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Biomechanics of Skeletal Muscle and the

Musculoskeletal System Hamill & Knutzen (Ch 3)

Nordin & Frankel (Ch 5), or Hall (Ch. 6)

Muscle Properties

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Muscle Properties (cont.) Functions of Muscle

  Produce Movement

  Maintain Postures and Positions

  Stabilize Joints

  Other Functions (not related to movement)  Support and protect visceral organs  Alter and control pressures within cavities  Help in maintenance of body temperature  Control entrances and exits to the body

Factors Influencing the Production of Muscular Tension  Muscle Size (cross-sectional area)  Electro-mechanical delay  Recruitment, Frequency,

Synchronization (activation level)  Length-Tension Relationship  Velocity-Tension Relationship

(Muscular Power)

Factors Influencing the Production of Muscular Tension

 Prior Contraction History  Elastic Elastic Energy (storage and

recoil)  Muscle Temperature  Muscle Fibre Type  Angle of pennation

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Muscle Cross Sectional Area

Hypertrophy - an increase in the size of a tissue such as muscle.

Hyperplasia - an increase in number of muscle fibers"

Strength vs Cross-Sectional Area

0 5 10 15 20 25

25 20 15 10 5

0

Cross Sectional Area (cm2)

Arm

Fle

xor S

tren

gth

(kg)

Males Females

Electromechanical Delay Stimulus Response of CC

Tension

Stimulus Strength Threshold

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Force-Time Profile

Forc

e

Time

Redrawn from Lieber et al., 1994.

5

Redrawn from Ralston et al., 1947.

Total Length-Tension Curve Text Fig.3-18

Brachialis

Biceps

Brachioradialis

Contractile Length

Concentric

Eccentric

Data from Edman, 1988.

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CC Force- Velocity Curve

Text Fig.3-20

3-D Plot Force – Length - Velocity

Power Power = Force x Velocity Maximum power occurs at about 30-33% of maximal velocity of shortening and about the same percentage of maximum concentric force.

Negative Power????

Power Force

Velocity

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Optimal Power

Athletes must find as high a level of power output as possible that can be sustained for the duration of their event.

Redrawn from Edgerton et al., 1986.

Torque vs. Forearm Flexion Angle

Torq

ue (N

m)

Angle (degrees) 0 20 40 60 80 100

80

60

40

20

0

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Force-Length Curve of Muscle

Lo

Tension

Length of Contractile element

Torque ≠ Force

…moment arms are very different

Force is equal ……but

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Brachialis

Biceps

Brachioradialis

50 100 150

100

75

50

25

0 Elbow Angle (degrees)

Moment Arm (mm)

Forearm Flexors Moment Arm vs. Elbow Angle Factors Affecting Muscle Torque

FORCE   force-length curve   force - velocity curve   activation profile   prior contraction

history   angle of pennation   freq., temp., etc.

FORCE ARM   insertion point   line of action of

muscle and joint angle

(i.e. force x force arm)

Isometric Exercise (static contraction)

Isotonic Exercise (constant resistance)

Concentric Contraction Eccentric Contraction

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Constant weight ≠ constant resistance   In a free weight exercise once you have got

the weight moving you will require less force to continue it.

  Furthermore to control the weight at the later stages of the lift you will have to all the weight to decelerate, hence your limbs will decelerate.

  How can we use the knowledge of muscle mechanics to better train muscles?

  Muscle strength at various angles   Inertial resistance of free weights?

As previously discussed in Chapter 5, work and machines; this is why are the pulley’s on weight

machines are often not round.

They are designed to better stress a muscular system throughout the

entire range of movement.

Variable Resistance (leverage {force x distance})

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Redrawn from Smith, 1982.

Max torque

Barbell

Nautilus

Adding Resistance with Chains

This system allows a gradual increase in resistance which will reduce the amount of deceleration that occurs later in the lift.

  This system combines the benefits of free weights with the benefits of variable resistance.

Isokinetic Concentric

Cybex

Isokinetic machines are designed so that the resistance is due to fluid viscosity

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Twitch Response

Tens

ion

Fast Twitch (FT) Fibre

Slow Twitch (ST) Fibre

Muscle Temperature Normal body temperature Elevated body temperature

Forc

e

Velocity

Warm Up – more than biomechanical   Decreased viscosity of blood   Enables oxygen in the blood to be delivered

at greater peed and volume   Increase of temperature in the muscles   Facilitates enzyme activity   Encourages the dissociation of oxygen from

haemoglobin   Decreased viscosity within the muscle   Greater extensibility and elasticity of muscle

fibres and associated connective tissue   Increased force and speed of contraction

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Physiological Cross- Section Area (PCSA)

30o

Ffibres Ftendon

Ftendon = Ffibres x cos 30

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Other Mechanical Factors

 Tendon Length  Joint Stability  The role of two joint muscles

Maximum Contraction

Resting Length Resting Length

Joint Stability Rotary Component

Dislocating Component

Stabilizing Component

Non-Rotary Component

Brachioradialis Origin: Humerous -

Lateral Condyle Insertion: Radius (Lateral

Distal) - Styloid Process

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Two-Joint Muscles Dynamic Stabilizers

  http://www.exrx.net/WeightExercises/Quadriceps/BBFullSquat.html

  Hamstrings   Gastrocnemius

Dynamic Stabilizers

  http://www.exrx.net/WeightExercises/PectoralSternal/BBBenchPress.html

  Biceps Brachii, Short Head

Antagonist Muscle Action   Which muscles are active during a squat exercise

(which requires hip and knee extension)?   Gluteus maximus, quadriceps and hamstrings.   Is this a contradiction as the hamstrings flex the

knee?   No…due to difference in moment arms.   Lombard’s Paradox   At the hip, the moment arm of rectus femoris is

much smaller than hamstring moment arm so hip extension is not reduced.

  Similarly at the knee the quadriceps moment are is greater than the hamstrings, so the knee extends.