 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
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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)
MicroscopicMacroscopic
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 (60120ms)
•Phasic units – larger,
fast twitch, poorly
capillarized, rely on
anaerobic metabolism,
high peak tension,
short time to peak (1050ms)
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 Contraction
Type 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 Positive work; muscle
= resistance; constant
moment and angular velocity
angular velocity; special case of joint in same direction
is isometric contraction
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 = force of contraction of muscle
fiber parallel to longitudinal axis of
muscle
Fa
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
Fa = force of contraction of muscle
fiber parallel to longitudinal axis of
muscle
Fa
Fm
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 Types
Type I
Slow-Twitch
Oxidative (SO)
Type IIA
Fast-Twitch
OxidativeGlycolytic (FOG)
Type IIB
Fast-Twitch
Glycolytic (FG)
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
Speed of
contraction
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