2.3.4. Muscle Mechanics I/II

Question 1

2 Functions of skeletal muscle

Answer 1

1. Produce Skeletal Muscle movement. Performs physical work and develops force, tension or stress
2. Maintain posture and body position

Question 2

What are the two functional elements of skeletal muscle and what are they in charge of?

Answer 2

1. Contractile Element - Consists of a cross bridge system that generates muscle force
2. Viscous Element/Elastic Element - Element that resists motion. Turns that energy into heat.

Question 3

Formula for calculating the elastic or viscous functional element force value?

Answer 3

F = -K*V

Question 4

What occurs with an isometric contraction?

Answer 4

The ends of the muscle are fixed (holding weight without moving it)

Two platforms: one fixed and one adjustable (but immovable during contraction)

Question 5

What occurs with an isotonic contraction?

Answer 5

Use passive stretch to set initial muscle length and apply a supermaximal electrical stimulus

Muscle shortens until there are just enough cross bridges to barely support the load, then motion stops

Question 6

What is a muscle twitch?

Answer 6

A muscle twitch, or fasciculation, is the tension (force) produced in response to a single muscle AP

Isometric Contraction

Question 7

How are twitch and tetanus related?

Answer 7

Twitch may also be used to describe the force and speed of contraction of a single muscle fiber. Total tension developed increases: can result in fused or unfused tetani

Question 8

Describe the difference between Fused vs. unfused Tetanus.

Answer 8

Unfused: Not “continuous” activation, but a summative result (bumpy graph)

Fused: Fused is a mechanical summation overall (smooth graph)

Question 9

What is muscle efficiency based on?

Answer 9

The ratio of mechanical work output to total metabolic cost (aka O2 consumption)

Question 10

Work equation

Answer 10

Work = F x D (joules)

Question 11

Power equation

Answer 11

Power = work/time (watts)

Question 12

Stress equation

Answer 12

Stress = Force / Area of muscle

Question 13

Is movement required to generate stress?

Answer 13

NO

Question 14

Voluntary activation of skeletal muscle

Answer 14

AP in α-motor neuron propagates to NMJ; AP releases transmitters into synaptic cleft; depolarization of sarcolemma; muscle AP; release of Ca; cross bridges cycle and muscle contracts

Question 15

How does muscle develop force?

Answer 15

Depends on:
Type
Length
Stimulus 
Number of cells firing

Question 16

What is the greatest determinant for the development of force?

Answer 16

Muscle length

Question 17

Contractile Element of muscles

Answer 17

Cross-bridge systems that generates muscle force

Question 18

Viscous element

Answer 18

Resists motion; acts like a shock absorber

Only produces force while in motion

Dissipates energy (turns it into heat)

Question 19

Series elastic element

Answer 19

Made up of tendon and connective tissue (small part made up of titin)

Question 20

Parallel elastic element

Answer 20

Largely made up of titin and connective tissue

Question 21

What does a bigger stimulus mean?

Answer 21

More fibers are activated (each individual fiber goes “all out” with each stimulus, large stimuli are just able to activate more than small ones)

Question 22

Place these muscles in increasing order of contraction time:

Gastrocnemius, Soleus, Occular

Answer 22

Occular < Gastrocnemius < Soleus

eyeballs don’t need a lot of force, but need to move quick < running and jumping < posture

Question 23

Difference b/w muscle twitches and APs

Answer 23

Twitches can be summed

Question 24

Mechanism behind fused tetanus

Answer 24

The viscous elements in the muscle don’t allow it to fully relax before the next stimulus arrives (intracellular Ca isn’t completely removed from the cytoplasm either)

Question 25

Is increasing the stimulus our primary means of controlling muscle force?

Answer 25

No (generally a fused tetanus has a magnitude of only 3-5 times that of an isolated twitch)

Question 26

Passive muscle strength

Answer 26

The muscle as a rubber band (no stimulation, no contraction of the muscle in questions) Stretch the muscle and measure the force produced

Question 27

How is force generated by passive muscle strength

Answer 27

By the muscle trying to get back to its resting length

Question 28

Total Tension

Answer 28

= Passive + Active Tension

Question 29

Sliding filament theory

Answer 29

Force is generated by making and breaking cross-bridges b/w the thick and thin filaments Sarcomere shortens by thick and thin filaments sliding past each other

Question 30

Active Tension Force Magnitude

Answer 30

Related to the number of cross bridges formed More cross bridges = greater force (related to thick and thin filament overlap and overlap is related to MUSCLE LENGTH)

Question 31

What is the thick filament made up of?

Answer 31

Myosin molecules pointed in opposite directions bound together in center of H-band by myomesin that forms the M-line

Question 32

What is the thin filament up of?

Answer 32

Actin, tropomyosin & troponin Attached to the z-disks

Question 33

Cross Bridge Cycling

Answer 33

Thick filaments globular heads form cross bridges with thin filaments; heads rotate inward toward the M-line pulling the thin filaments together; heads release from thin filament (need ATP); heads reset As long as Ca level is high, the muscle can keep contracting/crawling

Question 34

Too much thick/thin interaction

Answer 34

Thin filaments shield one another from thick filaments (decrease in cross bridges and less force) Thick filaments contact the Z line and resist contraction Tension falls to zero when initial length is set slightly before the thin filament contacts the Z line

Question 35

Skeletal muscle force is regulated by two things

Answer 35

``` Motor unti recruitment (primary) Firing pattern (secondary) ```

Question 36

Skeletal muscle relaxation

Answer 36

Isotonic relaxation followed by isometric relaxation

Question 37

Heart muscle relaxation

Answer 37

Isovolumetric

Question 38

Voluntary control of skeletal-muscle force

Answer 38

Length changes in skeletal muscle are not significant in force regulation Restricted by bones and ligaments (the muscle works near the flat peak of the active tension curve)

Question 39

Control of cardiac muscle force

Answer 39

Length changes are significant

Question 40

Motor Unit

Answer 40

All of the muscle fibers innervated by the motor unit Muscle fibers are not innervated by more than one motor neuron

Question 41

Small motor unit

Answer 41

Quick movement but little force

Question 42

Large motor unit

Answer 42

Slower movement but lots of force/strength

Question 43

Types of muscle fibers

Answer 43

Slow-oxidative Fast-oxidative Fast-glycolytic

Question 44

Type I

Answer 44

Slow Oxidative; slow myosin ATPase activity; high oxidative capacity Fatigue resistance; used continuously RED

Question 45

Type IIx

Answer 45

Fast glycolytic; fast myosin ATPase activity; high glycolytic Fast fatigue; used intermittently WHITE

Question 46

Type IIa

Answer 46

Fast oxidative; fast myosin ATPase activity; moderate oxidative capacity Moderately fatigue resistant

Question 47

Muscle organization

Answer 47

We have many small motor units and very few large motor units Largest motor units are recruited last, only during the highest forces