Postural thixotropy at the human hip.

In these experiments the person relaxed on his back with one leg free to swing in the horizontal plane into abduction or adduction; the weight was taken by a cord attached to the ceiling over the hip joint. Records of acceleration following a displacement showed a series of decrementing swings which could last for more than 10 s. The imposition of sinusoidal torques generated by a printed motor showed that the system was non-linear for when small torques were used the resonant frequency rose indicating stiffening. The use of a biasing force to move the mean position of the leg showed that the stiffening was not related to any particular position and was the result of the low level of movement. Under certain conditions the stiffening could be abolished by larger active or passive movements. Even a single active or passive movement could be effective. Such a history-dependent compliance was an indication of postural thixotropy. It was present and apparently normal in patients with artificial joints. Reasons are given for believing that the effects are a reflection of muscle properties.