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Why Don’t Most Runners Get Knee Osteoarthritis? A Case for Per-Unit-Distance Loads


Medicine & Science in Sports & Exercise: March 2014 - Volume 46 - Issue 3 - p 572–579
doi: 10.1249/MSS.0000000000000135
Applied Sciences

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 with nonrunners. This paradox suggests that running somehow blunts the effect of very high peak joint contact forces, perhaps to provide a load per unit distance (PUD) traveled that is relatively low.

Purpose This study aimed to compare peak and PUD knee joint loads between human walking and running.

Methods 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.

Results The peak load was three times higher in running (8.02 vs 2.72 body weight, P < 0.001), but the PUD load did not differ between running and walking (0.80 vs 0.75 body weight per meter, P = 0.098). The impulse of the joint contact force was greater for running than for walking (1.30 vs 1.04 body weight per second, P < 0.001). The peak load increased with increasing running speed, whereas the PUD load decreased with increasing speed.

Conclusions Compared with 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 that in walking. Waveform features other than or in addition to the peak value should be considered when studying joint loading and injuries.

Supplemental digital content is available in the text.

1Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON, CANADA; 2Human Mobility Research Centre, Queen’s University, Kingston, ON, CANADA; and 3Department of Kinesiology and Nutrition, University of Illinois, Chicago, IL

Address for correspondence: Ross H. Miller, Department of Kinesiology, Neuroscience and Cognitive Science Program, University of Maryland, College Park, MD 20742; E-mail:

Submitted for publication April 2013.

Accepted for publication August 2013.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Web site (

© 2014 American College of Sports Medicine