Last edited: 8/24/2021
MULTIPLE MOTOR UNIT SUMMATION
Muscle Mechanics | Multiple Motor Unit Summation | Part 2
OUTLINE
I) GRADED RESPONSE
II) SIZE PRINCIPLE
III) REVIEW QUESTIONS
IV) REFERENCES
PART 2
Medical Editor: Dr. Sofia Suhada M. Uzir
(B) TENSION GRAPH
How strength of stimulus corelates with tension. Figure 2
(1) Subthreshold potential/stimulus
o Will not produce any tension because it doesn’t
reach the threshold stimulus
o Won’t activate net potential
(2) Threshold potential potential/stimulus
I) GRADED RESPONSE
Produce muscle contraction and generate tension
Is affected by:
o Frequency of neural stimuli
o Strength of neural stimuli
 Subthreshold stimulus
 Threshold potential stimulus
 Maximal potential stimulus
(A) POTENTIAL GRAPH
o Generates action potential
o Contracts the muscle
o Is able to generate tension
(3) Maximal tension
o Corresponds to maximal potential stimulus
o Remains the same despite increase in voltage
because has reached the maximal threshold of
potential
o It is the most force the muscle can exert
Dependent on the stimuli to the nerve. Figure 1
(1) Subthreshold potential/stimulus
o Will produce no response
o Does not reach the minimal threshold needed to
produce a contraction
o Example: Shocking the muscle with 10mV while the
minimal threshold is 30mV
(2) Threshold potential/stimulus
o It is the minimum amount of voltage needed to
produce the first observable muscle contraction
o Example: Shocking the muscle with 30mV and a
contraction is seen
(3) Maximal potential/stimulus
o It produces prolonged and powerful muscle
contraction (muscle fibers extremely shortens) that
even with increasing voltage, will still produce the
same amount of contraction
o Increase in voltage will not increase the amount of
contraction
o Example: Shocking the muscle with 1000 mV
produces a very strong contraction, but when
shocking it again with 2000 mV the contraction
doesn’t increase. Hence, 1000 mV is said to be
maximal potential stimulus.
Figure 2. Tension graph [Chegg]
(C) IN THE MUSCLE FASCICLES FIGURE 4
The lesser the motor unit recruitment, the lower the
tension and contractions. Figure 3
o Fine movements
o Example: patting on the shoulders
The greater the motor unit recruitment, the greater the
actual tension and muscle contractions
o More recruitment of muscle fibers
o More force and tension
o Being able to carry out more profound reaction
o Example: slapping on the shoulders
Remember:
o Motor unit is motor neuron and all the muscle fibers
that are being supplied by it
o The more the voltage (within the maximal
potential/stimuli), the more the tension will be, hence
the more muscle fibers will be activated
Figure 3. Fibers in the fascicle [Chegg]
Figure 1. Stimulus voltage graph [Chegg]
Multiple Motor Unit Summation Part 2
MUSCULOSKELETAL: Note #0
1 of 3
Figure 4. Structure of a skeletal muscle [Wikipedia]
II) SIZE PRINCIPLE
(1) First order of recruitment
o Recruits smallest muscle fiber
 Most excitable motor neurons
 Very low threshold potential, hence reach the
threshold and contracts quicker
o Not maximal amount of tension will be produced
o Generate limited amount of force → causes very
weak contraction
o Example:
 Fine movements
• Tapping on the shoulder
(2) Second order of recruitment
o Recruits medium size muscle fibers
o Threshold potential is moderate
 Will reach the threshold later than the first order of
recruitment
o Produce moderate tension
 Still under the maximal tension
o Generate moderate amount of force → causes
moderate amount of contraction
o Example:
 Slapping the shoulder
(3) Third order of recruitment
o Recruits largest muscle fibers
 Threshold potential is higher
 Least excitable neurons
 Will reach the threshold later than the first and
second order of recruitment
o Maximal amount of tension generated → causes
highest amount of contraction
o Example:
 Slapping the shoulder using full force and the
whole length of arm
 Lifting extremely heavy weights
(B) MULTIPLE MOTOR UNIT SUMMATION
Also known as recruitment
Summation of all the activities together producing a
powerful contraction
In other words, it is the combined effect of multiple motor
units acting within a muscle at any given time
It is affected by the strength of the neural stimuli
o A weak stimulus causes lower tension than a
strong stimulus which causes higher tension
(1) Low tension, recruits smallest muscle fibers first
o Low threshold
o Produces weak contraction
(2) Moderate tension, recruits medium size muscle
fibers
o Moderate threshold potential
o Produces moderate contractile force
(3) Highest tension, recruits largest muscle fibers
o Highest threshold potential
o Produces highest contractile force
2 of 3
MUSCULOSKELETAL: Note #0
Multiple Motor Unit Summation Part 2
III) REVIEW QUESTIONS
‘Motor unit’ refers to
a. A single motor neuron and all the muscle fibers it
innervates
b. A single muscle fiber and all the motor neurons that
innervate it
c. All the motor neurons supplying a single muscle
d. A pair of antagonist muscles
All of the following are involved in producing graded
contractions of an entire muscle EXCEPT
a. Small motor units (those with fewer muscle fibers,
e.g., 5) are utilized for fine movements whilst large
motor units (those with more muscle fibers, e.g.,
600) are utilized for gross movements
b. Discharging many motor units at one time (spatial
recruitment) allows increased force to be generated
c. Discharging individual motor units at a more rapid
rate (temporal recruitment) allows increased force to
be generated
d. Contraction of only a portion of the muscle fibers
within a single motor unit during a finely graded
movement
Which of these statements is correct regarding
muscle contraction?
a. All motor units act together.
b. Muscle contraction continues for long periods after
nervous stimulation ceases.
c. The crossbridge bind to the actin and shorten the
sarcomeres.
d. Dystrophin is not needed to strengthen the
contracting muscle cell.
One of the following statements about muscular
responses is not true. Choose that one.
a. A muscle fiber contracts in an all-or-none fashion.
b. There is a slight latent period that occurs between
when the stimulus arrives at the muscle and when
the muscle contracts.
c. Muscles will add motor units to a contraction,
increasing the overall force of contraction.
d. When a person is fully at rest, none of her muscles
are contracting.
Contractions called _____ occur whenever the
forces applied to a muscle are increased, but the
muscle does not appear to be moving.
a. Isotonic
b. Isometric
c. Tetanic
d. Summation contractions
The constant contraction of a percentage of fibers
within a muscle is referred to as _____.
a. tetany
b. tonus
c. sustained contraction
d. summation
Summation of frequent muscle twitches to give
maximum contraction is called:
a. Motor unit summation
b. Twitch
c. Facilitation
d. Tetanus
CHECK YOUR ANSWERS
IV) REFERENCES
● One Learning Centre [Quiz]
http://novella.mhhe.com/sites/0070272468/student_view0/chapter9/
multiple_choice_quiz.html
● Christina N. Muscle Tissue. Chegg [Digital image]
https://www.chegg.com/flashcards/muscle-tissue-bb18d559-4a8a495e-be54-e409493ef8a0/deck
● Wikipedia. Muscle Fascicle. [Digital image]
https://en.wikipedia.org/wiki/Muscle_fascicle
● Sabatine MS. Pocket Medicine: the Massachusetts General
Hospital Handbook of Internal Medicine. Philadelphia: Wolters
Kluwer; 2020.
● Le T. First Aid for the USMLE Step 1 2020. 30th anniversary
edition: McGraw Hill; 2020.
● Jameson JL, Fauci AS, Kasper DL, Hauser SL, Longo DL,
Loscalzo J. Harrison's Principles of Internal Medicine, Twentieth
Edition (Vol.1 & Vol.2). McGraw-Hill Education / Medical; 2018
● Marieb EN, Hoehn K. Anatomy & Physiology. Hoboken, NJ:
Pearson; 2020.
● Boron WF, Boulpaep EL. Medical Physiology.; 2017.
Guyton and Hall Textbook of Medical Physiology. Philadelphia, PA:
Elsevier; 2021.
Which description of muscle contraction means that
all of the fibers within a muscle are fully contracted?
a. All-or-none law
b. Summation
c. Tetanic
d. Muscle twitching
Which of the following statements concerning
isometric contractions is true?
a. The length of the muscle changes.
b. Muscle tension decreases.
c. Joint movements are swift.
d. Muscle length remains constant.
A fascicle is a...
a. Muscle
b. Bundle of muscle fibers enclosed by a connective
sheath
c. Bundle of myofibrils
d. Group of myofilaments
Multiple Motor Unit Summation Part 2
MUSCULOSKELETAL: Note #0
3 of 3