1. The aim of this study was to describe the time-varying changes in t
he mechanical parameters of a multijointed limb. The parameters we con
sidered are the coefficients of stiffness, viscosity, and inertia. Con
tinuous pseudorandom perturbations were applied at the elbow joint dur
ing a catching task. A modified version of an ensemble technique was u
sed for the identification of time-varying parameters. Torques at the
elbow and wrist joints were then modeled with a linear combination of
the changes in angular position and velocity weighed by the matrix of
angular stiffness and the matrix of angular viscosity, respectively. C
ontrol experiments were also performed that involved the stationary ma
intenance of a given limb posture by resisting actively the applied pe
rturbations. Different limb postures were examined in each such experi
ment to investigate the dependence of the mechanical parameters on lim
b geometry. 2. The technique for the identification of limb mechanical
parameters proved adequate. The input perturbations applied at the el
bow joint elicited angular oscillations at the wrist essentially uncor
related with those produced at the elbow. The frequency of oscillation
is much higher at the wrist than at the elbow, mainly because of the
smaller inertia. The variance accounted for by the model was almost-eq
ual-to 80% under both stationary and time-varying conditions; in the l
atter case the value did not vary significantly throughout the task. I
n addition, the model predicted values of the inertial parameters that
were close to the anthropometric measures, and it reproduced the step
wise increase in limb inertia that occurs at the time the ball is held
in the hand. 3. The values of angular stiffness and viscosity estimat
ed under stationary conditions did not vary significantly with joint a
ngle, in agreement with previous results obtained under quasistatic po
stural conditions. The matrix of the coefficients of angular stiffness
was not symmetrical, indicating a prominent role for nonautogenic ref
lex feedbacks with unequal gains for elbow and wrist muscles. 4. A com
plex temporal modulation of angular stiffness and viscosity was observ
ed during the catching task. The changes in the direct coefficients of
angular stiffness tended to covary with those in the coupling coeffic
ients from trial start up to almost-equal-to 30 ms before impact time.
Around impact time, however, there was a complete dissociation: the d
irect terms peaked, whereas the coupling terms dropped. The direct ter
ms of angular viscosity also increased before impact, whereas the visc
osity coupling terms remained close to zero throughout. 5. Neural corr
elates of the changes in angular impedance were found by considering t
he time course of the changes in net electromyographic activity and st
retch reflexes during catching. Anticipatory muscle activity started 1
00-200 ms before impact and correlated qualitatively with anticipatory
changes in angular stiffness. The peaks of the direct terms of stiffn
ess and viscosity around impact could be accounted by the transient re
versal of the direction of short-latency stretch reflex responses. The
decrease of the coupling terms of stiffness around impact could be ex
plained by a transient decrease of the gain of heteronymous stretch re
flexes. 6. The matrices of the coefficients expressing stiffness and v
iscosity in the Cartesian coordinates of the limb endpoint were also c
omputed. From such matrices, the components corresponding to the vecto
rs of resistance offered by the hand to a virtual vertical displacemen
t were extracted. We found that the hand resistance is accurately modu
lated relative to the impact time. The magnitude of hand resistance ve
ctors increased consistently before impact, although with a different
time course for hand stiffness and viscosity. Also before impact, the
direction of viscous resistance vectors rotated closer to the vertical
, indicating that a larger component of reactive force is exerted in t
he direction of the expected perturbation. 7. The orientation of the v
ectors of hand viscosity was variably correlated with the orientation
of the vectors of hand inertia during catching. This result suggests t
he existence of a parallel neural control of different components of h
and impedance, that is inertia, stiffness, and viscosity. This paralle
l control is predicated on the availability of accurate internal model
s of the limb mechanical properties.