“It’s a name and a face that get people to understand what kind of a disease this really is,” she said. “It makes it more personal.”

Image Lou Gehrig during a sold-out tribute at Yankee Stadium on July 4, 1939. Credit... Murray Becker/Associated Press

A.L.S. in the N.F.L.

A link between professional football and A.L.S. follows recent discoveries of on-field brain trauma leading to dementia and other cognitive decline in some N.F.L. veterans. Dr. McKee and her group identified 14 former N.F.L. players since 1960 as having been given diagnoses of A.L.S., a total about eight times higher than what would be expected among men in the United States of similar ages.

However, the doctors cautioned, the existence of the increased number of A.L.S.-like cases should not create the same level of public alarm as the cognitive effects of brain trauma, which affect hundreds of former professionals and perhaps thousands of boys and girls across many youth sports.

Recent epidemiological studies have suggested that brain trauma in sports can be a risk factor for A.L.S.; for example, a 2005 paper found that Italian professional soccer players had developed the disease at rates about six times higher than normal. Studies have also linked service in the United States military to higher risk for A.L.S., possibly because of battlefield collisions and blast injuries.

The study, to be published Wednesday on the Web site of the Journal of Neuropathology & Experimental Neurology, represents the first firm pathological indications that brain trauma results in motor-neuron degeneration, and that the resulting disease (at least in the three men studied) is actually not A.L.S. It is a different disorder with different markings, specifically a pattern of two proteins in the spinal cord that compromise nerve function.

Dr. McKee had already found 12 deceased N.F.L. veterans to have had chronic traumatic encephalopathy, a progressive disease in brain tissue that results in cognitive impairment and eventually dementia. Two of those men  Wally Hilgenberg, a longtime linebacker for the Minnesota Vikings in the 1970s, and Eric Scoggins, who played only three games at linebacker for the 1982 San Francisco 49ers  also had A.L.S. diagnosed by their physicians.

When Dr. McKee examined the spinal-cord tissue of those men, as well as a former boxer who had A.L.S.-like symptoms, she found dramatically high levels of tau and TDP-43, two proteins known to cause motor-neuron degeneration. She said that they would appear in the cord as a result of blows to the brain, with the proteins probably traveling down the spinal cord, rather than direct injury to the spinal cord itself.