Watch Harlequins train and you get some idea of why they are back on top of the pile going into Saturday's rerun of last season's grand final against Leicester. They are not alone in this but their training usually ends with a session that does away with the natural pauses that are built into a game of rugby and goes flat out.
There is no chance of getting a breather by walking to scrums and lineouts because there aren't any. There isn't even time for a blow after scoring a try because the ball is immediately back in play – run from behind the try line. Stop concentrating, stop running and you're in a mess because everyone else is going at 100mph.
Mind you, when they play you can see why Harlequins drive themselves. To play the way they do, there can be no concessions when it comes to fitness. It's like paying money into a bank; if you put it in, there will always be something there when you need it most.
Someone once said that, if you train like a club player, then you'll stay a club player. However, train like an international and you stand a chance of becoming one. That became much truer when recent interpretations of the law of the game speeded up things immeasurably.
It started with the banning of kicks directly into touch from outside the 22-metre line. Initially that led to a backward step and those tedious aerial bombardments but once referees insisted that the man with the ball had to be released in the tackle, then defenders lost their fear of being caught in possession and played, even from deep in their own territory.
The ball stayed in play longer and longer so players had to get fitter and fitter to cope. Now it's reckoned that in Test matches a back will run up to 10km and a forward – even the very biggest of the big boys – will do between 6-7k while – and you can see where Harlequins are coming from here – there are fewer scrums and lineouts, which means fewer ambles to the set pieces.
Different sides found different ways of coping but weights have tended to make way for fitness based more on running. There will always be weight training, especially for the lighter and younger players, but in some respects training has gone full circle with echoes of how players got fit 30 or 40 years ago.
It was said then that the All Blacks got their strength from walking around the countryside with a rescued sheep slung across their shoulders. Well, now there are very few farmers in the New Zealand team, just as there are no coalmen or miners in the Wales squad, but fitness coaches have devised routines to mimic those work routines. Now we carry bags of bricks or sand instead of a sheep, pull tyres in sprint sessions, Sumo wrestle or wrestle with huge tractor tyres.
I've always maintained that the best training for rugby is anything that leaves you bent double, near to retching and with your lungs on fire – the "no pain, no gain" lobby if you like.
But does it work? You bet it does. A couple of weeks into the last World Cup and there were quite a few players in the Wales squad who were almost impossible to tire. You could run them off their feet but after 40 seconds recovery time they were ready to go again no matter what had gone before.
It was the result of having the squad together for three months and we could see a marked drop-off in fitness during the Six Nations. Wales still did the grand slam but we estimated that fitness levels had fallen by 10 to 15%.
We put that down to the heavy programme in Welsh rugby over the Christmas period, normally the season for local derbies. Too many games made the players match fit but left no time for the kind of quality training that would get them near those peak levels.
However, from the evidence of last season, when Harlequins stayed true to their all-court game with only that one slight wobble in the semi-finals against Northampton, it would seem to be a trap they learned to avoid. It certainly gave them the confidence to play the game their way and it will be fascinating to watch them at Welford Road on Saturday and see how things have moved on.