Is midsole thickness a key parameter for the running pattern?

Highlights

• We tested the effect of barefoot and five midsoles thicknesses on running pattern.

• Increase in midsole thickness only affected stance phase duration, increasing it.

• Barefoot and shod running conditions showed knee and ankle kinematic differences.

• Impact magnitude was not affected by shoe midsole thickness variations.

• Very thin upper and sole were sufficient to affect barefoot running pattern.

Abstract

Many studies have highlighted differences in foot strike pattern comparing habitually shod runners who ran barefoot and with running shoes. Barefoot running results in a flatter foot landing and in a decreased vertical ground reaction force compared to shod running. The aim of this study was to investigate one possible parameter influencing running pattern: the midsole thickness. Fifteen participants ran overground at 3.3 m s⁻¹ barefoot and with five shoes of different midsole thickness (0 mm, 2 mm, 4 mm, 8 mm, 16 mm) with no difference of height between rearfoot and forefoot. Impact magnitude was evaluated using transient peak of vertical ground reaction force, loading rate, tibial acceleration peak and rate. Hip, knee and ankle flexion angles were computed at touch-down and during stance phase (range of motion and maximum values). External net joint moments and stiffness for hip, knee and ankle joints were also observed as well as global leg stiffness. No significant effect of midsole thickness was observed on ground reaction force and tibial acceleration. However, the contact time increased with midsole thickness. Barefoot running compared to shod running induced ankle in plantar flexion at touch-down, higher ankle dorsiflexion and lower knee flexion during stance phase. These adjustments are suspected to explain the absence of difference on ground reaction force and tibial acceleration. This study showed that the presence of very thin footwear upper and sole was sufficient to significantly influence the running pattern.

Introduction

For over forty years, athletic footwear companies developed technical concepts aiming at preventing different injuries. Previous studies reported that 30% of runners suffer of an injury during a 13 weeks training period [1] and that most common running injuries were patella femoral pain syndrome, illiotibial band friction syndrome, Achilles tendinitis, shin splints and plantar fasciitis [2]. Knee injuries represent approximately 25% of injuries observed, muscles end tendons are the most affected tissues [3]. Among the most important factors causing running injuries are the magnitude and the repetition of impacts. In order to palliate this problem modern running shoes are composed of an important midsole thickness often made of viscoelastic materials located under the heel [4]. In addition midsoles often integrate others materials such as PU foam, gel inserts or air cushion.

However, it seems that the use of a modern shoe incorporating these concepts has altered the barefoot running pattern observed in people who never ran with “modern” shoes [5]. While 80–90% of recreational runners impact the ground with the heel [6], [7], various reasons suggest that heel–toe running is not the natural running pattern. Indeed the study of Lieberman et al. [5] showed that different populations such as the Kenyans from Rift Valley Province who usually run barefoot do not adopt the same foot strike pattern as “modern” society runners. Indeed habitually barefoot runners often land with the forefoot (forefoot strikers) or with a flat foot (midfoot strikers) [5]. Moreover the transient peak of the vertical ground reaction force (vGRF) usually observed during a heel–toe running stride tends to disappear during a forefoot strike. Transient peak and the associated loading rate are suspected to represent the harmful effects of repetitive impact during running [8], [9]. This suggests that running barefoot or in minimalist shoes may have real benefits over shod running when considering impact magnitude.

For habitually shod runners, many changes concerning biomechanical running pattern have been observed when comparing barefoot and shod running. Previous studies have shown during barefoot running an increase of stride frequency [10], [11] and leg stiffness [12], lower knee flexion range of motion (FRoM) [13], decreased impact forces [10], [14] and joint torques [15]. Moreover Bishop et al. [16] and Hamill et al. [14] observed ankle in dorsiflexion during shod running versus plantarflexion during barefoot running.

Although differences observed between barefoot and shod running for habitually shod runners seem to be less obvious than differences observed between habitually shod and habitually barefoot runners, it appears that significant adjustments exist between shod and barefoot running. Three main factors differentiate barefoot from shod running as a shoe is usually built with a thick and deformable sole, a difference of height between heel and forefoot (heel to toe drop), and an upper around the foot.

Midsole thickness is an important parameter concerning the plantar sensations and maybe a crucial parameter in the modification of foot strike pattern between shod and barefoot running. The study of Robbins and Gouw [17] suggested that modern shoes with thick and compliant midsoles attenuate plantar sensations at touchdown inducing the suppression of protective reflexes. Thus, the aim of the present study was to quantify the effect of midsole thickness on biomechanical responses during running for habitually shod runners.

It was hypothesized that a lower thickness would cause the foot strike pattern to be closer to the foot strike pattern observed in a barefoot condition, with a flatter foot at touch-down, therefore inducing lower impacts and lower net joint torques.