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Semple ran at Switzer and tried to rip her race number off to prevent her from continuing as an official competitor. In her memoir, she wrote:

Instinctively I jerked my head around quickly and looked square into the most vicious face I'd ever seen. A big man, a huge man, with bared teeth was set to pounce, and before I could react he grabbed my shoulder and flung me back, screaming, "Get the hell out of my race and give me those numbers!"

Semple's attack removed one of Switzer's gloves, but not her race number. When Switzer's slightly-built 50 year old coach Arnie Briggs attempted to protect Switzer, Semple knocked him to the ground. Switzer's boyfriend, Tom Miller, who was running with her, then put his shoulder into Semple causing him to fall down. Semple complained in a 1968 interview about Miller's success in stopping his attacks, saying, "That guy's a hammer thrower, for cripes' sake!"

Imagine how lucky this Semple guy is that Miller exercised restraint. Dude probably could have literally thrown Semple through the nearest building.

Also, from Semple's Wikipedia page...

Later in life, Semple reversed his position on women competing in the marathon. According to Marja Bakker (a later organizer of the race), "Once the rule was adjusted and women were allowed in the race, Jock was one of their staunchest supporters. He was very progressive." Semple later publicly reconciled with Switzer. "Old Jock Semple and I became the best of friends," she told a reporter in 2015. "It took a long time: six years. But we became best of friends."

Semple died of cancer of the liver and pancreas in March 1988 in Peabody, Massachusetts. He and Kathrine Switzer had become friends and she would visit him at the hospital where he was being treated for his cancer.

They became friends! It's nice when people can change for the better.