Ergonomics, seating for mobile equipment

Sitting has many advantages over standing, ie the weight is taken off the legs, energy consumption is reduced and fewer demands are placed on the blood circulation. Today, operators and drivers of mobile equipment are part of about three-quarters of all workers that have sedentary jobs.

However, sitting has major drawbacks. Prolonged sitting causes strain on the spine and the muscles in the back. Both badly designed seats and poor ergonomical layout of controls are a major cause of backache and fatigue in drivers.

The design of seats in ordinary motor cars and heavy goods vehicles has generally improved over the years. Unfortunately, the seats in some industrial vehicles, such as earthmoving equipment, mobile cranes and forklift trucks are still far from satisfactory.

The most common cause of backache is disc injury. An intervertebral disc is a cushion which separates two vertebrae and gives flexibility to the spine. Bad seating can lead to the deterioration of  the discs, they become flattened and in advanced cases the viscous fluid may be squeezed out. This impairs the mechanics of the spine and allows tissues and nerves to be strained and pinched, causing backache, sciatic troubles and even paralysis of the legs.

In addition, sitting for a prolonged period of time will inevitably bring about a sensation of pressure beneath the buttocks and the upper thighs.  In order to ease the discomfort, one starts to fidget. One may cross and uncross the legs, thereby changing the distribution of pressure beneath the buttocks which helps to restore the blood flow to the areas of compressed tissue.  In time, fidgeting will bring less relief and, eventually, one has to get up and walk to alleviate the symptoms.

Although the ergonomics of sitting is much the same for operators of mobile equipment as it is for the office worker, there are three important reasons why the prevalence of backache is higher in drivers. These are discussed below.

Fixed Posture

Drivers may spend considerable periods of time in a fixed posture with relatively little opportunity for fidgeting. Prolonged sitting in a seat that offers little or no support to the back may increase the pressure on the invertebral discs with subsequent injury, whilst a poorly designed seat pan will increase the risk, the design of seats in mobile equipment should comply with the following requirements.

• The backrest should have an inclination of between 100º and 110º with the horizontal plane in order to reduce the compressive loading on the intervertebral discs. Adjustability, with the ability to lock in any position is desirable.

• A backrest height of 460 to 530 mm, vertically above the seat surface is required and the upper part of the backrest should be slightly concave.

• The backrest must have a well formed lumbar pad, which should offer good support to the spine between the third vertebra and the sacrum, at a height of 100-200 mm above the seat surface.

• The seat pan should measure 400 to 500 mm across and 380 to 420 mm from back front. The seat pan should be padded, with a slight hollow.  The front end turned upwards at an inclination of 4º to 6º will prevent the driver from sliding forward.

Poor Workstation Design

The good ergonomical design of the driver’s seat cannot be overstressed. However, in many instances problems may result from the demands of the task, or the layout of the controls, rather than from the seat itself.

In some cases, poor visibility may require the driver to sit forward, thus losing the support of the backrest. In other cases the driver has to sit on the edge of the seat in order to operate the control levers which also results in no back support.

A seat that is fully adjustable in the forward and backward position, up and down, as well as the backrest inclination may alleviate some of these problems and allow for the accommodation of a wider range of body dimensions in the user population. Even more important is the consideration of good ergonomical principles in the design of the total workstation.

Vibration

The driver’s back is exposed to vibration and impact, and to dynamic stresses during acceleration, deceleration and cornering. Vibration levels in ordinary motor cars are relatively low, whilst levels in trucks and buses tend to be higher. The highest levels are found in tractors and earthmoving equipment such as bulldozers.

There are a number of health effects that may result from vehicle vibration. The most important are degenerative changes to the spine. One study has shown that more than 70 % of tractor drivers in their twenties showed such signs, compared with 29 % of artisans (average age 41) and 37 % of building labourers (average age 51).

A vehicle seat may either attenuate or amplify the vibration produced by the vehicle. Most car, truck and bus seats may attenuate vibrations by 20 % or more. Unfortunately, many of the vehicles with the highest risk of vibration induced spinal injury have seats that are bolted, or welded to the structure of the vehicle. An attenuation of 50 % or more may be achieved by mounting the seat on its own suspension mechanism.

One may therefore conclude that sitting is a hazardous occupation at best. Sitting increases the pressure on the intervertebral discs with subsequent degeneration and backache.  In drivers and operators of mobile equipment the problem is aggravated due to the long periods of time spent in a fixed posture, poor ergonomic layout of the cabin and controls, as well as vibration. AirCHECK specialies in the identification of these problems and is able to recommend practical and cost effective solutions where relevant. Please visit our Website for our detailed scope of services.