After Hawking was diagnosed with amyotrophic lateral sclerosis (ALS) at 21 years of age, he was forced to balance the stresses of academic life with physical illness. The disease negatively affects brain and spinal neurons, making patients lose control of voluntary motor function and movement.
During the first few years, his condition worsened rapidly. Soon he relied on a wheelchair. By 1974, Hawking was unable to feed himself. Fortunately, the progression of the disease slowed a bit, eventually becoming more gradual over the years. With time, however, he still began losing the use of his voluntary muscles, hands and certain facial expressions.
Hawking previously used his finger to control a computer and voice synthesizer. But once he lost use of his hands, he started depending on twitching a cheek muscle to communicate. Most computers designed for him rely on running lists of words. Whenever the cursor reaches a word or phrase he wishes to use, Hawking will twitch his cheek muscle to select it. Then he'll go on to the next word until he creates a sentence. In the 1990s, by selecting words with his finger, he could pick 10 to 15 words per minute. But with the difficulty of twitching a cheek muscle, he can select about one word per minute [source: Ferguson].
Because of this, most of Stephen Hawking's speeches and interviews are done in advance to save time.
Despite being too ill to attend his 70th birthday celebration, Hawking has remained incredibly active. In fact, some see his dismissive attitude of his condition as a refusal to let it get in the way of life.
For more information about Stephen Hawking and his work, check out the next page.