PHYSIOLOGY 1PHYSIOLOGY 1

LECTURE 21LECTURE 21

SKELETAL MUSCLE SKELETAL MUSCLE

MECHANICS - MODELMECHANICS - MODEL

Skeletal MuscleSkeletal MuscleMechanical ModelMechanical Model

A skeletal muscle fiber is connected to A skeletal muscle fiber is connected to elastic tendons and covered with a elastic tendons and covered with a coating of connective tissue called coating of connective tissue called endomysium. Therefore, as the muscle endomysium. Therefore, as the muscle contracts it must expend energy in contracts it must expend energy in stretching the elastic tendons to the point stretching the elastic tendons to the point where the tension developed in the where the tension developed in the tendon meets the load being placed on tendon meets the load being placed on the muscle before that load will begin to the muscle before that load will begin to move.move.

Skeletal MuscleSkeletal MuscleMechanical ModelMechanical Model

A simple way of illustrating this is to A simple way of illustrating this is to consider a rubber band attached to a can consider a rubber band attached to a can of soda pop and then pulled. The rubber of soda pop and then pulled. The rubber band will stretch until the tension it has band will stretch until the tension it has developed overcomes the force of friction developed overcomes the force of friction holding the can in place. When the can holding the can in place. When the can starts to move the rubber band stays the starts to move the rubber band stays the same length. The same thing happens same length. The same thing happens with the skeletal muscle tendons.with the skeletal muscle tendons.

Skeletal MuscleSkeletal MuscleMechanical ModelMechanical Model

We will consider two different types of We will consider two different types of muscle contraction.muscle contraction.

Isometric - The muscle fiber does not Isometric - The muscle fiber does not change length but muscle tension is change length but muscle tension is changing.changing.

Isotonic - The muscle fiber changes Isotonic - The muscle fiber changes length but muscle tension remains length but muscle tension remains constant (i.e. the muscle is lifting the constant (i.e. the muscle is lifting the load).load).

Skeletal MuscleSkeletal MuscleMechanical ModelMechanical Model

IsometricIsometric - (SAME - (SAME LENGTH) -LENGTH) -

Muscle contractile Muscle contractile machinery machinery contracts but the contracts but the tendons stretch an tendons stretch an equal amount, equal amount, therefore, the therefore, the muscle fiber does muscle fiber does not change length.not change length.

Skeletal MuscleSkeletal MuscleMechanical ModelMechanical Model

IsotonicIsotonic - (SAME - (SAME TENSION) -TENSION) -

The tension in the The tension in the muscle fiber remains muscle fiber remains constant but the constant but the fiber shortens as the fiber shortens as the contractile contractile machinery continues machinery continues to contract.to contract.

Skeletal MuscleSkeletal MuscleMechanical ModelMechanical Model

B. Tension development does not B. Tension development does not necessarily imply muscle contraction, necessarily imply muscle contraction, but may involve muscle lengthening but may involve muscle lengthening (Eccentric Contraction) or no (Eccentric Contraction) or no movement at all (Isometric movement at all (Isometric Contraction).Contraction).

C. Velocity of contraction is dependent C. Velocity of contraction is dependent on the load. The greater the load the on the load. The greater the load the slower the contraction.slower the contraction.

Skeletal MuscleSkeletal MuscleMechanical ModelMechanical Model

Mechanical ModelMechanical Model - - SEE - Series Elastic SEE - Series Elastic

Element (Tendons)Element (Tendons) PEE - Parallel PEE - Parallel

Elastic Element Elastic Element (Endomysium)(Endomysium)

CE - Contractile CE - Contractile Element Element (Sarcomere)(Sarcomere)

Skeletal MuscleSkeletal MuscleMechanical Model PhasesMechanical Model Phases

Any muscle contraction consists of Any muscle contraction consists of four phases.four phases.

1. Isometric Contraction Phase1. Isometric Contraction Phase 2. Isotonic Contraction Phase2. Isotonic Contraction Phase 3. Isotonic Relaxation Phase3. Isotonic Relaxation Phase 4. Isometric Relaxation Phase4. Isometric Relaxation Phase

Mechanical Model PhasesMechanical Model PhasesIsometric Contraction Isometric Contraction

PhasePhase

During the isometric contraction phaseDuring the isometric contraction phase SEE - Lengthens (Stretching tendons)SEE - Lengthens (Stretching tendons) CE - Shortens (Sarcomere shortens)CE - Shortens (Sarcomere shortens) PEE - Shortens (Endomysium is PEE - Shortens (Endomysium is

compressed)compressed) Whole Muscle - Stays the same lengthWhole Muscle - Stays the same length

Mechanical Model PhasesMechanical Model PhasesIsometric Contraction Isometric Contraction

PhasePhase

Mechanical Model PhasesMechanical Model PhasesIsotonic Contraction PhaseIsotonic Contraction Phase

During the isotonic contraction During the isotonic contraction phase phase

SEE - Stays the same lengthSEE - Stays the same length PEE - ShortensPEE - Shortens CE - ShortensCE - Shortens Whole muscle shortensWhole muscle shortens

Mechanical Model PhasesMechanical Model PhasesIsotonic Contraction PhaseIsotonic Contraction Phase

Mechanical Model PhasesMechanical Model PhasesIsotonic Relaxation PhaseIsotonic Relaxation Phase

During the isotonic relaxation During the isotonic relaxation phasephase

SEE - Stays the same lengthSEE - Stays the same length PEE - LengthensPEE - Lengthens CE - LengthensCE - Lengthens Whole muscle lengthensWhole muscle lengthens

Mechanical Model PhasesMechanical Model PhasesIsotonic Relaxation PhaseIsotonic Relaxation Phase

Mechanical Model PhasesMechanical Model PhasesIsometric Relaxation Isometric Relaxation

PhasePhase

During the isometric relaxation During the isometric relaxation phasephase

SEE - ShortensSEE - Shortens PEE - LengthensPEE - Lengthens CE - LengthensCE - Lengthens Whole muscle stays the same Whole muscle stays the same

lengthlength

Mechanical Model PhasesMechanical Model PhasesIsometric Relaxation Isometric Relaxation

PhasePhase

Sarcomere Length - Tension Sarcomere Length - Tension Relationship (Active Tension Relationship (Active Tension

Curve)Curve)

III.III. The sarcomere length tension relationship or active The sarcomere length tension relationship or active tension curve is generated by placing a muscle in an tension curve is generated by placing a muscle in an isometric condition, attached to a strain gage then isometric condition, attached to a strain gage then applying stretch to the muscle fiber. By stimulation a applying stretch to the muscle fiber. By stimulation a muscle contraction the tension developed for a given muscle contraction the tension developed for a given degree of sarcomere stretch can be measured. These degree of sarcomere stretch can be measured. These experiments result in a bell shaped curve where the experiments result in a bell shaped curve where the greatest tension developed for skeletal muscle occurs greatest tension developed for skeletal muscle occurs at the peak of the tension curve. The drops in tension at the peak of the tension curve. The drops in tension development on either the left or the right hand limb of development on either the left or the right hand limb of the curve has to do with the numbers of cross-bridges the curve has to do with the numbers of cross-bridges being formed, if the sarcomere is too long no cross being formed, if the sarcomere is too long no cross bridges etc.bridges etc.

Velocity of Shortening - Velocity of Shortening - Load RelationshipLoad Relationship

IV.IV. For any active muscle group it is clear that For any active muscle group it is clear that at zero load the muscle is capable of at zero load the muscle is capable of generating it”s greatest velocity of shortening generating it”s greatest velocity of shortening then it is also clear that at some point the load then it is also clear that at some point the load will become so great that the muscle will not will become so great that the muscle will not be able to lift it. The velocity of shortening to be able to lift it. The velocity of shortening to load relationship is not linear due to pathway load relationship is not linear due to pathway the weight must follow, changes in the angle the weight must follow, changes in the angle of the muscle to the bone, and the elastic of the muscle to the bone, and the elastic properties of the muscle.properties of the muscle.