Knee Jerk Reflex (Patellar Reflex)
The knee jerk reflex is one that you may have had tested at a check up at the doctor's
office. In this test, the doctor hits your knee at a spot just below your knee cap and your
leg kicks out. Try it! Have a partner sit with his or her legs crossed so that his leg can
swing freely. Hit his leg just below the knee with the side of your hand. DO NOT USE A
HAMMER!!!! The leg will kick out immediately (if you hit the right place).
The knee jerk reflex (seen in the figure to the right) is called a monosynaptic reflex
because there is only one synapse in the circuit needed to complete the reflex. It only
takes about 50 milliseconds between the tap and the start of the leg kick. That is fast! The
tap below the knee causes the thigh muscle to stretch. Information is then sent to the
spinal cord. After one synapse in the ventral horn of the spinal cord, the information is
sent back out to the muscle...and there you have the reflex.
Materials
How Fast are You?
Unlike the other activities on this reflex page, this project does not
test a simple reflex. Rather, this activity is designed to measure your
response time to something that you see.
Get a ruler (or a yardstick or candy bar). Hold the ruler near the end
(highest number) and let it hang down. Have another person put his or her hand at the
bottom of the ruler and have them ready to grab the ruler (however, they should not be
touching the ruler). Tell the other person that you will drop the ruler sometime within the
next 5 seconds and that they are supposed to catch the ruler as fast as they can after it is
dropped. Record the level (inches or centimeters) at which they catch the ruler (you can
convert the distance into reaction time with the chart below). Test the same person 3 to 5
times (vary the time of dropping the ruler within the 5 second "drop-zone" so the other
person cannot guess when you will drop the ruler).
Reflex reference
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Here is a table to convert the distance on the ruler to reaction time. For example, if you
caught the ruler at the 8 inch mark, then your reaction time is equal to 0.20 seconds (200
ms). Remember that there are 1,000 milliseconds (ms) in 1 second.
Distance
Time
2 in (~5 cm)
0.10 sec (100 ms)
4 in (~10 cm)
0.14 sec (140 ms)
6 in (~15 cm)
0.17 sec (170 ms)
8 in (~20 cm)
0.20 sec (200 ms)
10 in (~25.5 cm) 0.23 sec (230 ms)
12 in (~30.5 cm) 0.25 sec (250 ms)
17 in (~43 cm)
0.30 sec (300 ms)
24 in (~61 cm)
0.35 sec (350 ms)
31 in (~79 cm)
0.40 sec (400 ms)
39 in (~99 cm)
0.45 sec (450 ms)
48 in (~123 cm)
0.50 sec (500 ms)
69 in (~175 cm)
0.60 sec (600 ms)
If you want to be more precise with your calculations, use the following formulas:
Formula 1
Formula 2
Formula 1 provides you with the distance an object will fall in a
given amount of time. By rearranging Formula 1 into Formula 2,
you can get the amount of time it takes an object to fall a certain
distance...that's what you want to find out. All you have to do is
plug in the distance (in either centimeters or inches) that the
ruler fell into Formula 2 - this will give you the reaction time.
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In the formulas, t = time (in seconds); y = distance (in cm); g =
980 cm/sec2 (acceleration due to gravity). [Note: you can also
use inches in your distance measurement, but you must change g
to equal 385.8 in/sec2.]
This reaction time experiment required visual information (the movement of the ruler) to
travel to your brain. Then your brain sent a motor command ("grab that falling ruler") to
the muscles of your arm and hand. If all went well, you caught the ruler!!
1. Ankle Jerk Reflex - tests the medial popliteal nerve
a. Have the Subject stand on one leg alongside a chair, with the other leg bent at
the knee and its shin resting on the seat of the chair. The foot should project over
the edge of the chair.
b. Connect the SS2L leads to the calf (as seen in the picture above).
Lead Color
Signal
Position
Red
(+)
closest to knee
White
(-)
middle of calf
Black
(ground)
inside of ankle
c. Click on the "Start" button to begin recording.
d. Strike the Achilles tendon behind the ankle just above the heel and observe the
resulting muscle contraction.
e. Repeat 10 times and note reaction times in lab report.
Note: Reaction time is measured from onset of hammer strike to onset of EMG
activity.
f. Click on the "Stop" button to suspend recording.
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2. Knee Jerk Reflex - tests the quadriceps femoris tendon
a. Move the SS2L leads to the knee reflex electrodes (as seen in picture
above).
Lead Color
Signal
Position
Red
(+)
middle electrode
White
(-)
closest to waist
Black
(ground)
on the knee
b. Have the Subject sit with his or her legs hanging freely over the edge of
the chair.
c. Optional: Attach a goniometer (SS20L) to the outside of the knee (as
seen in picture above).

This will measure angular movement of the leg which is
proportionate to the force of the strike from the reflex hammer.
d. Before you continue recording, find the optimal spot on the subject's
knee that will cause a good reflex and mark the spot.
e. Click on the "Start" button to resume recording.
f. Strike the patellar ligament and observe the resulting reflex contraction.
g. Repeat 10 times and note reaction times and amplitudes in lab report.
Note: In order to get an accurate goniometer reading, it is crucial that the
leg come to a resting position between strikes.
h. Click on the "Stop" button to suspend recording.
1. The arm can be placed in either of 2 positions:
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a. Gently pull the arm out from the patient’s body, such that it roughly forms
a right angle at the shoulder. The lower arm should dangle directly
downward at the elbow.
Triceps Reflex, arm supported
b. Have the patient place their hands on their hips.
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Triceps Reflex, arm unsupported
2. Either of these techniques will allow the triceps to completely relax.
3. If you are certain as to the precise location of the tendon, strike this area directly
with your hammer. If the target is not clearly apparent or the tendon is surrounded
by an excessive amount of subcutaneous fat (which might dissipate the force of
your strike), place your index or middle finger firmly against the structure. Then
strike your finger.
4. Make sure that the triceps is uncovered, so that you can observe the response. The
normal reflex will cause the lower arm to extend at the elbow and swing away
from the body. If the patient’s hands are on their hips, the arm will not move but
the muscle should shorten vigorously .
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