This chapter focuses on the effects of using different muscle
groups in 6 DOF manipulation. In particular, it investigates human
performance differences in 6 DOF input control with and without
the involvement of the small muscle groups (fingers). The issue
of using different muscle groups in manual control has been studied
in low degree of freedom manipulation (e.g. Gibbs, 1962; Hammerton
and Tickner, 1966). 6 DOF manipulation poses a greater challenge
to the ergonomic design of input devices, however. If muscle group
differences have a minor effect on relatively easy 2 DOF control
tasks, they might affect 6 DOF control tasks much more significantly.
Neurophysiological studies have shown that various parts of the human body are represented in the brain disproportionately relative to their physical size and mass as illustrated in Figure 4.1. Of particular interest to this chapter is the fact that the representations of the fingers and the hands in both the somatosensory cortex and the motor cortex are much richer than those of the wrists, elbows and shoulders, as illustrated in the homunculus model of the somatosensory and motor cortex.
The homunculus model suggests that a potential performance enhancement
will result if fine muscle groups (i.e. fingers) are allowed to
take part in handling an input device. Indeed, this potential
has already been considered in the use of the Spaceball and the
EGG in this thesis, where subjects in Experiment 2 and 3 were
asked to use their fingers to grip the control handle during the
experiments. Interestingly, one of the most common types of virtual
reality input devices, the instrumented glove, such as the one
shown in Figure 4.2, does not utilise this potential advantage.
When using a glove, all translation and rotation operations are
carried out by the user's shoulder, elbow and wrist. i.e. the
gross joints and muscle groups in the human limb. The smaller,
finer joints and muscle groups on the fingers are not utilised.
Experiment 1 showed that the glove as an isotonic device performed
well in position control mode. Can an even better 6 DOF isotonic
device that also utilises small muscle groups, with a concomitant
increase in the number of effector degrees of freedom, be designed
and implemented? Will such a device in fact outperform the glove?
Before addressing these questions, the related literature on one
and two degree of freedom devices is reviewed.