| Dec 08, 2014
By Albrecht Dietze, M.D., and Scott C. Wearing, Ph.D.
Textbooks on sports medicine typically report that the heel pad is a thick elastic-adipose tissue which plays a critical shock-absorbing role during human locomotion. The concept of the heel pad as a shock absorber owes much to the work of McNeil-Alexander, Radin, Aerts and colleagues who, in the 1970s and 80s, used mechanically-simulated impacts to hypothesize that deformation of the heel pad attenuated peak force at impact, dissipated mechanical energy during heel strike and, ultimately, protected the calcaneus by lowering local stress.
In one of the few studies to investigate deformation of heel pad during locomotion, DeClercq, Aerts and co-workers demonstrated in the 90s that the heel pad actually offered minimal resistance to deformation at initial impact and ‘bottomed out’ during barefoot running. They hypothesised that deformation of the heel pad during barefoot running, therefore, mainly served to minimize local stress at the calcaneus. Given that shock absorption is related to energy dissipation over the entire load-deformation cycle, their study was unable to assess the shock absorbing capacity of the heel pad. So we decided to further examine the hypothesized function of the heel pad by undertaking an observational study which detailed the force-deformation behavior of the healthy human heel pad over the entire load-deformation cycle and at a lower gait speed.
Our most recent report, published in MSSE, is the first to assess the force-deformation properties of the heel pad in healthy subjects while walking barefoot at preferred speed using a dynamic radiographic imaging technique coupled with a pressure measuring platform. We confirmed previous observations and showed the heel pad had a distinct nonlinear behavior during walking, in which increasing force was associated with progressively less deformation. Initial stiffness of the heel pad was a tenth of its final stiffness and only about one joule of energy was dissipated by the heel pad with each walking step. The energy dissipated by the heel pad in our carefully controlled laboratory–based experiment was 5 to 10 times less than that reportedly dissipated by other structures such as the Achilles tendon and the ligaments of the medial longitudinal arch. In our view, this finding warrants a critical reappraisal of the relative shock reduction and energy dissipating role of the heel pad during locomotion.
In our experiment, we also found that the energy required to deform the heel pad during walking was only marginally less than that reported during barefoot running at moderate speed. Furthermore, we found that the peak deformation of the heel pad during walking was close to that predicted for the limit for pain tolerance. This finding has important clinical implications. For instance, to avoid potential pain and injury at higher barefoot gait speeds, movement of the rearfoot and soft tissues of the shank must increasingly contribute to energy dissipation during heel contact or gait adjustments must occur to ensure the contact energy during heel strike is comparable during barefoot walking. These findings also provide indirect support to the so–called ‘Robbins and Hanna’ hypothesis in which plantar foot sensation is proposed to moderate impact loading of the foot during gait. These findings also may, in part, account for the more plantar–grade foot strike pattern that occurs with barefoot gait at speeds faster than walking.
With the advent of treadmills containing inbuilt pressure sensor technology, our next step is to address some limitations associated with use of a one-dimensional analysis and fluoroscopic imaging of the heel pad to evaluate heel pad mechanics over a wider range of gait speeds— and the potentially mitigating effects of footwear.
Albrecht Dietze, M.D., is an orthopedic trauma surgeon with a special interest in foot and ankle surgery. His research focuses on clinical application of foot and ankle biomechanics. In particular, imaging techniques and pedobarographic analysis are the main methods applied to improve strategies and clinical outcome in the treatment of foot and ankle pathology.
Scott C. Wearing, Ph.D., is an experimental soft tissue bioengineer and researcher at the Institute of Health and Biomedical Innovation, Queensland University of Technology, Australia. His research focuses on the measurement of human soft tissue adaptation to exercise, pathology and disease and is targeted toward prevention, recovery and expedited rehabilitation of musculoskeletal injury.
This commentary presents Drs. Dietze’s and Wearing’s views on the topic of their research article which they and their colleagues published in the August 2014 issue of Medicine & Science in Sports & Exercise® (MSSE).
Viewpoints presented in Active Voice commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.