Trampoline injuries doubled in Christchurch after new trampoline parks opened - and the injuries were worse than those suffered bouncing at home.
Adults and older kids were more likely to hurt themselves at trampoline parks, usually injuring lower limbs.
Kids under 10 make up the bulk of at-home injuries, usually hurting arms and wrists falling off the tramp.
The study from the Canterbury District Health Board compared trampoline injuries in Christchurch in the 90 days before and after two trampoline parks opened in 2016, and in the same period a year earlier.
The research, led by Dr Lloyd Roffe and published today in the New Zealand Medical Journal, came after he and colleagues at the Christchurch Hospital Emergency Department noticed a worrying surge in people getting seriously hurt on tramps.
Mega Air Trampoline Arena in Hornby and Flip Out Trampoline Arena in Bromley both opened in August 2016. In the 90 days after both parks opened there were 602 ACC claims for trampoline-related injuries with 106 people going to hospital.
That's nearly double the same period in 2015, when there were 333 ACC claims, and nearly triple the number of hospitalisations (37 in 2015).
Over the 90 days 35 people went to hospital after being injured at one trampoline park, but just 15 from the other.
The park with more injuries let more than one person bounce on a trampoline at a time, according to the study.
Christchurch specialist emergency physician Dr Scott Pearson, who was involved with the study, said double bouncing sets up a weight difference so the lighter person can jump much higher.
When they land, they're more likely to hit someone, and will encounter a harder trampoline surface - which could be to blame for the increase in leg and spine injuries.
Flipout New Zealand franchise owner Kyle Dickinson said the centres have always had a one person per trampoline safety rule.
Dickinson said Flipout New Zealand had never had a spinal injury at any of its parks and in the 90-day period measured in the study had a very low percentage of injured customers: 0.0009 per cent.
Flipout New Zealand had a lower injury rate than two for every 10,000 jumpers - an overseas standard set by trampoline associations, Dickinson said.
Mega Air owner Doug Haselden declined to comment but in 2016 director Matthew Haselden said every customer was required to follow rigorous safety precautions, an induction including watching a safety video and were supervised at all times.
Mega Air does allow more than one person onto some of its longer trampolines - although it does not allow two people to jump into the foam pit at the same time and double bouncing is not allowed, according to its website.
New Zealand's first trampoline park opened in 2013 in Auckland, and there are now more than 10 around New Zealand.
ACC payouts for trampoline injuries went from 8,852 to 13,172 from 2013 to 2016, with the cost rising from $4.6m in 2013 to $7m in 2016.
But 2017 saw a dramatic fall in payout costs to $4.9m, despite a slight increase in payouts.
In Christchurch payouts fell to 717 last year, down from 787 in 2016, and costs dropped by a third.
•Canterbury DHB reports 28 trampoline park injuries in two months, including two broken necks
•The flip side of trampolining
•Teenage boy seriously injured at indoor trampoline park in Christchurch
After the spike in injuries at the Christchurch parks, WorkSafe issued new safety guidance and began inspecting all New Zealand's trampoline parks.
Pearson was pleased injury rates had plunged and stressed the researchers didn't want parks shut down.
"We're not the fun police. We want people to get out and enjoy themselves and exercise - not sitting at home on their iPads and smartphones. It's that question of how safe should we make things?
"But... this wasn't a trivial blip, it was quite a significant spike. We felt that those arenas must be able to do better and interestingly they have - they're clearly doing something different."
Improvements recommended by WorkSafe included first aid training, more padding and better safety practices.
The industry had been "positive and proactive" and all parks complied with WorkSafe's improvement notices, a spokeswoman said.
"Before WorkSafe's programme of trampoline parks visits in late 2016, it was receiving an average of 30-plus notifications a week relating to concerns about incidents at trampoline parks," she said.
"Within a matter of months, the volume of concerns reduced to an average of one or less notifications a week."