
"These motor units are not all excited simultaneously when a muscle is electrically excited and made to contract. In fact, the units are excited in a most asynchronous fashion by the trains of electrical impulses that come down the motor nerves from the spinal cord. As a result, while some of the motor units are contracting and shortening within the muscle belly, others will be relaxing and lengthening. The tremendous amount of overlap between motor units gives the appearance that the muscle is contracting smoothly overall.
"Strenuous exercise causes some of the motor units to drop out of service because of fatigue; it is this process that is ultimately responsible for the trembling you observe. Most of the fatigue probably occurs within the spinal cord at the level of the motor nerve cell and its neural connections, although some fatigue probably occurs also at the connections between this motor nerve and its muscle cells (the myoneural junction). Both these areas require the synthesis and release of particular chemicals to carry the electrical impulse across to either another nerve cell or a muscle cell. Researchers generally believe that the chemical cannot be manufactured and released fast enough to keep up with the level of activity, so it becomes depleted in this area of transmission. The depletion of these chemicals is a big part of what fatigue means in a biochemical sense.
"As more and more motor units become temporarily nonfunctional, the muscle contraction becomes dependent on fewer and fewer motor units. The dropout of the fatigued motor units causes the remaining individual contractions and relaxations to become on average more synchronous and less smoothly organized; the original overall appearance of a smooth contraction is replaced with a jerky, trembling movement now that many of the overlapping motor units have ceased to function. After adequate rest, the fatigued motor units return to normal, and the muscle again appears to produce a smooth contractile motion.