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Tennis Shoe Cushioning

Impact Testing To Compare Tennis Shoes

Crawford Lindsey, Tennis Warehouse, San Luis Obispo, CA
March 12, 2017


For tennis players, the primary factor determining purchase of a tennis shoe is comfort. For researchers and manufacturers, injury prevention is a main area of interest, concern and design. For both groups, heel and forefoot cushioning and stability is the common denominator between comfort and injury reduction. The objective of the experiment was to examine cushioning effects at different impact energies for both heel and forefoot, determine how stiffness affects cushioning and the player's response to it, and to compare multiple shoes on all parameters. Sixteen shoes from ten manufacturers were tested in both static and dynamic situations. Shoes were compared in the following parameters: Peak force, deceleration, and deformation; time to peak and contact duration, energy return and absorption, stiffness, load rate, and shoe thickness. The results showed that there are significant differences between shoes on all measures. Comparison tools were then programmed and uploaded to the internet to facilitate tennis shoe comparisons. Shoes will be continuously added to the database as they are introduced to the market and tested.

1. INTRODUCTION 1.1. Forces in Running

Much research has been done on the impact forces involved in running, primarily because high forces have been the prime suspects in contributing to acute and chronic lower leg injury and pain. Many studies have chronicled the occurrence of lower extremity injuries in running (and tennis) and causative relationships have been sought. To that end, mechanical tests on shoes have been performed in the lab and biomechanical data have been gathered from athletes running on force plates, shoes instrumented with pressure insoles, or from tracking markers attached to various positions on athletes legs and shoes. Video has also been used to augment the collection of kinematic and kinetic data. The two major areas of concern garnered from analysis of this data is the initial contact of the heel and surface in heel-toe running and the medial-lateral stability of the foot during the stance phase of the gait. Much research has been directed toward these topics, yet for years there has been no conclusive epidemiological and etiological evidence to conclusively connect these with injury (Nigg, 2001). At best, injury has been proven to be associated with impact shock and foot movement, but proven causality remains elusive. And to complicate things further, in spite of all the research and development that has gone into footwear, running injury rates have not declined (Lieberman, 2012).

Force plate signals acquired during heel-toe running reveal two ground reaction force (GRF) peaks — a first transient passive peak and a second larger and longer active peak (Figure 1). The smaller passive peak occurs during the first 50 ms of heel contact and is passive because it occurs during the period of muscle latency during which muscular reactions to impact have not yet been activated. This is pure impact force which initiates vibrations in the bones and joints of up to 20 Hz and sends a shock wave from the lower extremities all the way up to the head. In fact, deceleration at the foot is typically around 15 to 20 g and from 1-3 g at the head (Shorten, 2000). Impact will cause the ligaments, tendons and muscles to vibrate at lower frequencies. The active peak occurs during the propulsive portion of the gait after the weight has been shifted over the forefoot and the muscles are actively pushing off from the surface. The magnitude of the vertical GRF in heel-toe running typically varies between 1 and 3 times bodyweight (Cavanagh & Lafortune, 1980) and will depend on many factors including running speed, weight, lower extremity kinematics, surface, gradient, shoe, and athlete adaptations. The interpretation of how the high and low frequency vibrations are summed, analyzed, and interpreted also influences the determination on the "effective" impact magnitude (Nigg, 2001; Hennig, 2011; Shorten & Mientjes, 2011).

Ground reaction force vs time for running.

Figure 1 — Ground reaction force for running (adapted from Cavanagh, 1980).

The lateral (outside) portion of the heel typically strikes the ground first due to supination about the ankle joint. After contact, the ankle rotates medially and the foot rolls inward, or pronates. Too much pronation can transmit excessive forces to the angle, knee, and hip, causing pain and/or injury. This foot pronation is the primary instability that shoe manufacturers have tried to address in their designs.

Shoe designers have attempted to deal with the dual problem of impact and instability by adjusting the width, thickness and stiffness of the shoe midsole. For shock absorption, thickening and widening the heel will spread the impact force out in both space and time. Adjusting the stiffness of the heel and the medial and lateral midsole helps to control the rate of loading and rotation. Properly adjusting stiffness is important to prevent bottoming out during impact. Having a stiffer medial than lateral midsole will help minimize pronation. Adjusting stiffness is augmented by adjusting geometry. Varus and valgus wedges (thicker areas in the medial or lateral midsole) have also been used to control motion.

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