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The Backyard Brains Reflex Hammer
  • Human
  • Grade 9+

The Patellar Reflex: Reflexes vs. Reactions

Tap just below a kneecap and the leg kicks before you decide anything. In this experiment you’ll record that knee-jerk with the Human SpikerBox and Reflex Hammer, then race it against a reaction you choose to make.

About the Experiment

What Will You Learn?

  • How a reflex arc turns a stretch into a kick without asking the brain first.
  • How to record the patellar reflex with EMG and measure its speed in milliseconds.
  • Why a reflex beats a voluntary reaction, and what that says about spinal and brain pathways.

Background

When a reflex hammer taps the tendon just below your kneecap, it stretches the big quadriceps muscle on the front of your thigh. Stretch receptors in that muscle, the muscle spindles, fire off a message to your spinal cord, which sends a command straight back to the same muscle: contract! Your leg kicks before your brain has had a say. This shortcut is a reflex arc, and it protects you by keeping your muscles at the right length without a moment’s thought. A voluntary reaction is different: the signal has to travel up to your brain, be noticed and turned into a decision, then travel all the way back down. Which is faster, and by how much? Let’s measure it.

Procedure

  1. What You Need

    • Human SpikerBox with its orange electrode cables, sticker electrodes and electrode gel
    • Reflex Hammer (it plugs into the Human SpikerBox; it is not a stand-alone product)
    • SpikeRecorder on a computer, an Android device, or an iPhone or iPad with a USB-C connector (the Human SpikerBox connects to it over USB-C)
    • A table or high chair where your subject can sit with their legs dangling

  2. Electrode Placement

    1. Have your subject sit on the edge of a table so both lower legs hang freely.
    2. Place two sticker electrodes on the front of one thigh over the quadriceps, one on the inner muscle and one on the outer, and connect the red clips of channel 1 to them.
    3. Place a ground electrode on the back of the hand and connect the black clip to it.
    4. Plug the Reflex Hammer into the Human SpikerBox, connect to SpikeRecorder, and check that you see the muscle signal and the hammer’s trace.

  3. Record the Reflex

    1. Ask your subject to relax, close their eyes or look away, so they cannot anticipate the tap.
    2. Tap the tendon just below the kneecap with the Reflex Hammer. Its built-in accelerometer puts a mark in your recording the instant the hammer strikes.
    3. Record at least ten taps, a few seconds apart.
    4. For each tap, measure the time from the hammer’s mark to the start of the muscle signal: that is your reflex time.

  4. Reflex vs. Reaction

    1. Put a second pair of electrodes on the other thigh and connect them to channel 2.
    2. Ask your subject to kick with that other leg as soon as they feel the tap on the first.
    3. Record ten more taps and, for each, measure the time from the hammer’s mark to the start of the muscle signal on each channel.
    4. Average the reflex times and the reaction times for each person.

Results & Analysis

Compare your average reflex time with your average reaction time. Which is faster, and by how much? In Backyard Brains’ earlier version of this lab, reflexes landed around 15–30 milliseconds and voluntary reactions much later, around 130–200 milliseconds.

Draw two pathway models, one for the reflex (receptor, spinal cord, muscle) and one for the reaction (receptor, spinal cord, brain, spinal cord, muscle), and use them to explain your numbers. Is the difference between your two averages bigger than the spread within each?

How does a fast, automatic reflex help keep your body stable when you stumble? This is a classroom investigation, not a medical test.

NGSS alignment

Performance expectations

Reflex arcs as protective feedback maintaining homeostasis.

  • MS-LS1-8 Gather and synthesize information that sensory receptors respond to stimuli by sending messages to the brain for immediate behavior or storage as memories.

  • HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.

  • HS-LS1-3 Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis.

Science and engineering practices

  • 2 Developing and Using Models

    Draw and compare reflex-arc and voluntary-pathway models.

  • 3 Planning and Carrying Out Investigations

    Record at least ten patellar reflexes, then the two-leg reflex-versus-reaction protocol.

  • 4 Analyzing and Interpreting Data

    Measure reflex and reaction latencies from the recordings.

  • 5 Using Mathematics and Computational Thinking

    Average latencies in milliseconds and compare them.

  • 6 Constructing Explanations and Designing Solutions

    Use the two pathway models to explain the measured times.

  • 7 Engaging in Argument from Evidence

    Judge whether the difference between the two averages is bigger than the spread within each.

Crosscutting concepts

  • Cause and Effect

    A tap triggers the arc.

  • Systems and System Models

    Spinal vs. brain pathway.

  • Structure and Function

    Muscle-spindle stretch receptors.

Disciplinary core ideas

  • LS1.D Information Processing

    Spinal processing of some responses.

  • LS1.A Structure and Function

    Reflex circuitry.

Performance expectations, crosscutting concepts and core ideas from Hands-On Neuroscience Curriculum Overview, Unit 2, module 2.4, whose core lab this is. The practices are the ones this page asks students to do.