KINE 3301
Biomechanics of Human Movement
Muscle Excitation – Contraction
Coupling
Chapter 16
A single muscle fiber contains hundreds of
myofilaments (Actin & Myosin) which are bundled
together and surrounded by the muscle membrane
(sarcolemma).
A muscle fiber has a
diameter of 10 – 60
μm and a length of
10 – 300 mm.
• Troponin and tropomyosin are regulatory proteins.
• Under resting conditions they a crossbridge from forming
between actin and myosin.
• In the presence of Ca++ stops inhibiting muscular contraction by
rotating away from it’s resting position where it covers the actin
binding site.
Sarcomere: The Functional Unit of the Muscle Cell
Concentric Cross-bridge Cycle
• Ca++ released from SR
• Ca++ binds with troponin
• Tropomyosin moves away
from binding site.
• An Actin-Myosin crossbridge is formed.
• ATP downgraded to ADP +
Pi.
• Myosin does mechanical
work on Actin, Myosin arm
rotates shortening the
muscle fiber.
Eccentric Cross-bridge Cycle
• Ca++ released from SR
• Ca++ binds with troponin
• Tropomyosin moves away from binding
site.
• An Actin-Myosin cross-bridge is formed.
• ATP downgraded to ADP + Pi.
• Myosin attempts to rotate and shorten
fiber.
• External force causes the fiber to
lengthen, storing elastic energy in myosin
arm.
• The cross-bridge is broken (1-4 pN) while it
is still in the actively charged state, it
immediately forms another cross-bridge at
the next available Actin site without the
need for any additional ATP.
• This “break & make” cross-bridges will
continue until the length of the active
state is exceeded.
Isometric Cross-bridge Cycle
•
•
•
•
•
•
•
Ca++ released from SR
Ca++ binds with troponin
Tropomyosin moves away from binding site.
An Actin-Myosin cross-bridge is formed.
ATP downgraded to ADP + Pi.
Myosin attempts to rotate and shorten fiber.
External force causes the fiber to lengthen,
storing elastic energy in myosin arm.
• From this lengthened position the myosin arm
rotates and shortens the fiber.
Summary of Actin-Myosin Crossbridge Cycle
Contraction Energy
Type
Utilized
Force
Change in
Effect of Velocity on Force
Generated Fiber Length
Concentric
1 ATP
1 – 3 pN
Eccentric
1 ATP
n × 1-4 pN, Lengthened
n is # of
~5-20 nm
make &
break A-M
bonds
Force increases with increasing velocity:
3 pN for slow velocity
4 pN for fast velocity
Isometric
1 ATP
3 pN
none
Shortened
~5 nm
Relatively
Constant
Force decreases with increasing velocity:
1 pN for fast velocity
3 pN for slow velocity