Muscle Stimulation & FatigueStudent Protocol

Exercise: Muscle Fatigue

In this exercise, you will observe the decline in maximal force during a sustained contraction and will examine some properties of muscle fatigue. First, you will calibrate the Hand Dynamometer with respect to the volunteer’s maximal grip strength.

Equipment Setup and Calibration
  1. Connect the Hand Dynamometer to Input 1 on the front panel of the PowerLab (Figure 9).Open a new LabChart file. Turn off and collapse all Channels except Channel 1.

Figure 9. Equipment Setup for PowerLab 26T

  1. Use the following settings in Chart.

Fatigue Settings

Under Setup/Channel Settings, type Grip as the title for Channel 1. Close out of this dialog.

Input Settings: Input Amplifier

Sampling Rate:1000/s (Verify by clicking the blue Information icon in the upper right corner)

Range:20 mV
View: 20:1 compression (set by clicking the “mountain” icons in the lower right of the screen.)

  1. Click on Input Amplifier and activate the button for Differential recording. Examine the signal. If the signal is out-of-range, reset the Range to a larger number so that the signal appears on the screen.
  1. Have the volunteer loosely grip the Hand Dynamometer in the fist of their dominant hand, as shown in Figure 9.
  1. Squeeze the Hand Dynamometer as hard as possible and watch for a deflection of the trace in the Input Amplifier Box. Adjust the range so the deflection produced by a maximal contraction generates a large response on your screen.
  1. Close out of the Input Amplifier.
  1. Start recording. Have the volunteer squeeze the Hand Dynamometer as hard as possible for a second or two, and then relax their grip. After recording for a few seconds, have the volunteer repeat the maximum grip and then relax. Stop recording.
  1. Click-and-drag over the largest response to select a range of data that includes both the relaxed and maximum force signals (Figure 10). Select Units Conversion from the Grip pop-up menu in the upper right corner of the screen.

Figure 10. Data Selection for Units Conversion

  1. In the dialog box, select (click on) part of the trace where the force is zero, and click the arrow next to Point 1 (Red Arrow on Figure 11, next page)so that the voltage at the selected spot will be automatically entered in the first box. In the box to the right, enter “0”.
  2. Then select (click on) part of the trace at the peak, and click the arrow next to Point 2 (Figure 11). In the lower right box, enter “100” and then select % from the Units drop-down menu. Exit the Units Conversion box by clicking OK.
  1. Start recording and after a few seconds instruct your volunteer to produce a maximal contraction. You should see an obvious response. Stop recording.

Figure 11. Units Conversion Dialog

Procedure
  1. Click on the Down Arrow (blue in Figure 11) at the upper left corner of the Y axis which reveals the option to Set Scale.Adjust the scale for Channel 1 to show to 120% and -20%.
  2. Allow the volunteer to view the monitor. Start recording. After two seconds, ask the volunteer to maintain 20% maximal grip strength while watching the recorded trace. The Range/Amplitude display for Channel 1 shows the percentage force applied.
  1. After 20 seconds, tell the volunteer to relax. Stop recording. Add a comment with “20%.”
  1. Wait for 30 seconds to allow recovery of muscle function, and repeat steps 2-3 for contractions of 40%, 60%, 80%, and 100% of maximal grip strength. Allow the volunteer to rest for 30 seconds in between each contraction. Add a comment with the grip strength percentage each time.
  1. Have the volunteer rest for two minutes. Then have the volunteer turn away from the monitor so they cannot see the data trace.
  1. Start recording. Ask the volunteer to produce a sustained maximal contraction. After 10 seconds, or when the force has obviously declined, energetically instruct them to try harder! After another 10 seconds, repeat the vigorous encouragement. After five more seconds, tell the volunteer to relax. Stop recording. Allow the volunteer to rest briefly.

Note: Most volunteers can produce temporary increases in muscle force during a fatiguing contraction, when sufficiently motivated by verbal encouragement.

  1. Start recording again. Ask the volunteer to produce a sustained maximal contraction as before. Every 10 seconds, allow the volunteer to relax very briefly for ½ second, and then have them return to maximal contraction. Stop recording after 30 to 40 seconds.

Do brief periods of relaxation allow substantial recovery from fatigue?Is the recovery is only temporary?Consult Figure 12.

Figure 12. Fatiguing Contraction

  1. Turn the volunteer so they can see the monitor again. Start recording. Ask the volunteer to produce a 40% contraction while watching the data trace. Once the volunteers has produced at 40% contraction for 10 seconds, have the volunteer close their eyes and attempt to maintain exactly the same contraction force for the next 30 seconds. Don’t stop recording just yet.
  1. After the elapsed time, have the volunteer open their eyes and adjust the contraction force back to 40%. Stop recording.

Note: Almost all volunteers will show a declining force while their eyes are shut, which is very similar to fatigue. This is referred to as pseudo-fatigue. This is not true fatigue because the full 40% can be exerted easily, as can be seen when the volunteer’s eyes are opened again.

Exercise Observations

  1. Was the volunteer able to maintain 20%, 40%, 60%, 80%, and 100% of maximal grip strength in the beginning of the exercise?
  2. Was the volunteer able to increase contraction with encouragement?
  1. Could the volunteer maintain the same contraction force with their eyes closed? Were they able to return to the initial contraction with their eyes open again?

Once you’ve completed all analysis for one individual, exit from the LabChart Program and don’t save your file. Reload the program and perform the fatigue experiments on a second person, being sure to re-calibrate and re-set the scale to account for the difference in muscle strength of each subsequent subject.

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