Transcript easier - hcopgnt

Skeletal Muscle Structure and Contractile
Properties
The purpose of this chapter is to
describe muscle from the perspective
of its primary purpose as a motor.
Training has negligible
effects on the number of
fibers.
Basement
membrane is freely
permeable to
metabolites.
Plasma membrane: true cell boundary that
maintains acid-base balance.
Connective
tissue plays a
vital role in
transmitting the
force generated
via actinmyosin to the
tendon.
Titin connects myosin to the Z
disk and transfers the force
longitudinally down the fiber.
MHC
Isoforms
in order
of
ATPase
activity:
1. I
2. IIa
3. IIx
4. IIb
Myosin Heavy
Chain (MHC):
molecular weight
200 kilodaltons
Myosin Light
Chain: 16-28
kilodaltons
slow
% hybrid fibers
med
fast
Concentric Actin-Myosin Cycle
Eccentric Actin-Myosin
Cycle
Isometric Actin-Myosin Cycle
The capillary to
fiber ratio is the
most accurate
method of
quantifying
perfusion.
The capillary-tofiber ratio has been
shown to increase
5-20% in humans
subjected to 8-12
wks of endurance
training.
Each sarcomere is capable of generating a fixed amount
of force, called specific force. Specific force is approx.
22-28 N/cm2 of muscle fiber cross-sectional area.
When sarcomeres are arranged in series
the velocities become additive and the
overall velocity of shortening measured
at the tendons increases
Short length with high
cross-sectional area
yields high force.
Muscles with long fiber lengths
generate greater velocity and move
segment thru greater distance.
Optimal
length is
defined as
the length
with the
most
overlap of
actin &
myosin.
Relationship Between Total Muscle Force and Length
Total tension = Passive + Active
Passive muscle
tension (PE)
Active muscle
tension
developed by
sarcomeres (CE)
Force – Length & Muscle Architecture
The Force-Length relationship is effected by the muscle/tendon architecture and the changes in the moment arm.
Factors That Effect The Torque About A Joint
Weight easier to
lift
1. T = F * Perpendicular Distance.
2. Perpendicular Distance Changes.
3. Muscle Length-Tension Changes.
4. Distance and Tension are not optimal
at the same angle.
5. Each synergist has its own lengthtension, perpendicular distance curve.
Weight harder
to lift
Fiber type determines time to peak tension (TPT). A muscle
with a high % of IIb, IIa, or IIx will have a TPT of 12-15 ms,
whereas muscle with high % of type I will have a TPT of 5070 ms.
Fused tetanus, often called maximum isometric force or (Po) is
4-5 fold greater than twitch force (Pt). Fast twitch muscles
fuse at 150-200 Hz stimulation and slow twitch muscles fuse at
80-100 Hz stimulation.
A fast twitch
muscle has a
force-frequency
curve that is
shifted to the
right of a slow
twitch muscle.
This apparatus can be used to generate the force-velocity curve
of an isolated muscle.
Explosive
training raises
the forcevelocity curve
Force – Velocity
Relationship


Break & Make more bonds (n*3-4 pN)
Exceeds min time to
form A-M bond,
slipping occurs
A-M bonds turnover
quicker, with less
force (1-2 pN)



Eccentric force is greater than isometric.
Isometric force is greater than concentric.
Concentrically – an increase in velocity
results in a decrease in force, due to
reduced force per A-M bond (1-2 pN /
bond).
Concentric: As the velocity of shortening
increases the Xbridges turnover quicker,
resulting in fewer Xbridges attached. This
shortened attachment times reduces the
force transmitted by the Xbridge.
Eventually the rate of shortening equals the
rate of myosin arm rotation and the
Xbridges add little, nothing or actually
decrease the fiber speed [Merry-Go-Round
Effect].
Eccentrically – an increase in velocity
results in an increase in force, due to
breaking and making more A-M Bonds (n *
3-4 pN / bond).
At high velocity of stretch the minimum
time to form an A-M bond is exceeded and
the force declines, due to A-M bonds
slipping.
Isometric, Concentric, Eccentric & StretchShorten Contractions
Notice that the force goes down in the
concentric contraction and up in the
eccentric contraction, when compared to
the isometric contraction.
a) Isometric followed by concentric
contraction. The area under c of the ForceLength graph represents the work done in the
concentric phase.
Concentric
velocity is
the same.
b) Eccentric followed by
concentric contraction
(Stretch-Shorten Cycle).
During the eccentric
contraction energy is stored in
the muscle-tendon which
results in a more powerful
concentric contraction.
Compare the area under the c
portion of the Force-Length
graph. This additional work
represents the Force
Potentiation due to the SSC.
Apparatus Used By Bigland-Ritchie to Compare Energy Cost of
Eccentric vs Concentric Exercise From Bigland-Ritchie #1656
Subject pedals backward and
does eccentric exercise.
Subject pedals forward and
does concentric exercise.
Eccentric exercise
is metabolically
more efficient
than concentric
exercise. When
working at the
same work load
subjects burn
fewer calories.
Efficiency of Eccentric and Concentric Exercise
O2 uptake ( O2) for 1 subject
during final 2 min of baseline
exercise at 15 W during
heavy-intensity (HC, 330 W),
moderate-intensity (MC,
216 W), and low-intensity
(LC, 70 W) concentric
exercise, and during highintensity eccentric exercise
(HE, 330 W) for 6 min.
Breath-by-breath data have
been interpolated to 1-s
intervals and averaged across
3-4 repetitions for each
exercise condition. [From
Perrey #2304, J Appl Physiol
91 (2001)]
Evolution of integrated
electromyogram (iEMG) as a
function of time during MC, HC,
and HE exercise for the rectus
femoris (A) and vastus medialis (B)
muscles. Values are means ± SE for
6 subjects obtained in 2 repetitions
of each test normalized to the
individual values at 1 min under that
test condition. *Different from
minutes 1, 2, and 3, P < 0.05.
Muscle Tissue Damage Following Eccentric Exercise

Muscle membrane is damaged
 Z lines are torn apart.
Z Disk Streaming Following
Eccentric Exercise
Normal striation, Z disks are
perpendicular to myofibrillar
axis.
Streaming and smearing of
the Z disks following
eccentric exercise.