Academic journal article Human Factors

The Discrimination, Acquisition, and Retention of Aiming Movements Made with and without Elastic Resistance

Academic journal article Human Factors

The Discrimination, Acquisition, and Retention of Aiming Movements Made with and without Elastic Resistance

Article excerpt


Given the proliferation of machines and devices with which people interact in jobs and everyday lives, there are many occasions when people have to make discrete movements accurately without vision of their limbs. They do not see their limbs either because their eyes are occupied elsewhere, such as when attention is directed to a specific aspect of the environment during the manipulation of a control, or because the movement occurs in a dark environment. Reaching for an illuminated light switch in an otherwise dark room is a good example of the latter situation, whereas operating a crane is a good example of the former. The ubiquity with which so-called "blind positioning" and aiming movements are made on an everyday basis has tremendous implications for the design of work spaces and living spaces that allow controls to be effectively located and manipulated.

Despite the popularity of studying blind positioning movements in early motor control and learning research (see Howarth & Beggs, 1981; Laabs & Simmons, 1981; and Smyth, 1984 for reviews of much of this work), our current understanding of how we perceive and control the position and movement of our limbs without vision is less than adequate. Several issues must be resolved before guidelines that promote the performance and learning of blind movements can be formulated. One of these issues concerns the appropriateness of adding resistance to control mechanisms.

The use of most controls requires a combination of force to overcome resistance and displacement to position the control. Apart from free-position (isotonic) controls, nearly all controls have some resistance. The type of resistance depends on the task for which the control is designed and the aspect or aspects of performance that the designer wants to maximize. According to Bahrick (1957), there are four basic types of resistance: elasticity, viscous damping, inertia, and static and coulomb friction. It is hypothesized that elasticity improves the perception and control of spatial (positional) accuracy, damping improves the perception and control of rate of movement, and inertia improves the perception and control of acceleration. Because there is no systematic relationship between frictional forces and any aspect of control movement, static and coulomb friction are actually thought to degrade performance (Sanders & McCormick, 1993).

Perhaps the most widely accepted of the hypotheses outlined by Bahrick (1957) concerns the beneficial effects of elastic resistance to controls. Elastic resistance varies with the displacement of a control so that the farther the control is moved from a neutral position, the greater the resistance is. It is generally assumed that elastic resistance to movement is a desirable characteristic of a control mechanism because it enhances kinesthetic feedback and therefore increases the spatial precision with which movements can be executed and discriminated (e.g., Sanders & McCormick, 1993). Despite the intuitive nature of this premise, there is little evidence to support it. For example, Weiss (1954), Stelmach (1968), and Ellis (1969) have reported that adding elastic resistance to a discrete movement has negligible effects on positional accuracy, whereas a series of studies conducted by Bahrick and colleagues (Bahrick, Bennett, & Fitts, 1955; Bahrick, Fitts, & Schneider, 1955; Briggs, Fitts, & Bahrick, 1957) have produced equivocal results.

Two general arguments have been posited to explain why studies have often failed to find better performance when movements are made against elastic resistance. The first suggests that only an optimum ratio of relative force change to displacement improves the positional accuracy of movements (Bahrick, 1957). Although there is some evidence to support this hypothesis (Bahrick, Bennett, & Fitts, 1955; Jenkins, 1947), it is difficult to test because the optimum ratio should vary among individuals and as a function of possible changes in any one of a multitude of task and environmental constraints. …

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