Heart rate by accelerometer
This activity allows students to visualize heartbeats through small movements detectable by the accelerometer. It establishes a concrete link between technology and physiology.
Activity Summary
The student places their smartphone on their chest and uses the accelerometer to detect movements caused by heartbeats. By analyzing the resulting graph, it identifies the heart rate at rest, then after physical activity. The student can also try to identify secondary peaks corresponding to the different phases of the cardiac cycle.
Introduction
Every time your heart beats, it generates a tiny mechanical impulse that radiates throughout your body. These microscopic vibrations, invisible to the naked eye, carry a wealth of information about cardiac function. The technique of detecting heartbeats through body surface movements, called ballistocardiography, was invented in the 1930s by Isaac Starr using massive mechanical platforms. Today, the sensitive accelerometer embedded in every smartphone can achieve similar measurements with remarkable precision. The human heart beats approximately 100,000 times per day, each contraction generating forces strong enough to produce detectable chest wall motion of a few tenths of a millimeter. By simply placing a smartphone on the chest, students can transform their device into a rudimentary cardiac monitor, bridging the gap between modern sensor technology and human physiology. This experiment offers a tangible and personal connection to the physics of motion detection while exploring fundamental concepts of cardiac physiology.
Learning Objectives
- Determine the resting heart rate from accelerometer data
- Compare heart rate before and after physical exercise
- Identify the periodic nature of the cardiac signal in accelerometer recordings
- Understand the principle of ballistocardiography and its relationship to the cardiac cycle
Scientific Concepts
Instruments and sensors
Scientific instruments
- Z-axis acceleration (single-axis accelerometer measurement)
Sensors
- Accelerometer
FizziQ Features
- Experiment notebook — Used to save the resting and post-exercise recordings and compare the two heart rates
Required Materials
- - Smartphone or tablet with an accelerometer, using the FizziQ application for recording - Flat surface for lying down - Note: the protocol remains adaptable to any comparable accelerometer-based measurement tool.
Experimental Protocol
Open the FizziQ application and select the **Accelerometer** sensor. Choose the **Z-axis (vertical)** component for the clearest cardiac signal.
Lie down on a flat, stable surface. Place the smartphone **face down on your chest**, centered over the sternum, and remain as still as possible.
Start recording data in FizziQ. Breathe slowly and shallowly to **minimize respiratory movement** that could mask the cardiac signal.
Record for at least **30 seconds** while remaining perfectly still. You should see small periodic peaks on the acceleration graph.
Stop recording and examine the graph. Count the number of **distinct peaks** over a known time interval (e.g., 20 seconds).
Calculate the **resting heart rate** using the formula: HR (bpm) = (number of peaks / recording time in seconds) × 60.
Save this first measurement in your FizziQ experiment notebook as **"Rest"**.
Now perform **2 minutes of moderate physical exercise** (jumping jacks, running in place, or climbing stairs).
Immediately lie down again, place the smartphone on your chest, and start a new recording within **15 seconds** of stopping exercise.
Record for 30 seconds and calculate the **post-exercise heart rate** using the same method.
Save this second measurement as **"Post-exercise"** and compare both heart rates in your notebook.
If possible, zoom into the signal to try to identify **secondary peaks** that may correspond to different phases of the cardiac cycle (atrial contraction, ventricular ejection).
Expected Results
The resting accelerometer signal should show periodic peaks with an interval of 0.6 to 1.0 seconds, corresponding to heart rates of 60-100 bpm for most students. The acceleration amplitude of the peaks typically ranges from 0.01 to 0.05 m/s². After exercise, the heart rate should increase by 30-80% (to roughly 100-160 bpm), and the peaks may appear larger due to stronger cardiac contractions. The signal will contain noise from respiratory movements (slower, larger oscillations at about 0.2-0.3 Hz) and involuntary muscle movements. Students may find it difficult to distinguish individual peaks if there is too much body movement. The post-exercise signal may be noisier due to heavier breathing and residual trembling.
Scientific Questions
- How does the accelerometer signal differ from a clinical electrocardiogram (ECG)?
- What factors might cause variation in heart rate between different students?
- Why does heart rate increase during physical exercise?
- How could you improve the signal quality to reduce noise in the measurement?
- Could this technique be used for continuous heart rate monitoring? What are the limitations?
Scientific Background
The accelerometer of a smartphone can detect accelerations less than 0.01 m/s² with a sampling frequency greater than 100 Hz. When the heart beats, it causes very slight movements of the rib cage which can be captured by this device.
These movements correspond to the cardiac cycle: systole (ventricular contraction propelling the blood) and diastole (relaxation allowing the filling of the ventricles). The heartbeat creates a pressure wave that propagates throughout the body and generates tissue movement of just a few millimeters.
The accelerometer, placed perpendicular to the chest, detects these variations by converting them into an electrical signal. This technique, called ballistocardiography, was historically performed with complex medical equipment, but modern smartphones now make it simple to perform.
The normal resting heart rate varies between 60 and 100 beats per minute in adults (higher in children), and can increase significantly during physical exertion.
Extensions
- Record the cardiac signal from different body locations (chest, neck, wrist) and compare signal quality
- Investigate how caffeine consumption affects heart rate by measuring before and 30 minutes after drinking coffee
- Use the FizziQ frequency analysis tool to extract the heart rate from the Fourier transform of the signal
- Compare the smartphone measurement with a manual pulse count to evaluate measurement accuracy
Frequently Asked Questions
I cannot see any periodic signal on the accelerometer graph. What should I do?
Make sure you are lying on a hard, stable surface (not a soft bed) and that the phone is placed directly on the chest with its back touching your body. Breathe as slowly and shallowly as possible, and remain completely still. Try adjusting the graph scale in FizziQ to zoom in on small variations.
The signal is very noisy and I cannot identify individual heartbeats. How can I improve it?
Try placing a small book or weight on top of the smartphone to press it more firmly against your chest. Minimize all movement, including swallowing. If ambient vibrations are an issue (e.g., from traffic), try the experiment in a quieter location.
My heart rate seems abnormally high or low. Is the measurement accurate?
Verify by counting your pulse manually at the wrist for 15 seconds and multiplying by 4. The accelerometer method may miss beats or count noise peaks. A resting heart rate of 50-100 bpm is within the normal range for most people.
Why does the app show negative and positive acceleration values?
The accelerometer measures the direction of chest wall motion. Positive and negative values correspond to the chest moving toward and away from the phone during different phases of the cardiac cycle.
Detailed Description
The student places their smartphone on their chest and uses the accelerometer to detect movements caused by heartbeats. By analyzing the resulting graph, it identifies the heart rate at rest, then after physical activity. The student can also try to identify secondary peaks corresponding to the different phases of the cardiac cycle. FizziQ gives direct access to the smartphone's Z-axis accelerometer signal for this measurement, and its experiment notebook lets students save and compare the resting and post-exercise recordings.
Ready to start?
Download FizziQ and try this activity with your students.