Figure 1.

Schematic representation of the nerve conduction and muscle contraction studies. Nerve Conduction Velocity (NCV - left) measures the velocity and the quality of conduction of the electrical signal in a nerve. During the test, your nerve is stimulated, with an electrode attached to your skin. One or two more electrodes patches are placed on the skin over your nerve. The electrical impulse of the stimulated nerve pass from the stimulator to the other receiving electrode. The time (in milliseconds) spent by the impulse to move from a point to another, on the order of millimeters, represent the Velocity. In ALS, the impulse conduction is slower respect with control cases and is worsened by the increase of axonal degeneration. The electromyogram (EMG-right) measures the electrical activity of the muscles at rest and during contraction. There are two kinds of EMG: surface EMG and intramuscular EMG. In the first one the muscle activity is recorded by one or more electrodes patched on the skin and it asses the contractile response of superficial muscles. This approach presents several limitation since the result signal is influenced by the depth of the subcutaneous tissue at the site of the recording and by the discharges of adjacent muscles. With the intramuscular EMG, specific deep muscle activity is recorded by using one needle electrode inserted into the muscle. EMG and NCV tests are often done together to give more complete information.