Effect of Rest Periods on Mechanical Ageing of Running Shoes

Abstract

Running is a popular form of exercise, although runners are prone to injury from repeated impact. Running shoes can limit impact forces, but they deteriorate with use. Mechanical ageing typically involves repeatedly compressing the midsole while measuring the energy absorbed within compression cycles to assess degradation. Literature suggests mechanical aging often causes a higher rate of degradation than natural ageing. This work investigated the effect of introducing rest periods into mechanical ageing. Five shoes were mechanically aged using a sine plus dwell waveform (1.25 Hz, max. load 1.5 kN) for seven hours, equating to a simulated distance of 60 km. Three of the shoes were rested for 22 hours every 20 km. The shoes aged with rest periods absorbed more energy than their unrested counterparts for the first 10 km when testing recommenced. This finding has implications for the mechanical ageing, design and recommended lifespan of running shoes.

Fatigue mechanisms of a closed cell elastomeric foam: A mechanical and microstructural study using ex situ X-ray microtomography

“…Indeed, as evident from the whole graphs of the figure, it is interesting to notice that either the maximal stress, or the residual strain, or the absorbed energy, or the loss coefficient recorded during the interrupted cycling procedure follow those recorded during the continuous one, after a transient period of ≈ 1000 cycles, as if the continuous trends yielded to master curves of the interrupted ones, once a transient regime is achieved. It must be pointed out that similar memory effects were observed by Allen et al regarding the maximum absorbed energy on a 3 consecutive impact fatigue tests of 10000 cycles separated by a 22 h of a rest period [15]. The authors found that after the first resting time, the maximum absorbed energy value was comparable (within measurement error and sample consistency) to those reported at the start of the first ageing period and decrease in a similar manner.…”

“…Analyzing their fatigue (micro)mechanisms represents a challenging task as these systems are sequentially deformed at large cyclic finite strains and exhibit complex compressible elastoviscoplastic behavior [14]. Over the past 40 years, the midsole foams of sport running shoes are mainly made of ethylene-vinyl-acetate (EVA) [15,16], due to their low-cost and their fast and versatile manufacturing process. Thus, if we conducted a preliminary study on other systems such as thermoplastic polyurethane and polyamide-polyether copolymer closed cell foams [17], the fatigue of EVA foams are by far the most studied closed cell foams in the literature [18,19,20,21,22,23].…”

“…They observed a partial recovery of the peak force level during resting time, coupled with a global softening of the samples over the week. Allen et al compared a continuous and an interrupted procedure [15]. The authors found that samples partially recovered their energy absorption capabilities during a daily rest period and concluded that foams lose faster this property in a continuous fatigue procedure equivalent to 60 km.…”