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Muscle Fatigue
  • Human
  • Grade 6+

Muscle Fatigue: Recording EMG During Sustained Effort

Record EMG during a sustained muscle task and compare the recording over time. Where a dynamometer is available, measure task force separately. Discuss what changes in performance and electrical activity can—and cannot—tell you about fatigue.

Devices and Accessories5

About the Experiment

What Will You Learn?

What Will You Learn?

  • How to place EMG electrodes and record muscle activity.
  • How to compare EMG amplitude during a sustained contraction.
  • Why electrical activity, task force and fatigue require distinct measurements and interpretation.
Background

Background

Muscle fatigue is a reduction in the capacity to produce force or perform a task. Its mechanisms depend on the task and can involve both neural activation and muscle properties. Surface EMG may change during a sustained effort, but its direction and size depend on the contraction, posture, electrodes and signal processing. EMG alone does not show individual motor units dropping out or establish a metabolic cause.

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. Observe how the trace changes over time; it may increase, decrease or vary. Document posture changes and movement. The waveform alone does not identify which motor units were recruited or failed.

  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 Over Time In SpikeRecorder, compare the first and last 5 s of your recording and note each RMS value, documenting any movement or signal-quality problem. A change in RMS describes the recorded signal; it does not by itself prove recruitment, motor-unit failure or force loss. Compare independent force measurements when available.

2. Rate of Change in Recorded EMG Amplitude Calculate slope m = (ΔRMS) / Δtime using the elapsed time between the centers of your two measurement windows. Example: an RMS change from 6.8 mV to 2.1 mV between window centers 116 s apart gives m ≈ –0.04 mV s⁻¹. RMS uses voltage units; this slope describes voltage amplitude per second. It is not automatically a physiological fatigue rate, and it does not identify the cause of a change.

3. Compare Participants If comparing people, record the load, task duration, posture, electrode placement and independent force measurement. Differences in raw surface EMG alone do not establish strength, endurance or a sex difference. Discuss how the task and recording conditions affect a fair comparison.

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?

5. Model the Task and Evaluate Your Explanation Draw the pathway from your motor cortex to your biceps and label where you recorded. Use appropriate background sources to propose how energy supply and neural activation could contribute during brief intense and longer light tasks. Identify which parts of your explanation your measurements test and which require additional evidence. An RMS slope alone does not establish oxygen availability, ATP depletion or a metabolic cause of fatigue.

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

NGSS alignment

Performance expectations

Fatigue ties neural drive to ATP from cellular respiration.

  • MS-LS1-8 Gather and synthesize information that sensory receptors respond to stimuli by sending messages to the brain for immediate behavior or storage as memories.

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

  • HS-LS1-7 Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy.

Science and engineering practices

  • 2 Developing and Using Models

    Draw the pathway from motor cortex to biceps and model where the muscle's ATP comes from in a short lift and a long hold.

  • 3 Planning and Carrying Out Investigations

    Record biceps EMG during an isometric hold and forearm EMG during a maximal grip.

  • 4 Analyzing and Interpreting Data

    Read the change in RMS as motor units tire and new ones are recruited.

  • 5 Using Mathematics and Computational Thinking

    Calculate the fatigue slope m = ΔRMS/Δt.

  • 6 Constructing Explanations and Designing Solutions

    Use your ATP model to explain your fatigue slope.

Crosscutting concepts

  • Systems and System Models

    Nerve, muscle, energy supply.

  • Energy and Matter

    ATP limits contraction.

  • Patterns

    Fatigue curves.

Disciplinary core ideas

  • LS1.A Structure and Function

    Muscle structure.

  • LS1.C Organization for Matter and Energy Flow in Organisms

    Energy from respiration.

  • LS1.D Information Processing

    Motor signaling.

Performance expectations, crosscutting concepts and core ideas from Hands-On Neuroscience Curriculum Overview, Unit 2, module 2.1, whose core lab this is. The practices are the ones this page asks students to do.

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