How do I demonstrate muscle fatigue in a human volunteer?

Muscle fatigue can easily be recorded from the flexor digitorm muscles in the forearm while recording the force signal with the MLT004/ST Grip Force Transducer.
Here is the settings information from our good customers at Mohawk Vally Community College who developed this lab. You can use the following settings to set up your own with LabChart.
Sampling Rate: 200/s
View: 10:1 compression
Number of Channels: 3
Channel 2 should be “off;” channel 1 and 3 should be “on.”  Check the boxes to do this, if necessary.
Channel 1 Title: % maximum effort
Range: 0 - 800 Newtons
Low Pass: 20Hz
Invert: unchecked
Anti-alias: checked
Use default units: unchecked
Channel 2 Title: EMG activity
Calculation: Integral
Source: Channel 3
Integral Type: Absolute Value
Reset Type: Time Constant Decay 0.2 s
select Autoscale
Channel 3 Title: Raw EMG
Range: 2 mV
Input Settings: Bio Amp
Calculation: No Calculation
Save as a LabChart settings file with a suitable name, such as “Grip EMG Settings”. 
Grip force on channel 1, EMG on channel 3. channel 2 is the integrated EMG activity ("power") calculation of the raw EMG signal. The EMG Electrodes are placed over the flexor digitorum as seen in the attached pic.
First, zero the Grip Force Transducer, record the student's maximum grip force, and use the Units Conversion Channel Calculation dialog to change the units to "%" (two decimal places).  To learn more about the Units Conversion feature, view the instructional video located HERE. Next, have the students hold at 70% while watching the screen (A Miniwindow is very useful for this) for as long as they can. What you will see is increased EMG activity as the muscles fatigue even though force remains relatively constant.  Students should plot force on one y-axis, EMG activity on a second y-axis by selecting 1-2 seconds of data every 5 or 10 seconds over the course of their experiment.