For such a habitual success, Alastair Cook knows a fair bit about failure. Low scores are an inescapable fact of life for English openers and Cook has had plenty of leanish spells on his way to making 7607 Test runs and 25 hundreds. He is in one right now, with 83 runs in four Ashes innings. An average of 20.75 is slightly down on the 225 he was averaging after two Tests of the last Ashes.

It would surprise nobody if Cook made a huge score in the next Test at Old Trafford. He had an even worse start to the summit series against India in 2011, with scores of 12, 1, 2 and 5 in the first two Tests. In the next match he hit 294 at Edgbaston.

His Ashes mirabilis in 2010-11 followed the toughest summer of his career. However, the slightly absent-minded nature of a couple of his dismissals against Australia have induced the nagging and persuasive thought that Cook might just be starting to follow the pattern of England captains in the last 20 years: a spectacular start containing some of the best batting of their career - Cook was superhuman in India last year - followed by a slow decline as the incessant and varied demands of leadership take their toll.

Cook's overall average as captain is an outstanding 60.60. His average as full-time captain - since Andrew Strauss' resignation - is 52.60, four above his career average, but in 2013 it is has dropped to 37.69. Perhaps the novelty is wearing off. Or perhaps it is just a common-or-garden lean spell. Cook does things differently to most batsmen, and he will feel a far more relevant precedent is that of his mentor Graham Gooch, the last Englishman to have extended success as a batsman-captain. Gooch's improvement was extraordinary. When he took over in 1989-90, his Test average was 37.71. Over the next four years he averaged 58.72 before resigning the captaincy.

Gooch was an exception, rule-proving or otherwise. For most batsmen, particularly in England in modern times, captaincy has been the grimmest reaper. Its all-consuming nature compromises a batsman's relationship with his best friend: concentration. In A Captain's Diary, Graeme Smith - who has generally dealt extremely well with the twin demands - jokes he would like to lobby the ICC to extend the innings break from 10 to 15 minutes, such is the difficulty for captain-openers to leave the wider concerns at the pavilion gate. "You have so much eating away at you, so much still going on in your head."

Perhaps the best example of how the captaincy can affect a batsman came during the Ashes Test at Headingley in 2009. England's build-up on the first day was frenzied. They had to stand outside their hotel for almost an hour in the early hours because of a fire alarm; Matt Prior suffered a back spasm during a game of football, which led to the toss being put back ten minutes; there was an ongoing discussion as to whether Andrew Flintoff would be fit; the masseur, Mark Saxby, was smacked on the head during Australia's cricket practice. All this with the game due to start in less than half an hour. It was chaos, and Andrew Strauss could not focus on the smaller picture when he went out to bat. He should have been out lbw to the first ball of the match, and soon after, edged a loose drive to slip. The seam wasn't the only thing scrambled that morning.

That was an almost absurdly extreme example of how captaincy can impact, but it is always there. In Out Of My Comfort Zone, Steve Waugh wrote that captaincy "seemed to soak my spare time like a sponge". In that sense it is almost an extreme form of sporting parenthood - extreme as you have effectively given birth to decuplets. A captain must look after his ten team-mates, with their myriad concerns.

Then there are the toss, the media demands, the small talk with the mascots, the small talk with the Queen, the politics, the knowledge that your resting face and body language are being scrutinised at every moment, the angle of the man at fine leg. And that's only about 0.1% of the demands. What starts as exciting and novel eventually becomes mundane and trying; it's human nature. Changing your first nappy is one of the most memorable experiences of your life; changing the 2001st nappy is not. Then there is the pressure, the seeds of which are planted the day you take over and which grow over time.

There is a school of thought the middle should be the safest place for a captain: his equivalent of a parent's downtime, or a 22-yard sanctuary in which you can just bat, but it doesn't always work like that. Cook is better at compartmentalising than most, and seems to be a master of clearing extraneous thoughts, but captaincy will challenge that in ways he could not have imagined. In modern sport everything is done to protect the body. It is much more difficult to take care of the mind; to keep it clear and sharp.

That has been a recurring theme of England captains in the last 20 years. Most found the captaincy empowering rather than embattling at first. Mike Atherton, Alec Stewart and Strauss all had their most productive series leading the side in their first series as captain; Nasser Hussain's average of 61.66 in his first full series was the best of his tenure. Even short-lived or stand-in captains such as Kevin Pietersen and Marcus Trescothick scored centuries in their first and second games as captain respectively, while Flintoff batted superbly in India in 2005-06.

The exception is Michael Vaughan - but then he had no scope for improvement. When he assumed the captaincy he had scored seven hundreds in his last 12 Tests and had an average of 50.98. Vaughan was never the same player again; in 51 matches as captain he hit nine hundreds and averaged 36.02, a dreadfully unbecoming record for a man with a touch of genius. He resigned in tears, just like Hussain. It is inevitable that most captaincy careers will end in failure, and equally inevitable that most will struggle to maintain their output in the middle.

"At the start of my captaincy, not being able to spend time on my own game was a benefit because it prevented me from being too insular," wrote Atherton in Opening Up. "Initially, also, the added responsibility and pressure were empowering and resulted in better personal performances. Eventually, however, as pressure increased over time, my ability to cope clearly decreased. Now, I needed extra time to put my game in order and the captaincy was a hindrance. I was not the only captain whose game suffered."

His mate Hussain's certainly did. In his second year of captaincy Hussain could barely buy a run - he averaged 13.55 in a 12-Test period - and was increasingly obsessed with the idea that he was not worth his place. One night in Sri Lanka, when he could not sleep, he went to the hotel bar on his own at 1am.

"The barman was just packing up," he wrote in Playing With Fire, "but I managed to persuade to him to hang on, ordered myself a rum and Coke, lit up a fag [even though I don't smoke], and sat there, going through everything." This is what the captaincy came to do a man. A century in the next Test changed Hussain's life: he came to terms with it and had a second wind. Others were not so fortunate.

The problem does seem to afflict England more than most. In the last 20 years, England captains batting in the top six average 39.98 - above only New Zealand, Bangladesh and Zimbabwe, and 10 runs behind Australia.

"I always said I would be the only England captain not to go bald, but after days like today, it might not help that," joked Cook after the Trent Bridge Test. England need him to buck the trend of recent history. These are relatively insecure times for a batting line-up that was hitting 500 in its sleep only a couple of years ago. England cannot afford to lose their best batsman.