Drivers Beware:
Lumbar Support Can Be Harmful To Your Health!

 

Armrests

by Dennis Zacharkow, PT
© 2015

In all animals other than humans, the shoulders and arms serve to support the trunk. However, with our upright posture on two feet, the arms are no longer required to support the trunk. Instead, a postural reflex mechanism of the trunk muscles has evolved to provide stabilization for independent arm movement.1

Unfortunately, from constant sitting in a position of postural depression, this postural reflex mechanism of trunk stabilization has broken down. As can be observed at most computer workstations, human posture has actually reverted to that of the animal kingdom, with the arms functioning to support the collapsed trunk! This is accomplished by weight bearing heavily on the armrests of the chair, the front edge of the desk, or the wrist rest of the keyboard (Figure 1).

The bad news here is that the increased weight bearing on a wrist rest in these frequently observed postures increases the pressure directly over the carpal tunnel region and the underlying median nerve, thereby increasing the risk of developing carpal tunnel syndrome.

In addition, armrests and wrist rests should not be used when operating the keyboard. The reason is that armrest and wrist rest use eliminates the participation of all muscle groups above elbow level in the keying movement.2 The resulting increased workload placed on the forearms and hands may lead to muscle damage and tendinitis characteristic of cumulative trauma disorders.3

Proper keyboard operation involves using the entire upper extremities, starting at the shoulder joints, in a slight forwards and backwards gliding motion over the keys. This is accomplished without any arm or wrist support. Proper movement of the mouse and other pointing devices also involves using the entire upper extremity, with the upper arm kept close to the side of the body.4 In tasks where mouse use predominates over keyboard use, the keyboard should be placed off center, and the mouse should be given the prominent position in front of the body.

The good news here is that the Posture Activators™ of Sacral and Lower Thoracic Support, by promoting proper activation of the abdominals (transversus abdominis) and proper support to the lower thoracic spine to stabilize the rib cage, facilitate proper keyboard and mouse operation without arm support!

Proper unsupported arm posture at the keyboard is with the upper arms vertical and the forearms slightly below horizontal5,6 (Figure 2). Compared to keeping the forearms either horizontal or above horizontal, a downward slope of the forearms (slightly below horizontal) reduces the stress to the neck and upper back.7

The downward slope of the forearms will also facilitate maintaining a healthy, straight wrist posture when typing.8,9 This is important for reducing the risk of developing carpal tunnel syndrome.

Another frequent site of nerve compression with keyboard operation is the elbow region, involving the ulnar nerve. This is called cubital tunnel syndrome. A downward slope of the forearm, along with keeping the upper arm vertical and the wrist straight, will reduce the stress on the ulnar nerve.10

Therefore, proper keyboard height with a downward slope of the forearms will be just above the thighs, a position much lower than traditionally advocated. The keyboard or keyboard tray should have a slight downward slope (approximately 10 degrees) to match the downward slope of the forearms11 (Figure 3).

Important Note: If you need to look frequently at the keyboard for feedback when typing, this lowered keyboard position may increase the stress to your neck and upper back.8 The best compromise will then be a slightly higher keyboard and forearm position.

Although armrests on a chair should not be used when typing, they do have other important functions:

  1. They reduce stress to the hips, knees, and back when getting in and out of the chair.
  2. They provide intermittent support when taking rest breaks from typing.

References

  1. Keith, A.: Man's posture: its evolution and disorders. Lecture four. The adaptations of the abdomen and of its viscera to the orthograde posture. The British Medical Journal, 1: 587-590, 1923.
  2. Parsons, C.A.: Use of wrist rests by data input VDU operators. In Lovesey, E.J. (Ed.): Contemporary Ergonomics. London, Taylor and Francis, 1991, pp. 319-322.
  3. Pascarelli, E.P., and Kella, J.J.: Soft-tissue injuries related to use of the computer keyboard. Journal of Occupational Medicine, 35: 522-532, 1993.
  4. Harvey, R., and Peper, E.: Surface electromyography and mouse use position. Ergonomics, 40: 781-789, 1997.
  5. Straker, L.M., Pollock, C.M., and Mangharam, J.E.: The effect of shoulder posture on performance, discomfort and muscle fatigue whilst working on a visual display unit. International Journal of Industrial Ergonomics, 20: 1-10, 1997.
  6. Mosher, E.M.: The influence of habitual posture on the symmetry and health of the body. The Brooklyn Medical Journal, 6: 393-414, 1892.
  7. Avon, G., and Schmitt, L.: Electromyographie du trapeze dans diverses positions de travail a la machine a ecrire. Ergonomics, 18: 619-626, 1975.
  8. Stack, B.: Keyboard RSI: The Practical Solution. Hobart, Tasmania, Muden Publishing Company, 1987.
  9. Werner, R., Armstrong, T.J. Bir, C., and Aylard, M.K.: Intracarpal canal pressures: the role of finger, hand, wrist and forearm position. Clinical Biomechanics, 12: 44-51, 1997.
  10. Pechan, J., and Julis, I.: The pressure measurement in the ulnar nerve. A contribution to the pathophysiology of the cubital tunnel syndrome. Journal of Biomechanics, 8: 75-79, 1975.
  11. Hedge, A., and Powers, J.R.: Wrist postures while keyboarding: effects of a negative slope keyboard system and full motion forearm supports. Ergonomics, 38: 508-517, 1995.

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