Lecture 8: mechanical properties of skeletal muscle

Question 1

what are the** functions** of skeletal muscle?

Answer 1

• to move the body by creating motion
• to** provide strength** by generating force
• to protect joints by absorbing shock

Question 2

what are the specific conective tissue functions within the muscle?

Answer 2

• to provide gross structure to the muscle
• to generate** passive tension against strength**
• to transmit force to the bone & across the joint

Question 3

Describe the properties of a skeletal muscle fiber?

Answer 3

• their** extensibility** - their ability to be stretched / increase in length
  * elasticity- their ability to return to their original length after a stretch
  * contractibility - their ability to develop tension
  * irritability -their ability to respond to a stimulus eg action potential or mechaical force

Question 4

what is the fascia?

Answer 4

• the band of sheet of fibrous connective tissue (normally collagen)
• they surround muscles

Question 5

what is a muscle twitch?

Answer 5

the response of a muscle to a single stimulus

Question 6

what is the twitch tension?

Answer 6

the** force developed** as a result of a muscle twitch

Question 7

What are the** mechanics** of a single muscle fiber contraction?

Answer 7

• single muscle fiber contraction refers to the ‘turning on’ of the cross - bridge cycle
• the thin (actin) and thick (myosin) filaments do not shorten, but they increase their overlap

Question 8

After the activation of a muscle fibre, what are the 3 types of muscle contractions that can occur?

Answer 8

1. isometric contraction (iso=same, metric=distance) - the muscle generates tension but does not change length EG holding a weight
2. isotonic contraction/concentric - the muscle generates tension (force) and it shortens which moves a load - eg bicep curl
   3.** lengthening contraction/eccentric** - the muscle is trying to shorten by generating tension but in fact it is lengthening due to the overpowering external force being applied to the muscle EG lifting weight thats too heavy

Question 9

what does the movement around a joint require?

Answer 9

it requires** 2 antagonistic groups** of the muscles - 1 flexes the limb at the joint, the other extends the limb

Question 10

Describe how a muscle shortens/relaxes

Answer 10

• when a muscle fibre is activated, the process of a contraction begins with an AP passing across its length
• initial phase of acceleration during which the tension in the muscle increases until it equals that of the load to which it is attached
• the fibre the begins to shorten at a constant rate until it decelerates
• finally the muscle relaxes and the tension declines to zero

Question 11

What are the 3 types of skeletal fibers?

Answer 11

• Type I - slow oxidative twitch fibers (need oxygen)
• Type IIa - fast oxidative twitch fibers (intermediate) (fast but still use o2)
• Type IIb - fast glycolytic twitch fibers - anaerobic

Question 12

What are the 2 characteristics that determine the type of skeletal muscle fiber?

Answer 12

1. differences in maximal shortening velocites - due to different myosin enzymes with high or low ATPase activites
2. **diameter **- a larger diameter produces greater tension

Question 13

What is a motor unit?

Answer 13

• a single neuron and all the muscle fibers it innervates

Question 14

Can motor units contain a mix of all 3 types of fibres?

Answer 14

• No
• but most muscles do contain the 3 types, so we have many motor units

Question 15

Describe type I/ red muscle fibers

think the chicken leg - red

Answer 15

• red muscles contain predominately red muscle cells
• red muscle fibers/cells contain large amounts of mitochondria and myoglobin (protein containing oxygen)
• red muscle cells contain a myosin enzyme with low ATPase activity - hence a slow contraction
• examples - posture muscles -a sustained production of force is nessessary

Question 16

Describe **type II fibers/white muscle cells **

Note diff between type IIa and type IIb

Answer 16

• white muscles contain predominately white muscle cells
• they are** thicker** cells and contain less myoglobin
  * ATPase activity is high and hence contraction is fast
• type IIa (red) contain **many mitochondria **& and are available for both sustained contraction & short lasting contractions
• type IIb (white) contain a few mitochondria and they rely on anaerobic glycolysis to generate ATP needed for contraction

Question 17

Which muscle fibre type is most prone to fatigue?

Answer 17

type IIb

Question 18

What is recruitment?

Answer 18

• increasing the number of active motor units in a muscle
• slow oxidative (type I) fibers are recruited first, then fast oxidative, then fast glycolytic

Question 19

what is the size principle in relation to recruitment?

Answer 19

• the motor units are recruited according to size - ie type I first, then type IIa and then type IIb

Question 20

what effect does increasing motor unit recruitment have on the muscle?

Answer 20

it increases the velocity at which a muscle will move a given load

Question 21

what is the rate principle?

Answer 21

• increasing the frequency of action potentials in a muscle fiber
• this will increase the mechanical response ( tension and shortening ) up to the level of maximal tetanic shortening

Question 22

what **2 mechanisms **control muscle contraction?

Answer 22

1. recruitment
2. rate principle

Question 23

what is a tetanus muscle contraction ?

Answer 23

a sustained muscle contraction - action potentials are being emmitted from a nerve at a **high rate **

Question 24

what is the difference between a fused and unfused tetanus contraction?

Answer 24

• unfused tetanus occurs when muscle fibers** do not completely relax before the next stimulus** because they are being stimulated at a fast rate
• fused tetanus - the muscle fibers have reached maximum tension

Question 25

what is muscle fatigue?

Answer 25

the reduction of
* muscle force production capability
* contraction velocity

and prolonged relaxation of motor units between recruitment

Question 26

what does muscle fatigue depend on?

Answer 26

• the muscle itself
• exercise duration
• fibre type composition
  *

Question 27

what are the causes of muscle fatigue?

Answer 27

• inorganic phosphate which increases during fatigue due to breakdown of creatine phosphate
• reduction in the rate of intracellular Ca2+ release and uptake by sarcoplasmic reticulum
• inadequate oxygen availibility - NOT A LACK OF ATP

Question 28

what does long duration, low intensity exercise do to a muscle fiber?

Answer 28

increases the muscle fiber’s capacity for ATP production by increasing the no of mitochondria, blood vessels in the muscle which increases endurance

Question 29

what does short duration and high intensity exercise (eg weight lifting) do to a muscle fiber?

Answer 29

• increases fiber diameter as a result of increased synthesis of actin and myosin, resulting in increased strength

Question 30

Describe the length - tension curve

Answer 30

• optimal length - actin and myosin are at ideal positions for max cross bridge binding and therefore max tension
• increase or decrease in stretch from optimal length results in a deceased tension developed