During the performance of more demanding writing and pointing tasks, subjects exert more pen pressure towards the working [1]. This is a natural stiffness response to the noisier neuromotor signal in stressful conditions. Both physical and mental stress lead to such adaptive stiffness [3]. Higher stiffness, once the movement is initiated, can be achieved by speeding up the movement or by regulating the level of co-contraction of agonist and antagonist muscles. Acceleration is rather harmless and will only decrease the accuracy. Co-contraction can however be thought of as driving a car while applying the breaks. There is no return of the investment. Co-contraction also diminishes blood flow and thus the supply of nutrients and removal of carbon dioxide in the tissue. It could therefore lead to the beginning of RSI [2]. Being the most widely used input device for computers we designed a squeeze-mouse, sensitive to pressure applied to its sides to test the motor noise theory (NICI technical report 99-05) in the area of human-computer interaction. To measure grip forces, a steel construction, force receptors and an A/D measurement device were placed inside the mouse (Figure 1).

Calibration of the first prototype showed that no conversion of A/D-values to absolute force values could reliably be obtained. Within an experiment the baseline remained constant but was variable on longer time scales. Thus only within-experiment estimates could be obtained. In collaboration with the Hogeschool Enschede two main squeeze-mouse problems were identified. First, the characteristics of the mouse-material influence the measurements. Secondly, there is some type of noise in the signal that is hard to relate to any specific cause. Notwithstanding these problems the results of a first experiment with the mouse confirm the predictions made in the neuromotor noise theory [4].

Figure 1. Interior of squeeze-mouse

References

1. Galen, G.P. van; de Jong, W.P. (1995). Fitts' law as the outcome of a dynamic noise filtering model of motor control. Human Movement Science, 14, 539-571.
2. Galen, G.P. van; Smits-Engelsman, B.C.M.; Meulenbroek, R.G.J.; Bloemsaat, J.G. (1999). Over bewegen, stress en mogelijke mechanismen achter de muisarm en andere vormen van repetitive strain injury (RSI) [Movement, stress and other possible mechanisms as origin for mouse-arm-injury and other forms of RSI]. In: Jaarboek Fysiotherapie en Kinesitherapie 1999, pp.1-34. Houten/Diegem: Bohn Stafleu van Loghum.
3. Gemmert, A.W.A.; van Galen, G.P. (1997). Stress, neuromotor noise, and human performance: a theoretical perspective. Journal of Experimental Psychology: Human Perception and Performance, 23, 1299-1313.
4. Ven, J. van de; de Haan, A. (2000). Influence of pacing on mental load and computer-mouse handling. Accepted conference paper, International Ergonomics Association 2000, San Diego.

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