Wrist Rest Primer

Wrist Rest - An Ergonomic Primer
By David Gilkey, D.C., Ph.D, CPE
Director, Environmental & Radiological Health Sciences
Colorado State University

1)    Introduction

Wrist rests facilitate the support of the forearms when inputting data into computers.  Some computer users prefer wrist support during data entry and other users do not.  Data reveals that as much as 50% percent of users may prefer wrist support during computer use.  A well-designed workstation may be equipped with a wrist rest if it’s the user’s preference.  It is presently estimated that 45 million American workers spend some time each day using a computer and keyboard.  Approximately 30 million workers use the computer, keyboard, and pointing devices as their primary work equipment each day, all day, and up to 8 hours per day or more.  Computer use has been linked to several types of injuries known as “Upper Extremity Repetitive Stress Injuries” (UE-RSI's), “Cumulative Trauma Disorders” (CTDs), or “Work Related Musculoskeletal Disorders” (WRMSDs).  RSIs, CTDs, and WRMSDs are associated with the upper extremities (UE) or arms, forearms, wrists, hands, and fingers.  Common UE –RSI disorders associated with computer input devices include Carpal Tunnel Syndrome (CTS), Tendonitis, and Tenosynovitis effecting the hands, wrists, and forearms as well as Neck Tension Syndrome.  The Bureau of Labor and Statistics (BLS) reported that the incidence of such disorders has increased 770% in past decades (BNA, 1995).  Several studies have clearly shown a definitive link between UE-RSIs and computer use (NIOSH, 1997).  Ergonomics is the premier science, which concerns itself with humans at work and the many aspects of the “Human Computer Interface” (HCI).   Studies clearly demonstrate a scientific basis for ergonomic design of computer workstations.   The following is an overview of ergonomic issues related to wrist rests.

Definitions and Terms:
A)    Wrist rest – This equipment is designed to assist in the support of the weight of the forearm during computer use.  A wrist rest may be built into the keyboard, tray, computer desk, or added aftermarket to the workstation layout.

B)    UE Support  - This is the shared load transferred to the wrist rest and relieved from the muscles of the UEs.  Support can vary depending on the amount of contact pressure permitted by the user.

C)    Contact Pressure – This describes the pressure in “Pounds” or “Newtons” resulting from contact of forearms or wrists to the edge of the work surface or wrist rest.

D)    Static Loading – This is contraction of muscles to hold one’s body position without much motion.  Holding UE in the fixed position for computing is considered static loading. This is in contrast to dynamic loading, which is muscle contraction with good motion such as in walking. 

E)    Impaired circulation – This describes the loss of normal circulation to the hands and fingers resulting from direct contact pressure.

F)    Spinal Unloading – To decrease the weight on the spine.


3)    How Ergonomic Wrist Rests Address These Issues:

A)    The use of a wrist rest is completely an individual preference.  Studies suggest that approximately 50% of computer users enjoy the benefits of wrist rests.  Ergonomic wrist rests are designed to distribute weight from the UE over a broader area for those wishing to have support of upper limbs during computing.  Studies have shown that wrist rest users benefit by direct support of the UE weight, spinal unloading, and decreased muscle activity in shoulders.  Ergonomic wrist rests are often made of “hard foam”, “rubber” or “Jelly-like” substances that do not have sharp edges and will conform to and broaden the contact area.  Wrist rests may be designed straight, curved, or flexible to conform to keyboard design user’s need.  Some wrist rests are built into the keyboard, computer table, or added after the system is purchased.  The keyboard tray or typing table must have adequate room to place the writs rest near the keyboard.  The benefits of wrist rests include prolong endurance, increased comfort, faster recovery, and reduced risk of injury. 

B)    In previous studies ergonomic wrist rests have demonstrated their ability to bear between
15 N (or 3lbs.) to 35 N (or 8 lbs.) of the weight of the UE.  The reduced load to the shoulder muscles results in less fatigue and more comfort during keying.  Electromyography (EMG) of the UE and shoulders readily demonstrates that the muscles do not work as hard when wrist rests are used compared to when they are not.  Users do not report performance decrement.  The benefits of decreased muscle activity results in prolong endurance, increased comfort, faster recovery, and reduced risk of injury. 

C)    Ergonomic wrist rest is designed to broaden and soften contact areas minimizing focused pressure to the skin and blood vessels that might cause injury or impaired circulation.  Forces as small as one ounce per square inch have been shown to adversely affect circulation and predispose injury to UE.  Ergonomic wrist rests are designed to avoid contact related stress while supporting the weight of the UE.

D)    Supporting any portion of the UE will decrease the static load applied to shoulder muscles.  Static loads above 15% of maximum contraction may predispose the muscles to fatigue, breakdown, and injury.  A reduction of 3 lbs. to 8lbs. in forearm weight results in substantial reduction of shoulder stress.  Ergonomic wrist rests aid in the support of UE and reduce injury potential.

E)    Ergonomic wrist rests aid in maintaining normal circulation by decreasing static load to muscles contracted to lift and hold limb position during computing.  Muscles that contract vigorously quickly use their energy supply and starve for more oxygen and sugar.  Using a wrist rest reduces the amount of contraction necessary to hold the limb in position thereby reducing the use of oxygen and energy. 

F)    Studies suggest that supporting the weight of the UE results in decreased stress to the low back.  The low back supports the entire upper body weight and is also susceptible to static loading.   Shared loading by wrist rests adds to relief from static loading in the spine.

Summary:

A)    Wrist rests are a personal preference.  The data suggest there are several benefits including increased comfort, decreased muscle contraction, and static loading of the shoulders and spine.  Ergonomic designs optimize human interaction and preserve health and well-being.  Ergonomic wrest rests can be part of an overall workstation design that maximize HCI experience while minimizing chances for strain, stress, and injury to EU.

B)    Price –  ?

References:   
Bureau of National Affairs. (1995). 770 percent increase. Occupational Safety and Health Reporter, 36, p.1794.
Granjean, E. (1987). Ergonomics in Computerized Offices. Taylor Francis: NY.
Helander, M. (Ed.). (1994). Handbook of Human-Computer Interaction. (4th ed.) North-Holland: NY, NY.
International Business Machines. (1991). Human Factors of Workstations with Visual Displays.(4th. Ed.). IBM: Somers, NY.
National Institutes for Occupational Safety and Health. (1997). Musculoskeletal Disorders and Workplace Factors. NIOSH Pub No. 97-141.