England assistant coach Andy Farrell has praised the squad for their effort levels during the team's training camp in Denver.
Training at altitiude in Colorado, England are aiming to be one of the fittest sides at this year's Rugby World Cup.
Farrell said that the sessions had been informative for the coaches when it came to deciding who has what it takes to make the final 31-man group.
Describing the last week as "pretty brutal", the England squad took some time out to watch a baseball game between the Colorado Rockies and the Cincinnati Reds on Friday.
"The desire of the squad to get through one gruelling session together was absolutely awe-inspiring from a coach's point of view," said Farrell.
"They have been digging in together and suffering together and will come out a better team because of that.
"You get to know the players better because they've had to dig deep.
"We're not flogging them, but asking them questions about their ability to execute skills wise while under pressure and in extreme conditions. We've seen who can cope with that and who can't.
"The players couldn't have given any more. In fact we had to hold back on Friday because this week has been pretty brutal. Everyone says they've had the best pre-season of all time, but we're unbelievably happy with the plan and how it's going.
"The boys are 100 percent committed to the cause. The pain and excitement they have been going through together has been plain to see."