England scrum-half Ben Youngs has warned rugby's authorities that players are being pushed to the physical limit due to the growing number of games in a season.
Youngs was speaking at an England training camp from which international teammate Billy Vunipola withdrew after succumbing to a second long-term knee injury this year.
Premiership Rugby clubs have suffered a spate of injuries during the first four weeks of the season to a backdrop of discussions at the top of the game about expanding the domestic season and congesting England's fixture schedule at the Six Nations.
"I think we are at the limit in terms of the number games and the length of the season," said Youngs.
"Guys have been outspoken but that's because they care about the length of their careers. They want to finish on their terms when they want to finish. They don't want it to be four years shorter. That's important."
Before limping off during Saracens' 41-13 win over Sale on Saturday, Vunipola had said that players could be forced to strike if the number of games were not reduced.
Youngs admitted that players had the power to cause a lock-out but said a top-down solution which allowed rugby to continue to expand while easing its physical demands would be better.
"The guys do have that power," said the Leicester back, "but I don't think any player wants to get to that. Guys have talked about it but I am sure we will get to where we need to: making sure rugby is still hugely competitive and continues to grow but also that we are protected.
"It is up to the guys with a calculator to come up with the sums and work out what needs to happen; and the scientists and the doctors to understand the toll on the body."