Academic journal article Human Factors

The Effects of Work Pace on Within-Participant and Between-Participant Keying Force, Electromyography, and Fatigue

Academic journal article Human Factors

The Effects of Work Pace on Within-Participant and Between-Participant Keying Force, Electromyography, and Fatigue

Article excerpt


Cumulative trauma disorders are the most costly and severe disorders occurring in office work environments (Lyon, 1992). The number of recorded cases of repetitive motion injuries increased from 34 700 in 1984 to 372 300 in 1999 and represents 66% of all workplace illnesses (Bureau of Labor Statistics, 2000). Primary risk factors for the development of cumulative trauma disorders encountered while typing include force, static exertion, repetition, contact stress, and awkward posture (Armstrong et al., 1993; Kroemer, 1972).

Typing on a computer keyboard is a highly repetitive task. A worker typing at 60 words per minute (WPM) for 30 mm will produce 9000 exertions in that half hour. Hammer (1934) suggested that tendons cannot tolerate more than 1500 to 2000 exertions/h. Silverstein, Fine, and Armstrong (1987) rated jobs with more than 120 cycles/h as highly repetitive. Although the forces exerted during typing are much lower than those exerted in most industry jobs, the repetition rates are much higher.

Previous studies evaluating the relationship between work pace and muscle activity have shown mixed results. Sommerich, Marras, and Parnianpour (1996) found a moderate relationship between peak typing force and typing pace within participants when typing pace was externally controlled. However, these participants worked within a narrow range of their normal typing speed. Sommerich et al. (1996) also found that there was no relationship between typing force and typing speed when participants were allowed to choose their own typing pace. Arndt (1987) found that increases in work pace generally resulted in increases in electromyographic (EMG) activity and also noted that unsuccessful attempts to speed up or slow down produced similar increases or decreases in EMG activity. Thompson, Thomas, Cone, Daponte, and Markison (1990) assumed that EMG activity from the finger flexors and finger extensors was proportional to typing speed and that a doubling of typing speed would double the mean EMG activity recorded. Gerar d, Jones, Smith, Thomas, and Wang (1994) found a linear effect of typing speed on mean typing force with a static component added to the EMG activity at all typing speeds.

This study was intended to help increase understanding of the effects of force in low-force, high-repetition tasks such as typing on a computer keyboard. The goal of the current work was to determine how typing force and muscle activity are affected by work pace within participants and typing speed between participants. We hypothesized that finger flexor activity within participants is related to work pace, given that a faster typist strikes the keys more often in a given time interval.

We also hypothesized that the fastest and most skilled typists would require less finger flexor EMG activity to maintain a set typing speed than would the slowest typists. Faster typists would require less EMG activity because they would have less muscle cocontraction while typing. The faster typists would also be able to type at a lower relative pace of their maximum typing speed, compared with slower typists. This hypothesis was proposed before (Lundervold, 1958) but was not quantified. To test these hypotheses, we conducted an experiment in which within-participant typing pace and between-participant typing speed were controlled.


A laboratory study was conducted to determine the force, EMG activity, and subjective discomfort of experienced typists working at various typing paces. Participants typed for three 30-mm trials at each of three typing speeds: (a) self-paced, the speed the participant estimated would be comfortable typing throughout the day; (b) 50% of the participant's 1- min maximum typing speed; and (c) 100% of the participant's 1 -min maximum typing speed.


Eighteen touch typists (16 women, 2 men) were recruited through university newspaper advertisements, computer bulletin board postings, and a university temporary employment agency. …

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