1. BIOMECHANICS of HUMAN SKELETAL MUSCLE
ENT 214 Biomechanics
BIOMECHANICS of HUMAN SKELETAL MUSCLE
Mohd Yusof Baharuddin &
Assoc. Prof. Dr. Mohammad Iqbal Omar
(Biomedical Electronic Engineering Program)

2. Introduction
Muscular system consists of three muscle types: cardiac, smooth, and skeletal
Muscle types:
cardiac muscle: composes the heart
smooth muscle: lines hollow internal organs
skeletal (striated or voluntary) muscle: attached to skeleton via tendon & movement

3. 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
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

4. Lecture outlines: Composition & structure of skeletal muscle
Mechanics of Muscle Contraction
Force production in muscle
Muscle remodeling
Summary

5. I. Structural Organization of Skeletal Muscle
What is a muscle fiber?
Muscle fiber = muscle cell
Because of threadlike shape
single muscle cell surrounded by a membrane called the sarcolemma and containing specialized cytoplasm called sarcoplasm

6. Muscle Structure- contd
Structural unit of skeletal muscle is the multinucleated muscle cell or fiber (thickness: 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)

9. Connective Tissue Wrappings of Skeletal Muscle
Figure 6.1

10. Microscopic & Macroscopic Structure of Skeletal Muscle

11. Microscopic Anatomy of Skeletal Muscle
Figure 6.3a

12. Microscopic Anatomy of Skeletal Muscle
Myofibrils are aligned to give distinct bands
I band = light band
Contains only thin filaments
A band = dark band
Contains the entire length of the thick filaments

13. Microscopic Anatomy of Skeletal Muscle
Figure 6.3b

14. Bands of myofibrils A bands: thick filaments in central of sarcomere
I band
A bands: thick filaments in central of sarcomere
Z line: short elements that links thin filaments
I bands: thin filaments not overlap with thick filaments
H zone: gap between ends of thin filaments in center of A band
M line: transverse & longitudinally oriented linking proteins for adjacent thick filaments M Z Z
sarcomere

15. Microscopic Anatomy of Skeletal Muscle
Sarcomere—contractile unit of a muscle fiber
Organization of the sarcomere
Myofilaments
Thick filaments = myosin filaments
Thin filaments = actin filaments

18. Muscle Structure (continued)
Composition of sarcomere
Z line to Z line ( 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

19. Molecular basis of muscle contraction
Sliding filament theory: relative movement of actin & myosin filaments yields active sarcomere shortening
Myosin heads or cross-bridges generate contraction force
Sliding of actin filaments toward center of sarcomere: decrease in I band and decrease in H zone as Z lines move closer

22. ATP
ATP

23. 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

24. Muscle Tissue Behavioral Properties
Extensibility
Elasticity
Irritability
Ability to develop tension

25. Extensibility & Elasticity
What is extensibility?
Ability to stretch or to increase in length
How about elasticity?
Ability to return to normal length after stretch
Figure from

26. Muscle Tissue Behavioral Properties
Irritability or Excitability (also called responsiveness)
ability to receive and respond to a stimulus
Not necessarily a contraction
Ability to develop tension – Contractility
ability to shorten when an adequate stimulus is received

27. Elastic Behaviour Parallel elastic component (PEC)
Muscle membrane (epimysium, perimysium, endomysium, sarcolemma)
Supply resistance when muscle is passively stretched
Series elastic component (SEC)
Tendons
Act as spring to store elastic energy
Contractile component (CC)
Muscle property enabling tension by stimulated muscle fibers

28. Muscle Elastic Behavior Model

31. Structural Organization of Skeletal Muscle
Fast twitch (FT) fibers both reach peak tension and relax more quickly than slow twitch (ST) fibers.
Peak tension is typically greater for FT than for ST fibers.
Twitch Tension
Time
FT ST

32. 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

33. Motor unit
considered the functional unit of the neuromuscular system

36. 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

37. Size Principle- contd. Tension is reduced by the reverse process
Successive reduction of firing rates
Dropping out of larger units first

38. 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

39. 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)

40. 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)

41. Muscle Differentiation (types of fibers)
I (slow-twitch oxidative)
IIA (fast-twitch oxidative glycolytic)
IIB fast-twitch glycolytic
Contraction speed
Slow
fast
Myosin-ATPase activity
Low
High
Primary source of ATP production
Oxidative phosphorylation
Anaerobic glycolysis
Glycolytic enzyme activity
Intermediate
No. of mitochondria
Many
Few
Capillaries
Myoglobin contents
Muscle Color
Red
White
Glycogen content
Fiber diameter
small
Large
Rate of fatigue
slow
Fast

42. Contraction of Skeletal Muscle
Graded responses can be produced by changing
The frequency of muscle stimulation
The number of muscle cells being stimulated at one time

43. Types of Graded Responses
Twitch
Single, brief contraction
Not a normal muscle function

44. Types of Graded Responses
Figure 6.9a

45. Types of Graded Responses
Tetanus (summing of contractions)
One contraction is immediately followed by another
The muscle does not completely return to a resting state
The effects are added

46. Types of Graded Responses
Figure 6.9b

47. Types of Graded Responses
Unfused (incomplete) tetanus
Some relaxation occurs between contractions
The results are summed

48. Types of Graded Responses
Figure 6.9c

49. Types of Graded Responses
Fused (complete) tetanus
No evidence of relaxation before the following contractions
The result is a sustained muscle contraction

50. Types of Graded Responses
Figure 6.9d

51. Muscle Response to Strong Stimuli
Muscle force depends upon the number of fibers stimulated
More fibers contracting results in greater muscle tension
Muscles can continue to contract unless they run out of energy

52. II. Mechanics of Muscle Contraction
Neural stimulation – impulse
Mechanical response of a motor unit - twitch
T: twitch or contraction time, time for tension to reach maximum
F0: constant of a given motor unit
Averaged T values
Tricep brachii ms Soleus 74.0 ms
Biceps brachii 52.0 ms Medial Gastrocnemius ms
Tibialis anterior 58.0 ms

53. Generation of muscle tetanus
100Hz
10 Hz
Note: muscle is controlled by frequency modulation from neural input
very important in functional electrical stimulation

54. Motor unit recruitment
All-or-nothing event
2 ways to increase tension:
- Stimulation rate
Recruitment of more motor unit
Size principle
Smallest m.u. recruited first
Largest m.u. last

55. Muscle fatigue Drop in tension followed prolonged stimulation.
Fatigue occurs when the stimulation frequency outstrips rate of replacement of ATP, the twitch force decreases with time

56. III. Force production in muscle
Force production depends on
length
velocity
muscle composition & morphology

57. Terms used to describe muscle contractions
concentric: involving shortening
eccentric: involving lengthening
isometric: involving no change
isokinetic: no change in the resulting muscle length occurs

58. Types of contraction

60. 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 = resistance; constant angular velocity; special case 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

61. 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)

63. Fiber architecture There are 2 types of fiber architecture
parallel fiber arrangement
fibers are roughly parallel to the longitudinal axis of the muscle
example: sartorius, rectus abdominis, biseps brachii
pennate fiber arrangement
short fibers attach to one or more tendons within the muscle
example: tibialis posterior, rectus femoris, deltoid

65. Fiber architecture (cont’d)
parallel fiber arrangement
Muscle become shorten due to fiber shortening
pennate fiber arrangement
When muscle shorten, they rotate about their tendon attachment
This will increase the angle of pennation
Greater angle of pennation will induced smaller amount of effective force

66. 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

67. Fusiform Fiber Arrangement
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 Fa

68. 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

69. Pennate Fiber Arrangement
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 Fa Fm 

72. 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

73. 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).

74. 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

75. Histology of muscle Type IIA Type IIB Type I
Eye muscle (Rectus lateralis); Myofibrillar ATPase stain, PH 4.3

76. 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

77. 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  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

80. Muscles Roles Agonist: acts to cause a movement
Antagonist: acts to slow or stop a movement
Stabilizer: acts to stabilize a body part against some other force
Neutralizer: acts to eliminate an unwanted action produced by an agonist

81. Agonist Agonist – a muscle or group of muscles that causes the motion
Without this muscle (or group) the motion is o longer considered functional
The muscle contracts isotonically to produce a motion or isometrically to maintain a position
The isotonic contraction would be concentric since motion results from the shortening of the muscle
Example: The quadriceps, through concentric isotonic contraction, is the primary muscle-----responsible for knee extension

82. Antagonist performs the opposite motion of the prime mover
It contracts eccentrically or "relaxes" and lengthens to prevent, slow or control a motion
Examples: the quadriceps contracts eccentrically to slow down knee flexion, the motion
opposite to its prime mover motion of knee extension. This is seen in slow squats. Since motion is occurring, this example would be of an isotonic contraction.
Antagonistic muscle contractions may, however, be isometric
With elbow extension, the triceps would be the agonist and the biceps would be the antagonist. With elbow flexion, the biceps would be the agonist and the triceps would be the antagonist

83. Stabilizer
the muscle may contract to hold a body part immobile while another body part is moving.
The sustained stabilizing contraction is frequently isometric.
In most normal activities, proximal joints are stabilized by muscle contractions during movement of more distal joints - proximal stabilization.
For an isolated movement at one joint to occur, the muscles that control the joints proximal to that joint must stabilize the proximal joints so that no motion occurs there.
The antagonists for each motion at the proximal joint co-contract or contract against each other to prevent motion.
Example: the quadriceps may stabilize the knee in an extended position of permit plantar flexion of the ankle as when the individual rises to tip-toe in erect position
To perform isolated elbow flexion the proximal shoulder joint must be stabilized by flexors/extensors, abductors/adductors and internal/external rotators.

84. Neutralizer
prevents unwanted motion so muscle can perform a specific motion can occur.
Mostly dependant on the angle of pull.
Examples: the biceps can flex the elbow and supinate the forearm. If only elbow flexion
is wanted, the supination component must be ruled out. Therefore, the pronator teres,
which pronates the forearm, would contract to counteract the supination component of the
biceps, and only elbow flexion would occur. Neutralizers act to cancel out an unwanted
movement

85. Neutralizer (2)
With wrist ulnar deviation the flexor carpi ulnaris will cause flexion and lunar deviation of the wrist.
The extensor carpi ulnaris will cause extension and ulnar deviation.
If ulnar deviation is desired, these muscles would contract doing two things: they would neutralize each other's flexion/extension component, and they would act as agonists in wrist ulnar deviation
Stabilizers and Neutralizers both use co-contraction to prevent motion and have an relationship.
Stabilizers are associated with joints; Neutralizers are associated with muscle.

86. Two Joint and Multijoint Muscles
active insufficiency: failure to produce force when slack
passive insufficiency: restriction of joint range of motion when fully stretched

87. Active Insufficiency
Failure to produce force when muscles are slack (decreased ability to form a fist with the wrist in flexion)

88. Passive insufficiency
Restriction of joint range of motion when muscles are fully stretched (decreased ROM for wrist extension with the fingers extended)

89. Factors Affecting Muscular Force Generation
Velocity
Force
(Low resistance, high contraction velocity)
The force-velocity relationship for muscle tissue: When resistance (force) is negligible, muscle contracts with maximal velocity.

90. Force – Velocity Relationship
isometric maximum
The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum.

91. Length – Tension Relationship
Length (% of resting length)
Active Tension
Passive Tension
Total Tension
Tension
Length (% of resting length)
Active Tension
Passive Tension
Total Tension
The length-tension relationship: Tension present in a stretched muscle is the sum of the active tension provided by the muscle fibers and the passive tension provided by the tendons and membranes.

92. Force – Time Relationship
What is electromechanical delay (EMD)?
Myoelectric activity
Force
Stimulus
Electromechanical delay
EMD is the time between arrival of a neural stimulus and tension development by the muscle
Time needed for muscles reached maximum isometric is second after EMD

93. Muscular Strength
the amount of torque a muscle group can generate at a joint

94. How do we measure muscular strength?Ft
The component of muscle force that produces torque (Ft) at the joint is directed perpendicular to the attached bone.

95. Sample Problem 2
How much tork is produced at the elbow by biceps brachii inserting at an angle 60 degree on the radius when tension in the muscle is 400 N? α

96. Solution Find Fp which is perpendicular to bone Calculate tork
Tm = Fp x d

97. Factors affect muscular strength
tension-generating capability of the muscle tissue, which is in turn affected by:
muscle cross-sectional area
training state of muscle

98. Factors affect muscular strength
moment arms of the muscles crossing the joint (mechanical advantage), in turn affected by:
distance between muscle attachment to bone and joint center
angle of the muscle’s attachment to bone

99. Effects for differences angleB C
The mechanical advantage of the biceps bracchi is maximum when
the elbow is at approximately 90 degrees (A), because 100% of muscle
force is acting to rotate the radius. As the joint angle increases (B)
or decreases (C) from 90 degrees, the mechanical advantage of
the muscle is lessened because more and more of the force is pulling
the radius toward or away from the elbow rather than contributing
to forearm rotation.

100. Muscle Power
the product of muscular force and the velocity of muscle shortening
the rate of torque production at a joint
the product of net torque and angular velocity at a joint

101. Muscular Power
Force
Velocity
Power
Power-Velocity
Force-Velocity
The general shapes of the force-velocity and power-velocity curves for skeletal muscle.

102. Muscular Endurance
the ability of muscle to exert tension over a period of time
the opposite of muscle fatigability

103. Effect of muscle temperature (Warm up)
the speeds of nerve and muscle functions increase

104. Effect of muscle temperature (Warm up) cont’d
With warm-up, there is a shift to the right in the force-velocity curve, with higher maximum isometric tension and higher maximum velocity of shortening possible at a given load.
Normal body temperature
Elevated body temperature
Force
Velocity

105. IV. Muscle Remodeling Effects of Disuse and Immobilization
Immediate or early motion may prevent muscle atrophy after injury or surgery
Muscle fibers regenerate in more parallel orientation, capillariaztion occurred rapidly, tensile strength returned more quickly
Atrophy of quadriceps developed due to immobilization can not be reversed by isometric exercises.
Type I fibers atrophy with immobilization; cross-sectional area decreases & oxidative enzyme activity reduced
Tension in muscle afferent impulses from intrafusal muscle spindle increases & leading to increase stimulation of type I fiber

106. Effects of Physical Training
Increase cross-sectional area of muscle fibers, muscle bulk & strength
Relative percentage of fiber types also changes
In endurance athletes % type I, IIA increase
Stretch out of muscle-tendon complex increases elasticity & length of musculo-tendon unit; store more energy in viscoelastic & contractile components
Roles of muscle spindle & Golgi tendon organs: inhibition of spindle effect & enhance Golgi effect to relax muscle and promote further lengthening.

107. Summary Basic behavioral properties of the muscle unit
Relationships of fiber types and architecture to muscle function
Skeletal muscles function to produce coordinated movement of the human body
Effects of the force-velocity and length-tension relationships and EMD on muscle function
Muscular strength, power, and endurance

108. Thank you