Why do NRL players wear sports bras?

It's a question that would have the hardmen of the 70s and 80s squirming uncomfortably but it's nothing these days to see players parading around post-training looking eye-catching, fit and fearsome, even a bit metrosexual in their 'bras'.

The correct name is GPS Vests - so there goes any mystery or intrigue out the window with a regulation name like that.

But they are kind of "sexy" in their own way if you're into statistics, analysis, sports science and recovery methods. I've got my hand in the air – that's why I wanted to know more about them.

They carry a tracking device, which contains GPS antenna, accelerometer and heart rate receiver jammed into a device that is not much bigger than a matchbox.

The high-tech vests are crucial in helping coaching staff decide if a player is doing too much and possibly in danger of sparking a new injury, or igniting an old one.

Knowing more about how an individual's body reacts, or copes, with workloads helps head trainers like Cronulla's Andrew Gray better protect players.

"They provide a means for us to have greater control over the amount of work that every player performs in a given period," Gray, a former NSW Origin head trainer, told NRL.com.

"In that way, we can do our best to make sure that the training dose has been the ‘right’ amount to decrease the chance of injury and increase the chance of sustained high performance."

They also look nice and snug and comfortable – just ask Manly Sea Eagles second-rower Curtis Sironen.

"They are like a piece of underwear, to be honest," he told NRL.com.

Despite having suffered a pectoral tear in late 2017, Sironen says the vests provide no extra support for players recovering from sternum, shoulder or pec injuries.

"They are reasonably tight elastic sort of thing but they don’t really do too much in terms of extra support," Sironen said.

The vests are purely a training aid. But more often players wear them during matches.

"All players are now expected to wear them for games as well due to the fact that the most important piece of data is also game day," Gray said.

So what happened to the discs the size of an old iPod that slipped into the rear pocket on jerseys?

"The devices sit in there on game day so that the players do not have to wear a vest and a jersey if they don't want to," Gray said.

They don’t make you feel claustrophobic but they do take a little getting used to Curtis Sironen

"The more rigid the fit the better. If the unit moves around too much it definitely negatively affects the data."

The disc sits in the back of the GPS Vest, between the shoulder blades. It seems a personal preference on whether players wear the vests during a game, or opt for the jersey pocket.

After the PM's XIII game against PNG last September, Aaron Woods gave away his jersey and shorts to fans, leaving the field clad only in his 'sports bra' and underpants. Woods played in one despite the tropical heat and humidity of Port Moresby.

The vests do make players sweat more but Sironen says it's not a drama.

"No more than you’re going to heat up anyway in body temperature from playing," he said.

"They don’t make you feel claustrophobic or anything. But they do take a little getting used to."

You would think players in the Holden Women's Premiership would be all over this bra/vest thing. But it's apparently fairly new to them, too.

"Our women’s teams wear them only when injured at this stage," Gray said.

"But we are planning on rolling them out for games this year as well to gain a better understanding of the load involved in women’s games which will improve our ability to prepare our girls for high performance."

And for members of the public that like the fit or - dare we say it – the look of a sports bra, Gray says they are not readily available.

But the more players are photographed in them – looking steely and streamlined – the more demand will be created.

"Some tracking device companies are now releasing ‘Prosumer’ versions of these devices geared toward the high-end consumer or serious athlete," Gray said.

Times have definitely changed. You couldn't see the old hardheads from yesteryear like Arthur Beetson, Dallas Donnelly or Blocker Roach getting around in a sports bra no matter how much it helped their on-field performance.