Stokes will captain England into a five-Test series against India, starting at Headingley, but has struggled with fitness and injury problems over the last few years.
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Speaking exclusively to Radio Times, Broad said: "It's the fittest I've seen him looking. That doesn't necessarily correlate to being cricket fit, unfortunately – they do seem to be two very different things.
"He's very focused, he's trained incredibly hard and now his challenge will be controlling himself on the field, so not bowling eight- or nine-over spells, which is the reason his hamstring goes the next day.
"His job, to the best of his ability, is: can he bowl five overs before lunch, five overs after lunch, five overs in the final session and be done? Not 10 overs in one session. That will be his biggest challenge."
He said: "My job when I was playing was to control that a little bit, to get him off from bowling, or say, 'Not now,' but I don't necessarily buy into this thing that no one can tell him what to do or no one can stop him bowling once the ball is in his hand.
"Everyone around him has to buy into that as well, whether that's the physios or the players out there to literally, for the good of the team, tell him to stop bowling.
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