Roy Hodgson has responded to Roy Keane’s scathing remarks about Phil Jones and Chris Smalling by publicly supporting the two England and Manchester United defenders, insisting he will not be “moved or disturbed” by criticisms of his squad, on a day that demonstrated the extraordinary lengths the Football Association is taking to prepare for the heat and humidity of Brazil.

As photographs emerged from Coverciano of Italy’s players rehearsing for Manaus by training in a sauna, the FA was flying in a team of scientists from Loughborough University to its training camp in Portugal to embark on a programme entitled “Being comfortable being uncomfortable”.

Players were instructed to wear three different layers at the same time in training, with patches attached to three different parts of their bodies, so the specialists could gauge sweat levels and produce individually tailored recovery drinks when England play Italy on 14 June. England are understood to be the only nation in the World Cup to be using these methods, aimed at establishing the precise amount of fluids and different electrolytes each player needs to take.

Hodgson ended training by singling out Luke Shaw, Raheem Sterling and Ross Barkley and delivering a short speech to his three youngest players, emphasising the importance of not being overawed and reiterating why he had chosen them ahead of more experienced rivals. “I don’t detect any nerves at all,” he said later. “There has been a great feeling of enthusiasm and let’s start this adventure we have all been looking forward to.”

He then turned to the latest criticisms of Jones and Smalling. “That makes a change,” was his reaction, and there was a note of exasperation about Keane’s observation, as one of ITV’s World Cup pundits, that the two defenders had gone backwards and were fortunate to be chosen.

“I am very rarely taken aback by things people say and rarely moved or disturbed by it,” Hodgson said. “I work on the simple basis that everyone has an opinion. If you are a TV pundit you are required to give your opinion and I don’t expect everyone’s opinions to fall in with mine.

“If you say to me: ‘Has it been a great year for Man United and have Jones and Smalling starred?’ Of course they haven’t because they [United] have used so many players and their back-line has changed virtually every week. But as far as I am concerned they are England players. They have been England players throughout my two years and any games in particular that Smalling has played – because he has played more than Jones – he has certainly never let me down as a coach, and I don’t think he’s ever let down the nation, whether he has played centre-back or right-back. In my opinion, they fully justify their selection.”

Jones was restricted to a light workout as he recovers from his shoulder injury and John Stones, the back-up centre-half on England’s standby list, did not take part in the full session after a twist to a knee the previous day. Stones should return on Thursday and Jones hopes to be involved before the end of the week. Alex Oxlade-Chamberlain was able to take part and Wayne Rooney appears to be showing the benefits of spending the previous week working with two fitness coaches. At the end of the session Hodgson had to order Rooney off the pitch as he stayed back for extra shooting practice. “I think he should protect his thigh,” the manager explained. “If it had been up to him, he would have been having as many shots as all the others.”

The team from Loughborough has been introduced by James Collins, who normally works as Arsenal’s nutritionist but has been enrolled by the FA for the World Cup and has worked with the British Olympics team. There can be 99% humidity in Manaus and Hodgson thought back to the 1994 World Cup when his Switzerland team played twice in the blistering heat of Detroit.

“We thought we were being quite professional in our preparation but if you compare it with what we are doing now it was unbelievably amateurish. Football has moved on in sports science. The people here are the top people in their areas of expertise, chipping in to help the players get the very best out of their performances.”