Effect of Midsole Hardness on Soft Tissue Vibrations: An Ascendant Hierarchical Clustering on 133 Runners

Soft tissue vibrations (STV) can generate discomfort during running. Recent research has shown that footwear affects the amplitude of STV differently across runners but no studies have linked human characteristics and footwear construction yet. The purpose of this study was to investigate the runner specific STV responses to various midsole hardness and to identify functional groups, that is, groups of runners responding similarly to a given intervention. A total of 133 runners (56 females) with a wide range of anthropometrics and running habits performed an 8.5-min trial at a self-selected speed with three different midsole hardness (soft i.e., 46 Asker C, medium i.e., 58 Asker C and hard i.e., 71 Asker C). STV, kinetics, kinematics and electromyographic activity of gastrocnemius medialis muscle were recorded at minute 8. Mechanical properties of calf muscles were obtained through a standardized impact test on the calf muscles. An ascendant hierarchical clustering was computed to cluster individuals based on their STV amplitudes in the three conditions. Three functional groups with distinctive STV responses were identified. STV amplitude decreased for 61% of runners with the soft condition (SOFT group) and for 11% of runners with the medium condition (MED group), whereas 28% of runners demonstrated greater STV amplitude in the hard condition (HIGH-HARD group). The findings revealed differences between groups in term of body fat percentage, running volume per week, foot/ground angle at initial contact, ankle plantarflexion and flight time. It provides guidelines to recommend footwear reducing STV amplitude based on anthropometrics, running practice and biomechanical characteristics of the runners.