NGF

The NGF did not return a request for comment.

In 2005, when Tiger Woods was in his prime and the economy was humming, there were around 30 million golfers in the U.S., according to NGF.

More courses remained open in rural areas than in parts of the country with concentrated populations, the NGF said.

Dr. Cohen determined that this early development was spurred by a substance similar to NGF, which he called epidermal growth factor, or EGF.

NGF does this by binding to tyrosine kinase receptor A, which triggers a cascade of effects that include boosting the number of pain receptors and sensitizing nerves.