The farther away a keyboard was placed from the front edge of a work surface:
· The less the degree of wrist ulnar deviation
· The greater the degree of wrist extension
· The less the activity of the extensor carpi ulnaris and extensor carpi radialis muscles
· The greater the dynamic activity of the upper trapezius muscle
according to a laboratory study by Kotani et al.
Further, using a palm rest with the keyboard in the most distant horizontal position (15 cm from the work surface edge):
· Reduced wrist extension
· Increased wrist ulnar/radial deviation velocity and acceleration values
· Decreased activity of the extensor carpi ulnaris, extensor carpi radialis decreased
Keyboard horizontal distance, in general, did not affect wrist motion velocity or acceleration.
Twenty subjects (10 males and 10 females) volunteered to participate in the study. All were free of current discomfort or history of an upper extremity musculoskeletal disorder.
Computer Workstation Set Up
Subjects sat on an armless chair that was adjusted in vertical height such that the subjects’ feet were flat on the floor while their thighs were parallel to the floor. The work surface was adjustable in height and placed for each subject such that their elbow was parallel to the J-key (with the subject’s arms and shoulders relaxed).
Prior to placement of the keyboard, the subject was asked to position the chair at a comfortable horizontal distance from the work surface. The subjects were asked to keep their back upright against the chair backrest.
The keyboard was then placed on the work surface. Over the course of the experiment, the keyboard orientation was randomly altered in four different ways:
· Keyboard was located at the edge of the work surface (NEAR position)
· Keyboard was located 8 cm away for the edge of the work surface (MID position)
· Keyboard was located 15 cm from the edge of the work surface (FAR position)
· Keyboard was located 15 cm from the edge of the work surface with the addition of a palm rest in front of the keyboard (FWP position)
For each orientation, the keyboard was placed on the work surface with the alphanumeric portion of the keyboard centered to the volunteer’s body.
Two computer tasks were randomly performed for each keyboard orientation. The first task required continuous text typing into a word processing program. The reference text and text being typed were displaced in separate dialogue windows on the computer monitor. The second task required the subject to read a paragraph of text on the monitor and then answer five questions relative to the paragraph in a separate dialogue window. Each task took about five minutes to complete.
Upper Extremity Measurements
Right hand wrist postural positions (flexion, extension, ulnar deviation, radial deviation) were measured through use of a glove-based electrogoniometry system. Wrist postures were recorded continuously (20 samples per second).
Forearm position was recorded through use of an electro-magnetic motion sensor placed proximal to the wrist joint on the dorsal surface forearm (allowed for measurement of forearm pronation/supination, forearm elevation, and forearm internal/external position). Shoulder position (flexion and abduction) was recorded by placing a sensor proximal to the elbow joint. Data was collected at 10 samples per second.
Electromyographic activity was recorded from the extensor carpi ulnaris, extensor carpi radialis, anterior deltoid, and upper trapezius.
Following each task, the subjects completed a discomfort survey for eight body regions (back, neck, right/left shoulders, right/left elbows, and right/left wrists) on a 10 cm visual analogue scale.
When the keyboard was at the FAR position, the application of a palm rest (the FWP position) returned wrist extension to the position it assumed when the keyboard was in the NEAR position.
For the NEAR keyboard position, the elbow joint assumed a position near 90 degrees while the shoulder joint was in a neutral flexion position but slightly abducted. As the keyboard moved away from the work surface edge:
· shoulder flexion increased
· shoulder internal rotation decreased
· shoulder abduction decreased
· the inner angle of the elbow increased
Use of the palm rest in the FAR position did not significantly alter the shoulder or elbow joint positions.
Discomfort levels were markedly low with a 2.0 at the right elbow being the highest mean value reported (on the 10 cm visual analogue score with 0 = no discomfort). In the right upper extremity, there was a slight trend with the greatest discomfort reported in the NEAR keyboard position and the lowest in the FWP keyboard position.
The authors identified the following concerns:
1) The duration of the keyboarding tasks was short. Hence, study findings reflect short-term muscle activity response. Application of these conclusions to keyboarding tasks performed long term is unwise.
2) The changes noted in posture and muscle activity were minute. The impact of these small changes on injury risk are unknown.
3) This study focused on keyboard intensive tasks. The mouse was place adjacent to the keyboard but its use during this experiment was marginal.
4) Devices used to record findings (i.e., cables from EMG electrodes) may have altered how upper extremity motions occurred.
5) The trunk position was not continuously monitored over the course of the experiment.
Article Title: Effect of horizontal position of the computer keyboard on upper extremity posture and muscular load during computer work
Publication: Ergonomics 50:9 1419-1432, 2007
Authors: K Kotani, L H Barrero, D L Lee and J T Dennerlein
This article originally appeared in The Ergonomics Report™ on 2007-08-28.