1. The Biomechanics of Human Skeletal Muscle
Chapter 6
The Biomechanics of Human Skeletal Muscle
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.
© 2014 The McGraw-Hill Companies, Inc. All rights reserved.
McGraw-Hill/Irwin

2. Behavioral Properties of the Musculotendinous Unit
Normal resting length
Extended
After passive recoil
Contracted
Extensibility: ability to be stretched or to increase in length
Elasticity: ability to return to normal resting length following a stretch
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

3. Behavioral Properties of the Musculotendinous Unit
Components of muscle elasticity:
Parallel elastic component (PEC): passive elasticity derived from muscle membranes
Series elastic component (SEC): passive elasticity derived from tendons when a tensed muscle is stretched
PEC
SEC
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

4. Behavioral Properties of the Musculotendinous Unit
Parallel Elastic Component
Series Elastic Component
Contractile Component
From a mechanical perspective, the musculotendinous unit behaves as a contractile component (muscle fibers) in parallel with one elastic component (muscle membranes) and in series with another elastic component (tendons).
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

5. Behavioral Properties of the Musculotendinous Unit
What is the stretch-shortening cycle?
Eccentric contraction (in which the muscle is actively stretched) followed immediately by concentric contraction
Can you think of examples?
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

6. Behavioral Properties of the Musculotendinous Unit
Irritability: ability to respond to a stimulus
Ability to develop tension: the contractile component of muscle function
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

7. Structural Organization of Skeletal Muscle
What is a muscle fiber?
(Single muscle cell surrounded by a membrane called the sarcolemma and containing specialized cytoplasm called sarcoplasm)
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

8. Structural Organization of Skeletal Muscle
Sarcolemma
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

9. Structural Organization of Skeletal Muscle
What do we know about 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
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

10. Structural Organization of Skeletal Muscle
Sarcomere
The sarcomere is the basic structural unit of the muscle fiber. The alternating dark and light bands give muscle its striated appearance. The A bands contain thick, rough myosin filaments surrounded by six thin, smooth actin filaments. The I bands contain only thin actin filaments.
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

11. Structural Organization of Skeletal Muscle
What is a motor unit?
Single motor neuron and all fibers it innervates
Considered the functional unit of the neuromuscular system
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

12. Structural Organization of Skeletal MuscleFT ST
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.)
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

13. 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
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

14. Structural Organization of Skeletal Muscle
How are muscle fibers organized?
Parallel fiber arrangement: fibers are roughly parallel to the longitudinal axis of the muscle; examples are?
Pennate fiber arrangement: short fibers attach to one or more tendons within the muscle; examples?
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

15. Parallel fiber arrangements Pennate fiber arrangements
Structural Organization of Skeletal Muscle
Parallel fiber arrangements
Pennate fiber arrangements
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

16. Structural Organization of Skeletal Muscle
Relaxed
With tension development
The angle of pennation increases as tension progressively increases in the muscle fibers.
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

17. Skeletal Muscle Function
How are motor units (MUs) recruited?
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
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

18. Skeletal Muscle Function
What terms are used to describe muscle contractions based on change in muscle length?
Concentric: involving shortening
Eccentric: involving lengthening
Isometric: involving no change
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

19. Skeletal Muscle Function
What roles are assumed by muscles?
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
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

20. Skeletal Muscle Function
What are disadvantages associated with muscles that cross more than one joint?
Active insufficiency
Passive insufficiency
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

21. Skeletal Muscle Function
Active insufficiency: condition occurring when a two-joint muscle cannot shorten enough to cause full range of motion at both joints it crosses at the same time (decreased ability to form a fist with the wrist in flexion)
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

22. Skeletal Muscle Function
Passive insufficiency: inability of a two-joint muscle to stretch enough to allow full range of motion at both joints at the same time (decreased ROM for wrist extension with the fingers extended)
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

23. 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.
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

24. Factors Affecting Muscular Force Generation
The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum.
Velocity
Force
Isometric maximum
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

25. Factors Affecting Muscular Force Generation
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.
Total Tension
Active Tension
Tension
Passive Tension
Length (% of resting length)
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

26. Factors Affecting Muscular Force Generation
What is electromechanical delay?
force
EMG
EMD
(Time between arrival of a neural stimulus and tension development by the muscle)
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

27. Muscular Strength, Power, and Endurance
How do we measure muscular strength?
(The amount of torque a muscle group can generate at a joint)
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

28. Muscular Strength, Power, and Endurance
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.
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

29. Muscular Strength, Power and Endurance
What 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
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

30. Muscular Strength, Power, and Endurance
What 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
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

31. Skeletal Muscle Function
Torque produced by a muscle (Tm) at the joint center of rotation is the product of muscle force (Fm) and muscle moment arm (d ).
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

32. Muscular Strength, Power, and EnduranceB
The mechanical advantage of the biceps brachii 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.
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

33. Muscular Strength, Power, and Endurance
What is muscular 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
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

34. Muscular Strength, Power, and Endurance
Sprinting is one example of an activity requiring muscular power. What are others?
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

35. Muscular Strength, Power, and Endurance
Force
Velocity
Power
Power-Velocity
Force-Velocity
The general shapes of the force-velocity and power-velocity curves for skeletal muscle.
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

36. Muscular Strength, Power, and Endurance
What is muscular endurance?
The ability of muscle to exert tension over a period of time
The opposite of muscle fatigability
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

37. Muscular Strength, Power, and Endurance
The Ironman Triathlon, which begins at 7:00 am and ends with a mandatory cutoff time of 12:00 midnight, is an example of an endurance event.
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

38. Muscular Strength, Power, and Endurance
What is the effect increased muscle temperature with (warm up) activity prior to a competition?
(The speeds of nerve and muscle functions increase)
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.

39. Muscular Strength, Power, and Endurance
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.
Velocity
Force
Normal body temperature
Elevated body temperature
Basic Biomechanics, 7th edition
By Susan J. Hall, Ph.D.