MUSCLE PERFORMANCE AGING AND PATHOLOGY OF THE MUSCULAR

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 n Muscular dystrophy – a group of inherited muscle destroying diseases ¨ Affected muscles enlarge with fat and connective tissue ¨ Muscles degenerate n 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 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.