Experiment: The Patellar Reflex and Reaction
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Experiment: The Patellar Reflex and Reaction

Time 30 Minutes
Difficulty Intermediate

What will you learn?

We can use this demonstration to learn how reflexes work. Many important motor reflexes are processed by the spinal cord and occur without any input from our brain. Motor reflexes are involuntary, which we will see here. Make someone move without their control! (again)

Prerequisite Labs


Based on experiments and writing by Colorado State University student Breonna E. Holland, Associate Professor Brian L. Tracy, and CSU's Muscles Alive! outreach program.

Reflexes require no thought. They are automatic, fast, and of huge importance to a human's ability to successfully respond to their environment. Despite the magnificent information-processing power of the billions of neurons in our brain, we need a lot of stuff to be done automatically. Without reflexes, our brains would be overloaded with worrying about constantly updating the position of our unstable bodies to keep us upright. Without reflexes, our ability to engage in complex thought (black holes, neuroscience, what to do this weekend, how do I make an instrumented reflex hammer?) would be limited. Without reflexes, your reactions to painful stimuli would require thought, and... don't take it personally.. but you're WAY too slow a thinker-all humans are, to respond to dangerous painful stimuli. So we let our spinal cord do that fast work for us.

One example of a reflex is the patellar stretch reflex. Our spinal cord partners with sensors in our muscles, called muscle spindles, to keep track of where our bodies are in space and how stretched or contracted our muscles are. The way that these sensors interact with our spinal cord is through a reflex pathway. Stretching the muscle activates the muscle spindle at the end of the sensory neuron (embedded in your muscle) and starts the reflex.

As you can see, there is only one connection (a synapse) needed for the information from the sensory neuron to get to the motor neuron and cause a muscle contraction. Because of this single synapse, this can happen very fast. In a young, healthy person, it takes 15-30 milliseconds for the stretch stimulus to produce a muscle contraction, by comparison it takes 5-10 times that long to blink your eye in responses to a stimulus, or 150-300 milliseconds. This is super useful for correcting your muscle length in response to rapid changes such as a slip or trip. These situations require very fast corrections to prevent falling and injury. If you had to consciously flex your leg in response to the leg stretch (a reaction) it would be much slower than the 15-30 seconds of reflex.

Now let's try to measure this reflex! And perhaps let's measure a reaction too!

Before you begin, make sure you have the Backyard Brains Spike Recorder installed on your computer/smartphone/tablet. The Backyard Brains Spike Recorder program allows you to visualize and save the data on your computer when doing experiments. We have also built a simple lab handout to help you tabulate your data.



For this experiment we are going to look at the electromyographic signal in the rectus femoris muscle, one of the muscles in the quadriceps femoris group. We elicit a reflex using a tap to the patellar tendon. This is the same test that is done when you go to the doctor's office (the doctors use it as a quick test to determine spinal pathway health - problems can be symptomatic of neuromuscular diseases).

  1. Set up your Muscle SpikerBox Pro using a smartphone or computer with the Spike Recorder app.
  2. Connect the USB cable from the the Muscle SpikerBox Pro to the computer. Open Spike Recorder, and turn on your Muscle SpikerBox Pro. A USB icon should appear on the main SpikeRecorder screen. Click on it to connect to your Muscle SpikerBox Pro. (Note: If you are using a tablet, smartphone, and/or a Regular Muscle SpikerBox, you can connect to your tablet/smartphone/computer with the green smartphone audio cable.
  3. Have the subject sit on a sturdy surface high enough to allow their lower legs and feet to dangle freely.
  4. Have the subject contract their quadriceps (knee extensor) muscle so you can place two adhesive electrodes close together over the belly of the rectus femoris muscle as shown in below. Place a ground adhesive electrode on the back of the hand.
  5. Clip the red leads of your orange muscle cable of the electrodes on the muscle of your thigh, and the black lead on the ground on your outer palm.
  6. Getting a good quality muscle signal is really important for this experiment. Test to see if you get a good EMG signal with a fairly small contraction by asking the subject to straighten their knee against resistance. Poor signal? Adjust the vertical zoom on SpikeRecorder. If that still doesn't help, adjust the electrode placement.
  7. Locate the part of the patellar tendon just below the kneecap and above where it inserts into the shinbone. The best place to tap it is just below the kneecap. If you like this person, avoid hitting the knee cap because...ouch! Use a quick, firm tap to the tendon to elicit an involuntary kick out. Experiment with small changes in location and tap force. Find the sweet spot.
  8. . If you have good electrode placement and apply a smart tap to the patellar tendon, you should see something like this:
  9. Make a recording and zoom in over one of the reflex signals. You can see that the EMG signal looks much like other EMGs you have seen:
  10. Measure the average width of the clicking and dragging on the screen over everything we want to capture. You can see that this particular reflex EMG lasted approximately 28.8 milliseconds.
  11. But...Do you think you could you detect a time difference between the tendon tap and the EMG signal on the screen? Probably not. It seems like it happens immediately! But could we actually measure it?

    Advanced Version - Measure Reflex time

  12. Yes! We can measure the reflex time if we build a reflex hammer with an integrated button. You can buy these small buttons at your local electronics store or online.
  13. Affix the button to the hammerhead and solder on some wires of approximately 1 meter.
  14. We actually have prototypes of an expansion board for the Muscle SpikerBox Pro. Let us know if you would like one for this experiment.
  15. Have the two wires from the reflex hammer go into the event marker 1 screw pairs (also see video above).
  16. Connect your expansion board to the Muscle SpikerBox Pro.
  17. Now, every time you tap your reflex hammer to the person's patellar tendon, an event marker will appear, and you can measure the reflex time! Use the handout link above to have a form to allow you to easily tabulate the data.

    Advanced Version II - Now Measure Reaction time

  18. We can even measure the time different between "brain circuits" (reaction) and "spinal cord circuits" (reflex) in this patellar reflex!
  19. Tell the research subject to close their eyes and to intentionally kick when they feel the patellar tap. Make sure you are recording the file and tap the tendon. Repeat the tap 4-6 times, spaced about 5-10 seconds apart between taps.
  20. Open the recording file, and if you get a good patellar reflex in your subject, you should very easily see the difference between the reflex component and the reaction component. That's the time difference between your spinal cord and brain circuits!

Troubleshooting Suggestions

There is large variability between subjects in how well the EMG signal can be seen. As a last resort you may need to try the demonstration with a different volunteer.If you are still having trouble, it may be necessary to move the placement of the electrodes. The vastus medialis tends to work well if rectus femoris is not giving a good EMG signal. See below:

Science Fair Project Ideas

  • What are some other reflexes you think you could test?
  • Does the speed/amplitude of the response change depending on the state someone is in? For example, if they are tired, have a lot of energy, had coffee recently, etc. Why do you think these do or don't affect the reflex?
  • Don't hurt yourself or anyone else investigating this, but does the speed or amplitude depend at all with the force you strike the knee with? Why might this be or not be?
  • For this experiment, the person having their reflex activated normally keeps the leg relaxed, but what if they try to resist the reflex? Does that affect the speed or amplitude at all, or does the reflex even happen? Why might this be?
  • We have noticed two things worth further examination. The reflex time tends not to be very variant in a single person during a session (always +-2 ms), but the reflex time between individuals can vary from 13 ms to 35 ms (the two figures above showing the reflex are from two different people, 26 ms and 13 ms, for example). Since this is an unconscious reflex, what may be the reasons for such variability between individuals? Age, fitness, athletes (runners vs weightlifters, etc). Also the reaction component tends to be more varied trial to trial within a person (from 130 - 200 ms), as this requires conscious control and "attention" which increase time variability between trials. Start collecting data from all the people you know!


    If you have our very first original Muscle SpikerBox with 3 separate leads (not a single orange cable), the colors of the alligator clips are different. The Red and Black clips should go around the metal tabs on the popsicle sticks, while the white clip goes around the screw you are gripping with your hand.