Recruitment and Isometric and Isotonic Contractions [PowerPhys]

I got 100% on this but it took a while to do this report. I hope it helps those that want to understand better this type of contractions. Think before copying it. The best way to learn something is to understand it and make it meaningful. Have a great day!!
LABORATORY REPORT
 
Activity 3: Recruitment and Contractions

PREDICTIONS
1. When the arm goes from resting to flexing, the amplitude and frequency of sEMG
spikes will: increase.
2. During flexion, the amplitude and frequency of sEMG spikes will be greater than
during extension.
3. Recruitment of motor units will be greatest when the load is: 20 pounds.

MATERIALS AND METHODS
Comparison of motor unit activation during muscle tone and concentric and
eccentric isotonic contractions
1. Dependent Variable: amplitude and frequency of sEMG spikes.
2. Independent Variable: muscle movement.
3. Controlled Variables: total number of motor units present in muscle, muscle load.
Recruitment during isometric contractions
1. Dependent Variable: amplitude and frequency of sEMG spikes.
2. Independent Variable: muscle load.
3. Controlled Variables: total number of motor units present in muscle, muscle
movement, subject's physical condition.
4. What does the acronym sEMG stand for? Surface Electromyography.
5. During a muscle contraction, what is recorded on a sEMG? Amplitude (mV) and
frequency of sEMG spikes (motor unit action potentials).
6. Spike numbers were measured during a 200 msec sampling period.

RESULTS
See Table 3: Muscle Tone (Resting) and Isotonic Contractions
See Graph 1: sEMG Amplitudes and Frequencies at Rest and During Isotonic
Contractions
1. When the biceps brachii was at rest, were motor units activated as indicated by
amplitude and frequency of sEMG spikes? Yes, but at a very low rate (0.12 mV)
because normal resting muscles exhibit muscle tone due to the stimulation of a small
number of motor units. This asynchronous activation keeps all the motor units within a
muscle in shape.
2. What was the change, if any, of motor unit stimulation (reflected in amplitude
and frequency of sEMG spikes) during concentric contraction against a 5 pound
load? Yes, there was a change - from 0.12 mV at rest to 0.43 mV during a concentric
contraction against a 5 pound load. The increase of amplitude was of 0.31 mV. As for
the frequency of sEMG spikes, there was an increase from 1 (at rest) to 19 during
concentric contraction against a 5 pound load. The change was of 18 per 0.2 msec time
period.
3. Did motor unit stimulation, (reflected in amplitude and number of spikes per 0.2
sec time period) increase, decrease, or not change when biceps brachii went from
concentric to eccentric contraction with a 5 pound load? Motor unit decreased in
regards to the amplitude: it went from 0.43 mV in a concentric contraction to 0.32 mV in
an eccentric contraction with a 5 pound load. As for the frequency of sEMG spikes it did
not change from concentric to eccentric contraction with a 5 pound load. It stayed the
same: 19 per 0.2 msec time period.
4. Did the arm flex or extend during the eccentric contraction of the biceps brachii?
The arm extends during the eccentric contraction.
See Table 4: Isometric Contractions
See Graph 2: sEMG Amplitudes and Frequencies at Rest and During Isometric
Contractions under Different Loads
5. Describe how amplitude of sEMG spikes changed with increasing muscle load. A
gradual increase can be observed in the amplitude of sEMG spikes with increasing
muscle load. From rest to the isometric contraction with 0 lbs. a slight decrease in
amplitude can be observed (from 0.12 mV to 0.087 mV). But, by increasing the load
during the isometric contractions, the amplitude started increasing: for 2 lbs the
amplitude was 0.166 mV to 1.250 mV amplitude for a 20 lb weight.
6. Describe how frequency of sEMG spikes changed with increasing muscle load. As
the load started increasing so did the frequency of sEMG until it reached an optimal point
of 21 per 0.2 msec at the 10 lb threshold.
7. Based on changes in amplitude and frequency of sEMG, did motor unit activation
increase, decrease, or stay the same with increasing muscle load? The motor unit
activation increased with increasing muscle load because activated motor units are
contracting with greater frequency.
8. Do you think that the force of isometric contraction increased, decreased, or stayed
the same as muscle load increased? The force of the isometric contraction increased
as the muscle load increased.
9. Which would be a better predictor of increase in force of contraction, change in
sEMG amplitude of spikes or change in sEMG frequency of spikes? Change in
sEMG frequency of spikes.

DISCUSSION
1. Discuss the importance of muscle tone. Muscle tone is very important because it
allows the body to maintain its posture (e.g. keeping the head upright and preventing it
from falling forward). Tone means tension, i.e. the skeletal muscles undergo a small
amount of tension due to involuntary contractions of the motor units. Although the
skeletal muscles are kept firm by the muscle tone, this is not strong enough to produce
movement. Another area, where muscle tone plays an important role, is in smooth muscle
tissue (e.g. gastrointestinal tract or the blood vessels). In the GI tract the walls of the
digestive organs maintain a steady pressure on their contents, or in the blood vessels were
muscle tone helps in maintaining the blood pressure.
2. Discuss the importance of motor unit stimulation during eccentric isotonic
contraction? The importance of motor unit stimulation during eccentric isotonic
contraction is to try and keep the motor unit stable when muscle tension resists an action.
During an eccentric contraction, the tension exerted by the myosin cross-bridges resists
movement of a load and slows the lengthening process.
3. Discuss how increasing the number of motor units stimulated affects force.
Different motor units in a muscle are recruited in a specific order, depending on the
physical activity that needs to be performed. If a task that needs to be performed requires
only weak contractions in order to fulfill it, then only slow oxidative fibers are activated.
If more force is needed, then fast oxidative–glycolytic fibers are also stimulated. In order
to reach maximum force the help of fast glycolytic fibers is enlisted together with SO and
FOG fibers. Maximum force occurs when all motor units of muscle are stimulated and all
muscle fibers are contracting.
4. Discuss how increasing frequency of motor unit stimulation affects force. The
muscle tension developed by a contracting muscle is determined by the frequency of
stimulation of motor units and the number of motor units stimulated. If the contracting
motor units are stimulated again before the relaxation phase of a muscle twitch is
complete, then the next contraction will produce a greater force. Increasing the frequency
of muscle stimulation produces sustained force generation. Muscle tension depends on
the frequency of stimuli.
5. During arm flexion, what type of contraction would the triceps brachii be
exhibiting: eccentric isotonic contraction.
6. During arm extension, what type of contraction would the triceps brachii be
exhibiting: concentric isotonic contraction.
7. As muscle load is increased, which muscle fiber type is recruited first, second, and
last? The first muscle fiber type recruited is the slow oxidative; the second one recruited
is the fast oxidative-glycolytic and the last one is the fast glycolytic fibers.
8. Describe the importance of this order. Include affect on force of contraction and
fatigue. Recruitment is the process of increasing the number of active motor units to
increase force developed by a muscle. An increase in the number of motor units involved
in contraction increases the contraction force of the muscle. Usually, recruitment involves
activation of different motor units to help delay fatigue. The weaker, more fatigue
resistant motor units (slow oxidative motor units) are recruited first, with stronger motor
units (less fatigue resistant) added if more force is needed in response to greater muscle
load (weight moved). Motor units having many muscle fibers are capable of more
forceful contractions than those having only a few fibers. Increasing the number of motor
units contracting at the same time also increases the force generated. After long periods
of muscle contraction muscle fatigue sets in. The muscle cannot maintain force of
contraction. Fatigue results mainly from changes within the muscle fibers.
9. Restate your predictions that were correct and give data from your experiment
that support them. My predictions were correct. When the arm goes from resting to
flexing, the amplitude and frequency of sEMG spikes will: increase. It went from 0.12 to
0.43 mV and from 1 to 19 per 0.2 msec. Also, my second prediction was confirmed:
during flexion, the amplitude and frequency of sEMG spikes will be greater than during
extension. It went from 0.43 to 0.32 mV, but the frequency of spikes stayed the same (19
per 0.2 msec).

APPLICATION
1. Flaccid muscles do not exhibit muscle tone. Explain how muscles become flaccid.
Muscles become flaccid as a result of damaging the motor neurons that serve the skeletal
muscle. If the motor neurons are damaged or cut, then a state of limpness occurs and the
muscle loses its tone.
2. In the experiment “Muscle Tone and Concentric and Eccentric Isotonic
Contractions”, the muscle contracts isometrically prior to the concentric isotonic
contraction. Explain why the muscle contracts isometrically before it shortens. In
isometric muscle contraction the muscle maintains the same length, but the tension in the
muscle increases. It is similar to holding a weight in a static position for a prolonged
period of time. Although the length of the muscle remains fixed, the muscle is developing
tension. If movement occurs, then the muscle shortens and we get a concentric isotonic
contraction (during arm flexion).
3. The nerve that innervates the biceps brachii muscle is the musculocutaneous
nerve. Explain what effect damage to this muscle would have on contractile force
and recruitment of motor units in the biceps brachii muscle. The biceps brachii is
responsible for flexing the forearm at the elbow joint; supinate the forearm at radioulnar
joints, and flexing the arm at shoulder joint. Damage to the musculocutaneous nerve will
have repercussion on the biceps brachii muscle’s capability to perform different actions,
leading to loss of muscle strength, poor muscle tone, and sensory loss, weak or absent
biceps tendon reflex.