Academic journal article Human Factors

Keyboard Reaction Force and Finger Flexor Electromyograms during Computer Keyboard Work

Academic journal article Human Factors

Keyboard Reaction Force and Finger Flexor Electromyograms during Computer Keyboard Work

Article excerpt


Keyboard data entry systems are used widely in nearly all contemporary industries and occupations. Use ranges from a few keystrokes per hour to highly repetitive data entry, in which typists may perform as many as 200 000 keystrokes per day. Telephone operators, reservationists, and sales personnel perform fewer keystrokes but keep their hands poised over the keyboard throughout their entire work shift. Other workers, such as reporters and secretaries, may alternate between occasional light usage and extended bouts of heavy usage.

Pain and impairment of the upper limbs are growing causes of disability and compensation among keyboard users. Such disorders have also resulted in litigation against keyboard manufacturers and considerable media attention (Bureau of National Affairs, 1992; Galen, Mallory, Siwolop, & Garland, 1992). The mechanisms underlying these disorders are not well understood but are generally believed to be fatigue, ischemia, and irritation or compression of muscles, tendons, and nerves. These disorders are associated with multiple psychosocial and work-related factors occurring alone or in combination with one another (Faucett & Rempel, 1994). Possible work-related factors include repeated and sustained exertions, forceful exertions, extreme postures, and localized mechanical stresses (Armstrong, Radwin, Hansen, & Kennedy, 1986; Armstrong et al., 1993).

The American National Standard for Human Factors Engineering of Visual Display Terminal Workstations (ANSI/HFS 100-1988) characterizes the force requirements of keyboards in terms of key make force (actuation force) and recommends an upper limit of 1.5 N, a displacement of 2 mm to 4 mm, and a tactile feedback indicating when the key registers. However, it is not clear that these limits are appropriate to prevent fatigue and chronic musculoskeletal disorders.

In a recent study, Armstrong, Foulke, Martin, Gerson, and Rempel (1994) observed that the applied peak keystroke forces are 2.5 to 3.9 times greater than the key switch make force. These results call for further understanding of how force and finger movements are programmed in the typing task. Studies of the relationship between keyboard characteristics and performance began 50 years ago (for a review, see Alden, Daniels, & Kanarick, 1972; Potosnack, 1988); however, very few considered finger force exertion. Furthermore, the few studies that considered finger force exertion did not involve a "real" typing task (Guggenbuhl & Krueger, 1990); they analyzed only "median" muscular activity (Fernstrom, Ericson, & Malker, 1994) or did not specify key mechanical characteristics (Colombini, Occhipinti, Molteni, Semerano, & Grieco, 1989).

In the present study we were concerned specifically with developing ways to assess finger forces and muscle activity during keyboard work. Such methods are necessary to further understand how force and distribution of muscle load contribute to fatigue and musculoskeletal disorders and to evaluate alternative keyboard designs and individual work patterns. We evaluated the activity pattern of finger flexor muscles and finger force exertion and the relationship between keyboard reaction forces and electromyograms (EMGs) while keying. A windowing technique was used to examine EMGs and forces associated with each keystroke.


Two sets of experiments were performed to investigate the relationship between surface EMGs and keyboard reaction forces. First, we determined a relationship between surface EMGs and static flexion finger forces using a graded exertion of 0% to 100% maximal voluntary contraction (MVC). Second, we investigated the relationship between surface EMGs and keyboard reaction forces during keying.


Ten healthy people (five men and five women) who were students or staff at the University of Michigan Center for Ergonomics participated in the experiments. …

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