Clark KP, Weyand PG. Are running speeds maximized with simplespring stance mechanics?. Are the fastest running speeds achieved using the simple-spring stance mechanics predicted by the classic spring-mass model? We hypothesized that a passive, linear-spring model would not account for the running mechanics that maximize ground force application and speed. We tested this hypothesis by comparing patterns of ground force application across athletic specialization (competitive sprinters vs. athlete

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... nonsprinters, n ϭ 7 each) and running speed (top speeds vs. slower ones). Vertical ground reaction forces at 5.0 and 7.0 m/s, and individual top speeds (n ϭ 797 total footfalls) were acquired while subjects ran on a custom, high-speed force treadmill. The goodness of fit between measured vertical force vs. time waveform patterns and the patterns predicted by the spring-mass model were assessed using the R 2 statistic (where an R 2 of 1.00 ϭ perfect fit). As hypothesized, the force application patterns of the competitive sprinters deviated significantly more from the simple-spring pattern than those of the athlete, nonsprinters across the three test speeds (R 2 Ͻ0.85 vs. R 2 Ն 0.91, respectively), and deviated most at top speed (R 2 ϭ 0.78 Ϯ 0.02). Sprinters attained faster top speeds than nonsprinters (10.4 Ϯ 0.3 vs. 8.7 Ϯ 0.3 m/s) by applying greater vertical forces during the first half (2.65 Ϯ 0.05 vs. 2.21 Ϯ 0.05 body wt), but not the second half (1.71 Ϯ 0.04 vs. 1.73 Ϯ 0.04 body wt) of the stance phase. We conclude that a passive, simple-spring model has limited application to sprint running performance because the swiftest runners use an asymmetrical pattern of force application to maximize ground reaction forces and attain faster speeds. sprinting performance; musculoskeletal mechanics; ground reaction forces; gait; spring-mass model RUNNING SWIFTLY IS AN ATHLETIC attribute that has captivated the human imagination from prehistoric times through the present day. However, interest in running speed as an athletic phenomenon has probably never been greater than at present. A number of factors have heightened contemporary interest and focused it upon the determinants of how swiftly humans can run. These factors include the globalization and professionalization of athletics, the parallel emergence of a performancetraining profession, advances in scientific and technical methods for enhancing performance, and record-breaking sprint running performances in recent international competitions. Yet despite interest, incentives, and intervention options that are arguably all without precedent, the scientific understanding of how the fastest human running speeds are achieved remains significantly incomplete. At the whole-body level, the basic gait mechanics responsible for the swiftest human running speeds are well established. Contrary to intuition, fast and slow runners take essentially the Address for reprint requests and other correspondence: P. Weyand,