Clasp-Knife Response

What Is It, Causes, Assessment and More

Author:Georgina Tiarks

Editors:Alyssa Haag,Emily Miao, PharmD,Kelsey LaFayette, DNP, RN

Illustrator:Jessica Reynolds, MS

Copyeditor:Sadia Zaman, MBBS, BSc

What is a clasp-knife response?

The clasp-knife response describes an initial resistance when attempting passive movement (i.e., movement performed by another person) of the extremities, which is followed by a rapid decrease in resistance. The name “clasp-knife” comes from the resemblance between the motion of suddenly closing a clasp-knife after sufficient pressure is applied and the passive movement of a limb. It commonly presents due to damage to the upper central nervous system. 

Cartoon illustration of clinician attempting to flex and extend patient's arm.

What causes a clasp-knife response?

The clasp-knife response indicates an upper motor neuron lesion or damage. The upper motor neurons originate in the cerebral cortex (i.e., the outermost portion of the brain) and travel down toward the spinal cord. The lower motor neurons are subsequently located in the spinal column and after being innervated by the upper motor neurons, they send signals to the effector muscle. The upper motor neuron lesion or damage thus prevents muscle inhibition, which results in increased muscle tone. 

Originally, it was thought that the golgi tendon organs (GTOs) played a role in the clasp-knife reflex. GTOs are mechanoreceptors used to sense tension within skeletal muscle. When the tension becomes elevated, the golgi tendon reflex, also known as the inverse stretch reflex, produces an inhibitory action to protect the muscle from injury. In the clasp-knife reflex, it has been postulated that the GTO increases resistance until sufficient force is applied to overcome it. However, newer studies suggest that the golgi tendon organ may not be involved, and stretch sensory receptors may actually play an essential role. 

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How is the clasp-knife response assessed?

The clasp-knife response is assessed during a neurological exam by a healthcare provider. The provider will be able to visualize the reflex while passively attempting to flex a joint. After a few degrees of joint movement in the beginning, there will be excessive resistance to movement. However, after enough pressure has been applied, resistance will suddenly decrease, and the limb will move rapidly in the intended direction. More rapid movement of the joint will increase the degree of resistance while simultaneously decreasing its time of onset.

What are the most important facts to know about the clasp-knife response?

The clasp-knife response describes an initial resistance when attempting passive movement of the extremities, followed by a rapid decrease in resistance. It is often assessed during neurology examinations by healthcare providers and indicates upper motor neuron damage. This clasp-knife response is usually greatest in the flexors of the upper extremity and extensors of the lower extremity. While several mechanisms have been suggested, the cause of clasp-knife reflex is still largely unknown. 

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Related links

Muscular system anatomy and physiology
Muscle contraction
Hypokinetic movement disorders: Clinical practice

Resources for research and reference

Cleland, C. L., Hayward, L., & Rymer, W. Z. (1990). Neural mechanisms underlying the clasp-knife reflex in the cat. II. Stretch-sensitive muscular-free nerve endings. Journal of Neurophysiology, 64(4): 1319–1330. DOI: 10.1152/jn.1990.64.4.1319

Cleland, C. L., & Rymer, W. Z. (1993). Functional properties of spinal interneurons activated by muscular free nerve endings and their potential contributions to the clasp-knife reflex. Journal of Neurophysiology, 69(4): 1181–1191. DOI: 10.1152/jn.1993.69.4.1181

En-Academic. (2010). Clasp-knife response. In Academic Dictionaries and Encyclopedias. Retrieved November 17, 2021, from 

Physiopedia. (2022). Golgi Tendon Organ. In Physiopedia. Retrieved November 17, 2021, from 

Roberson, E.D. (2017). Treatment of Central Nervous System Degenerative Disorders. In Brunton L. L., Hilal-Dandan, R., & Knollmann, B. C. (Eds.). Goodman & Gilman’s: The Pharmacological Basis of Therapeutics (13th ed.). McGraw-Hill. 

Suneja, M., Szot, J. F., LeBlond, R. F., & Brown, D. D. (2020). The Nervous System. In DeGowin’s Diagnostic Examination (11th ed.). McGraw-Hill.