Ken Clark's research links running motion to ground forces using a two-mass model tested on sprinter David Oliver (10.2 m/s) and marathoner Ryan Hall (5.7 m/s). Sprinters generate forces up to five times body weight in under 100 milliseconds, while marathoners produce lower forces over longer durations. Just three motion parameters—contact time, aerial time, and lower-limb acceleration—predict all force patterns across running speeds.
Chapters
This is world champion and gold medalist in the 110-meter hurdles, David Oliver, running at 10.2 meters per second on the SMU High-Speed Force Treadmill. We simultaneously record ground reaction forces and video at 1,000 frames per second. To run fast, sprinters apply forces up to 5 times their body weight in less than 100 milliseconds on each leg. Here's distance runner Ryan Hall, the fastest American marathoner in history, running at 5.7 meters per second. 7 meters per second, a 2-hour and 3-minute marathon pace. He has a different force pattern, lower in amplitude and longer in duration than a sprinter. During a marathon, Ryan will impact the ground 25,000 times with a peak force of 3 times his body weight for each footfall. Running ground reaction forces thus have fundamental physical and biological significance. Is there an underlying principle that relates the force to the motion of a runner? There is. Newton's Laws of Motion. We determined that the motion of the body behaves as a two-mass system when running. Mass 1 consists of the lower contacting limb. Mass 2 consists of the rest of the body. An impulsive force results from the vertical collision of the lower limb with the running surface. The magnitude and timing depend on the vertical acceleration of the ankle. Simultaneously, an impulsive force results from the vertical motion of the rest of the body. The magnitude and timing depend on the contact and aerial time. The forces add together to form the total vertical ground reaction force. All force patterns are determined by just 3 motion parameters: contact time, aerial time, and lower limb acceleration. Our model accurately predicted forces across running speeds from 3 to 11 meters per second for 42 men and women with different foot strike mechanics. Applications of the model include gait analysis, robotics, prosthetics, orthotics, and shoe wear design. I'm Ken Clark. Further details of our research can be found in the Journal of Experimental Biology.
