Lab 5
Muscle Physiology
Lab 5
Human Muscle Physiology
At the beginning of lab you will have the opportunity for 2 bonus points! You must guess which
person in the class will have:
1) Maximum Grip Force
2) Longest time to half-max Force (longest time for muscle fatigue!)
Objectives:
1)
2)
3)
4)
To understand muscle physiology from the perspective of an isolated frog muscle.
To understand how isolated muscle physiology relates to whole-muscle performance.
To determine the relationship between muscle force and EMG activity.
To examine the effect of muscle size and EMG on muscle force by comparing maximum
grip force, forearm diameter, and EMG for the entire class.
Please read the PhysioEx portion of the lab manual prior to lab!
The classic experiment for muscle physiology uses a frog thigh muscle attached to an
electrical stimulator and a data-recording computer. However, execution of this lab involves
killing many, many, many frogs (~ 60 each semester for BIOL 201). Thus, computer simulations
have been developed that allow you to observe a simulated experiment involving the frog thigh
muscle. This computer simulation is very good and should effectively teach you some basic
principles regarding the physiology of an isolated skeletal muscle.
First, you will examine the effect of neural input (action potentials) on the development of
muscle force. When a motor neuron delivers a single action potential to a muscle, a mechanical
response occurs called a muscle twitch. Graphically, a muscle twitch shows how a single motor
unit develops force (i.e. tension) over time in response to a single action potential. A single
muscle twitch produces only a very small force; thus a single muscle twitch is not capable of
allowing you to do anything. At the level of the motor unit, muscles increase force production to
accomplish tasks by adding up individual muscle twitches. Treppe and summation are two ways
that individual muscle twitches can be physiologically combined to achieve greater force. Treppe
occurs when the frequency of action potential is such that successive NON-OVERLAPPING
muscle twitches achieve greater force. Summation occurs when the frequency of action
potentials produces OVERLAPPING muscle twitches that summate to produce a larger force.
Thus, increasing the frequency of neural signals to muscle cells can increase muscle force.
Muscles undergo two basic types of contraction: isometric and isotonic. Isometric
contractions occur when a muscle attempts to contract with MAXIMAL force. Under this condition
the length of the muscle fiber does not change (Iso = same, metric = length!). Isotonic
contractions occur when muscles contract to produce a submaximal force. This is usually
associated with muscle MOVING something. When muscles move stuff, they contract with a
constant force (i.e. tension, iso = same, tonic = tension). Think carefully about these two types of
contraction so that you do not confuse them! In isometric contractions, the muscle attempts to
move something that requires more force than it is capable of producing; thus the muscle
contracts and cannot shorten any further. Conversely, in isotonic contractions, the muscle
MOVES something and the something being moved is of a fixed weight. Thus, the muscle only
needs to produce a force slightly greater than the weight of the object to cause it to move!
ISOMETRIC CONTRACTION (OBJECT DOES NOT MOVE)
Force muscle < = Force object = Muscle cannot shorten and object DOES NOT MOVE
ISOTONIC CONTRACTION (OBJECT MOVES)
Force muscle > Force object = Muscle shortens (constant force) and object MOVES
NOW DO PHYSIOEX EXERCISE 2, Skeletal Muscle Physiology
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Lab 5
Muscle Physiology
Electromyogram (EMG)
Moving away from isolated muscle, we will next consider how an intact human (or any
other animal) uses muscles to produce force.
The electrical activity in the motor units during a muscle contraction can be recorded as
bursts of muscle action potentials that are collectively known as an electromyogram (EMG). The
duration of the bursts of electrical activity in the muscle is about equal to the duration of the
muscle contraction. When the electrical activity in the muscle begins, the muscle fibers begin to
contract. When the electrical activity stops, the muscle begins to relax. The strength of a muscle
contraction is related to the number of motor units in the muscle that are activated during the
contraction and the strength is directly proportional to the intensity of electrical activity in the
muscle.
Two primary factors determine differences in the ability to perform a task using the
musculoskeletal system: 1) muscle size and 2) neuromuscular coordination. For example, we all
expect that a person with larger arm and chest muscles will be able to lift more weight than a
person with smaller arm and chest muscles. This can be explained via muscle physiology studies
that show a direct, positive relationship between muscle mass and the ability of muscle to
produce force. Thus, individuals with more massive arms and chests will have greater muscle
mass in those areas and will ultimately be able to lift heavier weights. However, you may also
have noticed that two people that are about equally sized with about equal muscle mass are
capable of lifting different maximum weights! What explains this finding? Studies of
neuromuscular function have found that different people exhibit different levels of neural
coordination of muscle contraction. Remember the STRENGTH of a muscle contraction is
directly proportional to the number of motor units recruited, which is directly related to electrical
activity. Thus, someone who has trained lifting weights will develop a higher degree of neural
coordination, which results in more motor units acting simultaneously to produce a more forceful
muscular contraction. This allows the person to lift heavier weights, even if their muscle is the
same size or smaller than another person!
The strength of a muscle contraction is related to the number of motor units in the muscle
that are activated during the same time period. The electromyogram (EMG) recorded during a
muscle contraction is seen as a burst of spike-like signals, and the duration of the burst is about
equal to the duration of the muscle contraction. However, it is difficult to quantify the amount of
electrical activity in a muscle unless the raw EMG data is mathematically transformed. One of the
most common transformations used is the integration of the absolute values of the amplitudes of
the EMG spikes (called the absolute integral). The area under the graph of the absolute integral
of the EMG is linearly proportional to the strength of the muscle contraction.
.
In today’s experiment, you will use a hand dynamometer to measure a subject’s grip strength
while recording the EMG of the forearm muscles used to generate grip. The EMG activity will be
related to the grip strength by plotting the maximum grip strength as a function the area under the
absolute integral of the EMG activity during the muscle contraction. Data recordings will be made
from your dominant forearm and the relative strength and electrical activity of the muscles in the
forearm will be compared to its diameter (muscle size!). EACH PERSON IN THE CLASS MUST
PERFORM THE EXPERIEMTN ON THE DYNAMOMETER!
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Lab 5
Muscle Physiology
NOW, DO THE IWORX EXPERIMENT
The binder contains the information for setting up the iWorx equipment for these
experiments. Once you have set up the equipment, refer back to these instructions.
Experiment 1. MEASURING EMG AND MAXIMUM GRIP STENGTH
Procedure
1 Sit quietly with your dominant forearm resting on the table top. Explain the following procedure
to the subject:
“You will squeeze the hand dynamometer four times, with each contraction last two seconds
followed by two seconds of relaxation. Each successive contraction should be approximately
two, three, and four times stronger than the first contraction. The last contraction should
be a maximum effort, the hardest you can possibly squeeze!”
2 Click the Record button to begin the recording; then, press the Enter key on the keyboard to
mark the beginning of the recording. After the recording is marked, tell the subject to begin
squeezing the hand dynamometer following the procedure outlined in the step above.
3 In the relaxation period after the last contraction, click the Stop button,
4 Click the AutoScale buttons for the EMG and Muscle Force, and EMG Integral channels.
Your data should look similar to the example below.
Data Analysis
1 Use the Display Time icons to adjust the Display Time of the Main window to show the four
progressive muscle contractions on the Main window.
2 Click on the Analysis window icon in the toolbar.
3 Look at the Function Table that is above the uppermost channel displayed in the Analysis
window. There should be three columns of mathematical functions: 1) Abs. Area, 2) V2-V1, and
3) T2-T1. The values for Abs. Area, V2-V1, and T2-T1 on each channel are seen in the table
across the top margin of each channel.
4 Use the mouse to drag the cursors to the beginning and end of the first muscle contraction. The
values for Abs. Area on the EMG and Muscle force channels are the relative amount of the
electrical activity causing the contraction and relative strength of the muscle, respectively.
Record this value in the table on the next page. Repeat for each squeeze.
5 Use cloth measuring tape to measure the circumference of the dominant forearm at
approximately 3 centimeters below the elbow.
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Lab 5
Muscle Physiology
Dominant Forearm Diameter (mm):______
Relative Grip
Strength
Absolute Area of EMG Activity
Absolute Area under Force
Curve
Lowest
Higher 1
Higher 2
Highest
6 Record each person’s data in the excel spreadsheet at the front of the classroom. When
everyone is done with this experiment I will plot individual EMG and Arm Diameter vs. Grip Force,
so we can see relationships between these variables. The graph will be posted in WebCT.
Questions
1. For each person, individually, plot the absolute area of muscle contraction as a
function of the absolute area of the EMG signals for each muscle clinch. Is there a linear
relationship between the absolute area under EMG signals and the absolute area under
the muscle contraction?
2. Does the amplitude of the EMG signal and the force of contraction, as measured by
the absolute areas, increase because a finite number of fibers are firing more often, or
because more fibers are recruited to fire as the intensity of signals in the motor neurons
increases, or a combination of these two?
3. Make notes here regarding the relationships between EMG, Arm Diameter, and
Muscle force that I plot on the projector at the front of class. Although it will be posted on
WebCT, you may want to sketch this graph while taking notes.
4. Should there be a relationship between the circumference of the forearm and the
maximum force developed? What relationship would you expect? Does the class data
support this? Is the difference in the circumference of the forearms between people in the
class caused by a difference in the number of muscle fibers in the forearm or the
diameter of each muscle fiber in the forearm?
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Lab 5
Muscle Physiology
Experiment 2: Muscle Fatigue
Background
Physiologically, muscle fatigue means that muscles produce less force!
Please review Chapter 12 pgs 411-412 for a general discussion of the factors that contribute to
muscle fatigue. Also the following figure illustrates 9 factors that contribute to muscle fatigue
(from Enoka 2002, neuromechanics of human movement)
Neural Factors: Brain/CNS may be protecting body from injury
1) Reduced activity of the motor cortex in the brain
2) Central Nervous System to Motor Neuron Connections
Muscular Factors: Usually result of physical/chemical limitation of muscles ability
3) Motor Unit – certain fiber types become quickly fatigued (type 2-b), fewer
motor units activated
4) Neuromuscular Junction – Acetylcholine synthesis slower than rate of action
potentials being sent from CNS (DISEASE STATE)
5) Excitation-Contraction Coupling – most common factor, reduction in release
of Ca2+ from sacroplasmic reticulum > reduction in Ca2+ binding to troponin >
reduced ability of myosin-actin binding > less contraction.
6) Depletion of Metabolic Substrates (Fig 12-13 in your text),
Phosphocreatine (needed for ATP, power stroke of myosin head!),
ATP
Glycogen
7) Blood Supply - muscles increase the pressure during contraction, which can
squeeze closed the blood vessels needed to supply metabolites, remove waste,
and remove heat.
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Lab 5
Muscle Physiology
Procedure (Read steps 1-3 before beginning experiment)
1 Sit quietly with your dominant forearm on the table top.
The subject will squeeze the bulb of the hand dynamometer as tightly and as
long as possible in an attempt to fatigue the muscles of the forearm.
As time passes, the subject’s muscle force will decrease, but at a rate that is
dependent on the fitness of the subject.
When the subject’s muscle strength drops to a level that is below half of
the subject’s maximum muscle force at the beginning of the recording, the
recording will be stopped. This could take as little as 20 seconds or as long as a
few minutes.
IMPORTANT: Instruct the subject that he/she must maintain constant
pressure for the duration of the experiment. He/she should not briefly relax
and “re-squeeze.” The Muscle Force trace should steadily decline at some
rate (which will vary across subjects) during the experiment. There should
not be frequent amplitude increases in the Muscle Force recording; such
frequent amplitude increases indicate the subject is not maintaining
constant pressure on the dynamometer. Such a recording is not
demonstrating muscle fatigue.
It is your job as the experimenter to monitor the recording and ensure the
subject is properly carrying out the experiment as instructed!!!!!!
2 Click the Record button, followed by pressing the Enter key on the keyboard. Record a
baseline for ten seconds and then instruct the subject to squeeze and hold the hand
dynamometer with as much force as possible. Continue to record.
3 When the subject’s muscle strength (based on the Muscle Force trace) decreases to a
level that is below 50% of the maximum muscle strength at the beginning of the
recording, instruct the subject to release the bulb. Click the Stop button to halt the
recording.
4 Click the AutoScale buttons for the EMG and Muscle Force channels. The recording
should be similar to below.
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Lab 5
Muscle Physiology
Data Analysis
1 Use the Display Time icons to adjust the Display Time of the Main window to show the
complete muscle fatigue recording on the Main window.
2 Click on the Analysis window icon
3 Look at the Function Table that is above the uppermost channel displayed in the
Analysis window. The mathematical functions, Abs. Area, V2-V1, and T2-T1 should
appear in this table. The values for Abs. Area, V2-V1, and T2-T1 on each channel are
seen in the table across the top margin of each channel.
4 On the Muscle Force channel, use the mouse to click on and drag the cursors to
specific points on the recording to measure the following, record the value for each of
these in the table below:
The maximum muscle force. To measure this force, place one cursor on the
baseline before the muscle contraction and the second cursor on the peak
muscle force near the beginning of the contraction. The value for V2-V1 on the
Muscle Force channel is this amplitude.
The half-max muscle force. Divide the maximum muscle force by 2.
The half-max fatigue time. This is the time it takes for the muscle force to
decrease to 50% of the maximum during the fatigue experiment. Place one
cursor on the peak muscle force. Move the second cursor to the right of the peak
muscle force until the absolute value for V2-V1, seen on the Muscle Force
channel, is equal to the half-max muscle force
The Max and half-max EMG amplitudes.
1) Move the cursors so that they bound 1 SECOND AFTER your initial
maximum grip. Record the Abs Area from the EMG channel. This is your
Max EMG amplitude.
2) Move the cursors so that they now bound 0.5 SECONDS ON EITHER
SIDE of your half-max force grip. Record the Abs Area from the EMG
channel. This is your Half-Max EMG amplitude.
Enter your values in the table below.
Max Force
Hal-max Force
Half Max Time
Max EMG
Half Max EMG
Enter these values in the excel spreadsheet at the front of the classroom. I will generate
graphs (Half-max EMG vs. Half Max Force; Half Max time vs. Arm Diameter) of these
data for the entire class. Please sketch graphs below, as a study aid for the exam! They
will also be posted on WebCT.
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Lab 5
Muscle Physiology
Questions
How is the time to fatigue to half-strength related to the size of the muscles involved (i.e.
arm diameter)?
What factors could explain the relationship you find using information in this lab handout
and from you textbook (chapter 12 ps. 411-12).
What is the relationship between half-max EMG and half-max Force?
Is this similar to the relationship between max EMG and max Force found in the last
experiment? Why should they be similar, i.e. what does EMG tell you about muscle
producing force?
Aside for muscle size, what other factors contribute to muscle fatigue? Which factors do
you think are primarily responsible for differences among people in this class? Could we
easily measure these factors in lab on humans?
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