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Active vs. Passive Movements for Control Signals
  • Neuroengineering
  • Human
  • Grade 6+

Active vs. Passive Movements – Does the Brain Need to Initiate EMG Signals?

Can muscles spark electrical activity without direct brain commands? Explore the difference between voluntary (active) muscle contractions and externally induced (passive) movements to reveal some secrets of the EMG.

Devices and Accessories3Related Experiments2

About the Experiment

What Will You Learn?

What Will You Learn?

  • How EMG differs between active and passive muscle movement.
  • How the brain contributes to muscle electrical activity.
  • Ways to distinguish neurally driven from passive EMG responses.
Background

Background

EMG is all about the electrical chatter of muscles. But what if those muscles are moving without our input? Via the Human-Human Interface, you'll see whether EMG depends on neural drive.

Procedure

  1. Safety First:

    Both volunteers must understand the procedure and agree to take part. The Minion must not take part with a pacemaker or other implanted electronic device, metal implants, epilepsy, a heart condition, or pregnancy. Turn the HHI off before attaching, moving or removing stimulation electrodes; start at the lowest intensity and raise it slowly; stop at once if it hurts or either volunteer asks. Keep both stimulation electrodes on one forearm—never across the chest, throat, neck or head.

  2. Electrode Placement:

    1. Attach electrode patches to the Controller’s forearm and connect to the Human-Human Interface.
    2. Connect the stimulation electrodes to the Minion.
    3. Verify an EMG signal from the Controller causes the minion to flex.

  3. Active Condition:

    1. Flex the muscle voluntarily.
    2. Record EMG amplitude, frequency and pattern and see how much the Minion moves.

  4. Passive Movement:

    1. Completely relax the muscle.
    2. Have a partner passively move the limb while recording EMG.
    3. Compare the passive EMG with the active condition and see how much the Minion moves now.
Results & Analysis

Results & Analysis

Do EMG bursts show up if the arm is moved by someone else? Did the Minion still respond? How did the two conditions compare in terms of EMG power? Does lacking the brain’s intent‑to‑move signal also mean no EMG signal?

Draw the signal pathway for each condition. When the Controller flexes, where does the signal start? When a partner moves the arm, which part of the path is missing? Label where the EMG electrodes recorded.

Design Challenge

  1. No Ghost Commands

    A good neuroprosthetic moves only when its user means to move, never because someone bumped their arm. Can you design a Controller setup that ignores passive movement?

    1. Rank your criteria: no false triggers, a strong response to every real flex, and comfort for both volunteers. Either volunteer can call stop at any time.
    2. Test at least three setups: different electrode placements on the Controller’s forearm, or different settings of the button on the back of the HHI. For each, count how often the Minion moves during 10 real flexes and during 10 passive movements.
    3. Put your results in a table, combine the best features into one final setup, and test it again.
    4. Explain your trade-offs: for example, a setting that ignores bumps but needs a harder squeeze.
    5. Judge your final setup for a real user: what would it cost, is it safe to wear all day, and who could or could not use it—for example, someone whose arm shakes?

NGSS alignment

Performance expectations

  • MS-LS1-3 Use argument supported by evidence for how the body is a system of interacting subsystems composed of groups of cells.

    EMG appears only when the nervous system drives the muscle: two subsystems interacting.

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

    Draw where the signal starts in each condition: brain, nerve and muscle as one system, EMG depending on neural drive.

  • MS-ETS1-2 Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.

    Score at least three controller setups against ranked criteria in one table (design challenge).

  • MS-ETS1-3 Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.

    Test at least three controller setups, compare false triggers and real responses in a table, and combine the best features (design challenge).

  • HS-ETS1-3 Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics, as well as possible social, cultural, and environmental impacts.

    Rank no false triggers, a strong response and comfort, explain the trade-offs, and judge the final setup for a real user (design challenge).

Science and engineering practices

  • 1 Asking Questions and Defining Problems

    Define the problem: a controller that moves the Minion only when the Controller means to move (design challenge).

  • 2 Developing and Using Models

    Draw the signal pathway for the active and the passive condition and mark which part is missing when a partner moves the arm.

  • 3 Planning and Carrying Out Investigations

    Record EMG and Minion movement during active flexes and passive movements.

  • 4 Analyzing and Interpreting Data

    Compare EMG power and the Minion's response between the two conditions.

  • 6 Constructing Explanations and Designing Solutions

    Design a final setup from the test data and explain it (design challenge).

Crosscutting concepts

  • Cause and Effect

    Neural drive, not movement alone, produces EMG.

  • Systems and System Models

    Controller, device and Minion form one loop.

  • Influence of Science, Engineering, and Technology on Society and the Natural World Engineering connection

    A neuroprosthetic must respond only when its user means to move.

Disciplinary core ideas

  • LS1.D Information Processing

    Motor commands come from the brain.

  • ETS1.B Developing Possible Solutions

    Test several solutions and weigh false triggers against response.

  • ETS1.C Optimizing the Design Solution

    Use test results to pick and combine the best design.

Backyard Brains mapping using the book's method. Not in the book. Engineering claims rest on the design challenge on this page (Greg, 2026-10-06; real-user judgment step added 2026-10-08), modelled on the SpikerBot curriculum.