Atheatre director once told me that you should never ask what a play is about, just as you should never ask a prisoner what they’re in for: instead, you should ask, “What’s the inquiry?”. What’s the difference, you might wonder. Well, Bill Hayes’s Sweat is “about” a history of exercise, indeed, that’s its subtitle. Yet his inquiry would be better summarised: “What does my body and its acts and competencies say about who I am? What, for that matter, did Plato’s? And from everyone in between, what can I learn about the self?” “Libraries, like gyms,” he writes, “have always been a refuge for me, just as gyms, like libraries, have always been places of learning.” There is a playfulness in Hayes’s writing, which reaches from a rich topsoil of endearing wordplay (“pas de dads”, he calls the sight of two middle-aged men playing squash) to the deepest layers of curiosity and empathy. He takes a profound, historian’s pleasure in tropes that echo across centuries – “The ancient Greek word for ‘gym rat’… literally translates as ‘palestra addict’”, to build an enthusiasm it’s impossible not to share. In one chapter, he learns boxing; it’s physically arduous, Plato had a thing or two to say about it (didn’t he always?), the diary is tearing along with pace and wit when Hayes leads the reader casually into the relationship that made him want to box in the first place. His boyfriend Steve – protective, capable, “always there. In our apartment, at my side, at the other end of the phone, a presence” – dies, suddenly, of a heart attack at the age of 43. It’s like being punched in the face. Hayes, by his own account, ends up medium-good at boxing, but as a storyteller, he’s Joe Frazier. The focal point of Hayes’s research, which takes him from London to Paris as he tracks down academics and translators, is Girolamo Mercuriale, author of De Arte Gymnastica, the first comprehensive book on exercise, from 1573. Mercuriale was wrong about a lot of things – his treatise on sweat is wild – and right about more. Hayes is wrong about some things – he thinks Mercuriale looks like Shakespeare, which he doesn’t at all - and absolutely fascinating in his own, idiosyncratic fascinations. In between, he splices in his personal sporting journey, from swimming to boxing to running, to being a gym rat, to glancing medit – certainly the route isn’t chronological –ations on his father, and somehow he arrives at today’s conception of exercise. The author describes the modern consensus on fitness as cohering relatively recently, in the early 50s. The epidemiologist Jerry Morris, known as “the man who invented exercise”, more accurately described, per Hayes, as “the man who invented the field of exercise science” spearheaded Britain’s Society for Social Medicine. From his large-scale studies of transport workers, the difference in baseline health between conductors (who walked about) and drivers (who were sedentary) formed the basis for his conclusions about the benefits of keeping fit. In the same year, two American epidemiologists did a similar study on a different cohort, school children aged six to 19, which ultimately led to Eisenhower’s Council on Youth Fitness. The Society for Social Medicine had been established with an explicitly socialist, internationalist agenda, and the cross-pollination of the respective groups’ work was equally explicitly resisted by authorities in the McCarthy-era US (as Morris’s colleague, John Pemberton wrote: “When the immigration officer at New York read my introductory letter stating that the object of my visit was ‘to study the teaching of social medicine in the USA’ he said, with some emphasis, “We don’t want any of that filth here”). In public health terms, there is a fascinating left-right divide: the left uses the body as an index of systemic inequality, while the right uses the body as the ultimate outward sign of self-sufficiency and personal responsibility. And while Hayes is extremely alert to race and gender politics, never gulled by the exclusions of historical record (“I suddenly imagined Tolstoy and Einstein competing in a triathlon. Which man would win? Maybe a woman would instead”), he’s not so hot on these big ideological fissures. If that sounds like a criticism, it isn’t; rather, a way of saying, I would have liked more views, more observations, on more subjects – basically, I would have liked this book to go on for a lot longer. Erudite, ludic, eccentric, energetic and historically transporting, it’s like falling through a gym and landing in a joust.