Sons of Cuba (PG)

Ryan Gilbey is struck by the poverty of Cuba's child boxers.

There's a bitter-sweet moment in the documentary Sons of Cuba - one of many such instances - in which victory is shown to be as complicated as defeat. The Havana Boxing Academy, a school at the heart of Cuba's Olympic success in the ring (the country has notched up 63 medals for boxing in 40 years), is whittling down its intake to a kernel of under-12s that it will field in the national championships.

One pipsqueak, Cristian, is slumped on the floor in tears after a bout, his bony head bowed between gangly legs. But Cristian hasn't lost: the tears are for his opponent, whom he has had to subdue convincingly in order to qualify. Is such heartache restricted to children? I couldn't say, but YouTube is suspiciously light on clips of Mike Tyson crying a river for Razor Ruddock.

Cristian's father, the former boxer Luis Felipe Martínez, enjoyed Olympic glory in his day, and still trembles at the memory of receiving on his return a medal from Fidel Castro. Mar­tínez now resides in a sad room in a corrugated-iron cul-de-sac. "Sport is a flickering moment," he muses. "I shouldn't be living like this, should I?" The camera glances around the bare walls in silent agreement.

The film's director, Andrew Lang, doesn't dwell on the poverty of these people, but he doesn't have to; there's nothing he could have done, short of pointing his lens at the sky, to nudge it off-screen. The extent of the academy's facilities can be summed up by its sauna.

Fighters looking to sweat off those precious few kilos to reach their required weight are directed to crawl inside a set of portable wooden steps, where they huddle in the dense heat. Cristian's mother blames the sad look in his eyes on all the times he has seen her suffer, but pressure and fatigue must play their part: the boys are woken at 4am each day and have completed hours of punishing exercise before school starts.

Lang's approach feels hands-off, but detachment serves its own purpose. When you have secured permission to shoot the pre-dawn workout, accompanied not by the hi-NRG pulse of a gym but by the boys' chants of “Fatherland or death!", there's no need for commentary or intertitles. The material illustrates an underlying conflict. Does boxing provide a focus for children who would be adrift without this dream of sporting glory? Or, with its equation of personal fulfilment with an obligation to Castro, is it merely another impediment to freedom?

Lang rehearses the question in a series of domestic struggles. One child, Santos, repeatedly fails to lose weight. He protests that he's eaten nothing but that evening's main course. Later an invoice from a nearby bakery, exposing evidence of the child's empanada habit, is produced like something out of The Winslow Boy, and Santos cracks.

Junior, the third of the fighters who provide the film's main focus, also has his Kleenex moment, when he tries to conceal from his coach the true reason for his persistent absences. Junior's smile, brightest of all when he is providing an imaginary commentary to one of his own victories, is otherwise a constant here. During an adorable scene with his mother when he gazes fondly up at her like a baby in its crib, trailing his fingertips along her chin, you realise how little evidence Lang has provided of his subjects at play. These are boys whose childhoods have been curtailed, who bellow slogans when they should be dreaming up dirty rhymes.

Sons of Cuba doesn't press the political but, as with poverty, its evidence is ubiquitous. The production coincided with Fidel Castro transferring power to his brother Raúl, which in turn prompted the defection of three Cuban boxers. Throughout this, Lang fixes on the bewilderment of the children, who struggle to process psychologically the rejection, by men they idolise, of an ideology that's as dear to them as their own families.

But the conventions of the sports movie are hard to override, and the film ends naturally with the championship for which the boys have been preparing. Even here, normal pleasures are undercut. These bruisers are so slight that their weight is doubled when they put on their boxing gloves. Watching them fight doesn't supply the adrenalised kick of a Rocky movie. It's more like seeing butterflies tearing clumps out of one another's wings.

Ryan Gilbey is the New Statesman's film critic. He is also the author of It Don't Worry Me (Faber), about 1970s US cinema, and a study of Groundhog Day in the "Modern Classics" series (BFI Publishing). He was named reviewer of the year in the 2007 Press Gazette awards and is Film Critic in Residence at Falmouth University.