KNES 363 2

Second half of semester

What muscle structures are active & passive

Active - muscle
Passive - tendon, ligament, bone cartilage, etc.

Functions of muscle

To produce force
To create movement
To provide joint stability

To act as a passive force transmitter

Define the parts of the muscle starting from the outside moving in

Muscle =

Fascia
Epimysium
Perimysium
Sarcolemma
Endomysium
Myofibril

Muscle fasicles are surrounded by the

Perimysium

Myofibril, endomysium, & sarcolemma compose the

Muscle fibre (cell)

Name each section

Which band on the myofibril looks the same throughout?
Which does not?

I-band (isotropic band) - does look the same throughout
A-band (anistrotropic band) - does not look the same throughout

What is titin
Purpose?

Connects thick filament to Z line
Very large proteins

What is Nebulin
Purpose?

Actin-binding protein
Nebulin acts as a thin filament ruler that regulates the length during sarcomere assembly

What is Desmin?
Purpose?

Intermediate filaments
Stabilizes the sarcomere

What is a motor unit

A motor unit (MU) is a nerve cell (motor neuron) & all the muscle fibers innervated by this nerve cell

Afferent vs Efferent neurons

Afferent neurons - Arrive at the CNS
Efferent neurons - exit the CNS

Define neuro-muscular junction / motor endplate

A neuro-muscular junction or motor endplate is the junction b/w the motor neuron & muscle fibre

What is the neuromuscular junction

It is separated by?

Site where motor neuron meets the muscle fiber

Separated by a gap called neuromuscular cleft

What are the 16 steps of a Muscular Contraction

1. An action potential (an electric signal) moves down the motor neuron (nerve)
○ via efferent neurons

2. Reaches the pre-synaptic terminal

3. A series of chemical reactions take place which culminate in the release of acetylcholine (ACh) from synaptic vesicles located in the pre-synaptic terminal

4. ACh diffuses across synaptic cleft

5. ACh binds to receptor molecules in membrane of post-synaptic terminal

6. Causes an increase in permeability of the membrane to sodium (Na+) ions

7. Due to diffusion of the sodium ions the membrane is depolarized

8. If that depolarization reaches a critical threshold an action potential will travel along the stimulated muscle fiber

9. A special chemical process (acetylcholine → acetic acid + choline) prevents continuous stimulation

10. The action potential travels along the fiber, depolarizing the membrane
Once threshold is reached, it travels alongside the muscle

11. The depolarization reaches the interior of the muscle fibres at invaginations of the cell membrane called T-tubules

12. This causes the release of calcium (Ca2+) ions from sarco-plasmic reticulum into the sarcoplasm surrounding the myofibrils

13. Calcium ions bind to special binding sites on the troponin molecules of the thin myofilament, removing an inhibitory mechanism preventing cross-bridge formation
○ Inhibitory mechanisms prevents crossbridge
○ If there's no calcium, thick filament can't attach to thin filament

14. Cross-bridges attach to the active sites of the thin filaments

15. A breakdown of ATP → ADP + a phosphate ion provides energy to move the cross-bridge heads pulling thin filaments past the thick filaments

An ATP molecule attaches to the myosin portion of the cross-bridge so that the cross-bridge can release from its attachment site, return to its original position, ready for a new cycle

A motor unit w/ a large number of muscle fibers

A motor unit w/ a small number of muscle fibers

A motor unit w/ a large number of muscle fibers:
- Large increase in force for a given level of activation - Good for explosive or dynamics movements

A motor unit w/ a small number of muscle fibers:
- Small increase in force for a given level of activation
- Good for control of small or fine movements

Muscle w/ large number of MUs

Muscle w/ a small number of MUs

Muscle w/ large number of MUs:
- High resolution of control
- Finer control & adjustments

Muscle w/ a small number of MUs
- Sharper increase in force
- Less activation to get relatively more force production

Define Type I muscle fibers:

Speed
Fatigable?
Metabolism
Fiber size
Colour
Recruitment

Slow twitch
No
Oxidative
Small
Dark Red
Recruited first

Define type IIa muscle fibres

Speed
Fatigable?
Metabolism
Fiber size
Colour
Recruitment

Fast twitch
Resistant
Oxidative-glycolytic
Medium
Red
Second

Define type IIb muscle fibres

Speed
Fatigable?
Metabolism
Fiber size
Colour
Recruitment

Fast twitch
Fatigable
Glycolytic
Large
White
Last

What systems exist to help regulate our muscle contractions?

How do we know how much of our muscles have contracted or how far our muscles have been stretched?

Muscle spindles & golgi tendon organs

Afferent signals from sensory receptors, specifically movement related sensory receptors known as proprioceptors

What is muscle architecture?

The arrangement of muscle fibers relative to the axis of force production

Define longitudinal/fusiform muscle
Give an example

Fibers extend parallel to the muscle force generating axis: parallel, longitudinal or fusiform muscle

Biceps, sartorius, psoas major

Define unipennate muscle
Give an example

Fibers oriented at a single angle relative to the force generating axis

- Muscle fibers that are at an angle (all muscle fibers in one direction)
- Extensor digitorum (in forearm)
- Vastus Lateralis, Flexor Pollicis Longus, Palmar Interosseous

Define multi-pennate muscle
Give an example

Fibers oriented at several angles relative to the force generating axis: multipennate muscle

Gluteus Medius, Deltoid, Trapezius

What is physiological cross-sectional area

Sum of the cross-sectional areas of all of the fibres w/in a muscle

# of parallel fibers determines maximum force → ________ → ________

# of parallel fibers determines maximum force → larger CSA → larger force production capability

What is pennation angle?

Angle b/w muscle fibers & line of action

Activation of more fibers w/ smaller geometric cross-section tradeoff: loss of force per fiber in line w/ tendon also amount of length change?

Active force length relationship is established for ___________ __________

Active force length relationship is established for isometric contractions

Isometric
Isokinetic
Isotonic

Isometric - constant length

Isokinetic - constant rate of movement / velocity

Isotonic - same load/weight

What are the 3 main components of a muscle?

Contractile component
Series Elastic component
Parallel Elastic Component

What are Series Elastic Component

Structures in line w/ the contractile elements of the sarcomere

Structures like titin provide force when stretched as it acts like a spring in series w/ the CE

Tendon

What are Parallel Elastic Component

The structures aligned alongside or parallel to the CE & SE that also provide passive tension when stretched

Fascia, epimysium, perimysium & endomysium

Alongside active force generation, muscles also have the capabilities to generate tension through:

passive means

What is the Hill Type Muscle Model

Describes the mechanism of force production.

It is composed by different elements that describe the behaviour of the muscle (contractile, series elastic and parallel elastic element) and tendon.

Passive F-L: Hysteresis

Loss of energy
Damped system

Passive F-L: Stress Relaxation

Tissue held at constant length
Force (or stress) decreases over time but only to a certain extent

Passive F-L: Creep

Tissue held at constant stress
Tissue lengthens over time only to a certain extent

What structures are responsible for passive force?

Connective tissue:
- Fascia
- Endomysium
- Perimysium

Intra-fiber structures
- titin
- desmin

Intermediate filaments: _______ filaments

- Form __________ connections b/w the peripheries of successive __ ______
- Form a three-dimensional ________ around the _ _____
- Connect the entire contractile apparatus to the ________ & its _______

Intermediate filaments: desmin filaments

- Form longitudinal connections b/w the peripheries of successive Z lines
- Form a three-dimensional scaffold around the Z lines
- Connect the entire contractile apparatus to the sarcolemma & its proteins

The active force-length relationship describes muscle
behaviour at

The active force-length relationship describes muscle
behaviour at constant length (isometric contraction)

Muscle force decreases as movement speed _______

Muscle force decreases as movement speed increases

Muscle Shortening:
- _______ relationship
-Fmax is obtained at "_____" shortening velocity (______ ________)
- As the velocity of shortening increases, force ______
- The force generated at a shortening velocity is always ______ than the muscle's ______ ______
- The maximal shortening velocity corresponds to "_____" muscle _____

Muscle shortening:
- Hyperbolic relationship
-Fmax is obtained at "zero" shortening velocity (isometric conditions)
- As the velocity of shortening increases, force decreases
- The force generated at a shortening velocity is always lower than the muscle's maximal force
- The maximal shortening velocity corresponds to "zero" muscle force

Muscle force drops _____ as the velocity _____ intially

Muscle force drops rapidly as the velocity increases initially

In hill's equation, what does a and b represent

a = force constant
b = speed constant

Which is fast twitch and which is slow

Lengthening: eccentric contractions

Muscle is forced to lengthen during _________
Muscle can achieve high forces relative to its...
Force becomes ________ of the ______

Lengthening: eccentric contractions

Muscle is forced to lengthen during contraction
Muscle can achieve high forces relative to its maximal isometric force
Force becomes independent of the velocity

Lengthening:

Much of a muscle's normal activity occurs while it is actively lengthening; thus, eccentric contractions are ________ common

Muscle __________ thought to be associated more w/ eccentric contractions

Muscle _________ greater using eccentric training exercises

Much of a muscle's normal activity occurs while it is actively lengthening; thus, eccentric contractions are physiologically common

Muscle injury/soreness thought to be associated more w/ eccentric contractions

Muscle strengthening greater using eccentric training

The active force produced by a muscle depends on:

the number of attached cross bridges
the force produced per attached cross bridge

when the velocity of shortening increases, the active force _______ because:

when the velocity of shortening increases, the active force decreases because:
- number of attached cross bridges decreases
- the force produced epr attached cross bridge decreases

Why/how does active force decrease as velocity of shortening increases?

As the shortening velocity increases, the speed at which the thin filament slides past the thick filament increases, & so fewer & fewer cross bridges have time to attach

When the speed of sliding reaches the speed of attachment of cycling cross bridges, the cross bridges have no time to attach & thus force becomes zero

Lengthening:

Attached cross bridges are pulled: the force produced per cross bridge ______

Cross bridges do not detach, force does not ______

Lengthening:

Attached cross bridges are pulled: the force produced per cross bridge increases

Cross bridges do not detach, force does not decrease

Power is the...

Power = time rate of doing work

Which represents FT & ST fibers

Force-Velocity

Force-Velocity

Force-Velocity

Active F-L

Active F-L

Active F-L

Torque

quantity that describes the capacity of a force to produce rotation

aka moment of force / moment

If muscles generate sufficient force, bones ____ about joint axes

If muscles generate sufficient force, bones rotate about joint axes

Torque depends on

Force
Moment arm
Angle

Does torque stay constant when joint angle varies?

No b/c the moment arm changes

Strength is equal to ________ ________ ________ at a specific _____

Strength is equal to maximal isometric torque at a specific angle

What 3 things happen when training for strength

1. Greater force per cross-sectional area
2. Neural adaptations
3. Changes in muscle architecture

How does one obtain a greater cross-sectional area via strength training

Origin?

Hypertrophy:
↑ in muscle size & ↑ in fiber/myofibril size

Satellite cells (SC): activation

Satellite cell activation (2 processes)

Proliferation - division of satellite cells

Differentiation - formation of myoblasts to myofibrils

Explain neural adaptations for strength training (2)

Frequency (firing rates): increased stimulation
Improvement in motor unit recruitment: synchronization

Control of muscle force:

Temporal summation (frequency)
Spatial summation (recruitment)

Temporal summation

Tetanus

higher firing rate leads to quicker force production

Spatial summation

More motor units are recruited - syncrhonization

Larger force increments, quicker & larger force production

Changes in muscle architecture - what happens with pennation angle?

(during strength training)

There will be an increase in pennation angle b/c you can fit more muscle fibers in parallel

• More cross-sectional area, more force

What happens to fibers during strength training?

Increase in a type of fiber?

Increase in type IIb fibres in strength training

Strength training - time of improvement

first 1-2 weeks?
3-4 weeks?
4+?

Initial improvements (first 1-2 weeks)
learning effect – motor skill coordination

Subsequent improvements (3-4 weeks)
neural adaptations

Later improvements (4+ weeks)
muscle hypertrophy

What happens during endurance training?

Very small changes in muscle mass & fiber size
↑ Type I fibers
↑ number of mitochondria

Cardiovascular & respiratory adaptations:
- ↑ number of capillaries
- better supply of oxygen & nutrients

What is Specificity?

A muscle (fibre) adapts if it is recruited during exercise

Adaptations are therefore specific to the muscle trained

Adaptations are specific to the trained position

What causes the left shift in this graph?

If training has higher functional demands at a shorter muscle lengths, the tissue can and will adapt to ultimately produce more force over the range that is required by that training, resulting in a shifted curve like b in the graph above.

Increased functional demand

Strength & endurance training

Decreased functional demand

Reduction in the electrical & mechanical activities
- Neuromuscular diseases
- Spinal cord injury
- Denervation
- Immobilization
- ↘ weight bearing

Sarcopenia

Leads to decreased...

age related involuntary loss of muscle mass and
strength

decreased:
muscle cross sectional area
fiber size
fiber number
(greater loss in fast twitch fibers)

What is a joint

junction b/w 2 or more bones of the skeleton

Functions of joints

to permit & inhibit movement
to transfer forces b/w bones
to absorb shock through movement

Cartilaginous joint

Example
Where does it connect
Movement

Spinal vertebrae

cartilage

Small

Fibrous joint

Example
Where does it connect
Movement

distal/inferior tibiofibular joint

Fibrous tissue

small

Synovial joint

Example
Where does it connect
Movement

knee, hip

ligaments

small translation, large rotation

Synarthrosis joint - movement?

Immovable

amphiarthrosis joint - movement?

slightly movable

Diarthrosis joint - movement?

freely movable

What are degrees of freedom?

Number of variables (coordinates) needed to describe the movement of a body

A rigid body in 2-D space can

How many DOF?

translate in x direction
translate in y direction
rotate around z axis

3 DOF in 2-D space

A rigid body in 3-d space can

translate in x direction
translate in y direction
translate in z direction

rotate around x axis
rotate around y axis
rotate around z axis

6 DOF in 3-D space

Joints are often considered as ______
why?

Organs

changes in one structure affect all other structures:
- mechanically
- biochemically
- morphologically