For runners, pain is sometimes part of the process. Achy muscles after a particularly long run or shin splints when your shoes need replacing can be a gentle warning to ease off. But take training too far and bones can fail. ‘Stress fractures are thought to result from a fatigue failure process’, says Michael Baggaley from University of Calgary, Canada, explaining that the unfortunate injury can takes weeks to heal. But it wasn't clear whether running uphill and downhill might change the risk of developing a stress fracture in the shin bone. ‘How people run changes with surface gradient and speed’, Baggaley explains. To find out how much of a toll pounding up and down hills might take on runners’ shins, Baggaley and Brent Edwards (University of Calgary) invited 17 athletes to go running in the lab.
The researchers jacked up a treadmill so the runners could climb and descend 10 deg and 5 deg slopes, in addition to running on the flat, and set the athletes off at 12 km h−1, recording the forces exerted on the ground as their feet landed and pushed off, in addition to filming the runners’ movements in 3D, to accurately recreate their stride in a computer simulation. Then, Baggaley speeded up the downward sloping treadmill to 15 km h−1, to simulate how athletes run faster downhill, while also reducing the speed to 9 km h−1 as they laboured up both gradients. After recording the runners’ manoeuvres, Baggaley with University of Calgary colleagues Ifaz Haider, Olivia Bruce and Arash Khassetarash, began the painstaking task of reconstructing the forces exerted on the athletes’ lower limbs before applying them to a detailed computer model of a shin bone to find out exactly how much strain running uphill and downhill placed on the bone.
Surprisingly, the team found that running up and down slopes did not increase the strain on the bone, leaving the shin no more vulnerable to stress fracture than when running on the flat. In contrast, running 3 km h−1 faster while descending increased the strain on the bone by 9%, raising the chance of a stress fracture. ‘Changes in running speed, made when running up or down a hill, are likely to determine the damage potential to the shin’, says Baggaley.
So, running faster may be more of a risk to shins than running up and down hills, and Baggaley explains that the strength of each individual's bones is probably the main factor determining whether you'll suffer a shin stress fracture during your running career.