Why Don't Most Runners Get Knee Osteoarthritis? A Case for Per-Unit-Distance Loads.
ABSTRACT Peak knee joint contact forces ("loads") in running are much higher than they are in walking, where the peak load has been associated with the initiation and progression of knee osteoarthritis. However, runners do not have an especially high risk of osteoarthritis compared to non-runners. This paradox suggests that running somehow blunts the effect of very high peak joint contact forces, perhaps to provide a load per unit distance traveled (PUD) that is relatively low.
To compare peak and PUD knee joint loads between human walking and running.
Fourteen healthy adults walked and ran at self-selected speeds. Ground reaction force and motion capture data were measured and combined with inverse dynamics and musculoskeletal modeling to estimate the peak knee joint loads, PUD knee joint loads, and the impulse of the knee joint contact force for each gait with a matched-pair (within-subject) design.
The peak load was three times higher in running (8.02 vs. 2.72 bodyweights, p < 0.001) but the PUD load did not differ between running and walking (0.80 vs. 0.75 bodyweights·m, p = 0.098). The impulse of the joint contact force was greater for running than for walking (1.30 vs. 1.04 bodyweights·s, p < 0.001). The peak load increased with increasing running speed, while the PUD load decreased with increasing speed.
Compared to walking, the relatively short duration of ground contact and relatively long length of strides in running seem to blunt the effect of high peak joint loads, such that the PUD loads are no higher than in walking. Waveform features other than or in addition to the peak value should be considered when studying joint loading and injuries.