CrossFit uses diverse actions — dreaded push-up-to-leaping movements known as Burpees, sprinting, rowing, jumping rope, weight-lifting, and carrying odd objects — in ever-changing combinations to form prescribed “Workouts of the Day,” or WODs.

Despite his expertise and fitness (he also runs and plays squash), Gill, 47, recently aggravated an old back injury by lifting too much weight in a CrossFit class. He said he has treated a number of knee and shoulder injuries, mostly strained and torn cartilage, among CrossFit and Tough Mudder participants.

Tough Mudder started as a 500-person competition in Allentown, Pa., in 2010 and has morphed into a global phenomenon with an estimated 500,000 participants in events across the United States, United Kingdom, Australia, and Canada this year, according to the company.

Among them is Sebastian Buhaley, 36, a Jamaica Plain CrossFit devotee who recently completed his second Tough Mudder.

“I wanted to raise the intensity level,” said Buhaley, a former competitive runner who said he has not injured himself in his CrossFit workouts because the variation of the exercises has helped him avoid overuse injuries and get significantly stronger.

CrossFit has grown from 13 affiliates (trainers or gyms specially certified in the method) in 2005 to over 4,000 now, mostly in the United States, according to the company.

Data about the health benefits versus the risk of injuries in the new breed of high-intensity workouts is hard to come by.

A 2005 article by Greg Glassman, founder of CrossFit, noted at least five cases of “rhabdo,” short for rhabdomyolysis, a potentially lethal breakdown of muscle cells that can lead to kidney failure, associated with CrossFit workouts. All five people were hospitalized.

And there have been scattered media reports and blog postings of broken bones, hypothermia, and hospitalizations during Tough Mudder events in several states.