Chapter 2. Prehension

260 THE PHASES OF PREHENSION
might be mediating grip force responses. There was a lack of the
‘tracking response’ with digital anaesthesia as well, confirming the
requirement of continuous afferent input from both thumb and index
finger for grip force regulation in pad opposition. Contrary to their
initial expectations, there appeared to be independent force control for
each finger. The cutaneous strain at digital pulps resulting from
increased shear forces was believed to be the most relevant mechanical
stimulus of the digital mechanoreceptors. Johansson and colleagues
also note extension of the interphalangeal joints and flexion of the
metacarpophalangeal joint of the index finger as possible stimuli for
mechanoreceptors, since these appeared to aid grip force modulation
with digital anesthesia.
To investigate the influence of task instructions on loading or
unloading perturbations, Winstein, Abbs and Petashnick (1991), prior
to each block of trials, verbally told subjects to ‘hold’ or ‘let go’ in
response to any apparent change in object weight. Thus they had a set
to ‘hold’ or ‘let go’. The latencies of evoked grip changes, with a
mean of 66 (+13) ms and as early as 35 ms after perturbation onset,
were unaffected by either instructions or direction of loading. Grip
force responses to unloading were unaffected by instructions; in
contrast, with loading, the magnitude of early grip responses were
27% higher following hold than following let go instructions. In hold
conditions, the post-perturbation grip force level was sustained. In let
go conditions, a second grip force response was observed with
median latencies of 239-401 ms, across subjects. The latency of the
let go response did not vary with unloading or loading conditions.
They noted that the magnitude of the grip force responses were much
more marked (on average 3.6 times higher) to loading than unloading
forces of equivalent magnitude, suggesting this reflects functional
evolution of the hand in a gravity dominant world. Winstein et al.
note the importance of balancing two functional goals: first, to ensure
the object is not dropped or to maintain the object in stable grasp (not
too little gripping force) and second, to ensure the object is not
crushed or broken (not too much gripping force).
In determining the slip ratio in pad opposition, during the time
when individuals med to voluntarily release their grasp, they felt as if
their fingers adhered to the object (Johansson and Westling, 1987,
1990). It is as if the cutaneous afferents and peripheral biomechanics
to grasp are in contradiction to the efferent commands to release. The
effort required to overcome this magnet phenomenon was greater
using sandpaper as the surface structure, and became stronger as the
grip force approached the minimum grip force to prevent slips. They