Human Running On Surfaces Of Different Stiffness

Results of a study by a Harvard University research team appear in the February 2002 edition of the Journal of Applied Physiology

(March 12, 2002) -- Bethesda, MD -- The loss of a leg or severe leg injury can be devastating. In addition, even the most advanced prosthetic and orthotic devices have not adequately responded to changes in ground surface or walking speed.  But a new study that examines running on different surfaces could possibly assist in the development of advanced prosthetic devices that change stiffness in response to speed and ground variations. 

A Harvard research team conducted the study, entitled “Energetics And Mechanics of Human Running on Surfaces of Different Stiffnesses.” The investigators are Amy E. Kerdok and Thomas A. McMahon, both from the Division of Health Sciences and Technology and Division of Engineering and Applied Science, Harvard University, Cambridge, MA;  Andrew A. Biewener, Concord Field Station, Museum of Comparative Zoology, Harvard University, Bedford, MA; Peter G. Weyand, Concord Field Station, Museum of Comparative Zoology, Harvard University and the United States Army Research Institute for Environmental Medicine, Natick, MA; and Hugh M. Herr, affiliated with Division of Health Sciences and Technology, Harvard University, Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Cambridge and the Department of Physical Medicine and Rehabilitation, Harvard Medical School, Spaulding Rehabilitation Hospital, Boston, MA.  Their findings appear in the February 2002 edition of the Journal of Applied Physiology.

Background

For the Olympic runner in training, there is nothing more ideal than a "tuned track," running facilities designed with surface to enhance performance and decrease injuries.  Presently, Harvard University, Yale University, and Madison Square Garden all have these specialized tracks, resulting in a recorded three percent increase in running speed and a 50 percent decrease in injuries. 

The principles underlying the construction of these tracks rests with the expression of k_surf  values - a combined measure of decreasing foot-ground contact time, increasing stride length, and decreasing the initial spike in peak vertical ground reaction force.   Despite the success and popularity of these new tracks, the mechanisms resulting in enhanced performance are not clearly understood.

The team of Massachusetts researchers accepted the assumption that the running leg and surface could be represented as a simple spring and mass, with the leg spring having two stiffnesses (k_leg  and k_vert)). The former is the actual leg stiffness describing the leg's musculoskeletal system during the support phrase and is calculated by the ratio of the peak vertical ground reaction force (the reaction to the force the body exerts on the ground) to the compression of the leg spring.  The latter, k_vert, is the vertical stiffness of the runner, providing the mechanism by which the direction of the downward velocity of the body is reversed during limb contact.

Previous studies have found that the knee joint is the major determinant for k_leg   as a function of speed in human running.  Additional experimentation has revealed that a runner's center of mass deflection (maximum vertical displacement at the center of mass) remains constant regardless of the hardness of the surface, essentially the general principle of running mechanics.  Therefore, by adjusting the leg spring to different surface values, the runner maintains uniform support mechanics.

To date, no research study has related the performance enhancements of running on surfaces with different stiffnesses to metabolic costs (the generating force necessary to support body weight).

The goal of the research study was to relate human running biomechanics to energetics (changes in energy to a physical or chemical reaction) on surfaces of different stiffness.  Their expectation was to find a less flexed knee to account for a reduction in metabolic cost as well as an increase in  k_leg.    They further hypothesized that the metabolic costs of forward running reaches a minimum when the k_leg of the runner is maximized on surfaces of decreased stiffness.

Methodology 

Eight male subjects [mean body mass: 74.4 ± 7.1 (SD) kg; leg length: 0.96 ± 0.05 m] ran at 3.7 meters/s on a level treadmill fitted with track platforms of five different compliances. All subjects wore the same flat-soled running shoes. Subjects ran for five minutes on each compliant track platforms in a mirrored fashion (running on stiffest to softest and then softest to stiffest). Beaded strings hung from the ceiling to give the runner a tactile sign as to where he needed to run so that his mid step corresponded with the center of the track platform. Video was also used to ensure that the runner was centered and not stepping on both sides of the track simultaneously.   The ground reaction force (1000 Hz) was recorded using a force plate and kinematic data (60 Hz) using an infrared motion analysis system. Oxygen consumption data force plate and kinematic data were taken simultaneously, and oxygen consumption data were taken after three minutes of running so that the subject was at a steady state. Subjects participated in two separate trials so that they ran on each compliant surface four times. Averages were taken on each day and then averaged together for all variables measured. 

Results

The 12.5-fold decrease in surface stiffness resulted in a 12 percent decrease in the runner's metabolic rate and a 29 percent increase in their leg stiffness.  In every case, the support mechanics remained essentially unchanged over the four stiffest surfaces tested.

The experiment findings supported the authors' hypothesis that the metabolic cost of running at an intermediate speed is progressively reduced and that the leg's spring stiffness is increased as k_surf  changes without altering the body's overall mechanics. 

Conclusions

The study established a link between the mechanics and energetics of human running on different surfaces.  Both metabolic cost and k_leg  change when k_surf  is manipulated.  The metabolic reduction is due to the track's elastic energy return assisting the runner's leg spring.  This leg spring adjustment is key to the body sustaining constant support mechanisms over varying surfaces.

These findings will certainly benefit running shoe technology and new track design.  But their long term promise is to enable the physically challenged to move with greater ease and comfort, with devices incorporating the latest research in biomechanics.

Source: February edition of the Journal of Applied Physiology.

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The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.

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Editor’s Note: To set up an interview with a member of the research team, please contact Donna Krupa at 703.527.7357 (direct dial), 703.967.2751 (cell) or djkrupa1@aol.com.