The major purpose of this research was to determine whether kinesthetic/pro
prioceptive perceptions of the earth-fixed vertical axis are more accurate
than perceptions of intrinsic axes. In one experiment, accuracy of alignmen
t of the forearm to earth-fixed vertical and head- and trunk-longitudinal a
xes by seven blindfolded subjects was compared in four tasks: (1) Earth-Arm
- arm(humerus) orientation was manipulated by the experimenter; subjects a
ligned the forearm parallel to the vertical axis, which was also aligned wi
th the head and trunk longitudinal axis; (2) Head - head, trunk, and upper-
limb orientations were manipulated by the experimenter, subjects aligned th
e forearm parallel to the longitudinal axis of the head using only elbow fl
exion/extension and shoulder internal/external rotation; (3) Trunk - same a
s (2), except that subjects aligned the forearm parallel to the trunk-longi
tudinal axis; (4) Earth - same as (2), except that subjects aligned the for
earm parallel to the earth-fixed vertical. Head, trunk, and gravitational a
xes were never parallel in tasks 2, 3, and 4 so that subjects could not sim
ultaneously match their forearm to all three axes. The results showed that
the errors for alignment of the forearm with the earth-fixed vertical were
lower than for the trunk- and head-longitudinal axes. Furthermore, errors i
n the Earth condition were less dependent on alterations of the head and tr
unk orientation than in the Head and Trunk conditions. These data strongly
suggest that the earth-fixed vertical is used as one axis for the kinesthet
ic sensory coordinate system that specifies upper-limb orientation at the p
erceptual level. We also examined the effects of Varying gravitational torq
ues at the elbow and shoulder on the accuracy of forearm alignment to earth
-fixed axes. Adding a 450 g load to the forearm to increase gravitational t
orques when the forearm is not vertical did not improve the accuracy of for
earm alignment with the vertical. Furthermore, adding small, variably sized
loads (between which the subjects could not distinguish at the perceptual
level) to the forearm just proximal to the wrist produced similar errors in
aligning the forearm with the vertical and horizontal. Forearm-positioning
errors were not correlated with the size of the load, as would be expected
if gravitational torques affected forearm-position sense. We conclude that
gravitational torques exerted about the shoulder and elbow do not make sig
nificant contributions to sensing forearm-orientation relative to earth-fix
ed axes when the upper-limb segments are not constrained by external suppor
ts.