A Determination of the Mechanical Characteristics of the Human Heel Pad in Vivo

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Abstract

A group of 12 male runners and a group of 12 male nonrunners served as subjects in this study. Techniques of pendulum impacting, vibration testing, and three-dimensional stroboscopic photography were employed in order to determine the mechanical characteristics of the heel pad, and to determine if the heel pad characteristics differed between runners and nonrunners.

The pendulum impacting involved striking the heel pad of each subjects' restrained right leg with a free-swinging ballistic pendulum which was instrumented with a uniaxial accelerometer. Repeated trials were made at three different impact velocities, and trials were also made at all impact velocities with a tightly fitting rigid heel restraining device clamped about the heel region. Force-time characteristics and force-deformation characteristics were determined from the acceleration signal, and measures of peak force, % energy absorption, maximum deformation, and stiffness of the soft tissue of the heel pad were made. Double-exposure photographs were made simultaneously from three different directions during the impacting. The first exposure was made at the time the pendulum just touched the heel, and the second exposure occurred at a time approximating the time of peak force. The direct linear transformation technique was applied to determine the three-dimensional displacement of skin markers within the heel region between exposures.

The response of the heel pad to steady state vibrations was also measured using a mechanical shaker. The transfer function relating the acceleration of the shaker table to the acceleration of an instrumented loading plate which was in intimate contact with the heel pad was determined as a function of frequency at fixed excursions. This response was compared to the response of a three component discrete element model.

None of the tests made on the subjects revealed any differences between runners and nonrunners. The impacting revealed a distinct force-deformation relation which rose to peak force in two linear stages# and which exhibited high hysteresis. The peak force during impacting was found to increase from a mean of 223 N to 437 N as impact velocity increased from 0.8 m/sec to 1.2 m/sec respectively. The effect of the heel restraint was to increase the peak force by an average amount of 29.8 N. The amount of energy absorbed was high, ranging from 84% to 99%. The amount of energy absorption increased about 1% with each 0.2 increase in impact velocity, and decreased about 1% due to the presence of the heel restraint. The stiffness estimate made from the second linear portion of the hysteresis curve increased with increasing impact velocity. Both estimates for stiffness, 7910 N/m and 105,646 N/m when averaged over all conditions, were found to be greater than similar measures previously reported in the literature. Maximum deformation increased from 8.5 mm to 9.9 nm with increases from 0.8 to 1.2 m/sec in impact velocity. Maximum deformation decreased by 1 mm due to the presence of the heel restraint. It was found that this decrease was accounted for by the decrease that occurred in the initial portions of the impact as defined by the force-deformation curve. The photographs showed the skin markers to be displaced in all three dimensions, with the more proximal markers being confined largely to the cranial direction.

The measured frequency response revealed that the system being tested was extremely overdamped. There was no indication of a resonant frequency in the frequency range tested. Comparison of the frequency response with that of the proposed three component discrete element model showed the inappropriateness of the model for the representation of the heel pad.

It was concluded that the heel pad was a nonlinear viscoelastic substance which was capable of absorbing high percentages of energy. The reshaping of the heel pad in medial/lateral and in posterior directions was a mechanism for some of the energy absorption. This mechanism was affected slightly with the application of a tightly fitting heel counter.

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Cited By (9)

Year	Entry
1993	Scott SH, Winter DA. Biomechanical model of the human foot: kinematics and kinetics during the stance phase of walking. J Biomech. 1993;26(9):1091-1104.
1994	De Clercq D, Aerts P, Kunnen M. The mechanical characteristics of the human heel pad during foot strike in running: an in vivo cineradiographic study. J Biomech. October 1994;27(10):1213-1222.
1995	Aerts P, Ker RF, De Clercq D, Ilsley DW, Alexander RM. The mechanical properties of the human heel pad: a paradox resolved. J Biomech. November 1995;28(11):1299-1308.
2002	Miller-Young JE, Duncan NA, Baroud G. Material properties of the human calcaneal fat pad in compression: experiment and theory. J Biomech. December 2002;35(12):1523-1531.
2006	Erdemir A, Viveiros ML, Ulbrecht JS, Cavanagh PR. An inverse finite-element model of heel-pad indentation. J Biomech. 2006;39(7):1279-1286.
2007	Ledoux WR, Blevins JJ. The compressive material properties of the plantar soft tissue. J Biomech. 2007;40(13):2975-2981.
1997	Morag E. Structural and Functional Factors That Determine Regional Peak Pressures Under the Foot During Walking [PhD thesis]. State College, PA: Pennsylvania State University; May 1997.
2003	Miller-Young JE. Factors Affecting Human Heel Pad Mechanics: A Finite Element Study [PhD thesis]. Calgary, AB: University of Calgary; April 2003.
1999	Ledoux WR II. A Biomechanical Model of the Human Foot With Emphasis on the Plantar Soft Tissue [PhD thesis]. Philadelphia, PA: University of Pennsylvania; 1999.