Introduction: Repetitive sock loads are the results of interaction between human musculature system and the ground during daily tasks such as walking. The magnitude and the frequency of these loads are functions of muscle activity and shoe-ground stiffness parameters. Soft tissue vibration is one result of these loads and its characteristics could indicate how human body deals with the shock loads, and could be a mean to determining the appropriate footwear. Hence, in this study, the effects of vibration properties of walking shoe on the vibration of the lower extremities soft tissue were investigated.
Methodology: Shoe vibration parameters: four walking shoes were selected. A hammer test was performed on the shoes in order to determine their stiffness and damping parameters. A uni axial accelerometer (B&K Type 4507) was attached to the insole of the shoe. An impact then applied to the outer sole on the shoe heel using an impact hammer. The time history of impact as well as the resultant acceleration was recorded. The acceleration data showed an underdamped (0.9<ξ<1) behavior. Hence, a sinusoidal function with an exponential decay was fitted to the experimental data from which the stiffness and damping properties of the shoe were estimated. Walking test: a subject performed walking in his preferred speed wearing the tested shoes. Four walking trials were done for each shoe. A uni axial accelerometer (B&K Type 4507) was attached to the muscle belly of the lateral gastrocnemius. A wavelet analysis was performed on vibration data from the gastrocnemius to quantify the damping coefficient of the vibration.
Results: Three of the tested shoes were almost in the same stiffness (90<k<120 kN.m) and damping (300<c<350 N.s.m) area. These values for the fourth shoe were more (k = 300 kN.m, c = 660 N.s.m), indicating a harder sole material. Results of time-frequency analysis of the acceleration signals demonstrated that wearing the hardest shoe resulted in highest vibration intensity and the lowest damping coefficient, while the vibration intensity remained almost the same for other three shoes with similar damping and stiffness.
Discussion: given that the soft tissue vibration intensity and damping changed with different shoe sole stiffness, and considering that vibration damping in soft tissue is an indicator of the level of muscle activity, one should expect an optimum range of shoe stiffness and damping in which the vibration intensity and damping are both minimized.