The number of people hurting themselves at the gym is rapidly increasing and trainers are unsure why.

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Last year 52,964 claims were made to ACC for gym-related injuries, including 783 for fractures or dislocations, 177 for dental injuries and 13 for hernia.

The number of claims has risen steeply over the past seven years. There were just over 51,000 claims made in 2016, rising from only 17,694 in 2011.

Exercise Association chief executive Richard Beddie said it was still unclear why the number was on the rise.

"One reason we can explain which is around usage, so in the last five years there has been a more than 50 percent increase in the number of people exercising which is great."

However, the rate of injury was rising at a faste rpace than the increase in people heading to the gym, he said.

"So there is another factor or factors which are causing the growth and that's the bit we just don't know about."

The association was working with ACC to try and figure out why so many claims were being made.

"The biggest injury spots are some of the more obvious things like lower back, shoulders and knees, which are traditionally areas which get injured because they have potentially some weakness to start with," Mr Beddie said.

"20 to 29-year-olds have some of the lower injury rates when you factor in how many of them there actually are, and the highest rate is 40 to 49-year-old males."

Mr Beddie said the research might confirm an idea that older people were sustaining more injuries because they had not exercised in a while and were heading to the gym without warming up or knowing what was best for their body.

'We refer to them as cowboys'

Personal trainer Corinne Austin said it was time for the industry to be regulated by government and she had even written an open letter to the prime minister to consider the issue.

"Essentially anyone can go out and start a fitness business and run boot camps and personal training sessions, we top even rugby now for claims - not for the money value, but for the number of claims."

People did not have to have any qualification to work in a gym, Ms Austin said.

"There's a whole lot of people out there, we refer to them as cowboys, but they may have been to a fitness bootcamp or a weight loss challenge and they feel they have the knowledge to go out and do something, so they go out and start their own class.

"But the problem is there is a science and knowledge behind knowing how the body works ... [and] if you don't know what you're doing it could cause an injury and have a big impact on their life."

People could check if a trainer was qualified through the Register of Exercise Professionals, she said.

The Exercise Association said it hoped to have a good understanding of what the issues were by the end of this year so it could then look at making changes.