Muscle

Required readings: Biomechanics and Motor Control of Human

Movement (class text) by D.A. Winter, pp. 165-212

Next Class• Reading assignment

– Biomechanics of Skeletal Muscle by T. Lorenz and M. Campello (adapted from M. I. Pitman and L. Peterson; pp. 149-171

– EMG 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 Recording• The Use of Surface Electromyography in Biomechanics

• Exam on anthropometry• Turn in EMG abstract• Prepare short presentation on EMG research article• Laboratory experiment on EMG• Hour 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

Introduction• Muscular 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 Structure• Structural unit of skeletal muscle is the

multinucleated muscle cell or fiber (thickness: 10-100 m, length: 1-30 cm

• Muscle 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 sarcomere

– Z 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 sarcomere– Sarcomeres in series within myofibrils

Muscle Structure (continued)• Motor unit

– Functional unit of muscle contraction– Composed of motor neuron and all muscle cells

(fibers) innervated by motor neuron– Follows “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 frequency

•As frequency of stimulation of smallest MU increases, force of its contraction increases

•As frequency of stimulation continues to increase, but not before maximum contraction of smallest MU, another MU will be recruited

•Etc.

Size Principle• Smallest motor units recruited first

• Smallest motor units recruited with lower stimulation frequencies

• Smallest motor units with relatively low levels of tension provide for finer control of movement

• Larger motor units recruited later with increased frequency of stimulation and increased need for greater tension

Size Principle

• Tension is reduced by the reverse process– Successive reduction of firing rates– Dropping out of larger units first

Muscle Structure (continued)

• Motor unit– Vary in ratio of muscle fibers/motor neuron

• Fine control – few fibers (e.g., muscles of eye and fingers, as few as 3-6/motor neuron), tetanize at higher frequencies

• Gross control – many fibers (e.g., gastrocnemius, 2000/motor neuron), tetanize at lower frequencies

– Fibers of motor unit dispersed throughout muscle

•Motor Unit•Tonic 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 person– Twitch 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 Contraction• Shape and time period of voluntary tension curve

in building up maximum tension– Due to delay between each MU action potential and

maximum twitch tension– Related to the size principle of recruitment of motor

units– Turn-on times ≈ 200ms

• Shape and time period of voluntary relaxation curve in reducing tension– Related to shape of individual muscle twitches– Related to the size principle in reverse– Due to stored elastic energy of muscle– Turn-off times ≈ 300ms

Force Production – Length-Tension Relationship

• Force 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 Tissue• Parallel elastic component

• Tissues surrounding contractile elements

• Acts like elastic band

• Slack when muscle at resting length of less

• Non-linear force length curve

• Sarcolemma, endomysium, perimysium, and epimysium forms parallel elastic element of skeletal muscle

Series Elastic Tissue• Tissues in series with contractile component• Tendon forms series elastic element of

skeletal muscle• Endomysium, perimysium, and epimysium

continuous with connective tissue of tendon• Lengthen 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 Unit

• Tendon and connective tissues in muscle (sarcolemma, endomysium, perimysium, and epimysium) are viscoelastic

• Viscoelastic structures help determine mechanical characteristics of muscles during contraction and passive extension

Musculotendinous Unit (continued)

• Functions of elastic elements of muscle– Keep “ready” state for muscle contraction– Contribute to smooth contraction– Reduce force buildup on muscle and may

prevent or reduce muscle injury– Viscoelastic property may help muscle absorb,

store, and return energy

Muscle Model

Force Production – Gradation of Contraction

• Synchronization (number of motor units active at one time) – more force potential

• Size of motor units – motor units with larger number of fibers have greater force potential

• Type 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 increase– Smallest motor units recruited first and largest last

• Increased frequency of stimulation force of contraction of motor unit

• Low tension movements can be achieved in finely graded steps

• Increases frequency of stimulation recruitment of additional and larger motor units

• Movements requiring large forces are accomplished by recruiting larger and more forceful motor units

• Size principle – tension decrease– Last recruited motor units drop out first

Types of Muscle ContractionType of Contraction Definition Work

Concentric Force of muscle contraction resistance

Positive work; muscle moment and angular velocity of joint in same direction

Eccentric Force of muscle contraction resistance

Negative work; muscle moment and angular velocity of joint in opposite direction

Isokinetic Force of muscle contraction = resistance; constant angular velocity; special case is isometric contraction

Positive work; muscle moment and angular velocity of joint in same direction

Isometric Force 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 Relationship

• Difficult to study length-tension relationship– Difficult to isolate single agonist – Moment arm of muscle changes as joint angle

changes– Modeling may facilitate this type of study

Force Production – Load-Velocity Relationship

• Concentric 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 load

• Zero velocity occurs (no change in muscle length) when force of contraction equals resistance (no mechanical work)

Force Production – Load-Velocity Relationship

• Eccentric 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 Relationship

• In isometric contractions, greater force can be developed to maximum contractile force, with greater time

• Increased 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 Contraction

• Fusiform muscle– Fibers parallel to long axis of muscle– Many sarcomeres make up long myofibrils– Advantage for length of contraction– Example: sartorius muscle– Force of contraction along long axis of muscle

of force of contraction of all muscle fibers– Tends to have smaller physiological cross

sectional area

(see figure)

Fusiform Fiber Arrangement

Fa

Fa = 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 muscle– Fibers arranged obliquely to long axis of

muscle (pennation angle)– Uni-, bi-, and multi-pennate– Advantage for force of contraction– Example: rectus femoris (bi-pennate)– Tends to have larger physiological cross

sectional area

Pennate Fiber Arrangement

FmFa

Fa = 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 myofibrils

• Velocity and excursion (working range or amplitude) of muscle is proportional to length of myofiblril

Muscle Fiber TypesType I

Slow-Twitch Oxidative (SO)

Type IIA

Fast-Twitch Oxidative-

Glycolytic (FOG)

Type IIB

Fast-Twitch Glycolytic (FG)

Speed of contraction

Slow Fast Fast

Primary source of ATP production

Oxidative phosphorylation

Oxidative phosphorylation

Anaerobic glycolysis

Glycolytic enzyme activity

Low Intermediate High

Capillaries Many Many Few

Myoglobin content High High Low

Glycogen content Low Intermediate High

Fiber diameter Small Intermediate Large

Rate of fatigue Slow Intermediate Fast

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 fiber

• All fibers of motor unit are of same type

• Fiber type distribution in muscle genetically determined

• Average population distribution:– 50-55% type I– 30-35% type IIA– 15% 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 composition

• Natural selection of athletes at top levels of competition

Electrical Signals of Muscle Fibers• At 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 fiber

• Return to resting state (repolarization)• Electrical signals from the motor units

(motor unit action potential, muap) can be recorded (EMG) via electrodes