MUSCLE PERFORMANCE,
AGING, AND PATHOLOGY
OF THE MUSCULAR
SYSTEM
Increased use: strength training
Early gains in strength appear to be predominantly
due to neural factors…optimizing recruitment
patterns.
Long term gains almost solely the result of
hypertrophy i.e. increased size.
Threshold stimulus:
The minimum
amount of stimulus required for a muscle
contraction to occur
ATP
Aerobic Cellular Respiration
• Organisms take in oxygen (aerobic) and breakdown
glucose to make ATP, Carbon dioxide, and water
• Extra energy is stored in the form of glycogen in the liver
and muscles
• Respiration occurs in the mitochondria, where glucose is
broken down into ATP
Creatine
• Primary function of ATP is to transfer energy from one location to
another rather than long term storage of energy
• At rest, skeletal fibers produce more ATP than it needs. Under
these conditions, ATP transfers the extra energy to creatine.
Creatine + ATP
Creatine phosphate + ADP
Creatine
• During a contraction, each myosin head breaks down ATP
to make a phosphate group and ADP
• The energy stored in creatine phosphate “recharges” ADP,
converting it back to ATP:
Creatine phosphate + ADP
Creatine + ATP
Muscle Fatigue
• An active muscle is fatigued when it can no longer
continue to perform at the required level of activity
• If ATP demands can be met through aerobic metabolism,
fatigue will not occur until glycogen reserves are depleted
• This fatigue affects the muscles of long distance athletes
after hours of exertion
Muscle Fatigue
• Athletes who run sprints require immediate intense bursts
of energy.
• Majority of ATP provided for this is done through
glycolysis (converts glycogen into energy)
• Energy demands are high, and oxygen cannot diffuse into
muscle fiber fast enough to produce ATP
Recovery Period
• If contraction occurs at peak levels, lactic acid is
generated (anaerobic respiration)
• During recovery, muscles fiber conditions return to normal
levels
• Recovery can last anywhere from several hours to a
week, depending on the activity level
Oxygen Debt
• During recovery, the body’s oxygen demands remain
above normal resting levels
• The amount of oxygen required to restore the body to its
normal, pre-exertion conditions is called oxygen debt
• While oxygen debt is being repaid, breathing rate and
depth are increased, causing you to breathe heavily long
after exercise has ceased
Muscle Performance
• Two factors affect muscle performance:
• (1) types of muscle fibers in the muscle
• (2) physical conditioning or training
Skeletal Muscle Fibers
• Three main types:
• (1) fast fibers
• (2) slow fibers
• (3) intermediate fibers
Fast Fibers (White Muscles)
• Most skeletal muscles are fast
• Called fast due to the ability of muscle to
contract in 0.01 sec or less after stimulation
• Large diameter, densely packed myofibrils,
large glycogen reserves, few mitochondria
• Produce powerful contractions, but fatigue
quickly
• Comparable to “white meat” in chickens; breast
Slow/Red Fibers
• Half the diameter of fast fibers and require 3 times as long
•
•
•
•
to contract
Specialized to continue contracting for extended periods
More vascular and higher energy supply as a result
Contain myoglobin – red pigment that binds oxygen
molecules
Equivalent to “dark meat” – thighs in chicken
Performance (% of peak)
Performance Declines with Aging
--despite maintenance of physical activity
100
80
60
40
Shotput/Discus
Marathon
Basketball (rebounds/game)
20
0
10
20
30
40
50
60
Age (years)
D.H. Moore (1975) Nature 253:264-265.
NBA Register, 1992-1993 Edition
Number of motor units declines during aging
- extensor digitorum brevis muscle of humans
AGE-ASSOCIATED
ATROPHY DUE TO BOTH…
Individual fiber atrophy
(which may be at least
partially preventable and
reversible through exercise).
Loss of fibers
(which as yet appears
irreversible).
Campbell et al., (1973) J Neurol Neurosurg Psych 36:74-182.
Motor unit remodeling with aging
Central
nervous
system
•
•
Fewer motor units
More fibers/motor unit
AGING
Motor
neuron
loss
Muscle
Muscle injury may play a role in the development of
atrophy with aging.
• Muscles in old animals are more susceptible to contractioninduced injury than those in young or adult animals.
•
Muscles in old animals show delayed and impaired recovery
following contraction-induced injury.
•
Following severe injury, muscles in old animals display
prolonged, possibly irreversible, structural and functional
deficits.
Rigor Mortis
A few hours after a person or animal dies, the joints
of the body stiffen and become locked in place. This
stiffening is called rigor mortis. Depending on
temperature and other conditions, rigor mortis lasts
approximately 72 hours. The phenomenon is caused
by the skeletal muscles partially contracting. The
muscles are unable to relax, so the joints become
fixed in place.
Disorders of Muscle Tissue

Muscular dystrophy – a group of
inherited muscle destroying diseases
 Affected
muscles enlarge with fat and
connective tissue
 Muscles degenerate

Types of muscular dystrophy
 Duchenne muscular dystrophy
 Myotonic dystrophy
Duchenne’s
Muscular
Dystrophy
• Predominantly affects
males, because it is a sexlinked trait
Myasthenis Gravis
• A progressive muscular
paralysis that occurs due to the
loss of Ach receptors.
Autoimmune disorder where the
body attacks its own Ach
receptors. Genetic factors play
a role in predisposing individuals
to this condition.
Tetanus
Tetanus causes acetylcholinosterase to not break down the acetylcholine in
the synapse. This results in a person's muscles contracting and not relaxing.
A tetanus shot
must be
administered
shortly after
exposure to
the bacteria.
Once you develop
tetanus, there is no
cure.
Botulism
• Results from the consumption
of contaminated canned or
smoked foods that contain a
toxin. The toxin, produced by
bacteria, prevents the release
of Ach, leading to potentially
fatal muscular paralysis.