MUSCLE ENERGETICS AND FATIGUE
Summary of Mechanics
Muscles pull; they don’t push
Muscle lengthen by being yanked on by antagonists or gravity
Muscle force can be graded by recruitment of motor units
You activate small motor units first: the size principle
Muscle force can be graded by repetitive stimulation
Muscle force can be graded by changing length, but who cares
Muscle velocity is inversely related to muscle force: you can
be strong or fast but not both at the same time
8 Muscle power peaks at 1/3 maximum force
9 Pinnate muscle fibers develop more force at lower velocity
because of the angle
10 Muscles fatigue: they drop force on continued use
11 Muscles are heterogeneous based on contractile properties
1 Slow twitch (S)
2 Fast fatigue resistant (FR)
3 Fast Intermediate (FI)
4 Fast fatiguable (FF)
1
2
3
4
5
6
7
Summary of contractile mechanisms
1
2
3
4
5
6
7
8
Muscle cells are highly organized
Myofibrils consists of interdigitating hexagonal arrays of filaments
Thick filaments are mainly myosin: A bands
Thin filaments are actin + tropomyosin + TnI + TnC + TnT: I bands
Sliding filaments explains the length-tension curve
Cross-Bridge cycling couples ATP hydrolysis to force or shortening
Myosin isoforms have different turnover numbers
Costameres may transmit force from myofilaments to muscle
exterior through the cytoskeleton
Summary of Excitation-Contraction Coupling
1
2
3
4
5
6
7
8
9
10
11
Contraction begins with neuromuscular transmission
Muscle action potential depolarizes the T-tubule
T-tubule depolarization tickles the DHPR
DHPR tickles RyR1
RyR1 release loads of Ca, but SR is not empty
Ca saturates TnC
TnC-Ca disinhibits Acto-myosin interaction
Acto-myosin cycles the cross bridge
SR re-uptake shuts off contraction
Series-elastic elements explain twitch time course
Prolonged Ca transient with series-elastic elements
explains tetany
ATP is the energy currency of the cell; its free energy of hydrolysis
drives muscle contraction and energy output
Adenosine diphosphate, ADP
NH2
NH2
N
N
O-O
P
O-
OO
P
O
P O
O
N
N
CH2
O
O
H2O
N
N
O
O
-O P
O
OH
-
-
O
O P O
CH2
O
O
N
OH
OH OH
Adenosine triphosphate, ATP
+
O-O P OH
O
Phosphate
N
Acto-myosin cross bridge cycle: ATP hydrolysis is linked to force
generation or shortening
Table 1. Rate and amount of ATP needed for different track events.
Event
Rate of ATP consumption
(mol/min)
Amount of ATP needed (mol)
Rest
0.07
----
100 m sprint
2.6
0.4
800 m run
2.0
3.4
1500 m run
1.7
6
42200 m marathon
1.0
150
Adapted from Hultman, E. and Sjoholm, H. Biochemical causes of fatigue, in “Human Muscle Power”
(1986) Human Kinetics Publishers, Inc, Champaign Illinois.
Muscles are activated by brief trains of impulses followed by rest
periods
Table 2. Rate and amount of ATP available for contraction from various fuel sources
Source of energy
Rate of ATP production
(mol/min)
Amount of ATP available (mol)
ATP and creatine phosphate
4.4
0.7
Glycogen to lactate
2.4
1.6
Muscle glycogen to CO2
1.0
84
Liver glycogen to CO2
0.4
19
Fatty acids to CO2
0.4
4000
Table 3. Muscle Fiber Type Classification Schemes.
Classification Scheme
Muscle Property Used to
Classify Types
Fiber types
Burke
mechanical properties
S, FR, FI, FF
Brooke
myosin ATPase staining
I, IIA, IIB, IIC
Peter
metabolic capacity
SO, FOG, FG
Table 4. Comparison of Different Muscle Types.
Type I Muscle
Type IIa Muscle
Type IIb Muscle
Twitch
Slow
Fast
Fast
Fatigue
Resistant
Resistant
Fatigable
Resistant
Metabolism
Oxidative
Oxidative
Glycolytic
Oxidative
Mitochondria
Cardiac Muscle
+++
++++
+
SR volume
++
+++
++++
+
Glycogen
+
+++
++++
++
Myosin Heavy Chain
MHC-I
MHC-IIa
MHC-IIb, -IIx
MHC-, MHC-
Myosin Light Chain
MLC-1aS, -1bS
MLC-1f, -3f
MLC-1f, -3f
MLC-1v, -1a
SR Ca-ATPase
SERCA2a
SERCA1a
SERCA1a
SERCA2a
Phospholamban
-
-
fast
fast
RyR1
RyR1
RyR1
RyR2
Troponin C
TnC1
TnC2
TnC2
TnC1
Myoglobin
+++
+++
-
+++
-
+
++
-
Calsequestrin
RyR
Parvalbumin
++
++++
fast and cardiac
+
cardiac
Lactic acid levels increases with exercise intensity
Lactic acid is oxidized in mitochondria and carries in cytoplasmic
NADH equivalents
Lactic acid shuttles:
1. to the mitochondria
2. to oxidative fibers
3. to liver
Exercise increases GLUT4 population in muscle membranes and
increases glucose uptake independent of insulin
Muscle
glycogenoses
Signals for muscle hypertrophy
Nuclear domains in muscle
Table 5. The muscle fiber type continuum. Hybrid muscle fibers allow transitional forms intermediate
between Types I, IIa, and IIb. Humans do not make MHCIIb as in experimental animals, so the human
form in fast fatiguable muscles is named MHCIIx.
Muscle Fiber Type
Myosin Heavy Chain Expression
Muscle Fiber Description
Type I Pure Fiber
MHCI
Slow
MHCI>MHCIIa
Hybrid
Type IIa Pure Fiber
MHCIIa>MHCI
MHCIIa
Fast Fatigue Resistant
MHCIIa>MHCIIx
Hybrid
Type IIb Pure Fiber
MHCIIx>MHCIIa
MHCIIx
Fast Fatiguable
Fiber Type Switching