A study performed by Lee, McLoone and Dennerlein, found that computer users often exhibited finger extension postures, including lifting fingers completely off the mouse, when operating a commonly used two-button style mouse. The authors investigated hand positions assumed by users while performing both stationary (non-movement of the mouse or activation of mouse buttons) and dynamic mouse related tasks.
Prior research has associated mouse usage with forearm and wrist pain. Researchers postulate that repeat/long duration wrist/finger extension postural positions observed in mousing tasks, and the resulting static loading of musculoskeletal soft tissues, leads to symptom development. The findings of Lee et al. capture the prevalence of finger position and finger/wrist posture relative to mouse use. However, the authors did not investigate subject symptoms relative to these positions/postures.
The commonly used mouse has two buttons that become activated by flexing the index or middle fingers to produce a “click” sound/sensation. When the buttons are not being used (such as when moving the mouse to position the pointer on the monitor or when just grasping the mouse), the user’s fingers are frequently lifted off the buttons (through wrist/finger extension) to avoid unwanted button activation.
Unintended finger extension may also be produced through an enslaving effect, meaning force produced by one or more fingers can produce an involuntary force in adjacent fingers. If the index finger is flexed to activate the left button, the middle finger will also flex slightly. To avoid activating the right button via this middle finger motion, the user frequently extends the middle finger. The authors observed this phenomena in this study.
Key study findings are presented in Table 1.
|
Mouse Function
|
Percent of Subjects Who Lifted Finger Off the Mouse
|
|||
|
Index Finger
(2nd finger)
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Middle Finger
(3rd finger)
|
Ring Finger
(4th finger)
|
Little Finger
(5th finger)
|
|
|
Griping stationary mouse
|
19%
|
30%
|
4%
|
0%
|
|
Moving the mouse without activating the mouse buttons
|
28%
|
45%
|
15%
|
10%
|
|
Performing a dragging mouse activity
|
0%
|
48%
|
14%
|
11%
|
|
Performing a left click mouse activity
|
26%
|
43%
|
9%
|
2%
|
|
Performing scrolling functions
|
41%
|
46%
|
15%
|
8%
|
Table 1: Prevalence of select finger lifting off the mouse relative to common tasks.
Also, observation of wrist posture showed that:
- 2% of the subjects exhibited less than 15° of wrist extension
- 97% of subjects assumed a position of between 15° and 30° of wrist extension
- 1% of subjects had greater than 30° of wrist extension
Study Design
Subjects
One hundred subjects (all graduate students, 41 males and 59 females) were observed while performing computer activities in a micro-laboratory.
Computer Workstation Set Up
The micro-laboratory contained 20 identical computer workstations that were equipped with height adjustable keyboards, monitors and chairs. The mouse was not on an adjustable height surface and was above elbow height. The mouse was an Optical Wheel Mouse with standard right and left click buttons.
Observation of Subjects
At random times over the course of the day, the examiner (a trained occupational therapist who was also a member of the student body) occupied a workstation approximately 1.5 meters behind and to the right of the observed subject.
During a five minute observational period, the following items were identified:
- Gender
- Hand used
- Specific fingers used on each mouse button
- Wrist extension levels
- Level of wrist/forearm support on the workstation surface
- Overall mouse grip behavior
- Finger support during mouse use
- Finger postures across finger support during mouse use
- Subject identifier so that the same individual was not recorded more than once
Following the five minute observation, a checklist of findings was completed.
Finger support during mouse use was categorized as:
- Rested – finger on the button or against the mouse; or
- Lifted – non-physical contact with the mouse
Finger postures across finger support during mouse use were categorized as:
- Extended posture – a metacarpophalangeal joint (MCP) angle greater than 0° of flexion with the proximal interphalangeal joint (PIP) angle less than 15° of flexion.
- Neutral posture – a positioning of the MCP and PIP between the “extended” and “flexed” posture positions.
- Flexed posture – a MCP angle greater than 0° of flexion with the PIP angle greater than 60° of flexion.
- Clawed posture – a MCP angle that is hyperextended with the proximal or distal interphalangeal joints in any flexion angle position.
Measurement
The frequency count of observed identified variables (i.e., lifted index finger, middle finger, ring finger, or little finger) across a category of use (i.e., stationary mouse use activity, while performing a left click mouse activity) were presented as a percentage (prevalence).
Other Findings
Across all mouse activity (stationary and dynamic), the index and middle fingers mostly assumed a neutral postural position (65-79%) while occasionally exhibiting an extended postural position (19-23%).
Relative to using the work surface for upper extremity support:
· 77% had their forearms and wrists fully resting on the work surface
· 21% rested their wrist on the edge of the work surface
· 2% did not use the work surface for upper extremity support
Three percent of the subjects were observed as having a tense grip of the mouse while the remaining 97% had a relaxed grip. All subjects used their right hand to operate the mouse.
Study Concerns/Limitations
The authors identified the following potential shortcomings of this research:
- The study was performed using a limited subject group (graduate students).
- Study findings were generated while using a specific two button mouse with a scroll wheel. A different mouse design may produce different results.
- One researcher performed all observations raising concerns of data reliability.
Other points:
- There were no controls for subject injuries/disorders/impairments.
- This was an observational study. No instrumentation was used to determine hand position/joint angles.
- All data was obtained during a five-minute period. Postural positioning can change over time and a longer assessment time may produce different results.
- The prevalence of select concurrent lifted finger position (i.e., index and middle; index, middle, and ring; index, middle, ring, and little) and select concurrent lifted finger/extended wrist position is unknown.
Article Title: Observed finger behaviour during computer mouse use
Publication: Applied Ergonomics 39, 107-113, 2008
Authors: D L Lee, H McLoone, and J T Dennerlein
This article originally appeared in The Ergonomics Report™ on 2007-11-06.