Muscle Force
kiaa_p1
4340-NM-Force-04.pptxTypes of Muscle Contractions & Variations of Force
Chapter 1
Objectives
12. Describe the different types of muscle contraction, their force-to-resistance relationship, and how they differ in the excitation-contraction process.
13. Understand the different factors that influence the amount of force generated when a muscle fibers contract.
Types of Muscle Actions
The amount of force produce by a muscle compared to the resistance, will influence the type of contraction that occurs.
Concentric
Concentric Contraction
Are power strokes occurring?
Do the thin filaments slide inward?
Therefore, the sarcomeres ___________
Why…?
Because muscle force is _______ the resistance
1
2
3
Yes
Yes
Shorten
Greater than
Isometric
Isometric or Static Contraction
Are power strokes occurring?
Do the thin filaments slide inward?
Therefore, the sarcomeres ___________
Why…?
Muscle force is _______ the resistance
1
2
3
Yes
No
Don’t change length
Equal to
Eccentric
Eccentric Contraction
Are power strokes occurring?
Do the thin filaments slide inward?
What is happening to the thin filaments?
Therefore, the sarcomeres ___________
Why…?
Muscle force is _______ the resistance
1
2
3
Yes
No
They are being pulled outward
Lengthen
Less than
Eccentric Contraction
An eccentric contraction of the antagonist muscle restricts or controls the concentric contraction of the agonist; prevents excessive lengthening of the antagonist.
Example: triceps lowers dumbbell (agonists) while biceps (antagonist) ’controls’ the triceps.
Such eccentric contractions damage of myofibrils and other proteins
Muscle soreness
Strength gains
Generation of Force
low force
moderate force
high force
How is a muscle able to alter the amount of force it produces based on need or the circumstance?
Producing high force here is a waste of energy.
Producing low force here and he couldn’t curl the bar.
A Review of Muscle Force
Power strokes
Force
One power stroke produces several pico-newtons of force or several one-trillionths (10 to -12) of a Newton of force. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2290649/ One Newton is the force needed to accelerate 1 kilogram of mass at the rate of 1 meter per second squared. 1 billion = 1000 million. 1 trillion = 1 million million.
1. Motor Units
Low FORCE High
When 15% of all motor units are recruited, what type of motor units are they? How much force is produced?
When 90% of all motor units are recruited, what type of motor units are they? How much force is produced?
15
90
Size principle: motor units are recruited from smallest to largest;
the strength of the electrical stimulation
the number of motor units recruited
Exceptions to size principle
Time delay caused by recruiting smaller motor units may be detrimental to performance of a high-force activity
In such cases, high-threshold, fast motor units are recruited first to allow faster movement velocity
Asynchronous firing
2. Frequency of Stimulation: Rate Coding
Twitch = single action potential
Summation = secession of action potentials
Tetanus = secession of action potentials at high frequency
maximal tension
production
low Frequency high
= neuron action potential
Motor Unit Activation
MU1
MU2
MU3
2. Frequency of neuron stimulation
1. Number and Type of Motor Unit
IIb
MU1=S, MU2=FFR, and MU3=FF
Muscle Force
Also note how S motor units continue to fire even at maximal force production and that FFR and FF are added for additional force production.
Note how as the force increase so does the frequency of stimulation.
3. Muscle Fiber and Sarcomere Length
Consider three isometric contractions at three different joint angles.
1. Shortened muscle prior
to contraction
2. Mid-lengthen muscle prior
to contraction
3. Lengthen muscle prior
to contraction
An isometric contraction at which muscle length above (1, 2, or 3) would produce the greatest amount of force? Why?
3. Muscle Fiber and Sarcomere Length
The graphs is based on separate isometric contractions
2
2
3
3
1
1
Which sarcomere length below (1, 2, or 3) would allow for the greatest number of power strokes?
Which length would allow for the most power strokes?
Which length would produce the most force/tension?
3. Muscle Fiber and Sarcomere Length
Length-tension relationship
Optimal muscle/sarcomere length causes greater force
Shortened or lengthened sarcomeres prior to contraction results in less force
4. Speed of Contraction
Speed-force relationship
Concentric contractions: maximal force is produced at slower speeds of contraction. Why?
Slow
Fast
Low
High
Force
Speed of Concentric Contraction
[Eccentric: maximal tension develops at faster speeds of lengthening.]
4. Speed of Contraction
With slow contractions, more myosin heads can bind to actin resulting in more power strokes and therefore…
…more force.
Speeding up the contraction, speeds the sliding of the actin filaments,
Making it more difficult for myosin heads to bind,
Resulting in fewer power strokes and less force.
Peak Power Output*
*Note: Muscle Power and Muscle Force are not the same thing
Speed of Contraction
5. Muscle Size
Which muscle fiber, A or B, has the potential to produce the greatest amount of force? Why?
Muscle fiber A
Muscle fiber B
More filaments than B
5. Muscle Size
Which muscle has the potential to produce the greatest amount of force? Why?
Muscle A
Muscle B
More muscle fibers and filaments than A
Muscle B has three fibers compared to one.
More muscle fibers allow for more force production.
5. Muscle Size
Would muscle C produce more or less force than muscle A or muscle B? Why?
Muscle C
Larger fibers with more filaments than B
Muscle C has three large fibers which makes Muscle C larger than Muscle B.
Larger muscle fibers allow for more force production.
Know How Each of These Influence Force Production in a Muscle Fiber
Motor Units
Number and type
Frequency of stimulation
Sarcomere Length
Speed of Contraction
Muscle Size
Sample Question
Which of the following statements about muscle force is TRUE?
Muscle force is not produced during an isometric contraction.
Muscle force is greatest when a muscle contracts from a middle length or a neutral, resting position.
During maximal force production, only FF motor units are activated.
Fast concentric muscle contractions produce more force than slow concentric muscle contractions.
Can you explain why each answer is either true or false?
B
