Journal of Applied Physiology, Vol 62, Issue 4 1615-1621, Copyright © 1987 by American Physiological Society
Mechanism of thixotropic behavior at relaxed joints in the rat
A. W. Wiegner
When a relaxed joint is subjected to a small sinusoidal torque, the
amplitude of the steady-state displacement response is increased up to
severalfold by a transient larger perturbation. The original state, in
which the relaxed joint is unexpectedly stiff, is restored by several
seconds of inactivity. This thixotropic phenomenon has previously been
observed in a variety of human joints. We have now investigated the
mechanism of thixotropic behavior at relaxed joints in rats anesthetized
with pentobarbital sodium, by using a series of preparations including the
intact ankle joint, a blood-perfused soleus muscle preparation, an isolated
soleus muscle, and ankle joint isolated by severing all muscular
attachments. Thixotropic behavior was observed in all intact, isolated
muscle, and isolated joint preparations. The contribution of the joint to
thixotropic behavior was comparable to, and at times exceeded, the
contribution of muscle. We also analyzed the short-range stiffness
properties of relaxed, blood-perfused soleus muscles and found them to be
similar to thixotropy with respect to range of action (0.2-0.3% of muscle
length), elastic modulus (approximately 4 kg/cm2), and time course for
redevelopment (time constant = 2.5 s at 34 degrees C). Thus thixotropic
behavior at a relaxed joint may be attributed both to the joint structures
and to short-range stiffness of muscles acting at the joint.
