Ground Reaction Forces in Distance Running

1. Ground Reaction Forces in Distance Running Peter Cavanagh Mario Lafortune

2. Introduction • High incidence of lower extremity injuries in runners due to overuse • The design of footwear should depend upon knowledge of the force and pressure environment during running • Development of a shoe testing laboratory

3. Purpose of the Study • Document the ground reaction forces in running • Examine the center of pressure distrubution • Gain insight into the changes in velocity of the body center of mass

4. Methods - Subjects • 17 runners (10 males & 7 females) • Weekly running distances of 10-160 km

5. Data Collection • Kristal type force plate located in the middle of a 40 m runway • Photocells at hip height situated 5 m from the center of the force plate • Subjects were given practice trials to get accustomed to hitting the force plate with their right foot while running at 4.5 m/s (6 min/mile pace)

6. Data Collection • If the running speed was outside of the desired range (4.12 – 4.87 m/s) the trials were discarded • The subjects were required to run in a smooth fashion and hit the center of the force plate

7. Determination of Foot Placement • All subjects wore Etonic Km 505 running shoes • The surface of the force plate was covered with tape • The subjects ran through chalk prior to contacting the force plate • The position of the shoe on the plate was determined from the chalk imprint

8. Data Processing • Data from 5 right foot contacts were recorded • Fx, Fy, Fz, Mx and My were sampled at 500 Hz with a PDP 11/34 computer • Analog comparator with Fz = 50 N was used to identify GRF curves

9. Average Curves • Averaging curves with different durations and asynchronous peaks tends to attenuate peak values

10. Velocity Changes of CM

11. Center of Pressure

12. Results: Center of Pressure • Rearfoot strikers had a foot placement angle of 10.4 • By 42 ms into contact the COP had moved anteriorly to 50% of shoe length • It remained within 50-80% until the end of support, 146 ms later

13. Results: Center of Pressure • Midfoot Strikers: • First contact was made at 50% of shoe length • Following a brief anterior movement the COP migrates posteriorly and medially, reaching most posterior point at 20 ms • Followed by a rapid anterior movement reaching 65% of shoe length in 40 ms

14. .35 BW range .12 BW range .43 BW at 46 ms .45 BW at 11 ms .45 BW at 38 ms 2.2 BW at 23 ms 2.7 BW at 75 ms

15. Shoe Design • Prior to this paper, track authorities stated that heel-toe patterns should be used in long distance running and mid-foot patterns should be used in middle distance. • Protection from impact forces should extend from the heel to 60% of shoe length. • The mid-foot group requires additional mid-foot impact protection. • The backward slope of the force vectors indicates that braking occurs while the cop is as much as 70% of shoe length from the heel. As a result the outer sole should provide friction for braking throughout this region of the shoe.