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Physio > Lecture 8: Skeletal Muscle Mechanics > Flashcards

Lecture 8: Skeletal Muscle Mechanics Flashcards

1
Q

Isometric contraction

A

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

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2
Q

Isotonic contraction

A

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

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3
Q

Muscle tension

A

Force exerted by the contracting muscle on a load

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4
Q

Load

A

Force exerted by an object on the muscle

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5
Q

Tension vs load

A

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

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6
Q

Concentric contraction

A

When tension > load, the muscle shortens during contraction

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7
Q

Eccentric contraction

A

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

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8
Q

Muscle twitch

A

Mechanical response of muscle fiber to 1 action potential

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9
Q

Latent period

A

Delay between the action potential and corresponding increase in muscle tension

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10
Q

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

A

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.

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11
Q

Contraction time

A

Defined as the time from beginning of tension to peak tension

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12
Q

Muscle fiber types

A

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

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13
Q

What makes different fibers fast vs. slow?

A

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)

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14
Q

Differences between isometric and isotonic twitches

A

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

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15
Q

Effect of heavier load on an isotonic twitch

A

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

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16
Q

Maximum shortening velocity and maximum isometric tension

A

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

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17
Q

Why do heavier loads slow shortening velocity?

A

More load slows the forward movement of each power stroke

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18
Q

What determines max shortening velocity?

A

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

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19
Q

Muscle summation

A

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

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20
Q

Tetanus

A

Maintained muscle contraction due to repetitive stimulation

21
Q

Fused vs unfused tetanus

A

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

22
Q

Max tetanic tension vs isometric twitch tension

A

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.

23
Q

Passive muscle tension

A

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

24
Q

Optimal length L0

A

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

25
Q

Why does active tension decrease above L0?

A

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

26
Q

Why does active tension decrease below L0?

A

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

27
Q

Resting length of all muscle

A

Passive elasticity keeps most muscle fibers near L0 when relaxed

28
Q

How do muscles maintain high ATP levels?

A
  1. Phosphorylation of ADP by creatine phosphate (rapid)
  2. Oxidative phosphorylation in mitochondria (slower)
  3. Glycolysis ATP production in cytosol (slower)
29
Q

What supplies ATP for moderate muscle activity?

A

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

30
Q

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

A

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

31
Q

What supplies ATP for intense muscle activity?

A

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.

32
Q

Oxygen debt

A

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

33
Q

Muscle fatigue

A

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

34
Q

Metabolic changes in active muscle

A

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

35
Q

Effects of metabolic changes in active muscle

A

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

36
Q

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

A

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

37
Q

Central command fatigue

A

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

38
Q

Types of skeletal muscle fibers (twitch speed)

A

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

39
Q

Types of skeletal muscle fibers (energy)

A

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

40
Q

3 primary muscle fiber types

A

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

41
Q

Isometric tension differences between muscle fiber types

A

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

42
Q

Fatigue differences between muscle fiber types

A

Type I fatigue slowest, Type IIx fatigue fastest

Mainly due to energy metabolism profiles

43
Q

Properties of sarcomeres in series

A

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

44
Q

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

A

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

45
Q

Max tetanic tension without elasticity

A

Without elastic elements, max tetanic tension would equal twitch tension

46
Q

Total muscle tension

A

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

47
Q

Preload

A

Passive tension in the muscle before activation

48
Q

Afterload

A

Load lifted by the muscle (active + passive)

49
Q

How much will a muscle shorten for a given load?

A

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

Physio (20 decks)

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