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1. Mark J. Lake and Mario A. Lafortune.
   School of Human Sciences, Liverpool John Moores Uni., U.K;
   THE INFLUENCE OF UNANTICIPATED CHANGES IN ATHLETIC SHOE MIDSOLE HARDNESS ON LOWER LIMB MUSCULAR PREPARATION FOR IMPACT LOADING.
   
2. Alain Rakotomamonjy, M. Barbaud, M. Tronel, P. Marché
   Laboratoire Vision et Robotique, IUT de Bourges, France
   TIME-FREQUENCY ANALYSIS OF IMPACT SHOCK DURING RUNNING
   
3. Uwe Kersting, Brüggemann, G.-P.
   Institute for Athletics and Gymnastics, German Sport University - Cologne; Germany
   STEP TO STEP VARIABILITY OF IMPACT FORCES AND REARFOOT MOTION DURING TREADMILL RUNNING
   
4. Ian Wright, R.R. Neptune, A.J. van den Bogert, B.M. Nigg
   Human Performance Laboratory, The University of Calgary, Calgary, Canada
   THE REGULATION OF IMPACT FORCES IN RUNNING

Session 1: Impact
Session 2: Invited Speaker
Session 3: Insoles and Inserts
Session 4: Methods
Session 5:Clinical Aspects and Injury
Session 6:Sports
Session 7:Pressure Distribution
Session 8:Running and Running Shoes
Session 9: Structure and Function

Few studies have examined the short-term responses of the body to a modification in athletic shoe cushioning in a controlled manner. These responses might provide useful insight into the interaction between mechanical and neuromuscular factors that govern impact loading severity. Unexpected changes in footwear midsole characteristics were studied using a human pendulum apparatus which controlled the velocity and lower limb posture at impact. The severity of impact and pre-contact muscular activity were monitored for both a soft to hard footwear change and a hard to soft footwear change in ten subjects. Before each shoe exchange all subjects were accustomed to a specific type of footwear by undergoing two weeks of adaptation sessions consisting of pendulum impacts and treadmill running. The results indicated that the severity of lower extremity impact was modified due to the mechanical characteristics of the shoe midsole and short-term muscular adaptation effects were minimal or not present. Pre-contact muscular activity of selected lower extremity muscles was not modified after the change in footwear or after accommodation by treadmill running. This absence of change in muscular preparation prior to impact may suggest that the unexpected adjustment in impact severity was either not perceived by the subjects or they deemed such a change as unnecessary. The former hypothesis would submit that our ability to perceive differences in impact severity of a similar range to that experienced during locomotion is limited.

The aim of this study is to propose a new method for the analysis the impact shock during running. Time-Frequency methods show the evolution of the spectral power contents of a signal over the time. It is more powerful than the traditional Fourier Transform for non-stationary signal as Fourier Transform does not give any time information concerning the frequency of the signal. The time-frequency method we have used for impact shock analysis is the Wigner-Ville transform which is a quadratic time-frequency representation. It means that energy of the signal is spread over a time-frequency plane. Wigner-Ville Transform has been applied to tibial acceleration during barefoot and shod running. It has been showed that impact peak raises up to frequency higher than 40 Hz even if a major part of the impact shock energy belongs to the bandwidth (10-20 Hz). Relative energy of those high spectral contents of the impact have been calculated and we have found that they are inferior to -7dB. Comparison between barefoot and shod running allows to evaluate the cut-off frequency of a running shoe. It is obtained by calculating the higher frequency above a particular threshold during shod running. That threshold is defined with regard to the highest energy value during barefoot running.

Besides, all classical parameters defining the impact peak such as peak value or time to peak can be obtained with the Wigner-Ville Transform. But more informations concerning the impact peak can be retrieved with the time-frequency representation than with time or frequency analysis.

Several experimental studies and modeling approaches implicate a close connection between impact forces at touch down and rearfoot motion during running. However, recent publications showed that changes in shoe constructions designed to alter the pronatory movement in the subtarsal joint did not influence impacts substantially. To prove a direct coupling between rearfoot motion and impacts, a step-to-step analysis should demonstrate this relation clearly.

The purpose of this study was to examine the relationship between rearfoot motion and impacts over a sufficient number of consecutive steps.

The setup included an instrumented treadmill, an in-shoe goniometer, an in-shoe force sensor under the heel and a 2D video analysis system to record sagittal plane kinematics of the lower extremity. 10 subjects were equipped with standard running shoes. 25 steps were then recorded for further analysis.

Results showed inconsistent patterns between selected parameters among subjects. In particular maximum pronation, pronation velocity and total range of pronation showed almost no significant correlation with the impact peak or the force rate.

It thus cannot be concluded that a step-to-step variation of rearfoot motion directly influences impact forces during running. Other individual adaptation strategies might be responsible for the resulting inconsistencies.

Considerable effort has gone into increasing cushioning in shoes to reduce impact forces. However, only minimal changes in peak impact force have been observed with changes in shoe hardness. It has been hypothesized that increased rates of pronation regulate impact forces with harder shoes. The purpose of this study was to determine the contribution of rotations at the subtalar and talocrural joints to impact force regulation. A three dimensional model of the lower extremity was used to simulate impact in running, with two different shoe hardnesses. Initial joint positions and velocities for the simulations were taken from measurements of eight male subjects while heel-toe running. Simulations were performed first with normal ankle joint movement, then with the subtalar joint fused, and finally with the talocrural joint fused.

With no joint fusion, and with the subtalar joint fused, no consistent difference between the peak impact forces for the soft and for the hard shoe conditions were obtained. With the talocrural joint fused, peak impact forces for the hard shoe were higher than for the soft shoe for all subjects. Therefore, changes in rates of ankle plantar-flexion, rather than ankle pronation, were found to regulate peak impact force with changing shoe conditions.