The Efficacy of Subplantar Cushioning in Modulating Ground Reaction Force and Total Impulse in Normal Walking Gait

Abstract

Footwear is prevalent in modern society, and rarely does one undertake locomotive actions unshod. A detailed understanding of the long term influence of such orthotics is a prerequisite to attempting reduction of those geriatric conditions associated with gait, e.g. osteoarthritis. Whilst a considerable body of evidence exists to support the role of cushioning in modulating ground reaction force (GRF) and impulse during sporting activities, few studies consider the more common case of walking with normal gait. The present study evaluates the effect of wearing footware with reference to being unshod, and the effect of increasing the cushioning effect of footware by means of a removable insole. The presence of footware has been shown to modulate both GRF and impulse during normal gait, a finding which may have implications in terms of the treatment of degenerative joint and other geriatric diseases.

Introduction

Extensive research exists which relates to reducing key factors, including the GRF, in running shoes. This reflects a link which has been established between the properties of athletic footware and injury (Milani et al., 1997).
Nigg and Segesser identified three principle methods for reduction of overload-related injuries: cushioning, support, and guidance (Nigg and Segesser, 1992). The present study investigates the extent to which cushioning acts to reduce the GRF and impulse in the case of normal shoes of standard construction.
Much research has focused upon this topic in terms of running and other athletic activities (Bus, 2003; Fritz and Peikenkamp, 2003), neglecting the vast majority of persons who are not professional sportspersons, and spend most of their gait activity undertaking walking in normal shoes. The present study therefore focuses upon the influence of cushioning on everyday wear, something which may have implications in terms of lower back pain (Karahan and Bayraktar, 2004), osteoarthritis (Gill and O\'Connor, 2003), and other injuries.

Materials and Methods

Subjects
Two male and two female asymptomatic young adults (Mean age 23.25 years, Standard Deviation 3.2, n=4) volunteered to participate in this study, none of whom had reported history of trauma or disease to their lower limbs. Subjects had weight as assessed using Kistler Force Platform whilst attempting static stance Mean 736.84N (Standard Deviation 135.84, n=4). Shoe sizes were reported based upon subjects current footwear as mean 8 UK size (Standard Deviation 2.58, n=4). BMI values lay either within the normal (n=3) or overweight (n=1) ranges. No subjects were termed obese.

Footware
Subjects supplied shoes constructed of leather upper without heels. The shoes were those to which the subject was accustomed to wearing, to ensure results are indicative of long term use. For one experimental condition the supplied footwear was supplemented with a foamed polyester insole (Clarks Footwear, UK) cut to lay within the footwear selected. The insole consisted of parallel undulations at 8mm intervals perpendicular to the principle axis of the foot. The thickness ranged between 2.1 and 2.8mm.

Method
Each subject, in randomised order, undertook the study three times, once in barefoot, once wearing the shoes previously described, and once wearing both shoes and insoles.
Subjects were asked to walk a distance of 8 meters, across two centrally located Kistler Force Plates, upon which the subject weight had been previously determined. Subjects were instructed to look at an imagined point on the far wall, approximately at eye level. Subjects were monitored at all times visually, and inconsistencies in gait were recorded. Should the feet of the subject not be both fully incident upon the force plates, the starting position was modified by a distance no greater than 50cm such that incidence was achieved. The number of strides utilised, and the time taken to walk the 8m distance was recorded.
The walk was repeated as many times as necessary to obtain three repeats satisfying the acceptance criteria outlined below.

Acceptance Criteria
Only persons having more than 20 and fewer than 30 years were accepted for the study to avoid the influence of skeletally immature persons, and those who may be beginning to experience geriatric conditions, respectfully. Should the feet of the subject not be fully incident upon the force plate at any point within the study, that part of the study was repeated. If visual observation noted any discrepancy between initial gait and sequential repetitions, the subject was informed where required and the part of study repeated. If either the time to walk 8m or the number of strides in any given traversal differed by more than 10 percent of the initial value, this data was also rejected and repeated required.

Results

All subjects successfully completed all parts of the study, and the prerequisite number of acceptable results were obtained. The acceptance ratio was approximately 78%.


The peak GRF was calculated in each case and the means tabulated see Figure 2. A trapezoid approximation was used to determine the area beneath the GRF trace, giving the total impulse. The mean impulse values are displayed in Figure 1.

A two way ANOVA was conducted to evaluate whether the variations observed both in terms of Impulse and Peak GRF were statistically significant. In both cases different footwear conditions were statistically significant when tested at a 1% confidence interval.

Discussion

Figure 1 shows the total impulse associated with one step increased in all cases between barefooted and shoe-wearing persons. This is in agreement with the GRF data of Figure 2, with the exception of Subject 1. Subject 1 displayed a characteristic somewhat inverse to the rest of the study population, that of markedly decreasing GRF upon application of the insoles. Impulse itself is a complex measure, being the product of a great many factors, however, the cadence and mean velocity remained constant in all cases (p < 5% by single factor ANOVA).
Gill et al (Gill and O\'Connor, 2003) specifically identified the impulse as a risk factor for osteoarthritis, which makes the study of impulse a valuable undertaking. Cartilage degeneration is the principle characteristic of osteoarthritis, and is has been shown in an in vitro cell-seeded agarose model that chondrocytes suppress PGE2 and NO pathways, which are associated with the progression of osteoarthritis, when subjected to dynamic loading (Chowdhury et al., 2001). The influence of cushioning in increasing impulse may have implications in the modulation of these same pathways.
Gait is a highly complex process involving substantial neuromuscular control, both voluntary and involuntary. The way in which a person compensates a change in foot-ground interface is hence not entirely a biomechanical consideration. It has been reported that there is no statistically significant influence of prior knowledge regarding shoe construction on the GRF values recorded (Milani et al., 1997).
The study was not conducted as a blind trial, an omission which should be corrected in future studies, nor was the study population of a sufficient size to draw conclusions relating to the nature of GRF and impulse modulation via cushioning.

Conclusions

The presence of shoes acts to change GRF and impulse, and further padding further modulates change associated with the presence of footwear.
The average UK citizen walks in the region of 120km (HMSO, 2001) which implies that a minor change in impulse may well have lifetime implications in terms of accelerating the pathogenesis of arthritis, or other associated conditions.
It is the conclusion of the present study that cushioning is effective in modulating both GRF and Impulse during normal gait.

Reference List

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Fritz,M. and Peikenkamp,K. (2003) Simulation of the Influence of Sports Surfaces on Vertical Ground Reaction Forces During Landing. Med Biol Eng Comput 41, 11-17.

Gill,H.S. and O\'Connor,J.J. (2003) Heelstrike and the pathomechanics of osteoarthrosis: a pilot gait study. Journal of Biomechanics 36, 1625-1631.
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