A large new cohort study has challenged conventional thinking on how runners get hurt, concluding that the risk of overuse injury is driven less by weekly mileage than by a single session that jumps too far beyond recent training. The research, titled "How much running is too much?", tracked 5,205 runners over 18 months and has prompted a rethink of the long-held rules of thumb that govern training load.
Drawing on Garmin device data across 588,071 individual sessions, the authors recorded 1,820 running-related injuries, around 35 per cent of the cohort, and tested whether spikes in distance, over a single run or across a week, predicted injury. The headline finding is stark: injury rates rose significantly once a single session exceeded roughly 10 per cent of the longest run undertaken in the previous 30 days.
The size of the spike mattered. Compared with runners who held their longest recent distance, those who ran a small spike faced a 64 per cent higher injury rate, a moderate spike a 52 per cent increase, and a large spike more than double the risk. Crucially, the single-session measure proved a sharper predictor than the popular acute-to-chronic workload ratio, which compares one week against the previous three and has dominated load-monitoring advice for years.
The practical message is that the individual long run deserves far more caution than most training plans give it. A runner whose longest outing in the past month is 10 kilometres is, on this evidence, taking a meaningful risk by suddenly stepping up to 13 or 14, even if weekly totals look modest. The findings lend fresh weight to the familiar advice to build the long run gradually, but they reframe it as the single most important variable rather than one guideline among many.
The study is observational and cannot prove cause and effect, and the cohort skewed older and predominantly male, so the precise thresholds may not transfer cleanly to every runner. Even so, its scale and the granularity of its GPS data make it among the most persuasive examinations of training load yet published, and coaches are likely to fold its central lesson, respect the long run, into how they periodise mileage.