Skeletal Muscle Mechanics
Dr. Abdelrahman Mustafa
LECTUERER , Physiology
Department Basic Medical Sciences
Division of Physiology
Faculty of Medicine
Almaarefa Colleges
Learning Objecetives
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By the end of this lecture you should be able to
Define the muscle tension
List the factors that affect the muscle tension
Define the muscle twitch
List Phases of Twitch
Define Twitch Summation and Tetanus
Identify Motor Unit Recruitment
Identify Lever Systems
List the types of Contraction
Identify Muscle fatigue
Muscle Mechanics
• Muscle mechanics include
• .
Coordination of skeletal muscle.
• Muscle Tension and Muscle twitch
• Motor Unit Recruitment
Performance of skeletal muscle.
• Lever Systems
• Optimal Length and Types of Contraction
• Muscle Fatigue
Metabolism of skeletal muscle
• Source of energy
• Oxygen Debt
Skeletal Muscle Mechanics
• Muscle consists of groups of muscle fibers
bundled together and attached to bones
• Connective tissue covering muscle divides
muscle internally into bundles
• Connective tissue extends beyond ends of
muscle to form tendons
– Tendons attach muscle to bone
Muscle Tension
• Definition: The pulling force on the tendons that
muscle cells generate when contracting
• Production :within the sarcomere (contractile
component),as a result of cross-bridge cycling
resulting.in sarcomere shortening.
• Transmission: to move the bones, the tension
generated by these contractile component must be
transmitted to the bones via the connective tissue and
tendons.
Connective tissue + tendons = series elastic
components
Muscle Tension
• Factors influencing extent to which tension
can be developed
– Frequency of stimulation
– Length of fiber at onset of contraction
– Extent of fatigue
– Thickness of fiber
Muscle Twitch
Definition :A brief contraction-relaxation response to a
single action potential
Phases of Twitch
1. Latent period before contraction:
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the action potential moves through sarcolemma
causing Ca2+ release
2. Contraction phase:
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calcium ions bind
tension builds to peak
3. Relaxation phase:
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Ca2+ levels fall
active sites are covered
tension falls to resting levels
Muscle Twitch
Twitch Summation and Tetanus
• Single action potential in muscle fiber produces
twitch. Which is very weak & brief.
• Contraction of longer duration and greater
tension can be produces by repeated stimulation
of fiber.
• Two twitches from two action potential add
together , or sum, to produce greater tension in
the fiber.
• Twitch summation is possible because the
duration of action potential is much shorter than
duration of twitch(contraction)
Summation and Tetanus
Twitch Summation and Tetanus
• Tetanus
– Occurs if muscle fiber is stimulated so rapidly that
it does not have a chance to relax between
stimuli,
– Definition: smooth sustained contraction of
maximum strength occurs known as tetanus .
Motor Unit Recruitment
• Definition :the successive activation of the same
and additional motor units with increasing
strength of voluntary muscle contraction.
• Properties
1.Greater the number of fibers contracting greater the total
muscle tension
2.The number of muscle fibers contracting within a
muscle depends on the extent of motor unit
recruitment.
3.The pattern of motor unit recruitment varies, depending
on the inherent properties of specific motor neurons.
Motor Unit Recruitment
– Muscles that produce
precise, delicate
movements contain
fewer fibers per motor
unit e.g. external
muscles of eye
– Muscles performing
powerful, coarsely
controlled movement
have larger number of
fibers per motor unit
e.g. muscles of thigh
One motor neuron
innervates number of
muscle fibers but each
muscle fiber is
innervated by only one
motor neuron
Motor Unit Recruitment
Lever Systems
• A lever is a rigid structure that
can move around a fixed point
called a Fulcrum.
• A lever is acted on at two
different points by two
different forces:
– The Effort is the force due to
muscular contraction (which
causes movement)
– The load or (resistance) is the
weight that is moved or some
resistance an object to being
moved (which opposes
movement).
Types of levers
• There are 3 types of levers that differ on the
positions of the fulcrum, effort, and load.
– First-class levers are not common: the fulcrum is
between the effort and the load.
– Second-class levers are uncommon: the load is
between the fulcrum and the effort.
– Third-class levers are common: the effort is
between the fulcrum and the load.
Types of levers
Types of Contraction
• Two primary types
– Isotonic
• Muscle tension
remains constant as
muscle changes
length
– Isometric
• Muscle is prevented
from shortening
• Tension develops at
constant muscle
length
CONTRACTION
• Isometric contraction: occurs when muscle
contracts without shortening in length.
(iso= same, metric= measure or length)
Isotonic contraction: occurs when muscle
contracts with shortening of length but against a
constant load, thus, the tension on the muscle
remains constant (iso= same, tonic= tension)
Physiologic basis of Isometric & Isotonic contractions:
The same internal events occur in both isotonic and isometric
contractions:
Muscle excitation starts the sliding filament cycling; the cross bridges
start cycling; and filament sliding shortens the sarcomeres, which exert
force on the bone at the site of the muscle’s insertion.
During a given time, a muscle may shift between isotonic & isometric
contractions.
E.g. when you lift a book up it is isotonic contraction and when you
keep holding the book up while reading it is isometric contraction.
NOTE:
Since Work=Distance X Load,
Isotonic contractions do work where as Isometric do not.
MUSCLE FATIGUE
Definition:
Fatigue occurs when prolonged & strong stimulation of an
exercising muscle reaches a stage when the muscle is no
longer able to respond to the stimulation with the same
degree of contractile activity.
• Is of 2 main types:
1. Muscle fatigue: occurs in the muscle & is a defense
mechanism that protects the muscle by preventing it
from reaching a point where no ATP will be available.
2. Central fatigue: more psychological. Occurs when CNS
no longer activates the motor neurons supplying the
muscles. Person stops exercising even though the
muscles can still perform.
MUSCLE FATIGUE
CAUSES:
1. Depletion of Glycogen energy stores.
2. Accumulation of Hydrogen ions from lactic acidinterfere with cross- bridge functions.
3. Intracellular acidosis from lactic acid inhibits
glycolysis enzymes & slows ATP production.
4. NT depletion at the NMJ.
5. Central fatigue- lack of will & sleep.
6. Accumulation of extracellular K+
Q1
• Which is of the following factors DOSENOT
influencing tension:
• A)Frequency of stimulation
• B)Length of fiber at onset of contraction
• C)Extent of fatigue
• D)Type of muscle contraction
Q2
• Which of the following muscles has larger
number of fibers per motor unit
• A)Muscles of the eye
• B)Muscles of thigh
• C)Muscles of the mouth
• D)Muscles of the arm
Q3
• A lever system is:
• A)A rigid structure that can move around a fixed
point
• B) The pulling force on the tendons that muscle
cells generate when contracting
• C) A brief contraction-relaxation response to a
single action potential
• D) the successive activation of the same and
additional motor units with increasing strength of
voluntary muscle contraction.
Q4
• Physiological inability of the muscle to
contract is the definition of :
• A)Muscle tension
• B)Muscle fatigue
• C)Muscle twitch
• D)Muscle tetanus
References
• Human physiology by Lauralee Sherwood, 7th
edition
• Text book physiology by Guyton &Hall,12th
edition
• Text book of physiology by Linda .s
contanzo,third edition
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