The doctor said it was a "sciatic condition" but Marian called it a "broomstick up the arse"

First of all we looked at the X-ray. "This is your spine," said my doctor with a degree of assertiveness that suggested there had been occasions in the past when patients had been churlish enough to contradict even this initial part of his diagnosis. "You can see there's a minor malalignment of the spinous processes." His pointer indicated an area that looked not unlike an aerial shot of war-time Dresden.

"You'll also notice that the spine is very straight." I allowed myself a small smile of satisfaction. Friends might constantly draw attention to the ways in which my lifestyle has affected my general appearance, but on this evidence not one of them could accuse me of having been cavalier about my spine. There it was, backlit upon the wall: a veritable ramrod of a spine.

Even as my doctor paused to consult his notes, it occurred to me that I might have made more of this asset. Imagine how all those evenings in bed with my delightful friend Marian might have been transformed if, instead of constantly straining to pull in my stomach, I'd simply lain flat on my face and allowed her to revel in my near-perfect dorsal curvature.

There were also some powerful metaphorical connotations. My life might seem to a casual observer to have been full of ideological wavering: three years as a committed young communist, five years as a militant Trotskyist, 14 years as a half-hearted member of the Labour Party and now, in my latest political re-incarnation, six months as an intermittent purchaser of LM magazine. But how could any of this be regarded as evidence of treachery or pusillanimity when the very notion of backsliding or lack of backbone was so strenuously contradicted by the illuminated evidence of my X-ray?

"That, of course, may be your problem. Your back is too straight for your own good. What we like is a moderate lordosis, a minor curvature in the lower back, but as you can see your spine is absolutely straight. It's a condition we doctors sometimes call 'guardsman's back'. It gives you a military bearing but it can cause unnatural wear and tear on the lower discs and produce the type of sciatic condition that you report."

It's rather startling to discover as you move towards your twilight years that you've spent most of your life marching around like a demented sergeant-major. It does at least explain the failure of so many of my walking holidays, all those times when I've been half-way across a Yorkshire moor and looked back to discover that my former companions were not only several hundred yards behind me but also appeared to be giving each other mock salutes and simultaneously humming the opening bars of the Dambusters theme.

My "military bearing" must also have had something to do with my lack of acceptance by the hippy subculture in the early seventies. No matter how adept I proved myself at rolling joints or falling into a semi-trance during Dark Side of the Moon, there were bound to be suspicions about my total commitment when my companions saw me languidly rise from the bean bag and set off towards the kitchen with a gait that suggested I was not so much in search of chocolate cake as preparing to invade Poland.

So far I've only mentioned the diagnosis to my friend John. He was delighted with the news. Apparently, it confirmed a view of me with which he was already familiar. "People knew that I had inadequate lordosis?"

"Not exactly. But Marian once told me in confidence after you'd split up that she always thought you had a broomstick up your arse."