11/25/2014 Medical Physiology Muscle Mechanics Eric Olson

Question 1

What happens when skeletal muscle contracts?

Answer 1

It develops a force or tension

Question 2

Define isometric in the context of muscle mechanics

Answer 2

When a muscle develops tension, but stays the same length because its ends are attached to immovable objects, the contraction is said to be isometric

Question 3

Does the the length of the muscle stay constant during an isometric contraction?

Answer 3

Yes, though the tension increases

Question 4

Is external work performed during an isometric contraction?

Answer 4

No, since the muscle length is constant, and the load is not moved (W = F x ∆d)

Question 5

What is the power in an isometric contraction?

Answer 5

Zero (Р = dW/dt)

Question 6

Is the energy of ATP hydrolysis utilized whenever the muscle is exerting tension?

Answer 6

Yes, even though no external work is performed during an isometric contraction

Question 7

Cite a function of isometric contractions

Answer 7

Maintaining posture

Question 8

Describe what happens in an isometric contraction

Answer 8

During the development of tension, the degree of shortening of the contractile element (CE) equals the degree of elongation of the series elastic elements (SE), thereby keeping the overall length of the muscle fiber constant

Question 9

How does moving a book involve both isometric and isotonic contractions?

Answer 9

Combination of bicep undergoing isotonic contraction (production of movement) and back muscle undergoing isometric contraction (postural rigidity)

Question 10

Do isotonic contractions do all the work?

Answer 10

Yes

Question 11

Define isotonic twitch

Answer 11

When the force of contraction of the muscle is at least equal to the load so that the muscle shortens, the muscle is said to contract isotonically

Question 12

What does the tension curve for an isotonic twitch show?

Answer 12

A plateau during which the force or tension is constant (iso-same; tonic- tension)

Question 13

The ___ the load the ___ the plateau and the ___ the time lag between stimuli and the start of muscle shortening

Answer 13

greater; higher; greater

Question 14

What happens if the load exceeds the amount of force the muscle can generate?

Answer 14

An isometric twitch results

Question 15

Describe the four phases of an isotonic twitch

Answer 15

(1) isometric tension development
* tension develops without shortening until the force developed equals the load
(2) isotonic shortening
* shortening of the muscle at constant tension
(3) isotonic relaxation
* elongation to the initial length at constant tension
(4) isometric relaxation
* reduction of tension at the initial length

Question 16

Is external work performed during an isotonic contraction?

Answer 16

Yes. W=Fxd > 0

Question 17

In the tension-time curve, what does the slope represent?

Answer 17

The rate of development of tension (dF/dt)

*dF/dt is a function of both the active properties of the muscle and the passive properties of the tissue

Question 18

When is the velocity of shortening maximal?

Answer 18

At the beginning of shortening

Question 19

For a series of single isotonic twitches at a given initial length, as the weight (afterload) increases…

Answer 19

(1) the initial velocity of shortening decreases:
- “the muscle lifts a heavier load more slowly than a lighter load”
- more of the cross-bridges are needed to maintain tension against a heavier load, so less
are available for shortening

(2) the amount of shortening decreases:
- “the muscle lifts a heavier load less high than a lighter load”
- at a slower velocity with a heavier weight, the muscle will shorten less before relaxation occurs

(3) the duration of shortening decreases, i.e. the muscle drops the weight sooner
- “the muscle holds a heavier weight for less time than a lighter load”
- re-accumulation of calcium into the SR causes the tension to be less than the load sooner; hence, the weight is dropped sooner

Question 20

What happens at the point where the muscle cannot lift the weight at all?

Answer 20

The maximal amount of tension for the particular initial length is being generated

Question 21

When a muscle contracts isotonically under increasing loads the contractions display the following effects in the Distance shortened vs. time plot:

Answer 21

1. The latent period (time lag between stimulation and shortening) increases
2. The duration of shortening decreases
3. The velocity of shortening decreases.

Question 22

The Force-Velocity curve has two distinguishing points:

Answer 22

Vmax –> maximal velocity of contraction, extrapolated to zero load
* Vmax is a function of myosin ATPase in varying muscle types (slow, fast)

Po –> maximal isometric tension that can be generated at a given initial length, and a given state of contractility
* Po is a function of the number of force-generating cross-bridges that can be formed

Question 23

The larger the muscle, the ___ the number of cross bridges and the ___ is Po

Answer 23

larger; larger

Question 24

In skeletal muscle Po can ___ with frequency modulation, recruitment of additional motor units, and with initial lengths closer to the optimal initial length Lmax , but not by changes in ___

Answer 24

increase; contractility

Question 25

Is contractility defined for skeletal muscle?

Answer 25

No

Question 26

In cardiac muscle, Po can ___ with an increased initial length, and by increased contractility, but not by recruitment (since the heart contracts as a syncytium)

Answer 26

increase

Question 27

When a muscle is contracted isotonically under successively increasing loads the contractions display the following effects:

Answer 27

1. The latent period (time lag between stimulation and shortening) increases
2. The duration of shortening decreases
3. The velocity of shortening decreases

Question 28

Do all types of fibers have the same velocity of shortening?

Answer 28

No. Certain types of muscle fibers can shorten faster than others

Question 29

Define preload

Answer 29

Load required to stretch a muscle from its resting length

Question 30

For skeletal muscle, the afterload is equivalent to the ___

Answer 30

total tension