PHYSIOLOGY 1PHYSIOLOGY 1

LECTURE 17LECTURE 17

SKELETAL MUSCLE SKELETAL MUSCLE

MECHANICSMECHANICS

Skeletal MuscleSkeletal MuscleMechanicsMechanics

As a skeletal muscle contracts it As a skeletal muscle contracts it follows certain mechanical restraints.follows certain mechanical restraints.

Strength verses Speed of contractionStrength verses Speed of contraction Velocity of shortening to loadVelocity of shortening to load Muscle twitch (single twitch, multiple Muscle twitch (single twitch, multiple

twitch)twitch) Skeletal muscle motor unitSkeletal muscle motor unit Tension DevelopmentTension 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 MechanicsVelocity of Shortening to Velocity of Shortening to

LoadLoad

The maximum velocity with which any The maximum velocity with which any muscle fiber can contract is dictated by muscle fiber can contract is dictated by by the speed of crossbridge cycling. by the speed of crossbridge cycling. Crossbridge cycling rate is determined Crossbridge cycling rate is determined primarily by the type of Myosin ATPase primarily by the type of Myosin ATPase expressed by the muscle fiber. expressed by the muscle fiber. Furthermore, Furthermore, as load increases the as load increases the velocity of contraction must decline until velocity of contraction must decline until a load is reached which can not be a load is reached which can not be moved.moved.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsVelocity of Shortening to Velocity of Shortening to

LoadLoad

Skeletal Muscle MechanicsSkeletal Muscle MechanicsRelationship of Twitch to [Ca++] Relationship of Twitch to [Ca++]

and Force Developmentand Force Development

A single muscle action potential A single muscle action potential results in a submaximal force results in a submaximal force development due to the speed of development due to the speed of the Ca++ dependent Ca++ATPase the Ca++ dependent Ca++ATPase which removes Ca++ from the which removes Ca++ from the sarcoplasm to the sarcoplasmic sarcoplasm to the sarcoplasmic reticium.reticium.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsRelationship of Twitch to [Ca++] Relationship of Twitch to [Ca++]

and Force Developmentand Force Development

Skeletal Muscle MechanicsSkeletal Muscle MechanicsMulti Twitch Force Multi Twitch Force

DevelopmentDevelopment

As the frequency of action As the frequency of action potentials increases the Ca++ potentials increases the Ca++ ATPase can not remove cytosolic ATPase can not remove cytosolic Ca++ fast enough so the strength Ca++ fast enough so the strength of contraction increases to of contraction increases to maximum (Tetany).maximum (Tetany).

Skeletal Muscle MechanicsSkeletal Muscle MechanicsMulti Twitch Force Multi Twitch Force

DevelopmentDevelopment

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. Skeletal muscle fatigueSkeletal muscle fatigue 2. 2. Nerve fatigueNerve fatigue - In general except for - In general except for

some very unusual circumstances the some very unusual circumstances the nerves do not fatigue. Nervous fatigue nerves do not fatigue. Nervous fatigue implies depletion of neurotransmitter, implies depletion of neurotransmitter, not a normal occurrence except after not a normal occurrence except after several thousand high frequency firings.several thousand high frequency firings.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsFatigueFatigue

Skeletal Muscle FatigueSkeletal Muscle Fatigue Fatigue in skeletal muscle is not well Fatigue in skeletal muscle is not well

understood but the most likely understood but the most likely scenario is that fatigue is correlated scenario is that fatigue is correlated with Glycogen Depletion. In either with Glycogen Depletion. In either high intensity exercise or long exercise high intensity exercise or long exercise duration the muscle cells becomes duration the muscle cells becomes more reliant on the glycolytic pathway more reliant on the glycolytic pathway (Anaerobic Glycolysis) for energy. (Anaerobic Glycolysis) for energy.

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.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsMotor UnitsMotor Units

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 MechanicsMotor UnitsMotor Units

Skeletal Muscle MechanicsSkeletal Muscle MechanicsMotor UnitsMotor Units

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 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).

Skeletal Muscle MechanicsSkeletal Muscle MechanicsTension DevelopmentTension Development

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.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsTension DevelopmentTension Development

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.

Skeletal Muscle MechanicsSkeletal Muscle MechanicsTension DevelopmentTension Development

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

Skeletal Muscle MechanicsSkeletal Muscle MechanicsTension DevelopmentTension Development

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