Experiment: EMGs during Muscle Fatigue

Human

Grade 6+

Experiment: Muscle Fatigue

Ever wonder why your muscles start burning and shaking when you do those last few reps at the gym? In this experiment, you’ll explore muscle fatigue in real time by recording electromyograms (EMGs). Hold a weight at a fixed position or sustain a strong grip as you watch motor units drop out and your force output decline. Let’s find out why!

Background

Muscle fatigue happens when your muscles lose the ability to generate force. It can occur in the short term—like when you struggle to finish a set of heavy dumbbell curls—and over the long term, as in a marathon or multi-hour hike. The process behind muscle fatigue involves a complex interplay between motor unit recruitment, energy metabolism (ATP production), and the availability of oxygen. Short bursts rely on anaerobic energy sources, whereas endurance relies on continuous oxygen supply. With EMG, you can see larger motor units dropping out over time as fatigue sets in.

Procedure

Biceps: Isometric Hold

1. Place two electrode patches on your biceps and attach your red (recording) leads. Place one patch electrode on the back of your hand and attach the black (ground) lead.
2. Select a dumbbell at ~60% of your max (e.g., 10–25 lbs). Stand against a wall, bend your elbow to 90°, and hold the dumbbell isometrically—try not to move the joint.
3. Record your EMG in the Spike Recorder. Monitor the signal’s amplitude (peak height) and firing rate as you hold the weight. Note when you first sense fatigue and when you can no longer hold the weight.
4. Observe whether the EMG amplitude decreases over time, indicating larger motor units are 'dropping out' before failure.

Forearm Grip: Dynamometer

1. Place two patch electrodes along your inner forearm. Ground at the back of your hand.
2. Use a hand dynamometer or strain-gauge grip sensor. Squeeze as hard as you can and hold that maximum force as long as possible.
3. Track force changes every few seconds and watch your EMG data. You may notice quick drops or bursts as you 'recommit' to grip force.
4. Continue until you reach full fatigue. Collect the data for both the EMG amplitude and grip force.

Results & Analysis

1. EMG Amplitude & Motor Unit Drop-Out: Load the recording you made inside of Spike Recorder, select the first and last 5 seconds and write the RMS value of each. To select a section of recording on Windows right click+drag, on Mac two finger click+drag, on mobile hold+drag. The RMS value will be displayed once a section is selected.
You should see a decline in amplitude as motor units fatigue and drop out.

2. Rate of Fatigue (m): Calculate your slope (m) using two data points in time (y2 – y1) / (x2 – x1). For example, if RMS drops from 6.8 to 2.1 over 116 s, your rate is approximately –0.04.

3. Comparing Genders & Individuals: Collect multiple trials. You may find differences in initial force production vs. endurance. Research suggests that while men often produce higher peak force, women may fatigue more slowly.

4. Discussion Questions:
• How does anaerobic vs. aerobic muscle contraction affect your endurance?
• Do you see a difference between your dominant vs. non-dominant side?
• Why does marathon running fatigue happen over hours, while max lifts might fail in seconds?
• Which muscles in your body resist fatigue best (e.g., postural muscles)?

Try repeating your tests after a month of consistent training. Do you see improvements in how long you can sustain a contraction at the same load? This is how scientists measure changes in muscular endurance over time!

Shopping cart