Physiological Adjustments to Altitude of Football Players

Football is played worldwide in highly varied environmental circumstances. In some instances the climatic conditions are too hostile or are temporarily unsuitable for playing and there is a lull in the competitive programme.
As altitude increases the barometric pressure falls. At sea level the normal pressure is 760 mm Hg, at 1000m it is 680 mm Hg, at 3000m it is about 540 mm Hg. High School, altitude conditions are referred to as hypobaric or low pressure and the main problem associated with this environment is hypoxia or a relative lack of oxygen. Normally, the proportion of oxygen in the air is 20.93%, and the partial pressure of oxygen at sea level is 159 mm Hg. The partial pressure of oxygen decrease with increasing altitude; this corresponds to the fall in ambient pressure since the proportion of oxygen in the air is constant. As a result there are fewer oxygen molecules in the air at altitude for a given volume of air. Less oxygen is inspired for a given inspired volume and this ultimately means a reduction in the amount of oxygen delivered to the active tissues. As far as the uptake of oxygen into the body through the lungs is concerned the important factor is the tension of oxygen in the alveoli. Here the water vapour pressure is relatively constant at 47 mm Hg as is the pco2 of 35 - 40 mm Hg. The result of the fall in ambient pressure and consequently alveolar tension is that the gradient across the pulmonary capillaries for transferring oxygen into the blood becomes less favorable. Exercise that depends on oxygen transport mechanisms will be impaired at about 1200 m once desaturation occurs. This refers to the oxygen association curve of haemoglobin which is sigmoid-shaped and is affected by pressure. Normally the red blood cells are 97% saturated with O2 when pco2 levels drop at a point corresponding to this altitude. The O2-Hb curve is little affected for the first 1000-1500 m of altitude because of the flatness at its top. As the pressure drops further to reach the steep part of the curve, the supply of oxygen to the body's tissues is increasingly impaired. Nevertheless, at an altitude of 3000 m the saturation is about 90%. The immediate physiological compensation for hyoxia is an increase in ventilation. This is represented by an increased tidal volume and an increased breathing frequency. A consequence of this hyperventilation is that there is an increase in the CO2 blown off from blood passing through the lungs. The elimination of CO2 leaves the blood more alkaline than normal due to an excess of bicarbonate ions, CO2 being a weak acid in solution in body fluid. Over several days the kidneys compensate by excreting excess bicarbonate, so returning the blood to the normal pH level. However, the body's alkaline reserve is decreased and so the blood has a poorer buffering capacity for tolerating additional acids. Once at altitude, there is an increased production of the substance 2, 3-BPG (bisphosphoglycerate) by the red blood cells. This is beneficial in that it aids in unloading oxygen from the red blood cells at the tissues.

The oxygen carrying capacity of the blood is enhanced by an increase in the number of red blood cells. This process begins within a few days at altitude and is stimulated by the kidney hormone erythropoietin. This causes the bone marrow to increase red blood cell production: this requires that the body's iron stores are adequate and may indeed mean supplementation of iron intake prior to and during the stay at altitude. There is an apparent increase in haemoglobin in the first few days at altitude which reflects haemoconcentration due to a drop in plasma volume. Nevertheless there is a gradual true rise in haemoglobin which may take 10-12 weeks to be optimized. Even after a year or more at altitude the increase in total body haemoglobin and red cell count do not attain values observed in high altitude natives. As a result sea level natives will never be able to compete in aerobic events at altitude on equal terms with those born at altitude. They have to devise strategies to allow them to demonstrate their superior skills as well as prepare physiologically by acclimatizing.
Football players will experience more difficulty in exercising at altitude compared with sea level in spite of the physiological adjustments to hypoxia that take place. Changes in maximum cardiac output and in the oxygen transport system lead to a fall in maximal oxygen uptake at an altitude of 2300m, corresponding roughly to Mexico City, the initial decline in VO2max is about 15%. After 4 weeks at this altitude there is an improvement in VO2max bit it still remains about 9% below its sea level value. For sea level dwellers the initial decline in VO2max is 1-2% for every 100 m above 1500m.
Football play is mostly at sub maximal intensity, although periodically there are short maximal efforts. Maintaining a fixed sub maximal exercise intensity is more stressful at altitude than at sea level. The highest level of endurance exercise that can be sustained is determined by the intensity above which lactate accumulates progressively in the blood. This 'lactate threshold' is lowered at altitude although the percentage of VO2max at which it occurs is unaltered. In order to cope with the lack of oxygen, the active muscles rely on anaerobic processes and so football players will need longer low-intensity recovery periods during match-play, following from their bouts of all-out high-intensity efforts. Heart rate, ventilation and perceived exertion are all elevated beyond the normal sea level responses at any given sub maximal exercise intensity. As a result the pace of tolerable exercise is reduced. Football players should be prepared to pace their efforts more selectively during training sessions. This is especially important in the first days at altitude.
Successful adaptation to altitude results in a decreased tachycardia in response to sub maximal exercise compared with the heart rate on initial exposure. The heart rate response may approach sea level values after three to four weeks of exposure. Adaptations of skeletal muscles occur to aid their struggle against hypoxia. Improvements in maximum blood flow capacity and oxidative metabolism require a sojourn of many months at altitude. These long-term adaptations will not be of benefit to anaerobic processes. Anaerobic efforts such as sprinting may in fact be improved at altitude due to the reduced air resistance against which the body moves. Such conditions may in fact be favorable for improving running speed. The buffering capacity of muscle is improved with a prolonged stay at altitude and this complements the adaptations that occur in oxygen transport mechanisms.