Exercise in Hypobaric, Hyperbaric, and Microgravity Environments

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• Exercise in Hypobaric, Hyperbaric, and
  Microgravity Environments

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Definitions

• Hypobaric - low atmospheric pressure
• Altitude
• Hyperbaric - high atmospheric pressure
• Underwater
• Microgravity - low gravitational force
• Space

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Hypobaric Environment

• Altitudes of 1,500 m (4,921 ft) or more have a
  notable physiological impact on the human body.

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Hypobaric Environment

• Percentages of the gases in the air we breathe
  remain constant regardless of altitude.
• However, partial pressures of each of these gases
  varies with atmospheric pressure.

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Hypobaric Environment

• Reduced partial pressure of O2 leads to decreased
  performance at altitude, due to a reduced
  pressure gradient that hinders oxygen transport
  to the tissues.

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Hypobaric Environment

• Air temperature drops as altitude increases.
• Cold air can hold little water, so the air at
  altitude is dry.
• These two factors increase your susceptibility to
  cold-related disorders and dehydration when at
  altitude.

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Hypobaric Environment

• Because the atmosphere is thinner and drier at
  altitude, solar radiation is more intense at
  higher elevations.
• You also ventilate greater volumes of air at
  altitude because air is less dense.

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Hypobaric Environment

• Altitude training causes an increase in red blood
  cell count which increases blood oxygen-carrying
  capacity.

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Hypobaric Environment

• Total muscle mass decreases when at altitude, as
  does total body weight.
• Part of this is from dehydration and appetite
  suppression, which leads to protein breakdown in
  the muscles.

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Hypobaric Environment

• Other muscle adaptations include decreased fiber
  area, increased capillary supply and decreased
  metabolic enzyme activities.

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Hypobaric Environment

• The decrease in VO2MAX with initial exposure to
  altitude does not improve much during several
  weeks of exposure.

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Hypobaric Conditions

• Diminished oxygen supply.
• Increased pulmonary ventilation.

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Hypobaric Conditions

• Hyperventilation - clearing of too much carbon
  dioxide - respiratory alkalosis.
• Kidneys excrete more bicarbonate ions, so less
  acid can be buffered.

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Hypobaric Environment

• Hemoglobin saturation is reduced.
• Oxygen uptake is impaired.

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Hypobaric Conditions

• Plasma volume decreases causing an increase in
  red blood cell concentration and allowing more
  oxygen to be transported per unit of blood.

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Hypobaric Environment

• VO2MAX decreases.
• During submaximal work, cardiac output increases
  by increasing the heart rate.

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Hypobaric Environment

• During maximal work, stroke volume and heart rate
  are both lower, resulting in reduced cardiac
  output.

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Hypobaric Environment

• Endurance activity suffers the most in hypobaric
  conditions because oxidative energy production is
  limited.
• Anaerobic sprint activities that last less than
  one minute are generally not impaired at moderate
  altitude.

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Hypobaric Conditions

• The thinner air at altitude provides less
  resistance to movement.

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Hypobaric Environment

• You do increase your red blood cell number, which
  can provide a significant endurance advantage
  when exercising at sea level.
• However, these effects are transient and last
  only a few days.

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Hypobaric Environment

• Athletes who must perform at altitude should do
  so within the first 24 hr of arrival while the
  detrimental changes that occur have not yet
  become too great.

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Hypobaric Environment

• Alternatively, athletes who must perform at
  altitude could train at an altitude of 1500m
  (4921ft) to 3000m (9843 ft) for at least 2 weeks
  prior to performing.
• This allows their bodies time to adapt to hypoxic
  and other environmental conditions at altitude.

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Hypobaric Environment

• Acute altitude sickness typically causes symptoms
  such as headaches, nausea, vomiting, dyspnea, and
  insomnia.
• These usually appear in 6 to 96 hours after
  arrival at altitude.

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Hypobaric Environment

• The exact cause of altitude sickness is not
  known, but many researchers suspect the symptoms
  may result from carbon dioxide accumulation in
  the tissues.

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Hypobaric Environment

• Acute altitude sickness can usually be avoided by
  a gradual ascent to altitude, climbing no more
  than 300m per day at elevations above 3000m.
• Medications can also be used to reduce the
  symptoms.

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Hypobaric Environment

• Pulmonary edema and cerebral edema, which involve
  accumulation of fluid in the lungs and cranial
  cavity, respectively, are life-threatening
  conditions.
• Both are treated by oxygen administration and
  descent.

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Hyperbaric Conditions

• Conditions in which the external pressure is
  greater than at sea level.

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Hyperbaric Conditions

• Because volume decreases as pressure increases,
  air that is in the body before it goes underwater
  is compressed when the body is submerged.

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Hyperbaric Conditions

• Conversely, the air taken in at depth expands
  during ascent.
• More molecules of gas are forced into solution
  when the body is submerged, but with a rapid
  ascent they come out of solution and can form
  bubbles (bends).

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Hyperbaric Conditions

• Water reduces the stress on the cardiovascular
  system, reducing its work load.
• When the body is submerged, plasma volume also
  increases.

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Hyperbaric Conditions

• Because of these factors, resting heart rate
  drops even when the body is only partially
  submerged.

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Hyperbaric Conditions

• Hyperventilation is often practiced before
  breath-hold diving to increase how long you can
  hold your breath.
• Beware however, that it may lead to low oxygen
  levels which may cause you to pass out.

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Microgravity Conditions

• Most physiological changes that occur as a result
  of extended periods of exposure to microgravity
  conditions during space flight are similar to
  those seen with detraining in athletes and with
  reduced activity in the aging population.

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Microgravity Conditions

• Strength and the cross-sectional areas of muscle
  fibers decrease.
• Bone mineral loss approximating 4 from the
  weight bearing bones.

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Microgravity Conditions

• Microgravity removes most of the effects of
  hydrostatic pressure experienced in a 1-g
  environment, resulting in the bodys dumping a
  large percentage of its plasma volume.

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Microgravity Conditions

• While this allows excellent regulation of
  cardiovascular function at rest and during
  exercise in space, it presents major orthostatic
  hypotension problems on return to Earths
  atmosphere.

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Microgravity Conditions

• Exercise may be one of the most effective
  countermeasures during space flight to prepare
  astronauts for successful adaptation on their
  return to earth.