4. Functions of Skeletal Muscle

Question 1

What is a Motor Unit?

Answer 1

• A motor unit is the functional unit of a muscle.
• It consists of a motor neuron and all the muscle fibers that the motor neuron innervates.

Question 2

Where are Motor Neurons located?

Answer 2

The cell body of a motor neuron is located in the spinal cord.

Question 3

What is an Axon?

Answer 3

The axon of that motor neuron extends from the spinal cord to the target muscle which may be a few millimetres away or a few feet away

Question 4

Action Potential

Answer 4

• If the cell body of the motor neuron receives
  a strong enough stimulus, an action potential is generated.
• This action potential travels along the axon
  and all its bifurcations to stimulate each and every muscle fiber in that particular motor unit.

Question 5

What is the All-or-None Law?

Answer 5

The all-or-nothing law is an important principle that describes how nerve cells either fire at full strength or do not fire at all.

Question 6

Slow Twitch vs. Fast Twitch
Graph

Answer 6

Graph:

Question 7

3 Types of Twitches

Answer 7

three ways on the basis of metabolic characteristics:

• Slow twitch oxidative (SO)
  – also called “type I”
• Fast twitch oxidative-glycolytic (FOG)
  – also called “type IIa”
• Fast twitch glycolytic (FG)
  – also called “type IIx”

Question 8

Endurance vs Power Athletes Twitch Muscles?

Answer 8

Endurance athletes have higher than average proportions of slow twitch muscle fibers in the muscles used in their sport,

Power athletes have high proportions of fast-twitch muscle fibers.

Question 9

remember 4 - 5

Answer 9

Question 10

What are Hybrid Fibers?

Answer 10

Hybrid fibers, which express more than one type (I/IIa, IIa/IIx, I/IIa/IIx), may make up a significant proportion of fibres.

Question 11

what are motor units?

Answer 11

more force required means more motor units fired/used
a large muscle can contain up to 2000

Question 12

What is Concentric Contraction?

Answer 12

When the velocity is positive, then the
muscle is shortening.

Question 13

What is Eccentric Contraction?

Answer 13

If the velocity is negative, then the muscle is lengthening
(generates more force)

Question 14

What is Isometric Contraction?

Answer 14

When the velocity of shortening is zero,
there is no change in muscle length.

Question 14

What is Multiple Motor Unit Summation?

Answer 14

A large muscle may contain up to 2,000 motor units. A skeletal muscle can increase force production by activating more motor
units. A low force requires the activation of a small number of motor units while a higher force requirement progressively enlists more motor units.

Question 15

What is Frequency or Wave Summation?

Answer 15

If a single action potential travels down a motor neuron axon, the motor unit response is a twitch.
If many action potentials travel down the axon at a rate faster than the twitch response time of the motor unit, then the mechanical force response will summate.

At high motor neuron firing rates (i.e., many action potentials in a short period of time) this effect can generate on average about 5x the force of a single twitch

Question 16

What is the Size principle of motor unit recruitment?

Answer 16

There is a recruitment order of motor units as exercise intensity increases.

As the muscle force requirement increases, motor units with progressively larger axons are recruited.
Slow twitch motor units with the lowest activation threshold are selectively recruited during light to moderate effort.
More rapid, powerful movements progressively activate FOG motor units and then FG motor
units, until all the motor units of all types are activated

Question 17

What is the Muscle Length-Tension Relationship?

Answer 17

an isolated muscle can exert its maximal force or tension while in a resting stretched position. As the muscle shortens, less tension can be exerted.
When a muscle is stretched or shortened, the length of each individual sarcomere also increases or decreases, respectively. there is an optimal length which provides for the greatest possible number of cross-bridge formations.

Longer or shorter than this optimal length, the number of cross-bridge formations is reduced and thus active force production is reduced.
Muscles must be stretched from their resting length for the most effective action.

For example - flexing the knee, hip, and ankle joints before jumping.

Question 18

What is the Muscle Force-Velocity and Power-Velocity Relations?

Answer 18

When maximally activated, the faster a muscle shortens the less force it produces.

When the velocity of shortening is zero, this is defined as an isometric contraction (i.e., no change in muscle length).

If the velocity is negative, then the muscle is lengthening when it is activated. This is an eccentric contraction. An example of an eccentric contraction is when you slowly lower the barbell after doing a biceps curl.
You are still using your biceps, but your muscle is lengthening.

Question 19

What is the Angle of Muscle Pull?

Answer 19

In the intact human body, the muscles act on the bones about the joints to form a lever system.
When a muscle is pulling at an angle of 90 degrees to a bone, all of the muscle contractile force is acting to rotate the bone around the joint.
At angles greater than 90 degrees, the magnitude of the rotational component of the muscle pull force decreases while the magnitude of the stabilizing component of the force increases.
In summary, three factors that affect the expression of strength by a muscle are:
1. The initial length of the muscle fibers.
2. Speed of shortening.
3. The angle of pull of the muscle on the bony skeleton

Question 20

What is Sarcopenia?

Answer 20

• Greek word meaning “poverty of flesh”
• Defined as the age-related loss of skeletal muscle mass, strength, and function
  Physical characteristics of old age
• A significant decline in skeletal muscle mass, strength, and power
• Decreased Physical Activity
• Decreased Food Intake

Question 21

Causes of Sarcopenia?

Answer 21

Sarcopenia is caused by various factors:
* Neural Apoptosis – neuronal atrophy in areas of the brain responsible for motor control. There is reduced peripheral nerve regeneration and reduced neuromuscular junction remodeling resulting in loss of motor units and
subsequent atrophy of muscle fibers. These processes are slowed down by exercise programs.
* Imbalance of muscle protein synthesis/degradation
* Decline in mitochondrial function in skeletal muscle
* Physical inactivity – “If you don’t use it, you lose it!”
* Hormonal imbalance – decreased levels of testosterone, estrogen, growth hormone, etc
* Food intake – inadequate protein intake, Vitamin D intake, etc.

Question 22

When does Maximum Power Output Occur>

Answer 22

Maximum power output occurs at approximately
one-half of maximum velocity and
one-third of maximum concentric force.

Question 23

What 3 factors that affect the expression of strength by a muscle?

Answer 23

1. The initial length of the muscle fibers.
2. Speed of shortening.
3. The angle of pull of the muscle on the bony skeleton