PHYSIOLOGY 1 LECTURE 22 SKELETAL MUSCLE MECHANICS.

PHYSIOLOGY 1PHYSIOLOGY 1

LECTURE 22 LECTURE 22

SKELETAL MUSCLE SKELETAL MUSCLE

MECHANICSMECHANICS

Skeletal MuscleSkeletal MuscleMechanicsMechanics

I. Introduction:I. Introduction: Muscle mechanics has to do with how skeletal muscle Muscle mechanics has to do with how skeletal muscle

operates in order to perform their function. In general operates in order to perform their function. In general the movement of the body is a complex interaction the movement of the body is a complex interaction between the motor cortex of the brain, alpha motor between the motor cortex of the brain, alpha motor neurons, and skeletal muscle. It involves afferent neurons, and skeletal muscle. It involves afferent information from muscle, bone, and connective tissue information from muscle, bone, and connective tissue sent to the brain, integration by the motor cortex and sent to the brain, integration by the motor cortex and return efferent signals to the contracting skeletal return efferent signals to the contracting skeletal muscle. That signal carried by the alpha motor muscle. That signal carried by the alpha motor neurons produces a graded response in the activated neurons produces a graded response in the activated skeletal muscle which is dependent on the weight of skeletal muscle which is dependent on the weight of the load to be lifted.the load to be lifted.

Skeletal MuscleSkeletal MuscleMechanicsMechanics

Nerve stimulation Nerve stimulation of skeletal muscle of skeletal muscle can increase can increase almost infinitely almost infinitely but skeletal but skeletal muscle response muscle response maximizes (max maximizes (max tension tension development)development)

Skeletal Muscle MechanicsSkeletal Muscle MechanicsStrength verses Speed Strength verses Speed

In general the more sarcomeres a In general the more sarcomeres a skeletal muscle fiber has in parallel the skeletal muscle fiber has in parallel the stronger the fiber becomes. stronger the fiber becomes. (Hypertrophy or Hyperplasia)(Hypertrophy or Hyperplasia)

The more sarcomeres a skeletal muscle The more sarcomeres a skeletal muscle fiber has in series the faster its velocity fiber has in series the faster its velocity of contraction becomes.of contraction becomes.

NoteNote - - Velocity of contraction is limited by the Velocity of contraction is limited by the type of myosin ATPase expressed by the fiber.type of myosin ATPase expressed by the fiber.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsStrength verses SpeedStrength verses Speed

Skeletal Muscle MechanicsSkeletal Muscle MechanicsMuscle TwitchMuscle Twitch

A. Single Twitch - a single muscle A. Single Twitch - a single muscle twitch is the skeletal muscle twitch is the skeletal muscle contraction resulting from a single contraction resulting from a single action potential.action potential.– 1. Relationship of a single muscle twitch to 1. Relationship of a single muscle twitch to

the muscle action potential - cytoplasmic the muscle action potential - cytoplasmic calcium concentrations - and force calcium concentrations - and force developmentdevelopment

– Note - The electrical event must precede Note - The electrical event must precede the mechanical eventthe mechanical event


Single twitch is submaximal - Due Single twitch is submaximal - Due to the rapid action of the SR Ca+to the rapid action of the SR Ca++ATPase calcium concentration +ATPase calcium concentration falls before the skeletal muscle can falls before the skeletal muscle can develop it’s maximum tension.develop it’s maximum tension.


B. Multiple Twitch - Skeletal muscle B. Multiple Twitch - Skeletal muscle responds to several (Multiple) action responds to several (Multiple) action potentials delivered at a high frequency by potentials delivered at a high frequency by increasing it’s force of contraction up to increasing it’s force of contraction up to the maximum tension development. the maximum tension development. Therefore, a graded response can be Therefore, a graded response can be developed such that just the force needed developed such that just the force needed to lift the load is generated. There are two to lift the load is generated. There are two methods of generating this graded methods of generating this graded response.response.


1. Temporal summation - A series of 1. Temporal summation - A series of action potentials generated at a high action potentials generated at a high enough frequency to increase the enough frequency to increase the tension in a skeletal muscle fiber tension in a skeletal muscle fiber beyond that produced by a single action beyond that produced by a single action potential.potential.

2. Tetany - Maximal skeletal muscle 2. Tetany - Maximal skeletal muscle contraction produced by a series of high contraction produced by a series of high frequency action potentials.frequency action potentials.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsFatigueFatigue

In the human there are two types of In the human there are two types of fatigue.fatigue.

1. 1. Nerve fatigueNerve fatigue - - This is usually defined This is usually defined as a depletion of neurotransmitter quanta as a depletion of neurotransmitter quanta release to the point where the release falls short release to the point where the release falls short of a full quanta or becomes completely depleted of a full quanta or becomes completely depleted and fails to produce a response in the effector and fails to produce a response in the effector organ. Normally this will never occur as most organ. Normally this will never occur as most nerves store sufficient neurotransmitter for nerves store sufficient neurotransmitter for several thousand full quanta releases.several thousand full quanta releases.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsFatigueFatigue

2. Skeletal Muscle Fatigue2. Skeletal Muscle Fatigue Fatigue in skeletal muscle is not well understood but Fatigue in skeletal muscle is not well understood but

the most likely scenario is the following. Skeletal the most likely scenario is the following. Skeletal muscle fatigues at different rates depending on the muscle fatigues at different rates depending on the type of skeletal muscle fiber involved. Fatigue is type of skeletal muscle fiber involved. Fatigue is fairly tightly correlated with glycogen store depletion. fairly tightly correlated with glycogen store depletion. In exercise conditions over time aerobic ATP In exercise conditions over time aerobic ATP production declines, therefore, a greater degree of production declines, therefore, a greater degree of reliance must be placed on glycolysis to produce reliance must be placed on glycolysis to produce sufficient ATP to maintain exercise. As more and sufficient ATP to maintain exercise. As more and more ATP is produced from glycolysis lactic acid and more ATP is produced from glycolysis lactic acid and [H+] begins to build up inhibiting PFK.[H+] begins to build up inhibiting PFK.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsMotor UnitsMotor Units

Def. - The alpha motor neuron plus all Def. - The alpha motor neuron plus all of the skeletal muscle fibers it of the skeletal muscle fibers it innervates form a skeletal muscle innervates form a skeletal muscle motor unit.motor unit.

Motor unitsMotor units can be as small as a nerve can be as small as a nerve and one muscle fiber or as large as a and one muscle fiber or as large as a nerve and several thousand muscle nerve and several thousand muscle fibers.fibers.


1. Alpha motor neuron1. Alpha motor neuron 2. Muscle fibers innervated2. Muscle fibers innervated 3. Interdigitation of motor unit 3. Interdigitation of motor unit

fibers with other motor unit fibersfibers with other motor unit fibers 4. Graded response (Recruitment)4. Graded response (Recruitment)

Skeletal Muscle MechanicsSkeletal Muscle MechanicsGraded Response Graded Response

(Recruitment)(Recruitment)

A skeletal muscle contraction is a A skeletal muscle contraction is a graded response starting with the graded response starting with the smallest motor units, activated smallest motor units, activated first, larger and larger motor units first, larger and larger motor units are added on until the load is are added on until the load is lifted. This process is called lifted. This process is called recruitment.recruitment.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsGraded Response Graded Response

(Recruitment)(Recruitment)

Skeletal Muscle MechanicsSkeletal Muscle MechanicsTension DevelopmentTension Development

Any skeletal muscle contraction Any skeletal muscle contraction involves both an active tension involves both an active tension development due to crossbridge development due to crossbridge cycling and a passive tension cycling and a passive tension development due to compression development due to compression and stretch of elastic elements and stretch of elastic elements such as tendons (stretch) and such as tendons (stretch) and endomysium (compression).endomysium (compression).


Active Tension CurveActive Tension Curve - Sarcomere - Sarcomere shorteningshortening

Passive Tension CurvePassive Tension Curve - Tendons - Tendons and endomysiumand endomysium

Total Tension CurveTotal Tension Curve - The addition - The addition of the passive tension curve plus of the passive tension curve plus the active tension curve equal the the active tension curve equal the total tension curve.total tension curve.


Active Tension Active Tension Curve -Curve -

Generated under Generated under isometric conditionsisometric conditions

Resting sarcomere Resting sarcomere length in skeletal length in skeletal muscle occurs at muscle occurs at the peak of the the peak of the active tension active tension curve.curve.


Passive Tension Passive Tension Curve -Curve -

Generated by Generated by pulling on muscle pulling on muscle fiberfiber

Starts at resting Starts at resting sarcomere lengthsarcomere length


Total Tension Total Tension CurveCurve

Adding the Active Adding the Active tension curve to tension curve to the passive the passive tension curve tension curve gives the Total gives the Total tension curvetension curve

Skeletal Muscle TypesSkeletal Muscle Types

Skeletal muscle is composed of Skeletal muscle is composed of three different fiber types, Fast three different fiber types, Fast Glycolytic (White), Fast Oxidative-Glycolytic (White), Fast Oxidative-Glycolytic (Fast Red), and Slow Glycolytic (Fast Red), and Slow Oxidative (Red) fiber types. The Oxidative (Red) fiber types. The name is derived from the primary name is derived from the primary form of energy production which form of energy production which predominates within the fiber.predominates within the fiber.


Fast GlycolyticFast Glycolytic - - (White muscle fiber) - (White muscle fiber) - predominately expresses the enzymes involved in predominately expresses the enzymes involved in glycolysis. These are the power or strength fibers glycolysis. These are the power or strength fibers but lack endurance.but lack endurance.

Fast Oxidative-GlycolyticFast Oxidative-Glycolytic - - (Fast Red fibers) - (Fast Red fibers) - Express a balance between glycolytic and Krebs Express a balance between glycolytic and Krebs cycle enzymes. Intermediate fibers strong and fast. cycle enzymes. Intermediate fibers strong and fast. Most abundant fibers in the human.Most abundant fibers in the human.

Slow OxidativeSlow Oxidative - - (Slow Red fibers) - Express (Slow Red fibers) - Express primarily oxidative (Krebs Cycle) enzymes. These primarily oxidative (Krebs Cycle) enzymes. These are the endurance fibers but lack strength.are the endurance fibers but lack strength.


All types of skeletal muscle fibers occur All types of skeletal muscle fibers occur in every skeletal muscle group. in every skeletal muscle group. Classification is dependent on the Classification is dependent on the muscle fiber type in greatest muscle fiber type in greatest abundance. Therefore, a muscle group abundance. Therefore, a muscle group which predominantly expressed the which predominantly expressed the slow oxidativeslow oxidative fibers would be classified fibers would be classified as an oxidative muscle or sometimes as as an oxidative muscle or sometimes as an endurance muscle group.an endurance muscle group.

Skeletal Muscle TypesSkeletal Muscle TypesComparison of EnzymesComparison of Enzymes

ENZYMES F-G F-O-G S-OENZYMES F-G F-O-G S-O MyosinATPase 1a 1b 1cMyosinATPase 1a 1b 1c Glycolytic High High LowGlycolytic High High Low Krebs Cycle Low Med. HighKrebs Cycle Low Med. High Mitochondria Low Med. HighMitochondria Low Med. High Glycogen High High Med.Glycogen High High Med. Myoglobin Low Med. HighMyoglobin Low Med. High

Skeletal Muscle TypesSkeletal Muscle TypesAffects of ExerciseAffects of Exercise

While skeletal muscle types are genetically While skeletal muscle types are genetically determined they can be moved toward other determined they can be moved toward other muscle types by exercise training. For example an muscle types by exercise training. For example an individual who predominantly expresses the fast-individual who predominantly expresses the fast-oxidative-glycolytic fiber type if trained as a oxidative-glycolytic fiber type if trained as a marathoner will begin to express more oxidative marathoner will begin to express more oxidative enzymes in his FOG fibers. The FOG fibers are enzymes in his FOG fibers. The FOG fibers are much easier to move than the two extremes, much easier to move than the two extremes, therefore, exercise training focus on the FOG therefore, exercise training focus on the FOG fibers. In humans the FOG fibers are normally the fibers. In humans the FOG fibers are normally the most abundant and humans show better training most abundant and humans show better training effects than most other animals.effects than most other animals.

Skeletal Muscle TypesSkeletal Muscle TypesAffects of ExerciseAffects of Exercise

EXERCISE Predominant fiber type %EXERCISE Predominant fiber type % MarathonMarathon SOSO 42 42 SprintsSprints FGFG 39 39 Power LiftPower Lift FGFG 37 37 RowerRower FOGFOG 63 63 SwimmerSwimmer FOG FOG 65 65 NormalNormal FOGFOG 48 48

FGFG 22 22 SOSO 30 30

PHYSIOLOGY 1 LECTURE 22 SKELETAL MUSCLE MECHANICS.

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Transcript of PHYSIOLOGY 1 LECTURE 22 SKELETAL MUSCLE MECHANICS.