In vivo visco-elastic properties of the heel fat pad during gait: Is the heel pad at its physiological maximum during walking?

Abstract

The fat pad of the human heel is a highly specialised adipose tissue, which is widely regarded to function as a shock absorber during the impact phase of gait. Although altered mechanical properties of the heel pad have been implicated in the development of several overuse injuries, in vivo measurement of the visco-elastic properties of the heel pad during dynamic activities, such as walking, remain largely unexplored. This study used cine-radiography synchronised with a pressure platform to obtain stress-strain data for the heel pad of nine healthy subjects (age, 46 ± 12 years; height, 1.67 ± 0.11 m; weight, 80.1 ± 10.4 kg) while walking at their preferred pace. The initial thickness and compressive strain of the fat pad were estimated from dynamic lateral radiographs, while the compressive stress was derived from peak pressure data. Principle visco-elastic parameters of the heel pad, including peak strain, secant modulii, hysteresis and time constants were estimated from the stress-strain curves. Real in vivo transient loading profiles associated with walking induce rapidly changing strain rates in the heel pad and create irregular stress-strain curves that differ notably from those previously reported for standard materials tests. Moreover, in vivo viscous properties of the heel pad at physiologically relevant strain rates fall between those commonly cited by studies using conventional quasi-static and ballistic loading methods, despite similar absolute deformations. Given that deformation of the fat pad (11 ± 2 mm) in the current study is similar to that previously reported for barefoot running, it would appear that the heel pad operates near its physiological maximum, even during walking.