Strong enough is old enough: the science behind the NRL's age eligibility

Lower body strength will be critical in the timing of Roosters teenager Joseph Suaalii’s NRL debut according to Australian Catholic University strength and conditioning expert Rich Johnston. 

After a rigorous process of physical and psychological testing, the NRL has granted an exemption to the club to select the 17-year-old in the Roosters’ first grade side. NRL rules state players must be 18 to be eligible, however Suaalii has been cleared to play well in advance of his August 1 birthday. 

There is no definitive science to dictate when a young footballer is ready for first grade but Dr Johnston from ACU’s School of Behavioural and Health Sciences says there are at least four factors to consider ahead of his highly anticipated selection. 

“It sounds obvious but the most important thing to remember is he is 17. We want him to still be playing at 30,” Dr Johnston said. 


Research has shown that strength is vital for a variety of performance aspects, including error rates and tackling ability. 

An ACU research paper led by Dr Johnston and published in the International Journal of Sports Physiology and Performance showed lower body strength and relative squat strength was positively linked to fewer defensive errors and fewer attacking errors. 

“If these aren’t up to standard, then he is going to struggle with the physicality of the game,” Dr Johnston said. 

“Lower body strength really is the number one physical asset for footy players.” 


Suaalii is already taller than South Sydney Rabbitohs star Latrell Mitchell and just a few kilograms lighter. Patience will be important as the club balances physical demands with his development. 

“It’s likely he’s still growing, which is something to consider,” Dr Johnston said. 

Repeated bout effect 

Dr Johnston said: “There is something, too, to be said for being hardened for repeated bouts of contact, which is one of the distinguishing factors of NRL competition compared to the lower grades – they are more traumatic and more frequent. 

“So being prepared for this is important, so it is likely to take its toll on a body not used to it early on. It’s hard to replicate these demands anywhere else other than in the NRL.” 


Suaalii has already played against fully matured men in the NSW Cup but he can expect even higher intensity in the NRL. 

“He needs to be prepared for the speed of the game, which is likely higher than NSW Cup in both the average demands and the most intense periods,” Dr Johnston said. 

The Roosters have reportedly consulted The Kings School to ensure football commitments do not harm his academic performance.   

“Year 12 is a demanding year,” Dr Johnston said. “Easing him into the competition would be the most effective way to go over the next 2-3 years.”


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