For modern rugby sides, it is a Key Performance Indicator that cannot be ignored.
Ireland go into their second World Cup match, against Romania, tomorrow with another strong back row.
Sean O'Brien is on the bench and Peter O'Mahony is clicking his heels in the stands. And yet, the back row unit appears as strong as ever.
Joe Schmidt has been able to call on Jordi Murphy and Chris Henry to pack down with Jamie Heaslip. That is ball-carrying, robust tackling and breakdown meddling covered. Heaslip prides himself with doing the spade-work.
Asked about both players at today's Captain's Run, Heaslip gave a further insight to Ireland's gym sessions.
"Peter is kind of like Jordi, in terms of being very explosive.
"You can see that Pete in the gym. His Counter Movement Jump [CMJ] is the best in the squad."
O'Mahony's CMJ record, as per Mike Ross, is 63cm from a standing start. Having high CMJ numbers is a testament to a player's leg power and flexibility.
You can check out some of the Irish guys being measured for CMJs here:
https://youtu.be/wNYMZQaUV14?t=1m11s
The squatting record for the Irish squad is held by Cian Healy [300 kilos]. The loosehead also holds the bench press record.
For the front row, Ross holds the record for holding up a 15 kilo weight - five minutes.
Asked about Henry, the stand-in captain commented, "Chris is really good, out-and-out seven, who seems to get to every bloody breakdown and get involved.
"On the defensive side, he slows down lot of ball, gets lots of poaches and makes a general nuisance of himself. I hate playing against him, to be honest.
"He has a serious engines, his stats are pretty impressive - in terms of distances covered, number of rucks. He has a nice set of hands; he doesn’t get enough credit for that."
