Clasp-knife response

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.

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 ^[1]. 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^[1].

• Hypertonia

• http://www.lib.mcg.edu/edu/eshuphysio/program/section8/8ch3/s8ch3_21.htm
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