• My cart
Muscle Fatigue
  • Human
  • Grade 6+

Muscle Fatigue: Watching Motor Units Tire

Why do your muscles burn and shake during those last few reps at the gym? Record real-time EMG while holding a weight or gripping a dynamometer to see how your nervous system recruits more motor units as your muscle tires, and how force finally declines.

Devices and Accessories5

About the Experiment

What Will You Learn?

What Will You Learn?

  • How to place EMG electrodes and record muscle activity.
  • How sustained contractions tire motor units and how your nervous system compensates.
  • Why force production and endurance differ between muscles and individuals.
Background

Background

Muscle fatigue is the loss of force-generating capacity. During short, intense efforts—such as isometric dumbbell holds—muscle fibers burn through their quick fuel (phosphocreatine) and build up by-products, and the fast, easily tired fibers of larger motor units weaken first. Endurance tasks depend on continuous oxygen delivery and smaller, fatigue-resistant motor units. By monitoring EMG you can watch your nervous system respond: early in a hold, EMG amplitude often grows as extra motor units are recruited to replace tiring ones; as fibers fail near exhaustion, and in an all-out squeeze, it falls.

Procedure

  1. Biceps Isometric Hold

    Biceps Isometric Hold

    1. Place two electrode patches on the biceps and attach the red leads. Ground on the back of the hand and connect the black lead. 2. Choose a dumbbell at approximately 60% of your maximum (about 10–25 lb). Stand against a wall, bend the elbow to 90°, and hold without moving. 3. Record EMG in SpikeRecorder. Note the moment fatigue begins and when you must drop the weight. 4. Watch how EMG amplitude changes. Early in the hold it often grows as your nervous system recruits extra motor units to replace tiring ones; near exhaustion it falls.

  2. Forearm Grip — Dynamometer

    1. Place two electrodes on the inner forearm; ground on the back of the hand. 2. Squeeze a hand dynamometer (or strain-gauge grip sensor) as hard as possible and hold. 3. Record EMG and force simultaneously. Expect intermittent drops and bursts as you recommit to maximum grip. 4. Continue to failure, then save both EMG and force traces for analysis.

    Forearm Grip — Dynamometer
Results & Analysis

Results & Analysis

1. EMG Amplitude & Motor-Unit Fatigue In SpikeRecorder, highlight the first and last 5 s of your recording and note each RMS value. A rising RMS shows your nervous system recruiting more motor units to keep up the force; a falling RMS shows your muscle’s electrical output fading as fibers fail.

2. Rate of Fatigue Calculate slope m = (ΔRMS) / Δtime. Example: RMS from 6.8 mV to 2.1 mV over 116 s gives m ≈ –0.04 mV s⁻¹. Your slope may be negative, as in this example, or positive: early in a hold your nervous system often recruits more motor units, which raises the EMG; as fibers fail near exhaustion and in an all-out grip, it falls.

3. Compare Individuals & Sexes Repeat the test across classmates. Men often show higher peak force, while women may fatigue more slowly.

4. Discussion Prompts • How do anaerobic vs. aerobic pathways change endurance? • Dominant vs. non-dominant limb differences? • Why do marathoners fatigue over hours but max lifts fail in seconds? • Which postural muscles resist fatigue best?

Retest after a month of training—does time to failure improve?

What do you need?