BASIC PROPERTIES OF MUSCLE

BASIC PROPERTIES OF MUSCLE

Lecture Overview

• Muscles and motion

• Muscles • Muscle structure • Relevant properties • Force-length properties • Muscle states • Force-velocity relationship

• Muscle fiber types

• Isometric contraction

• Tendon (series elasticity)

• Tendon properties

• Activation

• Summary

• Review questions

Why?

MUSCLES AND MOTION

Newton’s First Law

If no external forces act on a body then the velocity of that body remains constant.

Muscle exert forces on our skeletal system so produce movement.

Resultant Joint Moment

The net effect of the moments about a joint due to all structures crossing that joint, including muscles,

ligaments, and bone forces.

∑ ∑ ∑ ∑ ++++ ∑∑∑∑ ++++ ∑ ∑ ∑ ∑ ==== ==== ==== ==== NL 1 i NC 1 i Ci Ci Li Li NM 1 i Mi Mi J r .F r .F r .F M Normally simplified to

∑

= = NM i Mi Mi J r F M 1 .

MUSCLES AND MOTION

Moment produced by a given muscle is a function of the muscle force and the moment arm of the muscle.

moment = moment arm of force x muscle force

T r F

₌

.

The moment arms of muscles

( )

r

, vary with the joint

angle, but what does the muscle force depend upon?

r

F

MUSCLES

The force produced by a muscle model (F_m) can be

described using the following equation

( ) ( )

f V f L f m a F F L F V F ₌ . _max. . Where f

a - normalized degree of activation of muscle fibers.

max

F - maximum isometric force muscle can produce.

( )

f

L L

F - normalized force length relationship of

muscle,

( )

f

V V

F - normalized force-velocity relationship of

muscle.

Look at structure to understand the sources of these properties.

MUSCLE STRUCTURE

Myosin

- protein forming thick part of myofibril

Actin

- protein forming thin part of myofibril

At the myofibril level it is the interaction of actin and myosin which generates force. (Cross-bridges.)

The more of these proteins the higher the force which can be generated (bigger muscles produce more force).

( ) ( )

f V f L f m a F F L F V F ₌ . _max. . CSA F_maxαααα Muscle Muscle Fiber

Bundle Muscle FiberSingle Myofibril

MUSCLE STRUCTURE

At the simplest level muscle is assumed to have the following structure.

Muscle-tendon complex the assumption is that there is no transition from tendon to muscle fibers.

Muscle Fibers

RELEVANT PROPERTIES

Passive Different Length

( )

f L L F

Muscle Fibers Active

Activation Dynamics (a_f ) Different Velocities

( )

f V V F

Tendon Passive Force/Length

Muscle Moment Arms Joint

FORCE-LENGTH PROPERTIES

As the length of the muscle fibers change so does the force they can produce.

Shortening

- cross-bridges interfere with one another, force reduced.

Lengthening

- some cross bridges are too far apart to form, so force is reduced.

[CF individual fibers and whole muscle.]

1.5 2.0 2.5 3.0 3.5 4.0 Sarcomere Length (_µµµµm) No rmalized For ce 4 3 2 1 [1] [2] [3] [4] 3.7 2.2 2.0 1.6

MUSCLE STATES

Isometric

– a muscle generating force without changing length.

Isokinetic

– a muscle generating force whilst

changing length at a constant velocity (a sub-class of isotonic).

Isotonic

– a muscle generating force whilst changing length.

Concentric Muscle Action

- a muscle shortening to produce force (+tive velocity).

Eccentric Muscle Action

- a muscle lengthening to yield to a force (-tive velocity).

FORCE-VELOCITY RELATIONSHIP

In same time period

• Smaller load greater shortening

• Larger load less shortening

Load Increases ⇒⇒⇒⇒ Force Increases ⇒⇒⇒⇒ Velocity decreases 0 0.2 0.4 0.6 0.8 1 1.2 0 5 10 15 20 25 Time (s) D egr ee of S h or te n ing (% ) Small Load Large Load

FORCE-VELOCITY RELATIONSHIP

• Limit to maximum velocity of shortening is caused by

limit to rate at which cross-bridges can cycle.

• As velocity increases the force decreases as time for

formation of cross-bridges is reduced (concentric phase)

• As negative velocity increases in magnitude the force

increases (eccentric phase)

100% 75% 50% 25% - Lengthen Shorten + VELOCITY Maximum tension F O R C E

MUSCLE FIBER TYPES

Muscle fibers can be divided into two major groups: fast twitch and slow twitch. These fiber types have different histochemical and biochemical profiles.

• Type I fibers have a long contraction time (slow

twitch), are well adapted for aerobic glycolysis.

• Type II fibers have short contraction times (fast

twitch).

• Type IIa fibers have a high capacity for anaerobic

metabolism but also have a capacity for aerobic metabolism.

• Type IIb fibers also have a high capacity for

anaerobic glycolysis and some limited capacity for aerobic metabolism.

Ø Faulkner et al. (1986) examining bundles of human

muscle fibers found that

• Type I fibers – max. velocity of shortening of 2 fl.s-1.

ISOMETRIC CONTRACTION

• Isometric means staying them same length.

• Contraction means reduction.

Question

: How can a muscle produce force during an isometric contraction?

The muscle must stay the same length BUT

ISOMETRIC CONTRACTION

Series Elasticity (tendon) Contratile Element (fibers)

No Force

High Force Low Force

TENDON (SERIES ELASTICITY)

• When activated muscle fibers develop tension which

is transferred to the skeleton via the elastic structures in series with the fibers – the tendon.

• During an isometric contraction, the muscle fibers

shorten producing tension, and the tendon stretches under this tension. The net length of the muscle

tendon complex stays the same.

• Tendons are composed mostly of the protein

collagen, it is this material which predominantly determines their properties.

TENDON PROPERTIES

Often assumed to be rigid, but is not, the force exerted on it by the muscle fibers will cause it to stretch.

Hysteresis

- difference between the curves during loading and unloading. This is small for a tendon because it is an efficient energy store.

Extension Force

TENDON PROPERTIES

The properties of tendon vary from muscle to muscles but as a general

⇒ Tendon tends to snap when stretched by

≈

8%

of its

resting length.

⇒ At maximum isometric force tendon stretched by

ACTIVATION

The force a muscle produces is modulated in two

ways:-• recruit more motor units (

recruitment

)

• increase the rate of discharge of the already active

motor units (

rate coding

)

What is the order of recruitment?

Henneman size principle (Henneman et al., 1965)

Ø Slow Twitch

Ø Fast Twitch Fatigue Resistant

ACTIVATION

• Active state and degree of activation are often used

synonymously.

• Muscle force depends on (using term activation)

• current level of activation which depends on

• previous level of activation

ACTIVATION

In figure maximum activation starts at 0 seconds, and ceases at 1 second.

Question:

What are nature of time delays?

Question:

When are these significant?

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (s) A ct ive st at e

SUMMARY

( ) ( )

f V f L f m a F F L F V F ₌ . _max. .

1) Maximum Isometric Force - proportional to amount of

contractile proteins present in the muscle.

2) Force-Length Relationship – dependent on

overlap of muscle cross-bridges.

3) Force-Velocity Relationship – force decreases as

muscle shortens at a higher velocity, but as it lengthens (yields) can produce more force.

4) Activation – there are significant time delays while muscles develop forces, or cease to develop forces.

REVIEW QUESTIONS

1) Explain the source of the maximum isometric force a muscle can produce.

2) What is the shape of the force-length relationship of isolated muscle fibers? What is this caused by?

3) What is the shape of the force-velocity relationship of isolated muscle fibers? What is this caused by?

4) What are the nature of the time delays caused by muscle activation? What is the significance of these delays?

5) What are key properties of tendon?

6) What are isometric, isokinetic, and isotonic

contractions? How is it possible to produce an isometric contraction?