SPORTS SCIENCE EXCHANGE MUSCLE ADAPTIONS TO AEROBIC TRAINING SSE

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SPORTS SCIENCE EXCHANGE MUSCLE ADAPTIONS TO
AEROBIC TRAINING SSE54-Volume 8 (1995) Number 1

• Ronald L. Terjung, Ph.D.Professor, Department of
  Physiology State University of New York Health
  Science Center Syracuse Syracuse, New
  YorkMember, GSSI Sports Medicine Review Board

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Figure 1. Time-course training/detraining
adaptations in mitochondrial content of skeletal
muscle. Note that about 50 of the increase of
mitochondrial content was lost after one unit,
i.e., 1 week, of detraining (a) and that all of
the adaptation was lost after five units of
detraining. Also, it took four units, i.e., 4
weeks, of training (b) to regain the adaptation
lost in the first week of detraining. Adapted
from booth (1977).
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Figure 2. Influences of exercise bout duration on
muscle adaptation. For a practical perspective,
one might assume that training program a was
conducted at an intensity of 40 of VO2 max, b at
50 VO2 max, c at 70 VO2 max, d at 85 VO2 max
and e at 100 VO2 max. Adapted from Dudley et
al., (1982).
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Figure 3. Influence of exercise bout intensity on
training-induced adaptations in muscle
mitochondrial content. As the training bouts
become more intense, more of the low oxidative
(type IIb) fibers are recruited and become
adapted to the training. Adapted from Dudley et
al. (1982).
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• KEY POINTS
• Muscle adapts to aerobic exercise training to
  become a more effective energy provider. An
  improved capacity for oxygen extraction from the
  blood supply and an altered cellular control of
  energy metabolism likely contribute to the
  improved muscle performance evident with
  training. Of course, performance is also enhanced
  by improvements in maximal cardiac output and
  other adaptations not related to biochemical
  changes in the muscles.
• 2. Training adaptations are induced specifically
  in the muscles actively used in the exercise
  these adaptations are sustained by continued
  activity and lost following inactivity. Both
  intensity and duration of exercise training
  sessions are important factors influencing muscle
  adaptations.
• 3. Although the development of optimal muscle
  adaptations is expected to enhance performance in
  competitive sport, meaningful adaptations
  developed in non-athletic populations by routine
  physical activity may also be important in
  promoting healthier living.

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SUMMARY While the adaptations to an endurance
type of training are very complex and
multifaceted, changes within the active muscles
are probably fundamental to the metabolic and
functional alterations that support the enhanced
endurance performance observed after training.
The adaptations that involve remodeling of the
muscle (e.g., enhanced mitochondrial content and
increased capillarity) are influenced by the
duration and intensity of daily exercise, require
an extended training period to achieve a
steady-state adaptation, and are lost with
inactivity.
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FAT METABOLISM DURING EXERCISE SSE59, Volume 8
(1995), Number 6

• Edward F. Coyle, Ph.D.Professor, Department of
  Kinesiology and Health EducationThe University
  of Texas at AustinAustin,Texas

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Key Points

• People store large amounts of body fat in the
  form of triglycerides within fat (adipose) tissue
  as well as within muscle fibers (intramuscular
  triglycerides).When compared to carbohydrate
  stored as muscle glycogen, these fat stores are
  mobilized and oxidized at relatively slow rates
  during exercise.
• As exercise progresses from low to moderate
  intensity, e.g., 25-65 VO2max, the rate of fatty
  acid mobilization from adipose tissue into blood
  plasma declines, whereas the rate of total fat
  oxidation increases due to a relatively large use
  of intramuscular triglycerides. Intramuscular
  triglycerides also account for the characteristic
  increase in fat oxidation as a result of habitual
  endurance-training programs.
• Dietary carbohydrate intake has a large influence
  on fat mobilization and oxidation during
  exercise when dietary carbohydrate produces
  sufficient carbohydrate reserves in the body,
  carbohydrate becomes the preferred fuel during
  exercise. This is especially important during
  intense exercise because only carbohydrate(not
  fat) can be mobilized and oxidized rapidly enough
  to meet the energy requirements for intense
  muscular contractions.

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Medium Chain Triglycerides

• Medium-chain triglycerides (MCTs) are fats with
  an unusual chemical structure that allows the
  body to digest them easily. Most fats are broken
  down in the intestine and remade into a special
  form that can be transported in the blood. But
  MCTs are absorbed intact and taken to the liver,
  where they are used directly for energy. In this
  sense, they are processed very similarly to
  carbohydrates.
• MCTs are different enough from other fats that
  they can be used as fat substitutes by people
  (especially those with AIDS), who need calories
  but are unable to absorb or metabolize normal
  fats.
• MCTs are also popular among athletes as a
  proposed performance enhancer, although there is
  little evidence as yet that they really work.

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Training Effects
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Carb Intake
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Summary

• People store large amounts of body fat in the
  form of triglyceride within adipose tissue as
  well as within muscle fibers. These stores must
  be mobilized into FFA and transported to muscle
  mitochondria for oxidation during exercise. Fatty
  acids from adipose tissue are mobilized into
  plasma and carried by albumin to muscle for
  oxidation.
• As exercise intensity increases from low (25
  VO2max) to moderate (65 VO2max) to high (85
  VO2max), plasma FFA mobilization declines.
  However, total fat oxidation increases when
  intensity increases from 25 to 65 VO2max, due
  to oxidation of intramuscular triglycerides,
  which provide about one-half of the fat for
  oxidation.
• Endurance training characteristically increases
  fat oxidation during moderate intensity exercise
  by accelerating the oxidation of intramuscular
  triglyceride without increasing the mobilization
  or oxidation of plasma FFA.

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Summary

• Similarly, during low-intensity exercise with
  little intramuscular triglyceride oxidation, the
  increased fat oxidation of trained people does
  not appear to be caused by increased mobilization
  of FFA into plasma, but rather by a greater rate
  of oxidation of the FFA removed from the blood
  during exercise. Therefore, it seems that
  untrained people have greater abilities to
  mobilize FFA than they do to oxidize it when they
  exercise in the fasted state.
• Carbohydrate ingestion during the hours before
  exercise, even in relatively small amounts,
  reduces fat oxidation during exercise largely
  through the action of insulin.
• Fat supplementation and special diets have
  limited ability to increase fat oxidation in
  people, especially during sport competitions.
  Therefore, fat from body stores and/or dietary
  supplementation cannot adequately replace muscle
  glycogen and blood glucose as fuels for intense
  exercise.

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