F. Lacquaniti et C. Maioli, INDEPENDENT CONTROL OF LIMB POSITION AND CONTACT FORCES IN CAT POSTURE, Journal of neurophysiology, 72(4), 1994, pp. 1476-1495
1. It has previously been demonstrated that a set of geometric and kin
etic parameters are invariant in cats standing at their preferred inte
rfoot distance and weight distribution. Thus the length and the angle
of orientation relative to the vertical of each limb axis remain appro
ximately constant when the supporting platform is tilted in the sagitt
al plane. The direction of the tangential contact forces is similarly
constrained in response to horizontal translations. The main aim of th
e present study is to assess whether or not the control of Limb positi
on is independent of the control of the contact forces at the feet. To
this end we have examined cat posture under a number of different con
ditions expressly designed to increase the range of postural variabili
ty. We considered that if the specification of limb position is a mere
byproduct of the neural control of contact forces (or vice versa), ge
ometric and kinetic parameters would covary interdependently. If inste
ad limb position and contact forces are controlled in parallel and ind
ependently of each other, they will tend to follow different laws of v
ariation. 2. Limb position and contact forces were measured in intact
cats standing freely on a support platform. In a first series of exper
iments the pitch angle of the platform was randomly changed, as were t
he interfoot distance and head orientation. In another series of exper
iments cats were tilted in the presence of an external load tending to
shift the weight distribution. The same load was applied in two diffe
rent manners: 1) it made contact with a very limited surface of the bo
dy, and 2) it was attached by means of a long vest that made contact w
ith most of the trunk and produced abnormal somesthesic cues to the bo
dy. 3. The range of different experimental conditions resulted in subs
tantial trial-to-trial variations of the length and orientation of the
axis of the limbs, as well as variations of the magnitude and orienta
tion of the net contact forces. We found that the changes of the orien
tation of the contact force vector are uncorrelated with the correspon
ding changes of limb orientation, thus providing a first line of evide
nce in favor of the existence of a separate neural control of geometri
c and kinetic parameters. 4. Another line of evidence is provided by t
he specific form of the laws of variation of geometric parameters and
tangential forces in different animals. Under normal (unloaded) condit
ions the values of the limb joint angles tend to covary linearly. The
best-fitting planar regression of the angular values requires an absol
ute reference to the vertical. The orientation of the plane is highly
consistent in all cats and it is essentially identical at the forelimb
s and at the hindlimbs, despite their large biomechanical differences.
5. The normal contact forces, as well as the strut component of the t
angential contact forces, are completely determined by the assigned po
stural geometry. The lever component (exerted by proximal muscles) of
the tangential contact forces is instead controlled independently of l
imb geometry. This force is accurately partitioned between forelimbs a
nd hindlimbs in all cats. The specific proportion of force partitionin
g, however, is idiosyncratic to each animal and highly variable among
animals. 6. The existence of a neural control of limb geometry indepen
dent of the control of limb kinetics is borne out also from the experi
ments involving the application of a load that shifted the center of m
ass of the body. When the load made contact with a limited surface of
the body, we found consistently that limb geometry(mean length and ori
entation of limb axis, and planar covariation of joint angles) was pre
served unaltered, whereas kinetic parameters ( magnitude of the contac
t forces and joint torques) were severely affected. 7. Limb geometry w
as altered, however, when the load was attached by means of a long ves
t that made contact with most of the trunk, resulting in anomalous som
esthesic stimuli. Cats did not maintain the limbs vertical as under no
rmal conditions, but rotated them almost in parallel with the rotation
of the table. Moreover, the orientation of the regression plane of li
mb joint angles was generally tilted. In contrast with the orientation
of the limb axis, the orientation of the contact forces did not chang
e relative to the control, thus providing another instance of independ
ent control of limb geometry and contact forces.