WARATAHS league recruit Timana Tahu says he has held himself back from running at full speed for almost seven years due to fears about his hamstrings.

The former Parramatta star was yesterday ruled out of the NSW squad for Friday night's Super 14 game against the Crusaders in Christchurch - to be named today - after a tear in his left hamstring failed to heal, and he may be sidelined for up to six weeks. The Waratahs have opted to spell him to permanently correct his hamstrings and reconstruct his running style, convincing Tahu that if he can modify his running technique to draw on his gluteal strength, there will be less stress on his hamstrings.

The 27-year-old said yesterday that since the age of "20 or 21" he had run at less than 100 per cent during games, going back to his early league days at the Newcastle Knights, which he joined as an under-19 recruit.

Tahu's hamstring issues have been well documented but only now has it been revealed how debilitating they have been during a career that has included State of Origin matches for NSW, Kangaroos Tests and a matchwinning try for the Knights in the 2001 grand final against the Eels.

"I never really opened up 100 per cent in sprinting since I was 20 or 21," Tahu revealed on the latest edition of NSW Rugby podcast Tah Talk. "All through the season, even my NRL seasons, I would always hold back because it's a fear I have got of tearing my hamstring again if I opened up to 100 per cent. If this works, hopefully it will make me more confident and might bring a new dimension to my game."

However, Tahu said he had found it easier to carry the injury in rugby league than in union.

"I usually held myself back, just took it easy or looked for support, just cruising. In union, you have to be running 100 per cent all the time," he said. "I didn't really need to run 100 per cent [in league]. I either had to try and break the line, look for support, or most of the time [I was] always looking for support. I never really ran out to my full length of stride."

Waratahs coach Ewen McKenzie has been impressed by Tahu's adaptation to the 15-a-side game but believes his input has been stifled by the hamstring complaint.

"We are getting maybe 90 per cent," McKenzie said. "We want to get him up to 100 per cent so we get the maximum value out of his skill set. He has a magnificent bunch of skills. We want to get maximum value out of them."

Once Tahu has fully recovered from the tear, McKenzie said biomechanists would assess his running style and try to make Tahu more reliant on his gluteal muscles. "Then there is less strain on the hamstring," McKenzie said. "At the moment, we haven't got enough gluteal activation. We need to get that engaged in the running process so we don't keep getting in a situation.

"We have to break that cycle. We have to do something radical and look right back at the running style … The good news is that it is fixable. And once it is fixed, it is an investment for the long term. Once we get the body right, it will be a lot easier because he will not be playing within himself. Once he feels free on the field and can run further and faster, it will feel a lot easier again."

Tahu was frustrated at missing the chance to face the Crusaders, who are undefeated and in first place on the Super 14 ladder after five rounds.

"That is one game I really wanted to play," he said. "Hopefully, these next weeks we can win a few more games … Hopefully, when I get back on board there will be a spot for me."

As for the long term? "I'm going to be here for four years. I'm not going anywhere."