John Duncan Semple (October 26, 1903 – March 10, 1988) was a Scottish-American runner, physical therapist, trainer, and sports official.

Switzer was officially entered in the race in accordance with the Boston Marathon's rule book, which at that time made no mention of sex.

[1][better source needed] Semple subsequently claimed that amateur rules banned women racing for more than 1.5 miles (2.4 km).

[3] Semple was born in Glasgow, Scotland, and emigrated to the United States in 1921 to work as a cabinetmaker in Philadelphia.

He had an established history dating back to at least 1957 of physically attacking Boston Marathon runners he perceived to be "non-serious" competitors, whether officially entered or running the course unofficially.

According to fellow race official Will Cloney: "He hurls not only his body at them, but also a rather choice array of epithets... Jock's method of attack is apt to vary."

In 1957, Semple narrowly escaped arrest for assault after attempting to knock down a marathon competitor who was wearing webbed snorkeler's shoes and a grotesque mask.

Although the Boston Marathon rule book made no mention of sex,[1] Semple later claimed her race registration was a result of an "oversight" in the entry-screening process.