Clasp-knife response

Overview
Clasp-knife response is one of the characteristic responses of a upper motor neurone lesion. It refers to a stretch reflex with a rapid decrease in resistance when attempting to flex a joint, usually during a neurological examination.

It gets its name from the resemblance between the motion of the limb and the sudden closing of a claspknife after sufficient pressure is applied.

Cause
Although seemingly a stretch reflex when flexing a joint, force from the muscle during the attempt to flex a joint is actually thought to be caused by the tendon reflex of the antagonistic muscle of that joint, which gets extended. In upper motor neurone lesions, muscle tonus may increase resistance of muscle to stretch increases. However, if sufficient force is applied, limb resistance suddenly decreases.