Biology 450 - Animal Physiology Lab
Fall 2006
Lab 5 – Measuring Muscle Activity Patterns in Vivo
Muscle groups measured in vivo can display many of the same properties as
individual muscle preparations examined in vitro. In this lab you will measure
activity patterns generated by muscles in human limbs to examine various aspects
of muscle function.
The goals of these exercises are:

To quantify muscle activity patterns in antagonistic pairs

To determine the delay between muscle excitation and contraction

To examine the relationship between motor unit recruitment and load

To determine the effects of muscle load on contraction velocity
Background
Overview of muscle contraction
Contraction of muscle is triggered by a membrane depolarization. This occurs when
an action potentialfrom a motor neuron reaches the motor endplate and triggers
the release of a neurotransmitter (acetylcholine), which in turn induces an action
potential in the surface membrane of the muscle fiber. The action potential
propagates away from the endplate in both directions, exciting the entire muscle
fiber membrane. This excitation sets into motion the entire sequence of events
leading to muscle contraction. Because excitation represents a voltage change
across the surface of the muscle cells, it can be measured directly and used to
determine when muscles are active, a technique known as electromyography
(EMG).
The following review describes the electrical sequence leading to membrane
depolarization and muscle contraction:
1. The surface membrane is depolarized by an action potential.
2. The action potential is conducted deep into the muscle fiber via the T
tubules.
3. The electrical signal carries from the T tubules to the sarcoplasmic reticulum
where Ca2+ is released.
4. The free Ca2+ concentration in the myoplasm increases to the necessary
concentration so that it binds to troponin. As a result, troponin changes its
configuration and allows cross-bridge formation.
5. Force is generated as myosin heads form cross-bridges with actin, undergo a
conformational change, detach from actin, and change conformation back to
its original state. This cycle of events continues, fueled by ATP, as long as
Ca2+ concentrations remain sufficiently high.
6. Ca2+ is taken up again by the SR, cross-bridge formation is inhibited, and the
muscle relaxes until the next depolarization.
Electromyography
As with action potentials in nerves, properly placed electrodes can detect muscle
membrane depolarization. The most effective way to measure this potential (that
is, to record an EMG) is to insert the electrodes directly into the muscle. This is
typically done using fine wire with a hooked end as the electrode. The wires are
run through a hypodermic needle, the needle is inserted into the muscle, and then
the needle is pulled off the wires at their free ends. Because only the very tip of
the electrode wire is free of insulation, recordings from these electrodes typically
detect the membrane potential of just a few muscle fibers.
Although the implantation method is the best way of detecting muscle activity,
surface electrodes will also detect an action potential. As was the case with
measuring action potentials in nerves using external electrodes (Lab 3), the
measured signals are not as strong or "focused" as with internal electrodes.
Instead, surface electrodes detect activity of whole muscles, with relative signal
strength being roughly proportional to the number of active muscle fibers. In this
lab, you will use surface electrodes to measure the electrical activity of your
muscles as part of contraction activation.
Excitation-contraction coupling
An important point to note about EMG's is that they measure the excitation phase
of the excitation-contraction process in muscles. Contraction occurs as a result of
excitation, but only after a delay of a few milliseconds. This delay can be
determined by measuring the time lag between the onset of EMG activity for a
muscle (excitation) and a signal generated by the resulting muscular contraction –
for example, pressing a button to close a circuit.
Force-velocity relationship
The velocity at which a single muscle fiber of a given length (number of sarcomeres
in series) contracts is determined by two factors: 1) fiber type (fast or slow) and 2)
load on the fiber. Similar factors affect the velocity of contraction in a whole
muscle: 1) the fiber types in the motor units in the muscle and 2) the load on the
whole muscle. However, in a whole muscle the number of motor units recruited
also affects the speed of a contraction; the nervous system can continue to recruit
motor units within the muscle in order to compensate for an increased load.
Therefore if you increase the load on a muscle in vivo, one of two things should
occur: either the velocity of the contraction will decrease, or the EMG of the
electrical activity of the muscle will become larger (see below) as more motor units
are recruited.
Load and motor unit recruitment
In general, as you increase the load on a whole muscle, it will keep activating
motor units until all are in use. If you continue to increase the load from this point,
you should perceive a slowing in the contraction velocity. As more motor units are
recruited, the total force produced by the muscle increases. Therefore the
increased magnitude of the EMG signal is an indicator of the increased muscle
recruitment, and an indicator of increased force. In this lab we will measure the
maximum amplitude of the EMG signal (distance from the origin to the top of the
signal) as an estimate of the muscle's force.
Antagonistic pairing of muscles
The action of a muscle in the body typically has a counteraction provided by some
other muscle or muscles. For example, the biceps muscle in humans flexes (bends)
the arm at the elbow, while the triceps extends (straightens) the arm at the elbow.
Thus most muscles in the body are part of antagonistic pairings of muscles – pairs
of muscles that act to opposite effect. When one muscle in an antagonistic pair is
contracting, the other muscle is usually inactive, and vice-versa.
Lab Procedures
Note – This lab requires applying surface electrodes to various parts of
your arms. The exercises will be much easier to perform if you wear or
change into a short-sleeved or sleeveless shirt. If you want to examine
the activities of leg muscles, you will need to wear shorts under a pair of
long pants.
Initial Setup
In these experiments, you will be applying surface electrodes (coated with a
gummy conducting substance on one side) to your skin over major muscles. The
electrodes will be wired to our differential amplifiers, which will be connected to the
PowerLab units. You will use Chart, recording at a relatively fast rate, to record
EMG's and other signals.
Exercise 1 – Muscle function & antagonistic pairing
In this exercise, you will verify your ability to collect useful EMG signals, and will
examine the activity patterns of antagonistic muscle groups. As with the
measurements of action potentials in nerves (in Lab 3), you will use the differential
amp to amplify the faint potentials generated at the skin by underlying muscles.
Procedure:
1. For each muscle you want to record EMG's from, you will need to hook up a
pair of EMG cables to one of the amp channels. One cable goes to the (+),
the other to the (-). Each amp channel you are using must then be
connected to a PowerLab channel. In addition, you will need one ground
cable (marked with green tape); this should be hooked to an unused channel
directly on the PowerLab.
2. On the amp, set the mode to "AC", the low pass filter to 1 KHz, the high pass
filter to 10 Hz, and the gain to 1000. Make sure both the (+) and (-) inputs
are on for each channel in use.
3. Use Chart to collect your data. You will need one channel for each muscle
EMG, and for each of these channels the "AC" mode should be selected from
the "Input Amplifier" dialog. For some exercises, you will also need another
channel to record a timing signal – "AC" should be deselected for this data.
For initial measurements, record at 1000 samples/sec. The voltage range
will probably have to be adjusted to suit the strength of the EMG signal. Try
500 mV as a starting point.
4. When the equipment is ready, test the setup by attempting to obtain a
biceps EMG. Apply two surface electrodes about 5-7 cm apart on the belly of
the muscle (i.e., the thickest part). Attach EMG cables to the electrodes and
use medical tape to secure the cables so they do not tear loose.
5. You will also need to apply a third electrode some distance away to act as the
ground. This electrode should be places to avoid muscular activity. The
inner side of the wrist is one possible location. Note that only one ground
electrode is needed regardless of the number of muscles being recorded.
6. When the cables are connected and the amp is on, make sure your biceps is
relaxed and click "Record". You should expect a little noise in the trace, but
if the signal is highly erratic or wandering there is likely a problem with your
setup. Otherwise, try contracting the biceps – you should see increased
signal amplitude (in both positive and negative directions) as the muscle is
activated.
7. You can examine an isometric contraction by attempting to lift up the lab
bench. An isotonic contraction can be achieved by lifting an object with a
more moderate weight, such as a book or backpack, with your elbow at your
side.
8. Use another set of EMG cables attached to amplifier channel 2 and place the
surface electrodes on the biceps antagonist, the triceps, and do the
following:

Try to generate isometric and isotonic contractions with the triceps.

Gently rest your arm on he lab bench and supinate your arm both
rapidly and slowly (i.e., move your hand as if you were turning a
screwdriver in one direction and then the other.

Try to lift the lab bench, and while pulling up hard with your biceps, try
to activate your triceps
9. Pick at least two other limb muscles. Generate a hypothesis of the function
of these muscles. Test these hypotheses using the EMG equipment.
10.You can also examine the function of some or all of these muscles during
simple tasks (e.g., walking slowly, stepping up, doing a push-up).
Exercise 2 – Load and motor unit recruitment
In this exercise, you will examine the relationship between the force production
requirements of a whole muscle (i.e., the amount of weight it must lift) and the
number of muscle fibers recruited during muscle activity. This is possible because
EMG amplitude is roughly proportional to the number of muscle fibers excited.
Procedure:
1. Locate a dumbbell and accompanying weights. Connect the EMG electrodes.
2. You will be examining the recruitment of additional motor units as the biceps
is required to generate more force to lift increasing amounts of weight on the
dumbbell. You should either save a new recording for each weight or add
comments to your recording so you know which EMG goes with which weight.
3. Start with a lightly loaded or unloaded dumbbell. Slowly lift the dumbbell in
an arm curl (bring your hand up to your shoulder).
4. Either now or at the end of the exercise, estimate the amplitude of the EMG
signal as the maximum voltage spikes (minus the baseline if it looks different
from zero).
5. Repeat this exercise with a number of different loads, up to about the
maximum you can lift. Be sure to perform the arm curl at approximately the
same slow speed during each trial (although you may slow down a bit at your
maximum weight). Find the amplitude of the electrical signal your muscle is
producing at each load.
Exercise 3 – Latent period between excitation and contraction
The delay between the spread of action potentials across a muscle’s surface and the
beginning of cross-bridge cycling will be determined by recording EMG activity of a
muscle on one A/D channel and a signal created by muscle contraction (in this
case, a button being pushed) on another channel.
Procedure:
1. Locate one of the push-button transducers and attach the cable to an open
input channel. Make sure that this channel is being recorded, and set the
voltage range to 10 V. Be sure that a voltage registers on your button
channel when the button is pushed.
2. For this exercise, you will want to increase the sampling speed to 2000
samples/sec or more to accurately measure the latent period.
3. You will push the button using your thumb, holding the body of the
transducer by wrapping your fingers around it. This movement of the thumb
is an example of flexion. With your hand empty, locate the muscle or muscle
group that will be responsible for the "button-pushing" action. Do this by
pressing your thumb hard against the index finger of a closed fist and
determining where contraction is occurring. Verify with one of the instructors
that you have located the correct muscle group.
4. Apply electrodes as needed to record the EMG from the muscle and make
sure you are getting a good signal.
5. Start recording and press the button several times. Allow a pause of a few
seconds between pushes. Try to push the button as rapidly as possible each
time to minimize the delay between muscle activation and the appearance of
the button’s signal.
6. Find a few button-pushes that give clear EMG's, then find the delay between
the start of each EMG and the voltage increase produced by the button’s
action.
Exercise 4 – Force-velocity relationship
As the load on a muscle increases, its shortening velocity should decrease. This
exercise investigates that phenomenon by measuring the time required to perform
a biceps contraction while lifting weights of different sizes.
Procedure:
1. Locate one of the flexion transducers and attach the cable to the bridge amp,
then ask an instructor to help you attach the transducer to your elbow. You
will also need to locate a dumbbell.
2. Place your arm as though you were going to lift the dumbbell. Place your
arm as you would about 1/3 of the way through a lift (or "curl") and find the
voltage, then do the same for a 2/3 position. You will use these positions to
determine relative velocity of contraction.
3. With your arm still in position, have your lab partner hand you a dumbbell
with weights attached to it. Begin with only a moderate amount of weight.
(You might even use just the bar.) Lift the dumbbell in a curl as quickly as
you can (without smashing the weight into your face). As with exercise 2,
save files or insert comments to keep track of which data go with which
weights.
4. Measure the time it took to move your arm between your two reference
points (i.e. between your two voltages) in seconds. The inverse of this value
is your relative velocity.
5. Repeat this exercise a number of times, adding more weight to the dumbbell
each time until you can just barely manage to bring the weight up to your
shoulder. Be sure to make the appropriate measurements of velocity for
each trial.