Lecture 8: Skeletal Muscle Mechanics

Question 1

Isometric contraction

Answer 1

Developing tension at a constant muscle length; represented by tension vs. length curve

Question 2

Isotonic contraction

Answer 2

Shortening/lengthening of muscle at constant tension; represented by change in length vs. time

Question 3

Muscle tension

Answer 3

Force exerted by the contracting muscle on a load

Question 4

Load

Answer 4

Force exerted by an object on the muscle

Question 5

Tension vs load

Answer 5

Moving a load requires total muscle tension to be just greater than the load

Question 6

Concentric contraction

Answer 6

When tension > load, the muscle shortens during contraction

Question 7

Eccentric contraction

Answer 7

When load > tension, the muscle lengthens during contraction (lengthening only bc of external forces)

Question 8

Muscle twitch

Answer 8

Mechanical response of muscle fiber to 1 action potential

Question 9

Latent period

Answer 9

Delay between the action potential and corresponding increase in muscle tension

Question 10

In isometric contraction, how can the muscle develop tension without changing length?

Answer 10

In isometric contraction, the filaments don’t slide. Instead, power stroke rotation is absorbed by the elastic elements of the fiber. Cycling cross-bridges repeatedly bind to the same actin molecule.

Question 11

Contraction time

Answer 11

Defined as the time from beginning of tension to peak tension

Question 12

Muscle fiber types

Answer 12

Fast twitch: contraction time ~10 ms
Slow twitch: contraction time 100 ms or longer

Question 13

What makes different fibers fast vs. slow?

Answer 13

Contraction time depends on 1) how long cytosolic Ca++ stays elevated and therefore amount of SERCA activity and 2) speed of cross-bridge cycle (myosin ATPase rate)

Question 14

Differences between isometric and isotonic twitches

Answer 14

Isotonic twitches have longer latent periods and are quicker mechanical events, because they need to first build enough tension to actually move the load

Question 15

Effect of heavier load on an isotonic twitch

Answer 15

Heavier loads means:
-Longer latent period
-Slower velocity of shortening
-Less distance shortened

Question 16

Maximum shortening velocity and maximum isometric tension

Answer 16

Max shortening velocity occurs with 0 load, max isometric tension means 0 velocity. Velocity then increases in lengthening with heavier loads until the muscle tears

Question 17

Why do heavier loads slow shortening velocity?

Answer 17

More load slows the forward movement of each power stroke

Question 18

What determines max shortening velocity?

Answer 18

Rate of myosin ATPase, which determines the speed of an individual cross-bridge cycle

Question 19

Muscle summation

Answer 19

Muscle twitches sum if a new stimulus is applied before the muscle completely relaxes. Higher frequency stimuli create greater peak tension, and eventually a smooth continuation of tension

Question 20

Tetanus

Answer 20

Maintained muscle contraction due to repetitive stimulation

Question 21

Fused vs unfused tetanus

Answer 21

In unfused tetanus, tension oscillates due to low frequency stimulation. Fused tetanus has no oscillations.

Question 22

Max tetanic tension vs isometric twitch tension

Answer 22

In tetanus, [Ca++] stays high so more cross-bridges are recruited and there is time to stretch out elastic elements. Max tetanic tension is much higher than 1 twitch, and is good for sustained work like maintaining posture.

Question 23

Passive muscle tension

Answer 23

Relaxed muscle fibers have passive tension due to titin. Passive tension increases with length of fiber

Question 24

Optimal length L0

Answer 24

Active tension depends on resting fiber length and is highest at some intermediate length L0, the length at which the fiber develops the greatest isometric active tension

Question 25

Why does active tension decrease above L0?

Answer 25

As the fiber lengthens, there is less thin/thick-f overlap and so fewer cross-bridge interactions available

Question 26

Why does active tension decrease below L0?

Answer 26

At shorter lengths, thin-f's from other sarcomeres overlap and interfere with X-bridges. Rigid thick-f's can also collide with Z-lines, creating internal resistance to further shortening

Question 27

Resting length of all muscle

Answer 27

Passive elasticity keeps most muscle fibers near L0 when relaxed

Question 28

How do muscles maintain high ATP levels?

Answer 28

1. Phosphorylation of ADP by creatine phosphate (rapid) 2. Oxidative phosphorylation in mitochondria (slower) 3. Glycolysis ATP production in cytosol (slower)

Question 29

What supplies ATP for moderate muscle activity?

Answer 29

At moderate intensity, most ATP is from oxidative phosphorylation (aerobic)

Question 30

Where does glucose for ATP come from for oxidative phosphorylation in muscle?

Answer 30

First 5-10 min: muscle glycogen stores Next 30 min: blood glucose and fatty acids After: More fatty acids, glucose use decreases

Question 31

What supplies ATP for intense muscle activity?

Answer 31

At intensities >70% max ATP consumption, oxidative phosphorylation maxes out and glycolysis contributes more as intensity continues to increase. Uses blood glucose and produces lactic acid as a byproduct. Anaerobic.

Question 32

Oxygen debt

Answer 32

Increased oxygen demand continues post-exercise to restore creatine-Pi/glycogen and metabolize accumulated lactate

Question 33

Muscle fatigue

Answer 33

A decline in muscle tension due to previous contractions. Also causes slower shortening velocity/rate of relaxation

Question 34

Metabolic changes in active muscle

Answer 34

[ATP] decreases [ADP], [Pi], [Mg++], [H+], O2 free radicals increase

Question 35

Effects of metabolic changes in active muscle

Answer 35

-Decreased Ca++ release, reuptake, and storage by SR -Decreased thin-f sensitivity to Ca++ activation -Direct inhibition of myosin X-bridge cycle

Question 36

Why does low intensity, long duration exercise demand disproportionately long recovery times?

Answer 36

RyRs become leaky -> constant high [Ca++] -> increase in protease activity on contractile proteins, requiring longer recovery for protein synthesis -Dehydration/glycogen depletion also mediate fatigue

Question 37

Central command fatigue

Answer 37

Phenomenon where cerebral cortex regions fail to send excitatory signals; possible feedforward response anticipating glucose depletion

Question 38

Types of skeletal muscle fibers (twitch speed)

Answer 38

Type I: slow twitch (slow myosin ATPase velocity) Type II: fast twitch (fast myosin ATPase velocity)

Question 39

Types of skeletal muscle fibers (energy)

Answer 39

Oxidative: many mitochondria for oxid. phos. and lots of myoglobin for O2 diffusion/storage; AKA red muscle fibers Glycolytic: few mitochondria, less vascularization/myoglobin, lots of glycolytic enzymes/glycogen; AKA white muscle fibers

Question 40

3 primary muscle fiber types

Answer 40

Type I (slow oxidative) Type IIa (fast oxidative glycolytic) Type IIx (fast glycolytic)

Question 41

Isometric tension differences between muscle fiber types

Answer 41

From least to most: I, IIa, IIx Due to different fiber diameters (filament density is equal, so bigger means more X-bridges), also variation in proportion of attached X-bridges and force per X-bridge

Question 42

Fatigue differences between muscle fiber types

Answer 42

Type I fatigue slowest, Type IIx fatigue fastest Mainly due to energy metabolism profiles

Question 43

Properties of sarcomeres in series

Answer 43

Sarcomeres in myofibrils are in series to enable faster/greater shortening at the cost of extra energy

Question 44

How much force does a myofibril generate based on the number of sarcomeres it has?

Answer 44

A myofibril generates the same amount of force as 1 sarcomere, because the sarcomeres are in series

Question 45

Max tetanic tension without elasticity

Answer 45

Without elastic elements, max tetanic tension would equal twitch tension

Question 46

Total muscle tension

Answer 46

Total tension = active + passive; these forces are in parallel

Question 47

Preload

Answer 47

Passive tension in the muscle before activation

Question 48

Afterload

Answer 48

Load lifted by the muscle (active + passive)

Question 49

How much will a muscle shorten for a given load?

Answer 49

The muscle will shorten down to where the total muscle force capability is equal to the load.