BIOL 373L : Laboratory #4 - Contractility and Fatigue of Skeletal Muscle

objective

to observe and measure muscle contraction and to understand the effects of fatigue on muscle strength

motor unit

A single motor neuron and all of the muscle fibers it innervates

the strength of a muscle contraction

- is related to the number of motor units that are activated during the same time period

EMG

- during the 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

strength of a striated muscle contraction

directly proportional to the amount of electrical activity in that muscle

how do you quantify the amount of electrical activity in a muscle

- use the absolute values of the amplitudes of the EMG spikes. the area under the graph of the absolute integral of EMG is linearlly proportional to the strength of the muscle contraction

how will EMG ac

- by plotting the maximum grip strength as a function the area the absolute integral of the EMG activity during muscle contraction.

equipment required

laptop
IXTA roam
C-AAMI-504 ECG cable and electrode lead wires
disposable electrodes
FT-220 hand dynamometer
alcohol swabs
bathroom scale and large pail
string
ruler

EMG cable and hand dynamometer setup

1. plug connector to hand dynamometer into A2 input
2. insert iwire b3g electrode harness into iWire1 plug which is purple
3. connect electrodes
4. apply red near elbow, black on forearm, green on wrist

software used

EMG-Gripstrength-iwireb3g

calibrating the hand dynamometer

1. fill pail with water
2. let hand dynamometer run for 10 seconds
3. continue to record as you place the pail on it, record for 10 more seconds
4. click autoscale on muscle force channel, use double display time
5. click on double cursors, place one cursor before pail was added and one after
6. open channel menu and put check mark in boxes in both "apply unit" boxes
7. cursor 1 set value to 0, cursor 2 type the weight of the pail and write "kg"

procedure 1: emg intensity and force in dominant arm

1. rest forearm on bench, squeeze hand dynamometer 4 times, each contraction is 2 seconds long followed by 2 seconds of relaxation. each contraction should be double the previous strength
2. record and autoscale for emg and muscle force channels

exp 1 data analysis

1. use display time icons to show the four contractions. place cursors on either side of a group of 4 contractions (start to start or end to end), click zoom between cursors
2. click on analysis
3. measure absolute integrals of each squeeze. drag cursors to the beginning and end of the contraction.

the values for Abs Int. on the EMG and muscle channels are the relative amount of electrical activity causing the contraction and relative strength of the muscle respectively.

4. measure the circumference of the forearm

is there a linear relationship between the absolute integral under the EMG signals and the absolute integral under the muscle contraction?

yes there is a linear relationship, as the emg activity increases the integral under force curve increases proportionally too

do muscle fibers have a refractory period like nerve fibers?

Muscle fibers also have a refractory period, during which a muscle cell is temporarily unable to respond to a subsequent stimulus, regardless of its strength. The refractory period plays a vital role in ensuring the orderly transmission of action potentials along nerve fibers and prevents the occurrence of overlapping or continuous signals

does amplitude of emg increase because fibers are firing more often or are more fibers being recruited or both?

The increase in the amplitude of the EMG signal and force of contraction can be attributed to both the firing frequency of motor units and the recruitment of additional motor units

exercise 2: EMG intensity and fatigue in dominant arm

to observe the relationship between the length and strength of a muscle contraction and EMG activity

procedure 2

1. subject will squeeze the bulb of hand dynamometer as tightly and long as they can to fatigue the forearm muscles
2. as time passes, muscle force will decrease
3. when muscle strength drops to below half the initial strength, stop the recording
4. click autoscale for both channels

data analysis for experiment 2

1. use display time icons to display both recordings. place one cursor on the relaxation period just before contraction and the other just after they release the bulb. click zoom between cursors
2. click analysis
3. place one cursor on baseline before contraction, and second at the peak near the beginning, this is your max value
4. the half max muscle force is the max value divided by 2
5. find the V2-V1 point which is half value and record the time it took to get to it

why does the dominant / stronger forearm have a greater circumference

The difference in size can be attributed to the same factors mentioned previously regarding grip strength. For instance, activities involving gripping or tasks demanding manual dexterity may impose higher demands on the muscles of the dominant forearm, resulting in gradual increases in muscle size and circumference