Knee Jerk Reaction (Origin)

by Craig Shrives

What Is the Origin of the Saying "Knee Jerk Reaction"?

The term "knee-jerk reaction" means a quick, automatic response.

The idiom "knee-jerk reaction" refers to the patellar reflex, which is an automatic extension of the leg that occurs when the tendon below the knee is struck.

The term has been used figuratively since the early 20th century as a metaphor for a sudden, unconsidered response. For example:

Selling my travel-industry shares was a knee-jerk reaction to the first Covid-19 lockdown.
Introducing the legislation was a knee-jerk reaction to the bombing.

Patellar Reflex

The patellar reflex is a deep tendon reflex, mediated by nerves in the spinal cord. The patellar reflex occurs when an abrupt change arises in muscle length. This is produced by the tendon stretching when the hammer stroke is applied.

Of note, the term "knee-jerk" should be hyphenated when it describes "reaction." From a grammatical perspective, it is a compound adjective.

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