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Biomechanics of the skeletal muscles
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Objectives Identify the basic behavioral properties of the musculotendinous unit. Explain the relationships of fiber types and fiber architecture to muscle function. Explain how skeletal muscles function to produce coordinated movement of the human body. Discuss the effects of the force-velocity and length-tension relationships and electromechanical delay on muscle function. Discuss the concepts of strength, power, and endurance from a biomechanical perspective.
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Structural Organization of Skeletal Muscle
Human body has approx. 434 muscles 40-45% of total body weight in adults 75 muscle pairs responsible for bodily movements and posture Muscle Fibers Motor Units Fiber Types Fiber Architecture
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Muscle Fibers During contraction, cross-bridges form
Sarcoplasmic Reticulum Transverse Tubules Endomysium Perimysium Fascicles Epimysium Variation of length and diameter within muscles seen in adults.
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Muscle Fibers Contain: sarcolemma sarcoplasm nuclei mitochondria myofibrils myofilaments Sarcomere Z lines M line A band myosin filaments I band actin filaments H zone
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Motor Units Motor unit: Axon Motor end plate Twitch Type Tonic Type
Summation Tetanus
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Behavioral Properties of the Musculotendinous Unit
Behavioral properties of muscle tissue: Extensibility Elasticity Irritability Ability to develop tension Behavioral properties common to all muscle: Cardiac, smooth, skeletal
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Extensibility and Elasticity
Two components: Parallel elastic component (PEC) Series elastic component (SEC) Contractile component Visoelastic
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Irritability and the Ability to Develop Tension
The ability to respond to electrical or mechanical stimulus. Response is the development of tension. Not necessarily a contraction
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Fiber Types Fast Twitch (FT) Type IIa Type IIb Slow Twitch (ST) Type I
Peak tension reached in FT in 1/7 time of ST ST and FT compose skeletal muscles Percentages of each range from muscle to muscle and individual to individual.
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Fiber Types Effects of training:
Endurance training can increase ST contraction velocity by 20% Resistance training can convert FT fibers from Type IIb to Type IIa Elite athlete fiber type distribution does not significantly differ from untrained individuals Affected by: Age and Obesity
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Fiber Architecture Parallel fiber arrangement
Resultant tension from shortening of muscle fibers Shortens the muscle Pennate fiber arrangement Increases the angle of pennation (attachment) to a tendon.
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Skeletal Muscle Function
Recruitment of motor units Change in length with tension development Roles assumed by muscles Two-joint and multijoint muscles
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Recruitment of Motor Units
CNS enables matching of speed and magnitude of muscle contraction to requirement of movement. Threshold activation ST activated first (low threshold) With an increase in speed, force, and/or duration requirement, higher threshold motor units are activated (FT fibers)
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Change in Muscle Length with Tension Development
Concentric Bicep shortening with the bicep curl (flexion) Isometric Body builders develop isometric contraction in competition Eccentric Acts as a breaking mechanism to control movement
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Roles Assumed by Muscles
Agonist Primary & Secondary Antagonist Stabilizer Neutralizer Agonists and Antagonists are typically positioned on opposite sides of a joint.
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Two-joint and Multijoint Muscles
Movement effectiveness depends on: Location and orientation of muscle’s attachment relative to the joint Tightness or laxity of musculotendinous unit Actions of other muscles crossing the joint Disadvantages: Active insufficiency Passive insufficiency
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Factors Affecting Muscular Force Generation
Force-Velocity Relationship Length-Tension Relationship Electromechanical Delay Stretch-Shortening Cycle
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Force-Velocity Relationship
Maximal force developed by muscle governed by velocity of muscle’s shortening or lengthening. Holds true for all muscle types Does not imply: It’s impossible to move heavy resistance at a fast speed. It’s impossible to move light loads at low speeds
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Force-Velocity Relationship
Maximum isometric tension Eccentric conditions Volitionally Represents contribution of the elastic components of muscle Eccentric Strength Training More effective than concentric training in increasing muscle size and strength.
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Length-Tension Relationship
In human body, force generation increases when muscle is slightly stretched. Parallel fibers at max just over resting length Pennate fibers at max with 120%-130% resting length. Due to contribution of elastic components of muscle (primarily the SEC)
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Electromechanical Delay
Electromechanical Delay (EMD) Varies among human muscles ( msec) Short EMDs produced by muscles with high percentage of FT fibers Associated with development of higher contraction forces Not effected by muscle length, contraction type, contraction velocity, or fatigue
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Stretch-Shortening Cycle
Stretch-Shortening Cycle (SSC) Elastic Recoil Stretch Reflex Activation Muscle can perform more work with active stretch prior to shortening contraction Less metabolic costs when SSC utilized. Eccentric training increases ability of musculotendinous unit to store and produce more elastic energy.
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Muscular Strength, Power, and Endurance
Muscular Power Muscular Endurance Muscular Fatigue Effect of Muscle Temperature
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Muscular Strength The ability of a given muscle group to generate torque at a particular joint. Two orthogonal components: 1) Rotary Component 2) Parallel to bone Derived from: amount of tension the muscles can generate moment arms of contributing muscles with respect to joint center.
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Muscular Strength Tension-generating capability of a muscle affected by: Cross-sectional area Training state Moment arm of a muscle affected by: Distance between the muscle’s anatomical attachment to bone and the axis of rotation at the joint center Angle of muscle’s attachment to bone.
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Muscular Power The product of muscular force and the velocity of muscular shortening. The rate of torque production at a joint Max. power occurs at: approx. 1/3 max. velocity, and approx. 1/3 max concentric force Affected by muscular strength and movement speed
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Muscular Endurance The ability to exert tension over a period of time.
Constant: gymnast in iron cross Vary: rowing, running, cycling Length of time dramatically effected by force and speed requirements of activity. Training involves many repetitions with light resistance.
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Muscular Fatigue Opposite of endurance Characteristics:
Reduction in force production Reduction in shortening velocity Prolonged relaxation of motor units between recruitment Absolute Fatigue Resistance: SO > FOG > FG Causes
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Effect of Muscle Temperature
Increased body temperature, increases speed of nerve and muscle function Fewer motor units needed to sustain given load Metabolic processes quicken Benefits of increased muscular strength, power and endurance Key point: Be sure to warm-up!
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Common Muscle Injuries
Strains Mild, moderate or severe Contusions Myositis ossificans Cramps Delayed-Onset Muscle Soreness (DOMS) Compartment Syndrome
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Summary Muscle is the only biological tissue capable of developing tension. Resulting actions can be concentric, eccentric, isometric for muscle shortening, lengthening or remaining unchanged in length Force production is the combination of many relationships (ex: force-velocity) Specific activity performance is related power, endurance, and strength
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The End
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