This is a pretty good explanation I think, taken from the NINDS website:
Normally, messages from nerve cells in the brain (called upper motor neurons) are transmitted to nerve cells in the brain stem and spinal cord (called lower motor neurons) and from them to particular muscles. Upper motor neurons direct the lower motor neurons to produce movements such as walking or chewing. Lower motor neurons control movement in the arms, legs, chest, face, throat, and tongue. Spinal motor neurons are also called anterior horn cells. Upper motor neurons are also called corticospinal neurons.
When there are disruptions in the signals between the lowest motor neurons and the muscle, the muscles do not work properly; the muscles gradually weaken and may begin wasting away and develop uncontrollable twitching (called fasciculations). When there are disruptions in the signals between the upper motor neurons and the lower motor neurons, the limb muscles develop stiffness (called spasticity), movements become slow and effortful, and tendon reflexes such as knee and ankle jerks become overactive. Over time, the ability to control voluntary movement can be lost.
Electromyography (EMG) is used to diagnose disorders of lower motor neurons, as well as disorders of muscle and peripheral nerves. In an EMG, a physician inserts a thin needle electrode, attached to a recording instrument, into a muscle to assess the electrical activity during a voluntary contraction and at rest. The electrical activity in the muscle is caused by the lower motor neurons. When motor neurons degenerate, characteristic abnormal electrical signals occur in the muscle. Testing usually lasts about an hour or more, depending on the number of muscles and nerves tested.
EMG is usually done in conjunction with a nerve conduction velocity study. Nerve conduction studies measure the speed and size of the impulses in the nerves from small electrodes taped to the skin. A small pulse of electricity (similar to a jolt from static electricity) is applied to the skin to stimulate the nerve that directs a particular muscle. The second set of electrodes transmits the responding electrical signal to a recording machine. Nerve conduction studies help to differentiate lower motor neuron diseases from peripheral neuropathy and can detect abnormalities in sensory nerves.
Back to me, not the website. Fasciculations and twitches suggest a lower motor neuron problem, but that doesn’t mean that there is more going on than MS. Some MSers have an abnormal EMG: it depends on where their lesions are, i.e. if they are affecting the spinal motor cells.