ReviewImpact attenuation during weight bearing activities in barefoot vs. shod conditions: A systematic review
Highlights
► Few studies on impact attenuation of shoes compared shod condition to barefoot. ► Methodological variations cause discrepancies across literature. ► Shoes in general reduce axial tibial acceleration during walking and running. ► Shoes in general did not affect vGRF or loading rate during walking and running. ► Athletic shoes reduced loading rate during running.
Introduction
With the large amount of research into shoe design, particularly athletic shoes, it could be expected that a substantial body of scientific evidence would be available regarding impact attenuation of shoes compared to barefoot. Weight-bearing activities have a risk of injury to the lower limb through repetitive impact and inadequate recovery [1]. There is an assumption that shoes can provide impact attenuation for the body, protecting the lower limb from repetitive loading during running and jumping through altering the vertical ground reaction force [2] and rate of loading to which the lower limb is exposed [3]. Indeed it has previously been stated that the primary objective of running shoes is to reduce the initial vertical ground reaction force impact transient [4], however the benefit of repetitive loading on the lower limbs for increasing bone mass cannot be ignored [5], [6], [7]. Despite systematic reviews being regarded as the highest level of evidence [8], a systematic review of the effect of shoes on impact attenuation has not previously been undertaken.
The concept that shoes reduce impact force has been implied in shoe advertising, but in the past decade there has been much debate on whether weight-bearing activities should be performed barefoot or shod. A recent trend has seen the appearance of minimalist footwear designs with thinner, flexible shoe soles for the everyday and competitive runner [9]. Altering the shoe design appears to alter shock attenuation, however there have been mixed results across the field.
Common variables used in the literature investigating impact attenuation are vertical ground reaction force, loading rate (rate of loading of vertical force over time), axial tibial acceleration (acceleration along the axis aligned through the shank) and plantar pressures, across the literature the nomenclature for the variables varies, therefore our definitions will be used in this review to provide consistency. Vertical ground reaction force can be collected at the impact transient or the peak value out of the whole stance phase can be used for analysis. The impact transient can be identified by ‘the first vertical impact force peak’ [10] which results from ‘passive’ [11] impact.
The standard method to measure the shock absorbing qualities of footwear is to mechanically compress the shoe at the rearfoot and forefoot sections using a durometer [12]. The shoe can be given a rating of hardness and resistance to compression using an appropriate hardness scale, most commonly the Shore A (used for normal rubber) or the Asker C (used for soft rubber and insoles of differing materials scales) with higher scores given to harder and more highly compression resistant footwear [13]. However, this fails to consider the human interaction with footwear. The studies by Robbins and colleagues examined the technique of landing on shoe midsole materials of different hardness properties [14], [15]. When landing on unfamiliar or uneven surfaces the body will be cautious on the first exposure then adjusts accordingly on subsequent occasions [14], [15], [16], [17], [18]. The body can prepare the lower limbs for impact, adjusting joint stiffness in relation to the perceived hardness of the landing surface [14], [15], [19], but when wearing shoes plantar sensory feedback [18], [20] and proprioception [21] are limited.
The vertical impact transient [22], which occurs just after initial ground contact, is claimed to be the crucial component of the gait cycle [3] as it has an association with repetitive strain injuries in the lower leg [23]. However, Shorten et al. [24] have suggested that previous studies, investigating force attenuation in shoes, are not explaining the differences in vertical ground reaction force adequately. Frequency analysis reveals footwear only contributes a small amount to the impact force at heel strike and load components from more proximal segments could influence the reduction in vertical ground reaction force [24]. While shoes are only one potential factor that could contribute to injury, shoes have the ability to modify biomechanical variables that may assist with reducing injury rates.
Although a previous systematic review found no studies investigating the effect of shoes on injury risk [25] due to strict inclusion criteria, studies have found an association between loading rate [26], [27] and anterior tibial acceleration [28] and the development of musculoskeletal injuries. Proper footwear [29] and impact attenuating boot inserts [30] have been shown to prevent stress fractures through added force attenuation to protect the feet from impact forces. Also, changing footwear was found to be the greatest predictor of foot pain reduction [31]. Repetitive impact force and high loading rates are risk factors for stress fractures particularly if inadequate rest is given between bouts to allow for bone and soft tissue remodelling [32]. However, there is no scientific evidence to demonstrate a direct causal link between impact force and injury, also it is important not to disregard the beneficial properties of weight bearing activity and how repetitive impacts can improve bone mineral density [5], [6], [7] as long as dose and frequency are appropriate.
Many studies have examined shoes and their various effects, but in order to understand the absolute effect of shoes a comparison must be made with the barefoot condition. A systematic review of the current literature is needed to dispel the confusion surrounding the ‘barefoot vs. shod’ debate and discern the true impact attenuation characteristics of shoes compared to the barefoot condition during any weight bearing activity including walking, running and jumping. This review aims to clarify the effect of shoes on the most commonly used variables believed to be associated with impact attenuation to the foot and lower leg. Much of the literature is based on impact attenuation theory with the assumption that more compliant shoes will reduce vertical ground reaction force and delay the time at which the force peak occurs [24]. Hence, impact attenuation will be defined as the capacity to reduce the magnitude of vertical ground reaction force, loading rate, axial tibial acceleration and/or local peak plantar pressures.
Section snippets
Search strategy
A search for the literature was performed in the following databases: Cinahl (1982–September 2011), Cochrane Library (all years), Medline (1950–September 2011), PubMed (all years–September 2011), Scopus (all years–September 2011), SportDiscus (all years–September 2011), Embase (all years–September 2011), Eric (1966–September 2011), Web of Science (1899–September 2011), Ausport (all years–September 2011), AMED (all years–September 2011) and Google Scholar (all years–September 2011). There were
Results
Searching identified 336 studies and 27 studies met the inclusion criteria (Fig. 1). The details of the included studies identifying participant characteristics and shoe conditions tested are shown in Table 1. The shoe types investigated were running/athletic shoes (n = 24), Oxford shoes (n = 5), rocker soled shoes (n = 1), and five-finger shoes (n = 1) [39].
The quality of the studies had a median score of ten out of 14 and ranged from four to twelve (Table 2). Randomisation of the shoe conditions was
Discussion
Initial expectation was the literature search would yield a large amount of studies investigating the impact attenuation properties of cushioned shoes. However, only a small number of these studies actually compared shoes to barefoot, and few studies examined the effect of shoes on weight bearing activities other than walking and running. Despite highly repetitive impacts on the lower leg representing an injury risk and shoes regarded as the prescribed treatment, there is insufficient evidence
Conclusion
There is insufficient evidence to justify a strong conclusion on the beneficial effects of cushioned shoes. The level of scientific evidence required for treatments of other health issues, such as drug trials, needs to be matched in this area of research. Although some studies have found that shoes appear to significantly reduce the impact force on the lower limb during walking and running, the overall stance of the current literature does not support this. Shoes in general reduced axial tibial
Conflict of interest statement
The authors declare no conflict of interest.
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