On a late December day in 1967, after the best season of his young career earned him the Cy Young award and a $30,000 raise, Boston Red Sox pitcher Jim Lonborg went skiing at Lake Tahoe. On his last run, Lonborg formed a ‘V’ with his skis to slow himself down. Rather than braking, he tumbled down the mountain. The crash resulted in torn ligaments in his left knee, a very unhappy baseball team and a new kind of sports contract provision that resonates today.

In undertaking an activity away from their playing field, professional athletes face the same risks as an average person, just with higher stakes. This weekend, Rory McIlroy ruptured an ankle ligament playing soccer and New York Giants defensive end Jason Pierre-Paul badly damaged his hand in a fireworks accident, two more reminders that the maladies that afflict usual weekend warriors can befall athletes with serious, and sometimes contractual, consequences.

McIlroy could miss the British Open next week, and Pierre-Paul reportedly lost the chance to sign a $60 million contract extension. Were they irresponsible, gambling with the bodies that earn their livings? Or were they unlucky, suffering misfortune in an everyday endeavor they shouldn’t be barred from because of their professions?

As an individual athlete, McIlroy will have to grapple with the debate himself. In team sports, contracts include stipulations that govern which activities athletes are responsible for, should an off-the-field injury strike. Every league’s uniform player contract includes a description of banned activities, and teams can add their own touches to all or individual contracts.

The clauses began showing up in baseball contracts, which were guaranteed thanks to the sport’s strong union. After Lonborg’s injury – which led indirectly to a shoulder injury and a series of wayward seasons – the prohibitions became baked-in to contracts.

For example, the standard NBA contract specifically prohibits boxing, professional wrestling, motorcycling, moped-riding, auto-racing, sky-diving and hang-gliding. How should a hang-gliding enthusiast bide his time until his NBA career expires? Well, the contract expressly permits golf, tennis, handball, swimming, hiking, softball and volleyball.

The NFL’s uniform player contract is vaguer, disallowing players from “any activity other than football which may involve a significant risk of personal injury.” Baseball teams tend to be more specific. The New York Yankees once banned log-rolling in their standard contract; the Washington Nationals prevent players from piloting a plane.

“Before Jim Lonborg fell off his skis,” sports agent Ron Shapiro said, “I don’t think there was a bunch of awareness.”

In the 1990s, Shapiro negotiated to give Baltimore Orioles shortstop Cal Ripken the right to play pick-up basketball, inserting a clause into his contract that covered him in case he suffered an injury while participating in an activity from which other players were banned. Ripken insisted his basketball habit was part of his conditioning program, and the Orioles couldn’t argue that the record holder in consecutive games played presented an injury risk. Ripken had a basketball gym built in his basement.

Outlawed offseason hoops helped change baseball history. In January 2004, New York Yankees third baseman Aaron Boone tore his anterior cruciate ligament playing pick-up basketball, which his one-year, $5.75-million contract forbade. The Yankees cut Boone, a newly minted postseason hero they planned to play all season. Without a third baseman and only scraps left on the free agent market, the Yankees swung a startling trade with the Texas Rangers for Alex Rodriguez.

Some of the same instincts that make athletes great – fearlessness, a sense of indestructibility –  lead them to take wild risks, even when contractually forbidden. One season after the Chicago Bulls made him the second overall pick in the NBA Draft, point guard Jay Williams wrecked his Yamaha motorcycle and suffered career-ending injuries. The Bulls cut ties with him, giving him $3 million for medical expenses. They would have paid him $10.3 million for two years had he remained healthy.

In March 2002, San Francisco Giants second baseman Jeff Kent broke a bone in his left wrist, endangering his status for the start of the season. He told Giants officials and reporters that he fell off the top of his truck while washing it himself. The story prompted jokes at first and then doubts: Eyewitnesses reported having seen Kent popping wheelies on his motorcycle. The truck-washing story had been concocted so Kent wouldn’t risk losing out on his $6 million salary. The Giants and Kent eventually settled the issue.

“There’s no secret that players’ contracts are structured to try and limit players from doing dangerous things and injuring themselves and hurt the team,” Kent told the San Francisco Chronicle at the time. “There’s probably about 50 or 100 things that players can’t do. If you’re going to restrict a player from living, then I think that’s unfair, too.”