Better By Design

Biomechanical aspects of safety shoes

Injuries to the upper ankle joint caused by stumbling, slipping and falling continue to be an unsolved problem in all areas of work resulting in high medical costs and a considerable loss of working hours. Work shoes are always being improved so that injuries can be prevented.

The knowledge of the position of the axes of movement and their interrelationship leads to a better understanding of the functional anatomy and kinematic action of the foot. This understanding can be used in the development of sports, leisure and work shoes.

A new concept for shoes has been developed at the GUT Clinic in which the movement axis of the shoe shaft has been adapted to suit the anatomical movement axes of the foot, in an effort to protect the external (fibular) ligament complex and make the shoe more comfortable.

Standards and guidelines have permanently changed occupational health and safety over the last century. Personal protective equipment is now available to every worker. In addition, innovations in recent years have led to a further minimisation of the accident figures. Figures from the Employers’ Occupational Liability Insurance (BG) from the year 2000 show, however, that more than half of all foot injuries from accidents subject to reporting (107,973) are caused by stumbling, twisting, slipping or falling.

Foot injuries are not restricted to specific professional groups. Foot injuries occur in all occupations insured by the BG:

• Transport 17%
• Construction 12%
• Mechanical engineering 9%

A high percentage of injuries such as sprains, fractures and torn ligaments affect the upper ankle joint.

Work accidents are also of a high socio-economic significance. They cost several billions of euros in medical care and continued wage payments every year. Facts and figures underline the necessity to look for solutions to develop effective protection against twisting in work shoes. The following questions arise in this context:

Mechanism of ankle joint injuries?

• Which biomechanical principles apply to the movement of foot/calf?
• Can a shoe design contribute to avoiding ankle joint injuries?
• What would a shoe design of this type look like?

Biomechanics and anatomy

The foot is a so-called multi-joint system, consisting of several large and small joints. These enable the foot to adjust as best as possible to uneven ground. The most important ankle joints are the upper and lower ankle joints. The upper ankle joint serves to lift and lower the foot (plantarflexion and dorsiflexion): the lower ankle joint turns the foot outwards (pronation) and inwards (supination).

We believe that the following biomechanical and anatomical principles are important to understand the development of the shoe:

• 1.Figure 1 shows a top view of the right foot. The diagonal line shows the functional axis (Henk’s axis) of the lower ankle joint which is turned approximately 20-30° to the body’s centreline. The anatomical structures of the ligaments (red circle) of the ankle joint are vertical to this axis and not to the foot.
• 2.Up to now it has been assumed in simplified terms that the axis of rotation of the calf is at a right angle to the longitudinal axis of the foot. However, when the knees are bent the knee joints move inwards to body centre (Fig 2).

Mechanism of the ankle joint injury

An ankle joint injury is caused by rotation of the forefoot around the functional axis of the lower ankle joint. If the foot is twisted the forefoot usually gets caught. It moves around the axis of the lower ankle joint until the maximum movement amplitude has been reached. This produces a lever effect on the ligaments and bone structures stabilising the upper ankle joint. If the force is large enough the structure will tear.

Demands on a work shoe

In addition to a functional protection from twisting to avoid ankle joint injuries, work-specific demands on the work shoe must be taken into consideration. These are reflected in different classes, such as S1, S2, etc.

Work processes are becoming more complex in the technical world and increasing demands placed on the concentration of the workers. Uncomfortable shoes can indirectly impair work performance.

A feeling of well-being can promote or maintain work performance.

The weight of a shoe and the foot climate play an important role in the acceptance for wearing work shoes. This can be guaranteed by use of functional materials. In addition, an appealing contemporary design has increasingly been taken into consideration in recent years.

Only when work shoes are worn can they provide appropriate protection.

Implementation of biomechanics in shoe design

When designing workshoes the natural movement axis of the ankle joints and of the calf should be taken into consideration.

In shoes with high shafts, folds typically develop at the bottom of the shoe shaft after the shoe has been ‘worn in’.

A loosening of the laces and the turning of the tongue outwards are known phenomena of shoes with high shafts. This is caused by the functional movement axis of the calf already mentioned in Point II.2. The symmetric shoe is deformed into the natural movement direction (to body centreline) by the movement of the calf. Over time this leads to a loss in stability of the shoe shaft due to material fatigue (Fig.3).

The new shoe concept is based on adjusting the shoe shaft axis to the functional movement axis of the calf.

The shoe shaft is enclosed by a flexible plastic cuff with a diagonally positioned axis (Fig.4).

The pivot point of the rotational axis is pushed forward on the outer side of the shoe and back on the inner side of the shoe.

The rotational axis of the cuff is therefore at a right angle to the rotating axis turned 20° inwards (Henk’s axis) and is now protectively positioned around the ligaments. This also produces a shorter lever of the forefoot on the rear part of the foot and results in a reduction in the force applied to the outer ligaments of the ankle. The diagonally positioned axis causes the plastic cuff to follow the natural movement of the calf. This enables the knee to make the anatomically designed inward movement without material resistance at the calf during bending and stretching.

As a result the shoe shaft maintains its stability and the shoe has a better rolling behaviour due to an optimum fit; this is also intended to lead to less fatigue. Slots at the pivots of the plastic cuff enable the cuff to be moved up or down (approx. 5mm), permitting the adjustment to bandy or knock knees.

Permanent stability in the shoe shaft is a necessity to counteract twisting of the foot. This may not impair comfort for the wearer. In the solution described here the natural movement of the foot is taken into consideration without additionally impairing the muscular activity of the foot. This counteracts a so-called ‘stress-shielding’ of the muscles which can lead to muscular instability with increased risk of injury, e.g. during leisure time when no stabilising shoes are worn.

However, the effectiveness of any such system will only be shown in long-term randomised field studies.

The demand for innovative functional personal protective equipment exists:

Sick leave of an employee in the car industry, for example, costs the company 500 euros a day and thus far much more than the price of a high quality safety shoe.

The investment in high quality and functional personal protective equipment is not only a good idea in larger companies, but also in small and medium-sized enterprises because skilled workers cannot be replaced directly due to the specific requirements placed on the workplace.

Other injury prevention measures

Unfortunately no worker is immune to foot injury. However, the hazards differ according to the workplace and the types of tasks the worker does. The first step in reducing foot problems is to identify the relevant hazards at the workplace. Such hazards should be assessed no matter how safe or dangerous it may seem.

What should I know about footwear?

Proper footwear is important, not only for foot comfort but also for one’s general well-being. Improper footwear can cause or aggravate existing foot problems. Being fashionable sometimes takes precedence over choosing well-fitting, supportive safety footwear. Although, many safety footwear manufacturers produce safety footwear that does look fashionable.

What should I know when I buy footwear for work?

Good footwear should have the following qualities:

• The inner side of the shoe must be straight from the heel to the end of the big toe
• The shoe must grip the heel firmly
• The forepart must allow freedom of movement for the toes
• The shoe must have a fastening across the instep to prevent the foot from slipping when walking
• The shoe must have a low, wide-based heel; flat shoes are recommended

General advice when purchasing footwear

• Have both feet measured when buying shoes. Feet normally differ in size
• Buy shoes to fit the bigger foot
• Buy shoes late in the afternoon when feet are likely to be swollen to their maximum size
• Ask a doctor’s advice if properly fitting shoes are not available
• Consider using shock-absorbing insoles where the job requires walking or standing on hard floors

When selecting footwear, one should remember that tight socks can cramp the toes as much as poorly-fitted shoes. Wrinkled socks, or socks that are too large or too small, can cause blisters. White woollen or cotton socks may be recommended since coloured socks cause skin allergies in some people.

What should I know about protective footwear?

The role of personal protective equipment is to minimise exposure to specific occupational hazards, not to eliminate them. Protective footwear does not guarantee total protection.

All working footwear, for both men and women, should provide comfort without compromising protective value.

Foot related injuries

Every day, 400 people injure their feet at work. National Safety Council.

66 percent of injured workers were wearing protective footwear, and 33 percent regular shoes, of those wearing protective footwear, 85 percent were injured because the object hit an unprotected part of the shoe or boot. Bureau of Labor Statistics

Hospital Episode Statistics, Department of Health, England, 2002-03.

• 90 percent of the injuries to ankle and foot required hospital admission in England 2002-03
• Many of these injuries are presumed work related. 60 percent of people visiting hospitals for injuries to the ankle and foot were men in England in 2002-03
• Six days was the mean length of stay in hospitals for injuries to ankle and foot in England 2002-03
• 90 percent of hospital episodes for dislocation, sprain and strain of joints and ligaments of ankle and foot required hospital admission in England 2002-03
• 100 percent of hospital episodes for injury of nerves at ankle and foot level required hospital admission in England 2002-03
• 67 percent of hospital episodes for injury of blood vessels at ankle and foot level required hospital admission in England 2002-03
• 95 percent of hospital episodes for injury of muscle and tendon at ankle and foot level required hospital admission in England 2002-03
• 96 percent of crushing injuries to the ankle and foot required hospital admission in England 2002-03
• 5.8 days was the mean length of stay in hospitals for crushing injury of ankle and foot
• 30 was the mean age of patients hospitalised for crushing injury of ankle and foot.
• 75 percent of hospital episodes for traumatic amputation of ankle and foot were men
• 6.7 days was the mean length of stay in hospitals for traumatic amputation of ankle and foot

Every year 22,000 people in the UK miss work for more than three days owing to foot injuries in the workplace. British Footwear Association

• A steel toe cap should cover the whole length of the toes from tips to beyond the natural bend of the foot. A soft pad covering the edge of the toecap increases comfort. If the toecap cuts into the foot, either the size or style of the footwear is incorrect
• Soles come in a variety of thicknesses and materials. They need to be chosen according to the hazards and type(s) of flooring in the workplace
• Uppers of protective footwear come in a variety of materials. Selection should take into account the hazards, and individual characteristics of the worker’s foot
• A steel midsole which protects the foot against penetration by sharp objects should be flexible enough to allow the foot to bend
• No one type of non-slip footwear can prevent the wearer from slipping on every surface type

Canadian Centre for Occupactional Health & Safety

For more information on Safety Footwear click here http:// http://www.osedirectory.com/product.php?type=health&product_id=18

Published: 01st Jan 2006 in Health and Safety International