Lab - Content Week 8 - Muscle

Question 1

The electromyogram (______), is a…?

Answer 1

Recording of the electrical activity in skeletal muscles as they contract.

Question 2

The EMG can be conveniently recorded from ____________ muscles by two surface ___________ placed on the _______.

These are then connected to a __________ that measures the…?

Answer 2

Superficial

electrodes

skin

voltmeter

potential difference between the two electrodes.

Question 3

In an EMG how is the output read? Wouldn’t the resulting frequency be too small from such small muscle contractions in positions like the resting state?

Answer 3

The output is amplified so it can be read.

It is also filtered to remove electrical noise and then displayed on a computer-based data analysis system that plots the changes in the potential difference against time.

Question 4

What is the set up for electromyography?

Answer 4

1 Thoroughly clean the skin overlaying the TRICEPS BRACHII on your supjects dominant arm using the alcoholic wipes provided.

2. Once the skin has dried, apply one of the gel-filled electrode pads over the proximal belly of the muscle. This point is easier to find if the subject contracts the muscle as you apple the pad. Make sure the pad is applied firmly and then apply a second pad to the distal belly of the muscle.
3. Repeat steps 1 & 2 to apply a pair of electrode pads over the belly of the biceps brachii muscle on the same arm.
4. Place the earth strap around the wrist of the same arm as the other electrodes and then wait a few minutes to allow the electrode pads to adhere to the skin.
5. Now connect the electrode pads on the biceps brachii muscle to the leads connected to channel 1 (CH1) of the data analysis system and the triceps brachii muscle electrodes to channel 2 (CH2) as shown in the diagram below. Finally connect the fifth electrode on the earth lead.
6. Adjust the subjects seat height so that they can sit comfortably with their forearm resting on the bench
7. Arrange the cables so that they are out of the way and won’t interfere with movements of the forearm

Question 5

List the process of forearm flexion

Answer 5

1. With your subject sitting comfortably and their forearm and elbow resting on the bench (palm facing up) start the data analysis system and record the RMG from both muscles for 10-15 seconds.
2. Now ask your subject to SLOWLY raise their forearm (leaving their elbow in contact with the bench) until it is vertical and then lower it back to the starting position. It should take around 5 seconds for the subject to raise their arm and then another 5 seconds to lower it again.
3. Repeat this cycle twice more (so that you have a total of three cycles) and then stop the data analysis system.
4. Use the auto scale feature to optimise the amplitude of the EMG recording from both muscles and adjust the horizontal scaling so that only the three cycles of movement are visible in the screen and then answer the following questions.

Question 6

What do you observe in the EMG recording from the biceps brachii during forearm flexion?

Answer 6

Recently, we demonstrated that there are unique phase differences between the biceps and triceps brachii during arm cycling. While the biceps brachii exhibits cyclical muscle activity, with large bursts in the EMG signal during the flexion phase and almost no activity in the extension phase, the triceps brachii demonstrates bursts of muscle activity in both phases

Question 7

What physiological events are being recorded in an EMG?

Answer 7

The EMG signal is a biomedical signal that measures electrical currents generated in muscles during its contraction representing neuromuscular activities. The nervous system always controls the muscle activity (contraction/relaxation).

Question 8

Why do you think the amplitude of the EMG is so small?

Question 9

What could you do to improve the signal to noise ratio for the EMG?

Answer 9

Increase the sensitivity of the recorder?

Question 10

What is the function of the low level of electrical activity observed in biceps brachii and triceps brachii even when the arm is not moving?

Answer 10

This low level electrical activity is the result of the brain maintaining posture during rest

Question 11

Skeletal muscles that act of the same joint are sometimes arranged in ___________ _______

Answer 11

Functional pairs

Question 12

Functional pairs: for any particular movement, one of these muscle has to ___________ while the other muscle _________ to enable the movement occur

Answer 12

Contracts

Relaxes

Question 13

The muscle that causes the movement by contracting is known as the ________ and the muscle that relaxes is known as the __________

Answer 13

agonist

antagonist

Question 14

In the agonist and antagonist experiment what did we do?

Answer 14

We used EMG to work out the agonist and antagonist for different types of forearm movement

Question 15

List the steps to agonist and antagonist EMG experiment

Answer 15

1. With your subject sitting comfortably get them to place their hand (palm facing up) on their upper leg and adjust their position (or seat height) so that their palm is just touching the underside of the bench. Start the data analysis system and record the EMG from both muscles for 10-15 seconds.
2. Now ask the subject to try and lift the bench off the ground using only their dominant hand. After 5 second get them to relax for 10-15 seconds and then try again. Repeat this once more (so you have three contraction phases) and then stop the data analysis system.
3. Now get your subject to rest their hand on the top edge of the bench (again with their palm facing up) and start the data analysis system and record the EMG from both muscles for 10-15 seconds. This time get the subject to try push the bench down in to the floor for 5 seconds before relaxing again. Get them to repeat this sequence twice more (again so you end up with three contraction cycles) before stopping the data analysis system.
4. Optimise the data analysis system so you can see all the data from this experiment and then answer the following questions.

Question 16

What can you conclude about the function of the biceps brachii from this experiment? Explain how you came to this conclusion

Answer 16

The biceps brachii function to flex the elbow joint. We can see this due to the electric activity in the EMG magnitude increasing when the flexion movement is being completed

Question 17

Based upon the results obtained in this experiment what can you conclude about a function of triceps brachii? Explain how you came to this conclusion.

Answer 17

The biceps brachii function to flex the elbow joint. We can see this due to the electric activity in the EMG magnitude increasing when the flexion movement is being completed

Question 18

Elbow flexion agonist = ?

Elbow flexion antagonist = ?

Elbow extension agonist = ?

Elbow extension antagonist = ?

Answer 18

Biceps brachii

Triceps brachii

Triceps brachii

Biceps brachii

Question 19

What type of muscle contractions are you studying in this experiment? (Elbow flexion/extension)

Answer 19

Concentric contraction:

Concentric contraction occurs when the total length of the muscle shortens as tension is produced. For example, the upward phase of a biceps curl is a concentric contraction

Question 20

Muscle loading experiment steps?

Answer 20

1. With your subject sitting comfortably and their forearm and elbow resting on the bench (palm facing up) start with data analysis system and record the EMG from both muscles for 10-15 seconds.
2. Now ask your subject to raise their forearm (leaving their elbow in contact with the bench) until it is vertical and then lower it back to the starting position. This should take around 5 seconds to complete.
3. During the subsequent 10-15 seconds rest period place a 1Kg dumbbell on the bench above their hand so that on the next cycle they can lift the weight into the vertical position and then lower it back to the starting position.
4. During the subsequent 10-15 seconds rest period replace the 1 Kg dumbbell with a 2 Kg dumbbell on the bench above your subjects hand so that on the next cycle they can lift the larger weight.
5. Repeat this cycle for 5Kg and then 10Kg (but only if your subject is able to lift the larger weights without difficulty).
6. Now optimise the data analysis system so you can see all the data from this experiment
7. In order to measure the magnitude of the EMG open a DATA PAD miniwindow and then click the middle of the tiny little window that opens. Choose Statistics and RMS from the top two panels on this setup menu and then biceps from the Calculation source drop down window.
8. If you now select portions of the EMG by placing the cursor in the timeline at the bottom of the main window you will be able to compare the magnitude of the EMF recorded in biceps brachii at various points during the experiment. Using this tool measure the average size of the EMG during the periods indicated in the table below and then use that information to answer some questions.

Question 21

Based upon these results how would you describe the relationship between muscle loading and the magnitude of the EMG?

Explain the physiology behind these observations.

What principle does this illustrate?

Answer 21

THe greater the load on the muscle the more the magnitude of the EMG increases.

As load increases action potentials increase in frequency and larger motor units are used.

The principle this illustrates is the orderly recruitment

Question 22

What type of muscle contraction are you studying in the muscle loading experiment?

Answer 22

Eccentric contraction:

During eccentric contraction, the muscle lengthens as the resistance becomes greater than the force the muscle is producing.

Question 23

In the mechanics experiments what do we measure?

Answer 23

The force generated by isolated skeletal muscles in response to electrical stimulation.

Question 24

Explain the setup for the Recruitment experiment

Answer 24

Choose the single stimulus experiment from the muscle simulation program and measure the effects on the twitch tension (called active force in this program) of stimulating the muscle at stimulus intensities between 1 and 10 volts and enter your data into the table below. NORMALISE your data by calculating the twitch. tension at each stimulus intensity as a percentage of maximal tension.

Stimulus Intensity (V)

Twitch Tension (g)

Percentage of Maximal tension

Question 25

How would you describe the relationship between stimulus intensity and twitch tension?

What is the cause of this relationship?

What type of muscle contraction are you studying in this experiment?

Question 26

Summation experiment:

Explain the setup

Answer 26

Choose the multiple experiment and first of all investigate the effect of a single 5 volt stimulus applied to the muscle. Now investigate the effect of stimulating the muscle at the same intensity but with multiple stimuli at a frequency of 30 stimuli per second.

Question 27

In the summation experiment, how does the magnitude of the tension produced with 30 stimuli per second compare to a single stimulus?

Why does this happen?

What is this effect known as?

Why does the tension produced by multiple stimuli at a frequency 30 stimuli per second gradually decline?

Question 28

Explain the setup for the Length-tension relationship and answer the following questions

Answer 28

Choose the isometric experiment from the muscle simulation program. Using a stimulus intensity of 8.2 volts, measure the twitch tension (active force) produced when the starting length of the muscle (i.e. before the stimulus is applied) is decreased from 85mm to 55mm in 5mm intervals and enter this data into the table below. Normalise your data by calculating the twitch tension at each length as a percentage of maximal tension.

What happens to the twitch tension produced as the muscle length increases?

What is the molecular mechanism thought to be responsible for this relationship?

In what kind of exercise is this type of relationship commonly observed?