Leaning too far forward when you run could lead to common overuse injuries.
Ailments such as runners' knee and Achilles tendonitis have long been the bane of regular runners, with around 80 per cent of injuries being of this type.
Now, researchers have discovered an unlikely culprit - trunk flexion - the angle at which a runner bends forward from the hip, a trait of running posture that can vary wildly.
A new study from the University of Colorado Denver (CU Denver) found that greater trunk flexion has significant impact on stride length, joint movements, and ground reaction forces - all of which can affect knee pain, medial tibial stress syndrome, or back pain.
"This was a pet peeve turned into a study," said Anna Warrener, PhD, lead author and assistant professor of anthropology at CU Denver. Warrener worked on the initial research with Daniel Liberman, PhD, in the Department of Human Evolutionary Biology at Harvard University. "When (Lieberman) was out preparing for his marathons, he noticed other people leaning too far forward as they ran, which had so many implications for their lower limbs. Our study was built to find out what they were."
Warrener and her team recruited 23 injury-free, recreational runners between the ages of 18 and 23. They recorded each participant running 15-second trials at their self-selected trunk position and three others: a 10-, 20-, and a 30-degree angle of flexion.
Her team found that the average stride length decreased 13 cm and stride frequency increased from 86.3 strides per minute to 92.8 strides per minute between 10 and 30 degrees.
"The relationship between strike frequency and stride length surprised us," said Warrener. "We thought that the more you lean forward, your leg would need to extend further to keep your body mass from falling outside the support area. As a result, overstride and stride frequency would go up. The inverse was true. Stride length got shorter and stride rate increased."
Increased angles led to a more flexed hip and bent knee joint and changed the runners' foot and lower limb position - altering the pressure on joints due to greater ground reaction forces (GRF).
With a greater number of strides and GRF, the pressure on joints and thus likelihood of overuse and repetitive strain injuries would increase.
"The big picture takeaway is that running is not all about what is happening from the trunk down--it's a whole-body experience," said Warrener. "Researchers should think about the downstream effects of trunk flexion when studying running biomechanics."
The study was published in Human Movement Science.