Ever wondered how it feels to control—or be controlled by—someone else? In this lab you’ll dive into the world of neural communication and motor control with our Human-Human Interface experiment.
Moving lets us explore the world—walking, dancing, or even controlling a robotic limb. Neurons in the motor cortex fire, signals travel down the spinal cord, and muscles contract. By linking two participants, you’ll experience how those signals can “borrow” another person’s muscles, revealing the power of functional electrical stimulation and the promise of neuroprosthetics.
Materials:
Setup: Two volunteers choose roles: Controller (sends the signal) and Minion (receives stimulation).
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.
Place two EMG patch electrodes on the forearm flexor muscle and a ground patch on the back of the hand. Clip the red leads to the muscle patches and the black lead to ground. Plug the orange cable into the HHI and turn it on. Ask the Controller to flex; all LEDs should light with a strong squeeze. If needed, use the button on the back of the HHI to change how strong a flex must be to trigger it.
Turn off the HHI. Place the two square stimulation electrodes over the ulnar nerve on the inner (pinky) side of the forearm, just below the elbow (adjust as needed). Connect the black FES cable to the HHI’s Minion port. When ready, power the device back on.
Power on the HHI with the intensity set low. The Controller flexes to light the red LED (threshold). The Minion keeps their forearm relaxed at 90° elbow flexion. Gradually turn the intensity knob until the Minion’s hand twitches—this is the threshold. If the Minion is comfortable, increase intensity to evoke a stronger motion. If stimulation is felt but movement is absent, adjust electrode placement.
Describe the sensations as Controller and as Minion. Measure reaction threshold (LED level vs. intensity dial) and note latency between flex and Minion movement. Discuss how functional electrical stimulation can bridge damaged neural pathways and how signal strength maps to muscle activation.
Draw the signal pathway from the Controller’s motor cortex to the Minion’s hand: motor cortex, spinal cord, the Controller’s forearm flexor muscle, the HHI, the Minion’s ulnar nerve and hand muscles. Label where you recorded the EMG and where the device takes over from the nervous system.
Engineers who build neuroprosthetics face a real trade-off: enough stimulation to move a hand every single time, but never more than the person wearing it finds comfortable. Can your team find the sweet spot?
MS-LS1-3 Use argument supported by evidence for how the body is a system of interacting subsystems composed of groups of cells.
One person's nervous system drives another person's muscles: nerve and muscle subsystems working together.
HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.
Draw the pathway from the Controller's motor cortex to the Minion's hand: brain, spinal cord, nerve and muscle working as one system across two bodies.
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 electrode placements 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 electrode placements, compare their data in a table and combine the best features into a final setup (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 reliability, comfort and speed, explain the trade-offs, and judge the final setup for a real user: cost, everyday safety, who could use it (design challenge).
1 Asking Questions and Defining Problems
Define the problem: write and rank criteria before testing (design challenge).
2 Developing and Using Models
Draw the signal pathway from the Controller's motor cortex to the Minion's hand and mark where the device takes over.
3 Planning and Carrying Out Investigations
Set the Controller's EMG threshold and find the intensity at which the Minion's hand moves.
4 Analyzing and Interpreting Data
Compare LED level with the intensity dial and note the delay between flex and movement.
6 Constructing Explanations and Designing Solutions
Design a final setup from the test data and explain it (design challenge).
Cause and Effect
The Controller's flex causes the Minion's hand to move.
Systems and System Models
Two people and a device form one control loop.
Influence of Science, Engineering, and Technology on Society and the Natural World Engineering connection
Functional electrical stimulation can bridge damaged neural pathways.
LS1.D Information Processing
Motor commands are electrical signals carried by nerves.
LS1.A Structure and Function
Nerves and muscles are specialized structures that work together.
ETS1.B Developing Possible Solutions
Test several solutions and weigh comfort against reliability.
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.
