Academic journal article Human Factors

Activation Force and Travel Effects on Overexertion in Repetitive Key Tapping

Academic journal article Human Factors

Activation Force and Travel Effects on Overexertion in Repetitive Key Tapping

Article excerpt

INTRODUCTION

This research is motivated by reports of upper extremity maladies among intensive keyboard users (Bernard, Sauter, Fine, Petersen, & Hales, 1994; McPhee, 1982; NIOSH, 1990; Rose, 1991). Force is one factor often cited as increasing the risk for localized fatigue and musculoskeletal disorders. Studies have shown that the force exerted by keyboard operators during keying markedly exceeds the force necessary to activate the keys (Armstrong, Foulke, Martin, Gerson, & Rempel, 1994; Feuerstein, Armstrong, & Hickey, 1994). The actual force that a typist exerts can be affected by numerous circumstances, including keyboard, workstation, and psychosocial factors, and individual factors, such as typing experience and proficiency. The current study is concerned with the effects of physical characteristics of key design on finger exertions in repetitive key tapping.

The physical characteristics of key switches are often described by their make and break points, which are measured from associated force and travel parameters. Key force is the force applied against the key cap. Because key force is usually opposed by a linear or nonlinear spring, when key force is applied, the key is depressed a corresponding displacement, defined as key travel. Make occurs when the switch makes electrical contact and the circuit is activated. Break occurs when the switch breaks electrical contact and the circuit is deactivated.

Because there is often hysteresis in switch mechanisms, the break point is not necessarily equivalent to the make point. Make point force is therefore the key force that must be applied in order to activate the key, and break point force is the key force that must be released in order to deactivate the key. Correspondingly, make point travel is the key travel necessary to activate the key. Over travel is defined as the maximum travel a key can be depressed beyond the make point travel until the key hits bottom. The actual key force and travel characteristics depend on both the mechanical and electrical designs of the particular key switch.

The American National Standard for Human Factors Engineering of Visual Display Terminal Workstations (HFES, 1988) contains requirements for the force necessary for activating keys and associated key displacement. The standard specifies key activation force ranges between 0.25 and 1.5 N with a key displacement between 1.5 and 6.0 mm and a preferred displacement between 2.0 and 4.0 mm. The force required for activating keys, however, does not necessarily reflect the actual exertions operators make when using a keyboard. Peak force during keying has been measured to be as much as 2.5 to 4.6 times the required activation force (Armstrong et al., 1994; Feuerstein et al., 1994; Martin et al., 1994).

Previous studies have shown that applied finger force increases with keyboards that have greater make forces (Armstrong et al., 1994; Rempel, Klinenberg, et al., 1994). Armstrong et al. (1994) compared three keyboards with similar layouts but different make force and key travel characteristics. Although different applied forces were observed among the keyboards, each one had specific key switch parameters, so these effects are confounded.

Rempel, Klinenberg, et al. (1994) performed a similar study and also found that applied key force was affected by make force, whereas other key switch characteristics (i.e., total travel and tactile feedback) were held constant. Neither of these investigations could account for the combined effects of keyboard force and travel characteristics on applied finger force.

The purpose of this study was to systematically investigate keyboard design factors that affect operator exertion. Key switch design parameters were investigated that maximize key-tapping speed while minimizing peak key force exerted. The make point force, make point travel, and over travel for the key switch covered a wide range of parameters currently recommended for keyboards. …

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