Perceptual mediators of force production


Therrien and colleagues (2010; 2012; 2013) observed a tendency for individuals to overproduce force in repetitive pinch grip and vocalization tasks when visual feedback is removed. Framed in models of feedforward control, these authors suggest that when visual feedback is not available, participants underestimate their self-produced force, thus leading to a systematic increase in force output. However questions remain, of both theoretical and applied relevance, as to whether this is a generalized effect or if it is present only when previously available visual feedback is removed as a repetitive task continues. From a functional perspective, for example, current ergonomic models do not consider availability of vision when acceptable force limits are being set for tasks where the hands cannot be seen. The purpose of this study therefore, is to identify whether the continuous availability of vision modulates the amount of force exerted to complete an insertion task. Participants completed a buckle-fastening task in 2 force directions (down and forward), both with and without direct vision of the task hand and the objects they were manipulating. Of particular interest was the maximum force produced as well as the total impulse of the insertion action. Results suggest that maximum pinch grip force increased by over 40% in the absence of vision (p<0.005). In addition, pinch grip impulse was 83% greater without visual feedback (p<0.001). The total force necessary to complete the insertion was significantly larger without vision, and mean insertion impulse more than doubled (p<0.001). The results of this study, suggesting that continuous vision is an important factor in attenuating force demands, provide insight into the perceptual mediators of kinetic output. They are discussed from theoretical and functional ergonomic perspectives.

Acknowledgments: Automotive Partnership Canada, NSERC