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SUNY Cortland EXS 387 - Muscle Mechanics

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1EXS 387 - BiomechanicsDr. MoranTuesday May 1, 2007Spring 2007Muscle MechanicsChapter 6EXS 387 2Review Questions• What are the five factors influencing muscle force development?• What unique feature separates muscle from other soft tissues?• What component of the musculotendon unit stores elastic energy?• Will the musculotendon unit immediately return to its resting length after a prolonged stretch? Explain why?• What is the “Size Principle?”• Can exercise alter muscle fiber-type composition?• What are the advantages/disadvantages of muscle pennation?• At what length does muscle generate maximum tensions?2EXS 387 3MuscleSome Quick Facts• Only tissue capable of developing force• Important Functions•• Move limbs•• Neuromuscular system stimulates to produce force• 434 muscles in human body (comprise % of BW)•• Rest for eye control and swallowingEXS 387 4Behavioral PropertiesMusculotendon Unit• Extensibility• Elasticity•• Ability to develop tension3EXS 387 5Behavioral PropertiesMusculotendon Unit• Extensibility - the ability to be stretched or to increase in length.• Elasticity -EXS 387 6Behavioral PropertiesMusculotendon Unit• Parallel elastic component (PEC) - muscle membranes that provide resistive tension when a muscle is passively stretched.• Series elastic component (SEC) -4EXS 387 7Behavioral PropertiesMusculotendon Unit• SEC: resides in tendon and acts as a spring to store elastic energy.•• Both SEC and PEC are time-dependent• This means that when a constant stretch is applied to a muscle group it will progressively lengthen over time•EXS 387 8Behavioral PropertiesMusculotendon Unit• Irritability -• Stimuli affecting muscles are either electrochemical, such as an action potential from the attaching nerve, or mechanical, such as an external blow to a portion of a muscle-tendon unit.•5EXS 387 9Functional OrganizationSkeletal Muscle• Muscle fibers - skeletal muscle fibers grow in length and diameter from birth to adulthood, with a fivefold increase in fiber diameter during this period.• The number of muscle fibers present is genetically determined and varies from person to person.•EXS 387 10Fiber Architecture• Parallel fiber arrangement• Pennate fiber arrangementThe tibialis posterior, rectus femoris, and deltoid muscles are pennate.6EXS 387 11Fiber ArchitectureContinued• When the fibers of a pennate muscle shorten, the rotate about their tendon attachments, progressively increasing the angle of pennation.• Although pennation reduces the effective force generated at a given level of fiber tension, this arrangement allows the packing of more fibers than the amount that can be packed into a longitudinal muscle occupying equal space.EXS 387 12Functional OrganizationSkeletal Muscle• Motor units -a single motor neuron and all fibers it innervates (less than 100  )• Precision Movements: MU innervate less fibers•• Fiber Types–– Fast Twitch (FT)» Type IIa: fast-twitch oxidative»• Online Tutorial 387 13Can Training Alter Fiber Type? • “Nature vs. Nuture: can exercise really alter fiber type composition in human skeletal muscle?”• Ingalls (2004)• 387 14Motor UnitsEffect of Age & Obesity• Age related reduction of the number of motor units and size of Type II fibers• Not related to training OR gender•8EXS 387 15MU Recruitment• Recruitment of motor units - slow twitch motor units always produce tension first, whether the final movement is slow or fast ( principle).• Summation -• Tetanus - state of muscle producing sustained maximal tension resulting from repetitive stimulation.EXS 387 16Muscle ForceFactors in the Development• Type of Motor Unit• Recruitment of Motor Unit• Size Principle (Henneman, 1974)•• Frequency (firing rate: max 50 times/sec)•• Length-Tension Relationship• Force-Velocity Relationship•9EXS 387 17Length-Tension Relationship-Force generation increases when muscle is slightly stretched--Due to SECEXS 387 18Force-Velocity Relationship• Hill

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