Exercise in Hypobaric, Hyperbaric, and Microgravity Environments
Definitions • Hypobaric - low atmospheric pressure • Altitude • Hyperbaric - high atmospheric pressure • Underwater • Microgravity - low gravitational force • Space
Hypobaric Environment • Altitudes of 1,500 m (4,921 ft) or more have a notable physiological impact on the human body.
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.
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.
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.
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.
Hypobaric Environment • Altitude training causes an increase in red blood cell count which increases blood oxygen-carrying capacity.
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.
Hypobaric Environment • Other muscle adaptations include decreased fiber area, increased capillary supply and decreased metabolic enzyme activities.
Hypobaric Environment • The decrease in VO2MAX with initial exposure to altitude does not improve much during several weeks of exposure.
Hypobaric Conditions • Diminished oxygen supply. • Increased pulmonary ventilation.
Hypobaric Conditions • Hyperventilation - clearing of too much carbon dioxide - respiratory alkalosis. • Kidneys excrete more bicarbonate ions, so less acid can be buffered.
Hypobaric Environment • Hemoglobin saturation is reduced. • Oxygen uptake is impaired.
Hypobaric Conditions • Plasma volume decreases causing an increase in red blood cell concentration and allowing more oxygen to be transported per unit of blood.
Hypobaric Environment • VO2MAX decreases. • During submaximal work, cardiac output increases by increasing the heart rate.
Hypobaric Environment • During maximal work, stroke volume and heart rate are both lower, resulting in reduced cardiac output.
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.
Hypobaric Conditions • The thinner air at altitude provides less resistance to movement.
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.
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.
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.
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.
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.
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.
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.
Hyperbaric Conditions • Conditions in which the external pressure is greater than at sea level.
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.
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).
Hyperbaric Conditions • Water reduces the stress on the cardiovascular system, reducing its work load. • When the body is submerged, plasma volume also increases.
Hyperbaric Conditions • Because of these factors, resting heart rate drops even when the body is only partially submerged.
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.
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.
Microgravity Conditions • Strength and the cross-sectional areas of muscle fibers decrease. • Bone mineral loss approximating 4% from the weight bearing bones.
Microgravity Conditions • Microgravity removes most of the effects of hydrostatic pressure experienced in a 1-g environment, resulting in the body’s dumping a large percentage of its plasma volume.
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 Earth’s atmosphere.
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.