Skeletal muscle mechanics
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Skeletal Muscle Mechanics. Review Types of contraction Static Dynamic Experimental models of contraction Muscle mechanics Static Dynamic. Capillarity. Capillaries don’t determine blood flow, they determine transit time Transit time = capillary volume/blood flow. Brooks et al.

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Skeletal Muscle Mechanics

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Skeletal muscle mechanics

Skeletal Muscle Mechanics

  • Review

  • Types of contraction

    • Static

    • Dynamic

  • Experimental models of contraction

  • Muscle mechanics

    • Static

    • Dynamic



  • Capillaries don’t determine blood flow, they determine transit time

  • Transit time = capillary volume/blood flow

Brooks et al.

Thin filament

Thin Filament

Resting State

Myosin Binding State

Cross bridge cycle

: strong binding

~: weak binding

f: cross-bridge exerting force

Pi: inorganic phosphate

Cross-bridge cycle

1. Before contraction begins, ATP is bound to myosin head and the ATPase cleaves it to ADP. This ADP remains bound. (steps 1-4)

2. Tm/Tp complex binds with Ca++, actin sites on actin are uncovered. (step 5)

3. The strong bond causes the hinge region to undergo a conformational change producing power stroke. The energy comes from the ATP that was cleaved earlier (step 6)

4. After powerstroke, then ADP is able to detached by the binding of a new ATP, causing detachment of myosin from the head of the actin. This adding of the ATP cocks the myosin head back again to restart the cycles. (steps 7-8)

One cross bridge cycle is the time a crossbridge first attaches to actin to the time when it binds again and repeats the process.

Gordon et al., News Physiol Sci 16: 49, 2001

Skeletal muscle mechanics

Thin filament












Cross bridge formation.









The power (working)




Cocking of myosin head.




Cross bridge


Cross bridge cycle1

Cross Bridge Cycle

Muscle strength and the h zone

Muscle Strength and the H-Zone

  • Skeletal muscle closely monitors the H-Zone to maintain maximal force production…

Muscle contration

Muscle Contration

  • If you are interested in a theory that opposes the sliding filament theory:

    • Actin as the generator of tension during muscle contraction

      • Schutt and Lindberg, PNAS 1992

Comment from last session

Comment from last session

  • Overall strength is not dependent on fiber type

    • Strength equates to cross-sectional area

    • Muscle architecture on CSA

  • Fiber type will affect power

Types of contraction

Types of contraction

Static contractions:

Isometric – best for force measurement

Dynamic contractions:

Concentric (shortening)

Eccentric (lengthening)

Experimental models of muscle contraction

Experimental models of muscle contraction

  • In vitro: muscle tissue removed from the animal or person (whole muscle, muscle fiber bundle, single fiber)

  • In situ: muscle remains essentially in place in the animal, but the entire muscle is not intact (e.g., distal tendon is detached and attached to a force transducer)

  • In vivo: torques are measured across joints in intact humans or animals

Length tension relationship

Length Tension Relationship

  • Pt – peak twitch tension

  • TPT – time to peak tension

  • ½ RT – time it takes to relax from peak tension to 50% of peak tension

  • Passive Force - connective tissue does not actively generate force but if it is stretched beyond its resting length it produces a passive, elastic force

Phases of muscle twitch

Phases of Muscle Twitch

Muscle twitch

Muscle Twitch

  • Three phases of a muscle twitch:

    • Latent period

      • the sarcolemma and the T tubules depolarize

      • calcium ions are released into the cytosol

      • cross bridges begin to cycle but there is no visible shortening of the muscle

    • Contraction phase

      • myosin cross bridge cycling causes sarcomeres to shorten

    • Relaxation

      • calcium ions are actively transported back into the terminal cisternae

      • cross bridge cycling decreases and end

      • muscle to return to its original length

      • Each different muscle has different actual time periods for each phase.

      • The speed with which the contraction phase occurs depends on

      • the weight of the load being lifted

      • the type of fibers contracting (slow-twitch fibers or fast-twitch fibers)

Muscle mechanics isometric contractions

Muscle mechanicsIsometric contractions

  • Length-tension relationship (Po, Lo)

  • Lo – optimal length

    • Sarcomere length that provides for optimal overlap of the thick and thin filaments

    • Length < Lo – maximal force is production impaired

    • Length > Lo – tension does not drop appreciably until the length is extended by 10-15%

  • Po – maximal isometric force

Brooks et al.

Muscle mechanics isometric contractions1

Muscle mechanicsIsometric contractions

Length-tension relationship (Po, Lo)

Stretch(elastic component)

Gordon, Physiology and Biophysics, Saunders, 1982

Muscle mechanics isometric contractions2

Muscle mechanics -Isometric contractions

Twitch (TPT, ½ RT)

Slow twitch vs. fast twitch

50-70 msec

12-15 msec

  • Twitch speed is determined by:

  • myosin ATPase activity (myosin HC isoform) - ↑ = cleaves faster

  • SR concentration – Ca2+

Brooks et al.

Fast fiber vs slow fiber twitches

Fast Fiber vs Slow Fiber Twitches

  • Fast Twitch Fibers

    • High ATPase activity on MHC

    • Increased SR

      • Short time to peak tension (TPT)

      • Short half relaxation time (½RT) = 12-15msec

  • Slow Twitch Fibers

    • Lower ATPase activity on MHC

    • Lower SR

      • Longer TPT

      • Longer ½RT= 50-70msec

Fast fiber vs slow fiber twitches1

Fast Fiber vs Slow Fiber Twitches

  • Fast - I.R. - internal rectus (eye muscle)

  • Intermediate - G – gastrocnemius

  • Slow - S - soleus

* Notice that the Peak Isometric Force (tension) is equal in all three fiber types

Does fiber type determine force?

Two ways to regulate force production

Two ways to regulate force production

  • Frequency modulation

  • Recruitment of different motor units

Muscle mechanics isometric contractions3

Muscle mechanics - Isometric contractions

Not fully relaxed

  • Increase frequency

  • increase force

  • decrease RT

Single Twitch

Temporal (Wave) Summation


Force production – same for fiber types

Brooks et al.

Rate coding

Rate Coding

Recruitment rule

– smaller units first followed by larger


Compare fine motor skills to maximal effort lift


Isometric contractions

Isometric contractions

Why is more force generated in slow fibers as stimulation frequency is increase?

Its all about the Ca++

Affects Relaxation Time

0 Hz

100 Hz

Brooks et al.

Dynamic contractions

Dynamic contractions

  • Po – max isometric tetanic tension. Occurs when force curve crosses the y-axis and velocity becomes zero

  • ↓force → ↑Velocity

  • Eccentric force is 50-100% due to more force needed to detach crossbridges

    • Also causes muscle damage

  • Power = Load x Velocity

    • “0” power when there is no load or when load is too heavy to be moved

  • Max force = loss in velocity

  • Max velocity = loss in force

McMahon, Muscles, Reflexes, and Locomotion, Princeton, 1984

The larger the load the less shortening

The larger the load, the less shortening











V max

No load

Small load

Medium load

Large Load

Very Large Load (no velocity)

Time (from onset of stimulation)

Muscle mechanics

Muscle mechanics

To review, determinants of force/power production by a muscle

1. # of motor units recruited (i.e., the cross-sectional area of the active muscle)

- recruitment rule – smaller units first followed by larger

2. frequency of stimulation (i.e., rate coding)

3. length of the fibers relative to Lo

4. velocity (shortening and lengthening)

a. myosin ATPase activity

b. SR concentration

5. muscle architecture (consider pennation)

a. orientation of fibers to the long axis

b. the # of sarcomeres in series

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