Evaluation and validation of a new designed test apparatus to test safety shoes preventing calcaneal fractures

Status:

completed 03/2010

Aims:

The project aims to develop a new test protocol for safety shoes that allows feasible evaluations with the prototype test apparatus for the evaluation of impact loads in the rear foot of the project HVBG FF0221 (Prevention of Calcaneal Fractures). The project is a subsequent project of the precursor "Contributing to the prevention of stumbling, slipping and fall accidents" named "evaluation and validation of a test apparatus for safety shoes to prevent calcaneal fractures".

Activities/Methods:

First evaluations of the University work group and the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) led to mechanical optimizations of the test apparatus. The implementation of a damping spring resulted in impact characteristics (peak impact, time-to-peak impact) closely matching the characteristics measured and extracted in former studies with real test subjects in simulated accidental situations. Further evaluations examined the potential of a pressure measurement system (Medilogic®, Schönefeld) with an individually accustomed sensor matrix. The system was then acquired in a next project step. Several test series were executed to estimate the reliability and validity of the test situation.

Results:

Test-re-test measurements of ground reaction forces (GRF) resulting from impacts of the artificial leg confirmed a highly reliable test situation (ICC: 0,99; Test-re-test-variability: 1,67%). Maximum impacts of 5245 N ±195 and a time-to-peak from initial ground contact to the time of maximum GRF values of 24 ms ±3 confirmed fracture relevant impacts and showed the congruence to real-life measurements with work personnel. The evaluations of rear foot peak pressure measurements showed inferior values for reliability measures (ICC: 0,83; Test-retest-variability: 19,3%). This can be attributed to the difference in sensor technique (resistive sensors).
The test protocol allowed the differentiation of different shoe constructions with the main outcome measure rear foot peak pressure. Data from real-life measurements of simulated accidental situations with test subjects was re-evaluated to define a threshold value that shoes should not exceed in the presented test protocol. The rationale for this threshold value definition was to pool all data from simulated falls where subjects were fully aware of the falling and subsequent landing task assuming full active muscular protection. In these situations, the average rear foot peak pressure value was 42 N/cm² (upper 95% confidence interval: 45 N/cm², lower 95% confidence interval: 39 N/cm²). In accidental situations which occur unexpected, these active compensation mechanisms may not protect the subject from injury and therefore passive measures like the shoe should step in to protect the person. Therefore shoes should not allow rear foot peak pressures in these situations above 39 N/cm²).
From all test series, the following has to be concluded: One test height of 20cm seems to be sufficient. Five trials per single shoe seem to be sufficient. It is reasonable to test five single shoes of one shoe model to evaluate a representative range of one shoe model. All shoes have to be tested for the main outcome criteria and they should not exceed peak rear foot pressures (out of a falling height of 20 cm) of 39 N/cm². Every single shoe tested has to stay below this threshold value.
Finally a proposal for a change of the existing official test protocol EN ISO 20344 - EN ISO 20347 was developed. This proposal can be used to advance the existing protocol and can serve as a basis for the German Social Accident Insurance (DGUV) to submit a European wide change of the test protocol of rear foot loads in safety shoes.

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