1. BIOMECHANICS of HUMAN SKELETAL MUSCLE
ENT 214 Biomechanics
BIOMECHANICS of HUMAN SKELETAL MUSCLE
Mohd Yusof Baharuddin
BBiomedEng(UM)
MBiomedEng(Melbourne)
Picture from MisterBisson Flickr pages

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

3. 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

4. Elastic Behaviour Parallel elastic component (PEC)
Muscle membrane
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

5. Muscle Elastic Behavior Model

6. Muscle Tissue Behavioral Properties
Irritability
ability to respond to a stimulus
Ability to develop tension
the contractile component of muscle function

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

8. Skeletal Muscle Silverthorn, Human Physiology,
Benjamin-Cummings, ISBN 0-
, Fig. 12-3, p. 392

9. Skeletal muscle Silverthorn, Human Physiology,
Benjamin-Cummings, ISBN 0-
, Fig. 12-1a, p. 390

10. Image taken from http://courses. cm. utexas

11. Muscle fibers
some fibers run the entire length of a muscle; others are shorter
skeletal muscle fibers grow in both length and diameter from birth through adulthood
fiber diameter can be increased through resistance training

12. Muscle Fiber (cont’d)

13. Muscle Fiber (cont’d)

14. Muscle Fiber (cont’d)

16. Myosin & Actin

21. Motor unit single motor neuron and all fibers it innervates
considered the functional unit of the neuromuscular system
There are 2 types
Fast twitch (FT)
Slow twitch (ST)

22. 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

23. Skeletal Muscle Fiber Characteristics
TYPE IIA
Type I Fast-Twitch Type IIB
Slow-Twitch Oxidative Fast-Twitch
Oxidative
Glycolytic Glycolytic
CHARACTERISTIC
(SO)
(FOG)
(FG)
Contraction Speed
slow
fast
Fatigue rate
intermediate
Diameter
small
large
ATPase concentration
low
high
Mitochondrial
concentration
Glycolytic enzyme

24. 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

26. 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

27. Sample Problem 1
How much force is exerted by tendon of pennate muscle when tension in fiber is 100 N, given that the angle of pennation is 60 degree. Ft α Ff

28. Solution
Ft = 100 cos 60
= 100 (0.5)
= 50 Ft α Ff

29. Recruitment of Motor Units (MU)
Slow twitch (ST) fibers are easier to activate than fast twitch (FT) fibers
ST fibers are always recruited first
Increasing speed, force, or duration of movement involves progressive recruitment of MUs with higher and higher activation thresholds

30. Terms used to describe muscle contractions
concentric: involving shortening
eccentric: involving lengthening
isometric: involving no change

31. 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

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

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

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

35. 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.

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

37. Length – Tension Relationship
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.
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.
Tension
Length (% of resting length)
Active Tension
Passive Tension
Total Tension
Tension
Length (% of resting length)
Active Tension
Passive Tension
Total Tension

38. 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

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

40. 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.

41. 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

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

43. 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

44. 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

45. 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.

46. 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

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

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

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

50. 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

51. 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