Chapter 2. Prehension

Chapter 6 - During Contact 259
50 - 70 ms after onset of load force perturbations, with a silent period
preceding the phasic burst, 30 - 40 ms after perturbation onset (in all
muscles recorded except extensor digitorum communis). Note the
similarity in latency to the ‘automatic’ adjustments in the grip-
force/load-force ratio to slips reported by Johansson and Westling
(1988a). Subjects never made an error in the direction of the
automatic, short latency, response, e.g., a rapid increase in load force
always yielded an increase in grip force. The increase in grip force
averaged 7.2 (f 2) N and was within 6 - 10% of the grip force needed
to lift the object. In contrast to the sustained response to object slip,
the grip force in response to load perturbation steadily decreased (to
the level necessary to maintain grasp). The grip force response to
perturbation was sensitive to the size and velocity of the load force
increase but not the preexisting grip force; thus, Cole and Abbs
concluded that the grip force was at a level to maintain grasp, i.e.,
proportional to the required load force, not at a specified level above
the object’s slip point. They note the impairment with digital
anesthesia, and consider the role of cutaneous, joint and muscle
mechanoreceptors in signalling the load changes, with emphasis on
cutaneous mechanoreceptors detecting shearing forces on the skin of
the finger pads.
Johansson and colleagues (Johansson et al., 1992a,b,c)
performed a series of load perturbation experiments, using pad
opposition. In the absence of instructions to initiate grip changes, they
showed that the latency of the initial grip force responses, termed
‘catch-up response’ was dependent on load force rate (e.g., 8W,
108+13, 138f27 and 174f39 ms for the 32, 8, 4, and 2 N/s rates
respectively). The latency of grip force response to different loads
(with a constant load force rate of 4 N/s) was 140 (f30) ms. The
amplitude of the grip force response was a function of both load force
and load force rate, but its time course was similar over all rates of
perturbing load force. For longer lasting loading phases, the catch-up
response was followed by a ‘tracking’ response, in which grip force
increased in parallel with load force, to maintain a slip ratio.
Interestingly, with digital anaesthesia, subjects had to voluntarily
attend to the task (usually automatic, without anesthesia), the grip
force latencies were prolonged (to several hundred milliseconds), with
less modulation of grip force, absence of grip force responses (absent
from 25 - 93% of trials, depending on subject), and there was a lack
of dependence of the ‘catch-up’ grip force responses on load force
rate. With digital anaesthesia, there were large individual differences,
suggesting that for some subjects, more proximal mechanoreceptors