Chapter 2. Prehension

256 THE PHASES OF PREHENSION
mechanoreceptive afferent information shown in Table 6.3 could be
from the joint and muscle mechanoreceptors in addition to the
cutaneous mechanoreceptors of primary interest to Johansson &
Westling (1988a,b; Westling & Johansson, 1987).
The tactile afferent information from glabrous skin has been
suggested as functional input for a neurally interfaced control system
for the restoration of upper limb motor functions in quadriplegia or
hemiplegia (Hoffer & Haugland, 1992). Such a closed loop control
system could be used in combination with Functional Electrical
Stimulation in paralyzed humans for transitions in these phases as
humans make and break physical contacts with objects and supporting
surfaces in the environment.
Johansson and Westling (1988b) reported EMG for elbow, wrist
and hand muscles during the loading, transitional, and unloading
phases of their lifting trials. These are shown in Figure 6.20. Several
points are of interest here. First, note that during the loading phase in
‘adequately programmed trials’ (those mals where the weight to be
lifted had been experienced in previous trials), all 4 pairs of antagonist
muscles contracted together, ‘providing the appropriate stiffness of the
armhand system’. Second, although lifting (change in position of the
object) may be due to motion about the elbow or wrist, none of the
EMG values seemed to correlate with the motion of lifting the object.
Third, on ‘erroneously programmed trials’ (e.g., lifts preceded by a
heavier weight), termination of the loading phase was triggered by
sensory signal related to the moment of lift off, associated with
functional adjustments in EMG. When the lift occurred earlier than
expected for the grip forces produced (i.e., lifts preceded by a heavier
weight), somatosensory signals elicited by the start of movement
triggered changes in the EMG of the intrinsic hand muscles in 100-1 10
ms and of the wrist and elbow muscles 20 ms earlier. For lifts
erroneously programmed for a lighter weight (lift off doesn’t occur
when expected), the load forces are especially discontinuous, until the
required load force is achieved. For the unloading phase, contact with
the table triggered changes in the EMG of intrinsic hand muscles in
60-70 ms, and again 20 ms earlier for wrist and elbow muscles.
Johansson and Westling attributed these changes associated with
breaking and making contact with the table primarily to FA11 units
(Pacinian corpuscles), contrasting them with musculotendinous
receptors. Pacinian corpuscles in the interphalangeal joints might be
considered as possible candidates for this function.
For more detailed analyses of EMG activities during the dynamic
phases of applying forces, we refer the reader to excellent references