Celtic may have played their 67th and last match of a long season yesterday. But for many of their players, it isn't over yet. Not by a long shot.

Stuart Armstrong has benefited most from Celtics increased emphasis on fitness. Photograph: SNS
Stuart Armstrong has benefited most from Celtics increased emphasis on fitness. Photograph: SNS

Gordon Strachan expects them to reach the levels they have attained at Celtic when Scotland host England a week on Saturday, with players from Parkhead potentially making up more than half the team.

Strachan is comforted to know their fitness data is guaranteed to be impressive when they are monitored on their return to the fray after a few days of post-cup final downtime.

Sign up to our Football newsletter

There were just two Celtic players in the side when Scotland last featured in competitive action so late into a summer, against Republic of Ireland two years ago. But out of those who started in Dublin it still meant Scott Brown and Charlie Mulgrew were members of the biggest continent from a single club.

Ronny Deila. Photographs: SNS

However, that is dwarfed by the Celtic half-dozen many expect will be in Strachan’s team to face England. Rarely in recent times has a Scotland manager relied on not only the goodwill of a club – Celtic manager Brendan Rodgers gave Brown his blessing to rescind his retirement – but also their strength and conditioning programme, which in Celtic’s case has been proved to be first-class under the new management team.

Strachan knows it will sound harsh since to compliment the current regime, means having to denigrate the previous one. But as much as he liked Ronny Deila, Strachan isn’t able to interpret it any other way. He can’t sugar coat it. Celtic are fitter now under Rodgers than they were last season, which is good news for Scotland.

It won’t be so pleasing to hear for Deila, who has regularly had to cope with such unflattering comments since returning to Norway just more than a year ago. With such an array of instruments to monitor fitness, there is no hiding place for a player – or manager – these days. But Strachan insists the evidence is there just from watching them perform.

“It is very hard on Ronny (Deila) and his background staff that went before but that is fact,” said Strachan this week. “We have seen it in front of our very eyes. As I say, fitness allows you to do things: the pressing, keeping the ball.”

Gordon Strachan recalls running out of steam in the Rous Cup at Hampden. Photograph: Getty Images

Strachan believes Stuart Armstrong, who made such an impressive Scotland debut in the 1-0 win over Slovenia in March, has gained the greatest benefit from Rodgers’ methods.

“What Stuart does, the way he beats people, that is all down to his fitness,” said Strachan. “He has always had that technique, it is adding that extra all round fitness that makes you a better player. It is (Jurgen) Klopp’s mantra, (Mauricio) Pochettino too. You have to be fit.

“Pochettino got rid of the older players and look at them now, they run all over place,” he added. “Unfortunately, as an international manager, I can’t work on their fitness. You can’t get them any fitter in two days, it doesn’t work. In fact if you try and push them it can be a negative. Before you know it they have strained something.”

Strachan knows it’s not just Celtic. Players are getting fitter all the time, particularly in cash rich English football as training programmes evolve and new methods come to light. “It is a good weapon, yeah,” said Strachan, with reference to his players’ strong running qualities. “And also we get it from (Andrew) Robertson (at Hull City) and (Ryan) Fraser (at Bournemouth). You need that and you need to push yourself to another level.

Ronny Deila. Photographs: SNS

“My advice to a kid 19, 20, if the fitness coach says your heart rate is there well make it even better,” he added. “Push it, push yourself further than anyone else in the squad. You will probably carry someone else as well.

“We are getting too many kids now, 18, 19 who say my fitness coach told me heart rate had to be that, I need to do that. Push on, defy them, be better than them.

“Take that Celtic approach, get your fitness level and you can do a lot more things. You will all have played sport and know you only make mistakes when you are tired.”

Strachan is speaking from experience. He played against England at the tail end of the 1984-85 season, when he played over 50 matches for Manchester United. By his own admission, he struggled in the Hampden Rous Cup fixture, which Scotland won 1-0 courtesy of Richard Gough’s towering header.

Gordon Strachan recalls running out of steam in the Rous Cup at Hampden. Photograph: Getty Images

“I had played a lot of games, yes,” he recalled. “It was a shame because the season had gone fantastically well for myself and my team, but I played against Robbo (Bryan Robson) and I was useless.

“Matter of fact Richard Gough scored a goal with a header at the back post and if you have a look at it again, you will see I am standing behind him thinking: ‘go and head it because I am having a stinker here!’”