Knee jerk -method of elicitation

Knee jerk is an example of deep tendon reflex of the lower limb. It is a stretch reflex of the quadriceps femoris muscle, because the tap on the tendon stretches the muscle
Method of elicitation of knee jerk
Methods of Eliciting Knee jerk.It can be done in the following methods

• Both knees flexed and resting on the forearm of the examiner
• Sitting position with the knee hanging down
• Sitting position with legs crossed

There are 2 commonly used method of eliciting knee jerk
1.Conventional method of elicitation

• Patient lies supine with both the knee joints flexed. 
• Now place your forearm below both knees (preferable), or pass your hand below' one knee (to be tested) and place it upon the opposite flexed knee. 
• Feel the quadriceps tendon and apply a sharp tap there. 
• The quadriceps muscle contracts with extension of the knee.

2.Special method of elicting knee jerk

• Patient sits on a chair with the legs hanging free side by side. 
• After tapping the patellar tendon, look for the pendulous movement of the legs. 
• This is classically seen in cerebellar lesion and sometimes in chorea (indicate hypotonia). Hung-up reflex may be seen by this special method .
• The patellar' hammer derives its name from knee jerk (because the patellar tendon is tapped)
• as it was the first tendon reflex which was regularly practised for neurological examination.

Always perform the  jendrassik's manoeuvre  before declaring a tendon reflex absent .

Knee Jerk root value
Innervation  of knee jerk -L3, L4
How will you grade knee jerk?
The knee jerk reflex is an example of a deep tendon reflex (DTR) in a neurological exam and is graded on the following scale:

• 0 (absent),
• 1+ (hypoactive),
• 2+ (brisk, normal),
• 3+ (hyperactive without clonus),
• 4+ (hyperactive with mild clonus), and
• 5+ (hyperactive with sustained clonus).

Abnormalities of knee jerk 
Absent knee jerk 
Pendular Knee Jerk
Hung up Knee jerk
Brisk knee reflex