Human Subcalcaneal Fat Pad Mechanical Properties

Open Access
Author:
Gales, Daniel Joseph
Graduate Program:
Kinesiology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 20, 2013
Committee Members:
  • John Henry Challis, Dissertation Advisor
  • John Henry Challis, Committee Chair
  • Stephen Jacob Piazza, Committee Member
  • Neil Sharkey, Committee Member
  • Andris Freivalds, Committee Member
Keywords:
  • Biomechanics
  • Mechanical Properties
  • Heel Pad
  • Fluctuation Analysis (DFA)
  • Humans
  • Material Testing System (MTS)
  • Running
Abstract:
The human heel pad is located inferior to the calcaneus and functions to decrease local stress on this bone, decrease the impact of heel strike transients, and act as a shock absorber. It is approximately 15 mm thick and consists of dense fibrous septa which are filled with closely packed adipocytes to perform these functions. The purpose of this dissertation was to examine the mechanical properties of the human heel pad to gain a greater understanding of the functional role of this tissue. Three experiments were completed to address several issues: 1) are the mechanical properties of the human heel different when experiencing physiological rest times compared with non-physiological rest times, 2) are the mechanical properties of the human heel pad different when experiencing physiological forces compared with non-physiological forces, and 3) what effect does heel pad containment and exposure have on the mechanical properties of the human heel pad. Experiment 1 examined the effect of physiological rest times associated with running on the mechanical properties of the human heel pad. Data collected on a group of runners revealed that these rest time intervals, the stride time intervals, demonstrated long-term correlations. When loadings were applied to cadaveric heel pads using a material testing system, these timing fluctuations significantly changed the deformation and stiffness of the heel pad compared with the results from non-physiological loadings. The results indicated some evolutionary adaptation in heel properties to the nature of physiological rest intervals between the loadings they experience. Experiment 2 examined the effect of physiological forces on the mechanical properties of the human heel pad. Data collected on a group of runners revealed that physiological forces during heel impact demonstrated long term correlations. When physiological forces were applied to cadaveric heel pads using a material testing system, the forces did not significantly affect the mechanical properties of the human heel pad. It was hypothesized that the heel pad mechanical properties would exploit the variability found in physiological forces of runners, but this was not the case. Experiment 3 examined the effect of various amounts of heel pad containment and exposure on heel pad mechanical properties, in a similar fashion to different shoe designs. For the purpose of this experiment, containment was defined as the act of heel pad confinement by an external device. Exposure was defined as the amount of heel pad that is not supported by an external device. Using a custom external containment system heel pads were tested in a material testing system by loading with forces approximating those that the heel pad experiences during running, and using rest intervals reflecting those from gait. For ten cadaver heel pad the peak deformation, hysteresis, and stiffness were determined. These results indicate that the behavior of the human heel pad is altered in the presence of external containment. Heel pads demonstrated significantly less maximal deformation expressed as a percentage of heel pad thickness and greater stiffness at body weight when compared with uncontained heels. From the exposure perspective, the behavior of the heel pad is also altered by exposure. The findings of this experiment have implications for footwear design. The findings from these experiments indicate that the mechanical behavior of the human heel pad is different when experiencing physiological rest times during running gait, and that heel pad mechanical behavior is influenced due to the type of constraints found in shoes and orthotic devices. The results of this study have implications for shoe and orthotic design and for the further examination of the behavior of the human heel pad under different simulated gait patterns.