Insufficient acceptance of safety shoes in the workplace challenges optimization of this footwear category (18). Prejudices against safety footwear are inadequate comfort, little cushioning abilities or high weight. Moreover, overuse injury is directly related to poor quality or function of safety shoes (1, 10, 14, 24).
Over-use injury at work like tendinopathy are evoked by repetitive overload19. Inappropriate footwear might contribute to this development. Optimization of safety footwear has therefore the potential to reduce injury development and to increase wear compliance and safety of workers.
This eventually leads to less expense in health care5, 8. Refinement of footwear is thought to be achieved with inserts3, 21. Many studies on orthotic inserts with enhanced cushioning properties show prospectively an apparently reduced rate of overuse injury16, 21. Admittedly, many studies are uncontrolled and do not follow evidence-based criteria.
A recent Cochrane review on the efficiency of interventions preventing lower limb soft tissue injury shows no evidence for insoles to be able to reduce the aforementioned type of injuries25. Methodological problems like outcome measures based solely on the subjective evaluation of subjects, the absence of control groups or a short followup period are major reasons for confounding results. Objective measures of biomechanical shoe characteristics (acute and longterm) and how they may or may not influence the safety shoe wearing subject in workplace settings have not been studied extensively.
Depending on work place demands, high impact situations like the stepdown from a height higher than a normal step require specific properties of safety footwear. Muscular activity of joint stabilizing muscles before initial contact provide preparatory tendomuscular stiffness in step-down and landing movements to adequately absorb external load and to regulate stiffness during ground contact15.
This control mechanism is thought to be centrally pre-programmed9. Footwear is required to not alter these body-inherent compensation mechanisms for optimal shock absorption. Therefore footwear modifications like changes of impact cushioning have to be evaluated regarding mechanical alterations at the foot-shoe interface as well as regarding muscular compensation mechanisms of shoe wearers. Plantar pressure distribution measurements as well as surface electromyography offer probate methods to analyze these changes over time2, 22. The purpose of this study was therefore to investigate plantar pressure distribution and pre-impact muscular activity of workers wearing two different safety shoe models in a step-down movement simulating a work place situation over a period of 6 months.
48 workers were recruited for this study. All subjects agreed to take part in the study voluntarily and signed an Informed Consent Statement conforming to the principles of “Good Clinical Practice (GCP)” outlined in the Declaration of Helsinki7. The study was approved by the ethics commission of the local University. Subjects served either as warehousemen in a wholesale of electrical installation material or as control personnel of a combined heat and power plant. Both workplaces required the use of safety shoes (category S1 according to the European Safety Standards EN344- 345: “shoes to protect wearing subject from injury: characteristics: strong upper, Steel toe cap, shoe sole flexible and not puncture-resistant) and included a high amount of standing and walking (70% of work shift). No acute or chronic injuries or complaints of the lower extremity were reported by the subjects at time of study inclusion. All subjects were randomly assigned to either a prototype shoe group (P: age: 44±13 years; height: 1.78±0.05m; weight: 80±11kg) or a standard shoe group (S: age: 41±13 years; height: 1.76±0.07m; weight: 79±15kg). The S group was equipped with a standard safety shoe with a combined mid and outer sole out of polyurethane foam (PU, shore 50) with a leather upper and a steel toe cap (Elten®, Uedem, Germany). The P group was endowed with the same safety shoe with modifications (Elten®, Uedem, Germany). The modifications consisted of an additional 3mm plastazote (shore 35) cushioning layer under a flexible suede insole. To further enhance cushioning properties, compression molded inserts out of plastazote (shore 35) with a three-dimensional shape (molded to a standard foot shape) provided cushioning under the complete plantar surface. For S seven dropouts were registered. All of them reported insufficient cushioning of shoe S and refused to further wear their assigned footwear. Out of group P, three dropouts were reported. Two of the subjects changed their job position and one subject reported insufficient fit of his shoe in the toe area and refused to further wear shoe P.
After emission of safety shoes, all subjects performed a two week accommodation period to rule out insufficient adaptation to the shoes for the first measurement. Subjects wore the new footwear during all work days for the time of their shifts. Following the accommodation phase, a first biomechanical measurement was applied. After preparation of measurement equipment, (see below) all subjects executed five steps down from a stage (height: 0.29m, respectively the height of two Europool-paletts (height: 0.144m, length: 1.2m, width: 0.8m: standard pallet of the European transport system according to the European Pallet Association EPAL) with their preferred leg in their assigned footwear (P or S) and barefoot (BF). The order of barefoot and shoe condition were randomly selected.
Subjects were briefed to step with the first step to the edge of the stage (Fig.1, I.) and the second step had to be the step-down movement with an initial heel touchdown (Fig.1, II.). The resulting step length was between 0.35-0.55m. After two more steps, trials were completed (Fig.1, III.). Trials were repeated till 5 successful trials have been recorded. This situation was thought to be an artificial but standardized test to simulate a typical load situation of the job requirement of warehousemen Afterwards subjects were asked to fill out a comfort rating form. Then all workers had to continue to wear the assigned footwear for the next 6 months. Continuous wear was controlled weekly by workers’ supervisors.
The biomechanical measurements and comfort ratings were repeated three (measurement M2) and six months (measurement M3) after the initial test (measurement M1). Data from comfort ratings was not a primary outcome measure and was used only for detection of drop out reasons.
Data acquisition and analysis
Plantar pressure distribution was recorded at the interface between foot and shoe/ground using a Pedar Mobile® System (Novel®, Munich, Germany; 50Hz, 1 sensor per 0.0002m2 resulting in 99 sensors per measurement insole). The sensors were calibrated using the manufacturer’s air bladder technique (Trublu Novel®, Munich, Germany) in a range from 0 to 600 kPa intended to be used in gait analysis. All steps with initial heel contact were visually controlled for plausibility and then averaged to get one representative step for each condition (barefoot and shod).
“48 workers were recruited for a study, S group was equipped with a standard safety shoe, P group was endowed with the same safety shoe with modifications”
Peak pressures in the rear foot and fore foot (using Novel®’s 4-mask standard division dividing the foot in a toe region, fore foot, mid foot and rear foot) were calculated from average steps. Data processing was made using the manufacturer’s software packages pedar-x expert® (vers. 10.2.24), novel projects® and novel multimask® (vers. 12.3.34) (Novel®, Munich, Germany). Muscular activity was measured using bipolar surface-electromyography (EMG) (EISA, Freiburg, Germany, measurement frequency: 1000Hz; pre-amplification of signal directly after signal deduction in shielded cables; amplification in A/D converter with a total gain of 1000) of the Musculus tibialis and Musculus gastrocnemius lateralis.
Preparation of the muscles was done by the method described by Winter and Yack23. Before the electrodes were applied (disposable electrodes, AMBU Medicotest®, Denmark, Type N-00-S, interelectrode distance: 2cm), the skin was shaved and slightly roughened to remove surface epithelial layers. In addition, skin resistance was controlled (<5kΩ). Heel contact was detected with a light grid (Hellack®, Rottenburg, Germany) in place of the step-down area and an analogue signal of the light grid was collected with the A/D-converter to synchronize EMG-Signals with the step-down movement.
Signals were full wave rectified and visually inspected for artefacts. Preactivity comparison and the detection of possible influences of footwear on barefoot and shod foot strike pattern were realized by processing 150 millisecond intervals before foot contact from 5 trials for mean activity.
Those trials were averaged to get a representative muscular activity pattern for each condition6. For intersubject Article | Safety Footwear comparability of the preactivation amplitudes (-150ms to touchdown) all amplitudes before touchdown were normalized to the mean amplitude of the entire movement (150ms before touchdown to 500ms after touchdown).
Statistics included descriptive analysis with presentation of means and corresponding 95%-confidence intervals. In addition two-way repeated measures ANOVAs (factor 1: group P/S, factor 2: Measurement 1, 2, 3) for peak pressure in the rear foot as well as in the fore foot were calculated. A Bonferroni correction with a resulting α-level of .025 examined group differences at the probability error level of 5%. Analysis of group differences (P/S) per measurement day was made by a post-hoc-test (Tukey Kramer). These mean comparisons were corrected (Bonferroni) with a resulting α-level of 0.01 to test on the 5%-level.
The comparison of barefoot data between P and S showed no differences, due to random allocation of subjects (p>0.05). Therefore, further presentation of data took place with a combined view of barefoot data, since differences between shod conditions were the main focus of interest. The analysis of peak pressure values in the rear foot area showed a reduction in peak pressure values in both shoes compared to barefoot (p<0.05), but no differences were measured between the two shoes during the whole study period (M1, M2 and M3) (Fig.2). Analysis of peak pressure values in the fore foot area revealed a reduction by P (-20 to 25%, p<0.01) compared to BF and S, whereas BF and S showed identical values. This reduction of peak pressure remained constant throughout the whole observation period (Fig.3).
A possible influence of altered cushioning properties on the muscular system at foot strike was analyzed by measuring the preactivation EMG of the M. tibialis anterior and M. gastrocnemius lateralis. Both muscles are performing a co-contraction right before touchdown to stiffen the ankle joint and to absorb impact at heel contact. The different shoe constructions did not result in altered pre-setting of both muscles. Preactivation amplitudes of both muscles were only slightly higher in S (p>0.05), whereas P demonstrated the same values than barefoot EMG at all measurements (Fig.4 and Fig.5). Overall normalized amplitudes in preactivation were lower (20-25% of mean M. tibialis anterior activity, 10-15% of mean M. gastrocnemius lateralis activity) than the mean activity level of the whole movement. The main activity of both muscles occurred after initial contact during stance.
The goal of this study was to investigate plantar pressure and muscular activity in a high impact situation in workers wearing two differently constructed safety shoes as well as the long term evaluation of possible differences.
The chosen experimental situation had its limitations in a way that the step-down movement required an initial heel touch down. This was chosen to standardize the situation but resulted in a quite artificial movement. Normally, a first contact with the fore foot is realized as soon as a certain height is reached during step-down movements. Nevertheless, this movement was chosen to guarantee a standardized test exercise.
Rear foot peak pressure was not reduced by shoe P with a prototype cushioning construction. Although enhanced cushioning was already proven with this footwear modification during walking movement (4). Similar peak pressure values in P and S might be due to the severity of impact in the step-down movement with initial heel contact. The cushioning properties were able to reduce impact compared to barefoot, but no further shoe differences could be revealed. The step-down height of 0.29m probably led to a penetration of the impact body through the material resulting in similar impacts (17).
On the other hand, body inherent compensation mechanisms might have led to similar peak pressures. Admittedly, this body inherent compensation was not seen in pretouch down activity of the analysed muscles. It was shown in literature that hard surfaces lead to higher muscular activity, compensating for the hard surface while long-term standing on different surfaces (13). Changes in midsole material and construction did not result in changes of pre-impact EMG. Preparatory cocontractions of joint stabilizing muscles are used to provide appropriate tendomuscular stiffness for landing and to protect passive structures during impact (11, 20). This pre-programmed motor control is thought to be influenced by afferent feedback (9). The described shoe modifications seemed not to provide changes in afferent feedback because of the absence of changes in muscular activity levels. It can be concluded that enhanced fore foot cushioning did not result in changed motor control and did not afford higher muscular work. The longterm evaluation showed a stable performance of the midsole material throughout the whole observation period. A descriptively evaluated peak pressures reduction in P as well as in S on measurement 3 implied a slight breakdown of midsole material after six months.
Similar results are shown in literature only for insert materials but not for shoe elements. Insert materials (also plastazote) have shown long-term cushioning abilities for up to 12 months (12). It can only be speculated if the prototype footwear in this study has preventive potential. The high drop out rate (n=7 out of 24 in group S) might point out insufficient compliance in the group with common safety footwear. All drop outs named inadequate cushioning for their refuse to further wear their assigned footwear. Here, the new construction can at least contribute to a better wear compliance. The preventive effect of enhanced cushioning shoe properties on the development of overuse complaints and injury would require prospective studies with sufficient subject numbers (25). Hence, the question of possible preventive effects of the tested optimized safety shoes remains open.
The study showed that using adequate material, cushioning of safety shoes can be enhanced. Especially fore foot peak pressure can be reduced. Therefore cushioning on the whole plantar surface has to amend the traditional interest of heel impact cushioning. These results were demonstrated not only during acute testing but also during long term evaluation up to six months. Improved cushioning was not associated with an increase in muscular activity. It is concluded that optimized safety footwear has the potential for better wear compliance in workers and eventually may help to reduce health care costs.
This study was supported by Schuhfabrik van Elten GmbH, Uedem, Germany. Subjects were recruited from J.W. Zander GmbH & Co. KG, Freiburg, Germany and from the combined heat and power plant (CHP) of the University Hospital Freiburg, Germany.
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Published: 01st Jan 2010 in Health and Safety International