knee jerk

Last reviewed 01/2018

The knee jerk reflex is supplied by the L3 and L4 nerve roots.

The examiner:

  • slides his one arm under the patient's knees so they are supported and slightly bent
  • lets the tendon hammer fall onto the infrapatellar tendon
  • an intact reflex arc results in contraction of quadriceps and extension of the knee

If the reflexes appear to be absent then ask the patient to interlock his fingers and then to pull them hard apart - 'reinforcement'. While the patient is doing this, the examiner tests for the reflexes again.