1. Chapter 6: The Biomechanics of Human Skeletal Muscle
Basic Biomechanics, 4th edition
Susan J. Hall
Presentation Created by
TK Koesterer, Ph.D., ATC
Humboldt State University

2. Objectives
Identify the basic behavioral properties of the musculotendinous unit.
Explain the relationships of fiber types and fiber architecture to muscle function.
Explain how skeletal muscles function to produce coordinated movement of the human body.
Discuss the effects of the force-velocity and length-tension relationships and electromechanical delay on muscle function.
Discuss the concepts of strength, power, and endurance from a biomechanical perspective.

3. Behavioral Properties of the Musculotendinous Unit
Behavioral properties of muscle tissue:
Extensibility
Elasticity
Irritability
Ability to develop tension
Behavioral properties common to all muscle:
Cardiac, smooth, skeletal

4. Extensibility and Elasticity
Two components:
Parallel elastic component (PEC)
Series elastic component (SEC)
Contractile component
Visoelastic

5. Irritability and the Ability to Develop Tension
The ability to respond to electrical or mechanical stimulus.
Response is the development of tension.
Not necessarily a contraction

6. Structural Organization of Skeletal Muscle
Human body has approx. 434 muscles
40-45% of total body weight in adults
75 muscle pairs responsible for bodily movements and posture
Muscle Fibers
Motor Units
Fiber Types
Fiber Architecture

7. Muscle Fibers Contain: sarcolemma sarcoplasm nuclei mitochondria
myofibrils
myofilaments
Sarcomere
Z lines
M line
A band
myosin filaments
I band
actin filaments
H zone

8. Muscle Fibers During contraction, cross-bridges form
Sarcoplasmic Reticulum
Transverse Tubules
Endomysium
Perimysium
Fascicles
Epimysium
Variation of length and diameter within muscles seen in adults.

9. Motor Units Motor unit: Axon Motor end plate Twitch Type Tonic Type
Summation
Tetanus

10. Fiber Types Fast Twitch (FT) Type IIa Type IIb Slow Twitch (ST) Type I
Peak tension reached in FT in 1/7 time of ST
ST and FT compose skeletal muscles
Percentages of each range from muscle to muscle and individual to individual.

11. Fiber Types Effects of training:
Endurance training can increase ST contraction velocity by 20%
Resistance training can convert FT fibers from Type IIb to Type IIa
Elite athlete fiber type distribution does not significantly differ from untrained individuals
Affected by:
Age and Obesity

12. Fiber Architecture Parallel fiber arrangement
Resultant tension from shortening of muscle fibers
Shortens the muscle
Pennate fiber arrangement
Increases the angle of pennation (attachment) to a tendon.

13. Skeletal Muscle Function
Recruitment of motor units
Change in length with tension development
Roles assumed by muscles
Two-joint and multijoint muscles

14. Recruitment of Motor Units
CNS enables matching of speed and magnitude of muscle contraction to requirement of movement.
Threshold activation
ST activated first (low threshold)
With an increase in speed, force, and/or duration requirement, higher threshold motor units are activated (FT fibers)

15. Change in Muscle Length with Tension Development
Concentric
Bicep shortening with the bicep curl (flexion)
Isometric
Body builders develop isometric contraction in competition
Eccentric
Acts as a breaking mechanism to control movement

16. Roles Assumed by Muscles
Agonist
Primary & Secondary
Antagonist
Stabilizer
Neutralizer
Agonists and Antagonists are typically positioned on opposite sides of a joint.

17. Two-joint and Multijoint Muscles
Movement effectiveness depends on:
Location and orientation of muscle’s attachment relative to the joint
Tightness or laxity of musculotendinous unit
Actions of other muscles crossing the joint
Disadvantages:
Active insufficiency
Passive insufficiency

18. Factors Affecting Muscular Force Generation
Force-Velocity Relationship
Length-Tension Relationship
Electromechanical Delay
Stretch-Shortening Cycle

19. Force-Velocity Relationship
Maximal force developed by muscle governed by velocity of muscle’s shortening or lengthening.
Holds true for all muscle types
Does not imply:
It’s impossible to move heavy resistance at a fast speed.
It’s impossible to move light loads at low speeds

20. Force-Velocity Relationship
Maximum isometric tension
Eccentric conditions
Volitionally
Represents contribution of the elastic components of muscle
Eccentric Strength Training
More effective than concentric training in increasing muscle size and strength.

21. Length-Tension Relationship
In human body, force generation increases when muscle is slightly stretched.
Parallel fibers at max just over resting length
Pennate fibers at max with 120%-130% resting length.
Due to contribution of elastic components of muscle (primarily the SEC)

22. Electromechanical Delay
Electromechanical Delay (EMD)
Varies among human muscles ( msec)
Short EMDs produced by muscles with high percentage of FT fibers
Associated with development of higher contraction forces
Not effected by muscle length, contraction type, contraction velocity, or fatigue

23. Stretch-Shortening Cycle
Stretch-Shortening Cycle (SSC)
Elastic Recoil
Stretch Reflex Activation
Muscle can perform more work with active stretch prior to shortening contraction
Less metabolic costs when SSC utilized.
Eccentric training increases ability of musculotendinous unit to store and produce more elastic energy.

24. Muscular Strength, Power, and Endurance
Muscular Power
Muscular Endurance
Muscular Fatigue
Effect of Muscle Temperature

25. Muscular Strength
The ability of a given muscle group to generate torque at a particular joint.
Two orthogonal components:
1) Rotary Component
2) Parallel to bone
Derived from:
amount of tension the muscles can generate
moment arms of contributing muscles with respect to joint center.

26. Muscular Strength
Tension-generating capability of a muscle affected by:
Cross-sectional area
Training state
Moment arm of a muscle affected by:
Distance between the muscle’s anatomical attachment to bone and the axis of rotation at the joint center
Angle of muscle’s attachment to bone.

27. Muscular Power
The product of muscular force and the velocity of muscular shortening.
The rate of torque production at a joint
Max. power occurs at:
approx. 1/3 max. velocity, and
approx. 1/3 max concentric force
Affected by muscular strength and movement speed

28. Muscular Endurance The ability to exert tension over a period of time.
Constant: gymnast in iron cross
Vary: rowing, running, cycling
Length of time dramatically effected by force and speed requirements of activity.
Training involves many repetitions with light resistance.

29. Muscular Fatigue Opposite of endurance Characteristics:
Reduction in force production
Reduction in shortening velocity
Prolonged relaxation of motor units between recruitment
Absolute Fatigue
Resistance:
SO > FOG > FG
Causes

30. Effect of Muscle Temperature
Increased body temperature, increases speed of nerve and muscle function
Fewer motor units needed to sustain given load
Metabolic processes quicken
Benefits of increased muscular strength, power and endurance
Key point: Be sure to warm-up!

31. Common Muscle Injuries
Strains
Mild, moderate or severe
Contusions
Myositis ossificans
Cramps
Delayed-Onset Muscle Soreness (DOMS)
Compartment Syndrome

32. Summary
Muscle is the only biological tissue capable of developing tension.
Resulting actions can be concentric, eccentric, isometric for muscle shortening, lengthening or remaining unchanged in length
Force production the the combination of many relationships (ex: force-velocity)
Specific activity performance is related power, endurance, and strength