Required readings:Biomechanics and Motor Control of HumanMovement (class text) by D.A. Winter, pp. 165-212

Next ClassReading assignmentBiomechanics of Skeletal Muscle by T. Lorenz and M. Campello (adapted from M. I. Pitman and L. Peterson; pp. 149-171EMG by W. Herzog, A. C. S. Guimaraes, and Y. T. Zhang; pp. 308-336 http://www.delsys.com/library/tutorials.htm Surface Electromyography: Detecting and RecordingThe Use of Surface Electromyography in BiomechanicsExam on anthropometryTurn in EMG abstractPrepare short presentation on EMG research articleLaboratory experiment on EMGHour assigned

Advanced Biomechanics of Physical Activity (KIN 831)

Muscle Structure, Function, and Electromechanical Characteristics

Material included in this presentation is derived primarily from two sources: Jensen, C. R., Schultz, G. W., Bangerter, B. L. (1983). Applied kinesiology and biomechanics. New York: McGraw-Hill Nigg, B. M. & Herzog, W. (1994). Biomechanics of the musculo-skeletal system. New York: Wiley & Sons Nordin, M. & Frankel, V. H. (1989). Basic Biomechanics of the Musculoskeletal System. (2nd ed.). Philadelphia: Lea & Febiger Winter, D.A. (1990). Biomechanical and motor control of human movement. (2nd ed.). New York: Wiley & Sons

IntroductionMuscular system consists of three muscle types: cardiac, smooth, and skeletal

Skeletal muscle most abundant tissue in the human body (40-45% of total body weight)

Human body has more than 430 pairs of skeletal muscle; most vigorous movement produced by 80 pairs

Introduction (continued)Skeletal muscles provide strength and protection for the skeleton, enable bones to move, provide the maintenance of body posture against gravity

Skeletal muscles perform both dynamic and static work

Muscle StructureStructural unit of skeletal muscle is the multinucleated muscle cell or fiber (thickness: 10-100 m, length: 1-30 cmMuscle fibers consist of myofibrils (sarcomeres in series: basic contractile unit of muscle)Myofibrils consist of myofilaments (actin and myosin)

Microscopic-Macroscopic Structure of Skeletal Muscle

Muscle Structure (continued)Composition of sarcomereZ line to Z line ( 1.27-3.6 m in length)Thin filaments (actin: 5 nm in diameter)Thick filaments (myosin: 15 nm in diameter)Myofilaments in parallel with sarcomereSarcomeres in series within myofibrils

Muscle Structure (continued)Motor unitFunctional unit of muscle contractionComposed of motor neuron and all muscle cells (fibers) innervated by motor neuronFollows all-or-none principle impulse from motor neuron will cause contraction in all muscle fibers it innervates or none

Smallest MU recruited at lowest stimulation frequencyAs frequency of stimulation of smallest MU increases, force of its contraction increasesAs frequency of stimulation continues to increase, but not before maximum contraction of smallest MU, another MU will be recruitedEtc.

Size PrincipleSmallest motor units recruited firstSmallest motor units recruited with lower stimulation frequenciesSmallest motor units with relatively low levels of tension provide for finer control of movementLarger motor units recruited later with increased frequency of stimulation and increased need for greater tension

Size PrincipleTension is reduced by the reverse processSuccessive reduction of firing ratesDropping out of larger units first

Muscle Structure (continued)Motor unitVary in ratio of muscle fibers/motor neuronFine control few fibers (e.g., muscles of eye and fingers, as few as 3-6/motor neuron), tetanize at higher frequenciesGross control many fibers (e.g., gastrocnemius, 2000/motor neuron), tetanize at lower frequenciesFibers of motor unit dispersed throughout muscle

Motor UnitTonic units smaller, slow twitch, rich in mitochondria, highly capillarized, high aerobic metabolism, low peak tension, long time to peak (60-120ms) Phasic units larger, fast twitch, poorly capillarized, rely on anaerobic metabolism, high peak tension, short time to peak (10-50ms)

Muscle Structure (continued)Motor unit (continued)Weakest voluntary contraction is a twitch (single contraction of a motor unit)Twitch times for tension to reach maximum varies by muscle and personTwitch times for maximum tension are shorter in the upper extremity muscles (40-50ms) than in the lower extremity muscles (70-80ms)

Motor Unit Twitch

Shape of Graded Contraction

Shape of Graded ContractionShape and time period of voluntary tension curve in building up maximum tensionDue to delay between each MU action potential and maximum twitch tensionRelated to the size principle of recruitment of motor unitsTurn-on times 200msShape and time period of voluntary relaxation curve in reducing tensionRelated to shape of individual muscle twitchesRelated to the size principle in reverseDue to stored elastic energy of muscleTurn-off times 300ms

Force Production Length-Tension RelationshipForce of contraction in a single fiber determined by overlap of actin and myosin (i.e., structural alterations in sarcomere) (see figure)Force of contraction for whole muscle must account for active (contractile) and passive (series and parallel elastic elements) components

Parallel Connective TissueParallel elastic componentTissues surrounding contractile elementsActs like elastic bandSlack when muscle at resting length of lessNon-linear force length curveSarcolemma, endomysium, perimysium, and epimysium forms parallel elastic element of skeletal muscle

Series Elastic TissueTissues in series with contractile componentTendon forms series elastic element of skeletal muscleEndomysium, perimysium, and epimysium continuous with connective tissue of tendonLengthen slightly under isometric contraction ( 3-7% of muscle length)Potential mechanism for stored elastic energy (i.e., function in prestretch of muscle prior to explosive concentric contraction)

Isometric Contraction

Musculotendinous UnitTendon and connective tissues in muscle (sarcolemma, endomysium, perimysium, and epimysium) are viscoelasticViscoelastic structures help determine mechanical characteristics of muscles during contraction and passive extension

Musculotendinous Unit (continued)Functions of elastic elements of muscleKeep ready state for muscle contractionContribute to smooth contractionReduce force buildup on muscle and may prevent or reduce muscle injuryViscoelastic property may help muscle absorb, store, and return energy

Muscle Model

Force Production Gradation of ContractionSynchronization (number of motor units active at one time) more force potentialSize of motor units motor units with larger number of fibers have greater force potentialType of motor units type IIA and IIB force potential, type I force potential

Force Production Gradation of Contraction (continued)Summation increase frequency of stimulation, to some limit, increases the force of contraction

Force Production Gradation of Contraction (continued)Size principle tension increaseSmallest motor units recruited first and largest lastIncreased frequency of stimulation force of contraction of motor unitLow tension movements can be achieved in finely graded stepsIncreases frequency of stimulation recruitment of additional and larger motor unitsMovements requiring large forces are accomplished by recruiting larger and more forceful motor unitsSize principle tension decreaseLast recruited motor units drop out first

Types of Muscle Contraction

Type of ContractionDefinitionWorkConcentricForce of muscle contraction resistancePositive work; muscle moment and angular velocity of joint in same directionEccentricForce of muscle contraction resistanceNegative work; muscle moment and angular velocity of joint in opposite directionIsokineticForce of muscle contraction = resistance; constant angular velocity; special case is isometric contractionPositive work; muscle moment and angular velocity of joint in same directionIsometricForce of muscle contraction resistance; series elastic component stretch = shortening of contractile element (few to 7% of resting length of muscle)No mechanical work; physiological work

Force Production Length-Tension RelationshipDifficult to study length-tension relationshipDifficult to isolate single agonist Moment arm of muscle changes as joint angle changesModeling may facilitate this type of study

Force Production Load-Velocity RelationshipConcentric contraction (muscle shortening) occurs when the force of contraction is greater than the resistance (positive work)Velocity of concentric contraction inversely related to difference between force of contraction and external loadZero velocity occurs (no change in muscle length) when force of contraction equals resistance (no mechanical work)

Force Production Load-Velocity RelationshipEccentric contraction (muscle lengthening) occurs when the force of contraction is less than the resistance (negative work)Velocity of eccentric contraction is directly related to the difference between force of contraction and external load

Force Production Force-Time RelationshipIn isometric contractions, greater force can be developed to maximum contractile force, with greater timeIncreased time permits greater force generation and transmission through the parallel elastic elements to the series elastic elements (tendon)Maximum contractile force may be generated in the contractile component of muscle in 10 msec; transmission to the tendon may take 300msec

3-D Relationship of Force-Velocity-Length

3-D Relationship of Force-Velocity-Length

Effect of Muscle Architecture on ContractionFusiform muscleFibers parallel to long axis of muscleMany sarcomeres make up long myofibrilsAdvantage for length of contractionExample: sartorius muscleForce of contraction along long axis of muscle of force of contraction of all muscle fibersTends to have smaller physiological cross sectional area(see figure)

Fusiform Fiber ArrangementFaFa = force of contraction of muscle fiber parallel to longitudinal axis of muscle

Fa = sum of all muscle fiber contractions parallel to long axis of muscle

Effect of Muscle Architecture on Contraction (continued)Pennate muscleFibers arranged obliquely to long axis of muscle (pennation angle)Uni-, bi-, and multi-pennateAdvantage for force of contractionExample: rectus femoris (bi-pennate)Tends to have larger physiological cross sectional area

Pennate Fiber Arrangement FmFaFa = force of contraction of muscle fiber parallel to longitudinal axis of muscle

Fm = force of contraction of muscle fiber

= pennation angle

Fa = (cos )(Fm)

Fa = sum of all muscle fiber contractions parallel to long axis of muscle

Effect of Muscle Architecture on Contraction (continued)Force of muscle contraction proportional to physiological cross sectional area (PCSA); sum of the cross sectional area of myofibrilsVelocity and excursion (working range or amplitude) of muscle is proportional to length of myofiblril

Muscle Fiber Types

Type ISlow-Twitch Oxidative (SO)Type IIAFast-Twitch Oxidative-Glycolytic (FOG)Type IIBFast-Twitch Glycolytic (FG)Speed of contractionSlowFastFastPrimary source of ATP productionOxidative phosphorylationOxidative phosphorylationAnaerobic glycolysisGlycolytic enzyme activityLowIntermediateHighCapillariesManyManyFewMyoglobin contentHighHighLowGlycogen contentLowIntermediateHighFiber diameterSmallIntermediateLargeRate of fatigueSlowIntermediateFast

Muscle Fiber Types (continued)Smaller slow twitch motor units are characterized as tonic units, red in appearance, smaller muscle fibers, fibers rich in mitochondria, highly capillarized, high capacity for aerobic metabolism, and produce low peak tension in a long time to peak (60-120ms).Larger fast twitch motor units are characterized as phasic units, white in appearance, larger muscle fibers, less mitochondria, poorly capillarized, rely on anaerobic metabolism, and produce large peak tensions in shorter periods of time (10-50ms).

Muscle Fiber Types (continued)Nerve innervating muscle fiber determines its type; possible to change fiber type by changing innervations of fiberAll fibers of motor unit are of same typeFiber type distribution in muscle genetically determinedAverage population distribution:50-55% type I30-35% type IIA15% type IIB

Muscle Fiber Types (continued)Fiber composition of muscle relates to function (e.g., soleus posture muscle, high percentage type I)Muscles mixed in fiber type compositionNatural selection of athletes at top levels of competition

Electrical Signals of Muscle FibersAt rest, action potential of muscle fiber -90 mV;caused by concentrations of ions outside and inside fiber (resting state)With sufficient stimulation, potential inside cell raised to 30-40 mV (depolarization); associated with transverse tubular system and sarcoplasmic reticulum; causes contraction of fiberReturn to resting state (repolarization)Electrical signals from the motor units (motor unit action potential, muap) can be recorded (EMG) via electrodes