Chapter 2. Prehension

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THE PHASES OF PREHENSION
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Figure 6.17 Force coordination during the initial part of lifing
trials. Load force, grip force, vertical position and ratio between
grip and load force are shown as a function of time. (a) Influence
of different weights: 800g, 400g and 200g. Constant suede surface
structure. Average data from 24 trials (single subject). (b)
Influence of different surface structures: finely textured silk-most
slippery, suede-less slippery, and sandpaper-least slippery. Weight
constant at 400g. Average data from 120 trials (9 subjects). In
both a and b, arrows indicate mean-slip ratios for the three
structures. Illustrations represent standardized conditions in which
the previous trials were carried out with the same weight and
friction, respectively (from Johansson & Westling, 1984b;
reprinted by permission from Johansson & Westling, 1990).