You Asked for It
June 2003

Question:

Hi Sharon,

I read your article Just Right in the Canadian Occupational Safety magazine. Congratulations! (Also read in your newsletter that you are now certified- not to be confused with certifiable! -congrats on that achievement as well!) I was wondering what you were referring to when you mentioned "palm support/pad" to support the heel of the hand and a flat wrist when using the mouse.

Are you referring to a mouse 'wrist rest'? We have found that these increase anchoring and a 'windshield wiper' action at the wrist and have discouraged them, but have not found too many studies on this topic one way or the other.

If you could share your thoughts on this we'd appreciate it. Also, have you come across any studies on the topic of wrist pads for use at the mouse?

Sue
Ergonomics Consultant, Ontario

Answer:

I know the issue of support for mousing is contentious. I would be thrilled if the workers could achieve arm support through the chair, but the unfortunate reality that most of my clients face is that the arm rests of the chair are too far apart (unless they have a Steelcase or Haworth chair where the arm rests move in over the seat pan). Because of the amount of control required to the do the task, a point of contact is required so that the large muscle groups of the shoulder and upper back (intended for gross motor control) do not have to continuously stabilize the arm - resulting in static contractions.

So, there are two alternatives in my mind.

1. Support the arm, under the fleshy part of the arm, with the upper arm hanging at the side of the body and the shoulder relaxed. This requires an investment in a high end chair such as the Leap, Drive or Improv if the arm rests need to move in over the seat pan. Despite having support at the arm rest, I still find that people will instinctively seek support at the level of the hand. This results in them dropping the wrist to the desk surface, with the resultant extension pulling on the extensor tendons while applying direct pressure to the carpal tunnel. From this extended (and locked) position, the workers then use the same windshield wiper motion that you describe. (I usually describe mousing as a fine motor task... one which you cannot complete successfully with the eyes closed. A colleague recently challenge me on that statement. She countered that then, successfully swinging a baseball bat could be considered a fine motor task - it could not be done with the eyes closed. To a degree she is right - but the minute adjustments to speed, pitch, angle, etc. are made long before the bat connects to the ball. At the time of completing the task, the eyes could very well be closed. At the time of task completion, it is a gross motor task. )

2. Support the hand so that the wrist remains flat so that the tendons can work in a straight line. This is where I use the term "palm support". I use the word "palm" instead of wrist, because I want people to place the heel of the hand on the padded surface. I find that changing the word helps people to understand how the tool should be used. When the wrist is supported, there is still potential to apply pressure to the carpal tunnel. When the palm is supported, I find that they are more likely to move their hand. (Also, at the keyboard I use the term "palm rest" rather than "support" so that they will get the idea of using the device for resting on rather than as a means of support.)

So, in the end, I agree with you to a point. Locking the wrist onto a palm pad is bad because it promotes using the mouse with radial and ulnar deviation. But if used with the palm of the hand in contact, workers may find that they have the level of support required for the complexity of the task and still gently move the hand over the surface of the palm pad. As with most tools, giving the worker an explanation of how to use it correctly goes a long way.

As for studies... I mostly write from my experience and exposure to people's habits in the workplace and go back to basic principles of motor learning and biomechanics. I did however attend a short seminar put on by Peter Kier (York University) in which I specifically asked him about supporting the hand for mousing. He confirmed that supporting the heel of the hand reduced pressure in the carpal tunnel (compared to allowing the wrist to fall into extension and contact the desk surface). I know he did write it up and the study was published in a journal, but don't recall where.

I hope that helps somewhat. If nothing else, maybe some grad student will take on the task of trying to prove me wrong!!

st

(NOTE: the inquirer is going to ask her contact at York if he can recall where the article was published, but if any other readers are aware of this study, please let me know.)

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