Clasp-knife response

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Clasp-knife response is one of the characteristic responses of a upper motor neuron 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.

1 Cause
2 See also
3 References
4 External links

[edit] 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 is an extensor muscle that becomes stretched^[1]. In upper motor neuron lesions, muscle tonus may increase and resistance of muscle to stretch increases. However, if sufficient force is applied, limb resistance suddenly decreases^[1].