PT245 Muscle Mechanics

University of Scranton - Department of Physical Therapy
Edmund M. Kosmahl, PT, EdD
send e-mail to Dr. Kosmahl copyright 2000 Edmund M. Kosmahl

MUSCLE MECHANICS

Objectives:

Define the types of muscle contraction.
Explain the effects of cross-sectional area, fiber orientation, age, and sex, on force of muscle contraction.
Define the length-tension relationship, and explain its effect on force of muscle contraction.
Explain how leverage and the length-tension relationship interact to influence force of muscle contraction.
Define the passive tension curve and explain how passive stretch conserves energy.
Define and explain the following concepts: isometric torque curve, isokinetic torque, muscle excursion, passive insufficiency, active insufficiency, "lag", tenodesis, and muscular power.
Explain how the force-velocity relationship influences force of muscle contraction.

MUSCLE CONTRACTION TYPES

Isometric - constant length (no movement)
Isotonic - constant load (not really possible in human lever systems, but commonly used to refer to lifting weights)
Isokinetic - constant speed of movement (but accommodating resistance)
Concentric - shortening
Eccentric - lengthening
Agonist (prime mover), Antagonist (muscle that causes movement in the direction that is opposite to the movement direction caused by the agonist), Synergist (a helper or stabilizer - sometimes controls (eliminates) movement in an unwanted direction)

KINEMATIC CHAIN

Open - distal component moves
Closed - proximal component moves
Modifies "origin - insertion" concept

See Kinematic Chain Picture - 17K

FIBER ORIENTATIONS

Parallel great excursion
Pennate great tension

See Fiber Orientations Picture - 44K

CROSS SECTIONAL AREA

Cross sectional area is proportional to tension (strength)
Area is measured perpendicular to fiber length
4 kg / cm2
e.g. quadriceps cross sectional area = @175 cm2, about 500 - 700 kg max ( 1100 -1600 lb)

See Cross Sectional Area Picture - 41K

INFLUENCE OF AGE AND SEX ON MUSCLE TENSION

Tension capability increases until 20 - 30 yrs, decreases thereafter
After puberty, males > females due to greater muscle mass

LENGTH - TENSION RELATIONSHIP

Active tension development capabilities change with changes in elongation
Resting length in vitro (the length a muscle assumes when it is detached from bone)
Maximum tension @ 110% of in vitro resting length
Most useful range for developing tension @ 75% to 110% in vitro resting length
Too short tension "wasted" taking up slack, actin overlaps, myosin meets Z disk
Too long fewer actin / myosin binding sites - less tension developed

See Length-Tension Relationship Picture - 53K

PASSIVE TENSION CURVE

Passive elongation vs. tension development
Stress strain curve
Explains how passive stretch "takes up the slack" (conserves energy)

LEVERAGE INTERACTIONS

Torque arm (perpendicular distance from axis to muscle attachment) - changes during movement
Longer torque arm = greater in vivo effective force
Leverage and length/tension curve interact to produce effective force

ISOMETRIC TORQUE CURVES

Display the torque produced at various points in the range (in vivo)
Can help to define the predominance of mechanical or physiologic factors
Used in the design of exercise equipment

MUSCLE EXCURSION

How far can the muscle lengthen beyond "resting length"?
Mean = 50%
Range = 34% to 89%

PASSIVE INSUFFICIENCY

The inability to complete a range of motion (passively) because the antagonistic muscle cannot be elongated further
Two joint antagonist elongated maximally prohibits agonist movement of joint (e.g. tight hamstrings prevent full extension of knee)

ACTIVE INSUFFICIENCY

The inability to produce maximal measurable tension (actively) because the muscle has been placed in a slackened position
Slackened muscles produce less tension (see length - tension relationship)
e.g. extrinsic finger flexors produce less tension when wrist is flexed as compared to wrist extended (when extrinsic finger flexors are elongated to near 110% of resting length)

"LAG"

Inability to complete active range of motion in spite of full available passive range
"Lag" usually connotes pathology

Adhesions preventing contractile structure excursion (e.g. can't extend knee when patello-femoral joint is adherent)
Pathological lengthening of contractile structure (e.g. can't extend knee when quadriceps tendon has been over stretched after rupture and repair)

TENODESIS

Developing and using passive tension in a two (or more) joint contractile structure (by moving one of the joints) to produce movement at another of the joints
Can be used to substitute for lost function (paralysis)

e.g. developing a functional grip by extending wrist when finger flexors are non-functional (remember, extrinsic finger flexors cross the wrist and finger joints - when the wrist is extended, the fingers are pulled closed)

Can be incorporated in a dynamic splint

FORCE - VELOCITY RELATIONSHIP

Greater tension can be developed at slower speeds
Eccentric > isometric > concentric
"Slower" contractions allow for more actin / myosin binding and more passive elongation "taking up the slack"
Less energy expenditure during "negative" (eccentric) work

See Force-Velocity Relationship Picture - 45K

MUSCULAR POWER

Rate of doing work
P = F x d / t
Greatest at about 30% of maximum load

ISOKINETIC TORQUE

Isokinetic = fixed speed, variable (accommodating) resistance
Allows definition of maximum torques throughout range for various speeds
Allows testing and training at "functional" (fast) speeds

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