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

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.

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

Extensibility and Elasticity

• Extensibility• Elasticity• Two components:

– Parallel elastic component (PEC)– Series elastic component (SEC)

• Contractile component• Visoelastic

Irritability and the Ability to Develop Tension

• Irritability– The ability to respond to electrical or

mechanical stimulus.– Response is the development of tension.

• Not necessarily a contraction

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

Muscle Fibers

Contain:

sarcolemma

sarcoplasm

nuclei

mitochondria

myofibrils

myofilaments

Sarcomere

Z lines

M line

A band

myosin filaments

I band

actin filaments

H zone

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.

Motor Units

• Motor unit:– Axon– Motor end plate

• Twitch Type• Tonic Type• Summation• Tetanus

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.

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

Fiber Architecture

• Parallel fiber arrangement– Resultant tension from shortening of

muscle fibers• Shortens the muscle

• Pennate fiber arrangement– Resultant tension from shortening of

muscle fibers• Increases the angle of pennation

(attachment) to a tendon.

Skeletal Muscle Function

• Recruitment of motor units

• Change in length with tension development

• Roles assumed by muscles

• Two-joint and multijoint muscles

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)

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

Roles Assumed by Muscles

• Agonist– Primary & Secondary

• Antagonist• Stabilizer• Neutralizer• Agonists and Antagonists are typically

positioned on opposite sides of a joint.

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

Factors Affecting Muscular Force Generation

• Force-Velocity Relationship

• Length-Tension Relationship

• Electromechanical Delay

• Stretch-Shortening Cycle

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

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.

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)

Electromechanical Delay

• Electromechanical Delay (EMD)• Varies among human muscles (20-100 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

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.

Muscular Strength, Power, and Endurance

• Muscular Strength• Muscular Power• Muscular Endurance• Muscular Fatigue• Effect of Muscle Temperature

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.

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.

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

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.

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

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!

Common Muscle Injuries

• Strains– Mild, moderate or severe

• Contusions– Myositis ossificans

• Cramps• Delayed-Onset Muscle Soreness (DOMS)• Compartment Syndrome

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