7 muscular-force (1)

note

1. objectives
•ToDistinguish types of contraction: isometric, isotonic
concentric, and eccentric contraction
•To define relation between velocity of contraction and
Load (Hill equation)
•To know the factors contribute to the strength and
maximum duration of a muscle contraction
•To define Fenneffect
•To know Energy mechanism for Muscle Contraction
•To define work and efficiency of muscle contraction
•To say Motor fiber types and define motor unit

2. Isometric Versus Isotonic Contraction.
A muscle contraction at constant length is termed isometric. Force is measured in
Newton (N). In muscles, the traditional expression for force is stress or tension in
N per cross-sectional area of the muscle (N m
-2
), which is actually pressure (Pascal,
Pa).

3. Isometric Contraction. Isotonic Contraction.
Isometric contraction: Length of muscle fibers remain constant.
Velocity of muscle shortening is zero.
Isotonic Contraction: Force of contraction remains constant during
contraction.
Velocityof muscle shortening is decreasing as load increases.
https://www.youtube.com/watch?v=T3OiOJ6-x34

4. Hill Equation: Force-Velocity
Askeletalmusclecontractsextremelyrapidlywhenitcontracts
againstnoload—toastateoffullcontractioninabout0.1second
fortheaveragemuscle.Whenloadsareapplied,thevelocityof
contractionbecomesprogressivelyless
Velocity
of
shortening
Contractionat thispointis
isometric
when the load increased to the maximum force(Fo)that the muscle can exert, the
velocity of contraction becomes zero and no contraction results, despite activation of
the muscle fiber

5. Vmax= b Fo/ a
Fo: number of active cross-bridges and Ca amount
Vmax: Cross-bridge cycle rate
V
maxpoint has been shown to be closely correlated with the
actin-myosin ATPase activity of the muscle and is thought to
indicate the maximum possible rate of interaction between thick
and thin filaments within the sarcomere
latency
Vmax

6. An increase in which of the followings(with the othersheld
constant) will result in an increase in the Vmax?
a. PreLoad
b. Contractility
c. Recruitment of additional motor units
d. Numberof activeCross-bridges
e. Cross-bridgecyclerate
http://flylib.com/books/en/4.5.1.6/1/
isolated cardiac muscle;shows the
effect of norepinephrine

7. As load increases, the slower the velocity and shorter the duration of
contraction.
Recruitment of additional motor units increases velocity and
duration of contraction.
Adaptations to Exercise
Aerobicexercise promotes an
increase in capillary penetration,
the number of mitochondria, and
increased synthesis of myoglobin,
leading to more efficient
metabolism, but no hypertrophy.
Resistance exercise, such as
weight lifting or isometric exercise,
promotes an increase in the
number of mitochondria,
myofilaments and myofibrils, and
glycogen storage, leading to
hypertrophied cells.

8. Muscle power is the product of its force and velocity. At Fo(V = 0)
and Vo (F = 0), the power is zero, but at every other Fi and Vi, there is
a positive power output. The power curve peaks at about 0.25 Fo.
MusclePower
Muscle power(or work rate) equals the product of muscle force and shortening
velocity
Power (W) = Force (N) * Velocity (m s
-1
).
https://www.youtube.com/watch?v=ddMnSGy
yPX0

9. Work Output during muscle contraction
When a muscle contracts against a load, it performs work. This
means that energy is transferred from the muscle to the external
load to lift an object to a greater height or to overcome resistance to
movement.
W=Load x Distance
A muscle’s power output is the product of its force and velocity:
P=Force x Velocity (distance/time)
P=Fx V= N x m/s=J/s = Watt (The power generated by
contraction)
Ex: The maximum poweroutput(work rate)of human muscles is
reached at a contraction velocity of 2.5 m s
-1
. If the force is 300 kN,
what is the maximal work-rate?
The maximal poweroutputis thus (300 kN*2.5 m s
-1
) = 750 kW per
square meter of cross sectional area.

10. At the onset of and during muscle contraction, energy is supplied by
the hydrolysis of ATP, with one mole of ATP generating approximately
48 kJ of energy.
However, only about 40%–50% of this chemical energy is converted
to mechanical energy (work done), with the remainder dissipated as
heat, the latter specifically referred to as initial heat.
ChemicalEnergy(E)=Q+W
Efficiency=mecanicalwork/freeenergy(energyinput)=W/W+Q
Efficiencyof contraction

11. Heatin muscle
•Resting heat
•Heat liberated during contraction (Initial heat)
–Activation heat: energy required for excitation-
contraction coupling
–Shortening heat (Fenn)
–Maintenance heat
–Relaxation heat
•Recovery heat

12. The following factors contribute to the strength and maximum
duration of a muscle contraction:
1.Type of muscle fiber
2.Frequency of stimuli
3.Strength of stimulus
4.Initiallengthof muscle fiber (length-tensionrelation)
5.Muscle fatigue.
6.Type of contraction

13. Characteristics of Isometric Twitch
Phasesofamusclecontraction
1.Thelatentperiodisthetimerequired
forthereleaseofCa
2+
.
2.Thecontractionperiodrepresentsthe
timeduringactualmusclecontraction.
3.Therelaxationperiodisthetime
duringwhichCa
2+
arereturnedtothe
sarcoplasmicreticulumbyactive
transport.
4.Therefractoryperiodisthetime
immediatelyfollowingastimulus.This
isthetimeperiodwhenamuscleis
contractingandthereforewillnot
respondtoasecondstimulus.Since
thisisoccurringatthesametimeas
thecontraction,itdoesnotappearon
themyogramasaseparateevent.

14. 1. Type of muscle fiber
Characteristics of Isometric Twitches Recorded from
Different Muscles.
isometric contractions of three types of skeletal muscle: an ocular muscle, which has a
duration of isometric contraction of less than 1/40 second; the gastrocnemius muscle,
which has a duration of contraction of about 1/15 second; and the soleus muscle, which
has a duration of contraction of about 1/3 second.
the energetics of muscle contraction
vary considerably from one muscle to
another. Therefore, the mechanical
characteristics of muscle contraction
differ among muscles.

15. 2. Frequencyof Stimuli
Staircaseeffect, Frequency(wave) summation, Tetanus
•Staircaseeffect(treppe)is
producedifeachsuccessive
stimulusoccursafterthe
relaxationperiodofthe
previousstimulus.
•Wave(temporal)summation
occursifconsecutivestimuli
areappliedduringthe
relaxationperiodofeach
preceding muscle
contraction.
•Incompletetetanus,occurs
whenthefrequencyof
stimuliincreases.
•Completetetanus,occurs
whenthefrequencyof
stimuliincreasesstillfurther.

16. 3. Strength of stimulus
•Stimulationofmoremotorunits
leadstomoreforcefulmuscle
contraction.
•Sizeprinciple:Henneman’ssize
principlestatesthat,inorderto
moveaload,motorunitsare
recruitedfromsmallestto
largest

17. Mechanics of Skeletal Muscle Contraction
Muscle Contractions of Different Force—Force Summation.
•by increasing the number of motor units contracting
simultaneously, which is called multiple fiber summation
•by increasing the frequency of contraction, which is called
frequency summation and can lead to tetanization.

18. 4. Length of muscle fiber contraction
Tension depends upon the degree of overlap between thick and thin filaments.
Resting length
•Sarcomere length 2 to 2.2 μm
•Maximum myosin & actin
overlap is possible
•Maximum tension is produced
Sarcomere length > or < than
resting length
•Actin & myosin overlap not
optimal
•Less tension produced
http://michaeldmann.net/mann14.html
Musclelength(normalisedtoinitiallength, Lo)

19. 5. Muscle Fatigue
Causes:
In muscle
•Lack of nutrients and glycogen
•Lack of oxygen
•Accumulation of lactic acid
•Conduction failure along ‘T’ tubules -
blockage of Ca++release for
sarcoplasmic cistern
In Neuromuscular junction
•Depletion of Acetyl choline
In CNS
•CNS cannot send excitatory signals to
the contracting muscles
•Generally psychological
•Fatigue reverses by taking rest

20. The power of striated muscle can be calculated
✓if we multiply the exerted force with the actual velocity of the
contraction.
if we multiply the velocity of the contraction with the length of the
contraction.
if we multiply the mass of the muscle with the average velocity of the
contractions.
if we multiply the exerted force with the actual time of the
contraction.
The striated muscle utilizes the chemical energy
with less than 5% efficiency.
with 10-20% efficiency.
with 20-40% efficiency.
✓with more than 50% efficiency.

21. Effect of Muscle Length on Force of Contraction in the
Whole Intact Muscle.