Physiological Principles of Exercises & Sports
- A brief discussion -
- Part 2 -

By Dr. Hanjabam Cha Barun *

Our body contains about 2.05 liters of stored oxygen which can to be used for aerobic metabolism even without breathing any new oxygen. This stored oxygen is used up within a minute or so in heavy exercise, also it takes a few times for the circulation to deliver the extra oxygen required by the working muscles.

During this period of non availability of oxygen, ATP is primarily produced by anaerobic mechanisms (phosphagen & lactic acid system). This causes oxygen-deficit at the beginning of exercise. This oxygen deficit is repaid after the stoppage of exercise in the form of oxygen-dept which is approx.11.5liters (2.05 liters for the storing of oxygen in the body+9.45 liters to reconstitute phosphagen & lactic acid system) & hence even after the exercise is over, the oxygen uptake still remains above normal, at high level for the 1st 4 min & at a lower level for another 52 minutes.

Physical performance or fitness is inversely related to the oxygen-deficit, & physical training & warm up decreases the oxygen-deficit; therefore warm up & physical training increases physical performance & fitness. Also when adequate amounts of energy are available from oxidative metabolism, a large portion of the lactic acid is converted into glucose mainly in liver which replenish the glycogen stores of the muscles; the remaining lactic acid is converted back into pyruvic acid & oxidized to release energy.

Warm up is very important & is essential part of every sportive event. Warm up effects:
(a) Increase the blood flow & nutrients to working muscles.
(b) Increase level of mitochondrial enzymes & energy stores causing lesser use of anaerobic work.
(c)Prevents heart damage during 1st few seconds of heavy exercise; otherwise there will be inadequate blood flow to the heart.
(d) Prevents muscular or connective tissue injuries.

Physical training is, as discussed above, one of the most important determinant of athletic performance & fitness. Physical training effects:
(a) Improvement in psychology of the athlete, & the decrease in psychic stimuli to vasomotor & respiratory centers.
(b) Cardio-respiratory response reaches a steady-state early with optimal blood flow distribution.
(c) Greater fats are used for energy, sparing glycogen, which lead to increase endurance of the athlete as physical performance is a direct function of the glycogen stores.

This is due to the decrease in respiratory quotient, RQ, which is the ratio of the volume of carbon dioxide produced by the volume of oxygen consumed during a given time, because of the aerobic training (RQ for 100% fats utilization is 0.7 as compared to 0.83 for proteins & 1 for carbohydrates).

(d) Higher Vo2max.(explained later)can be achieved. This is due to increase in maximal cardiac output; increase in arteriovenous oxygen concentration difference; decrease of peripheral resistance & less increase in both systolic & diastolic blood pressure; less increase in pulmonary ventilation & less stimulation of respiratory centre; & lastly more increase in diffusion capacity of lungs for oxygen due to increase pulmonary capillary density.

In addition to stored glycogen, muscles also utilize glucose from blood (released from stored glycogen in liver) as source of energy. These two are the energy nutrients of choice for intense muscle activity. This also explains the usefulness of glucose solutions given to players during the sportive events which provide as much as 30-40 % of the energy required during the events.

Apart from carbohydrate energy source, muscles also use other source of energy. Muscles use large amounts of fat for energy in the form of fatty acids & acetoacetic acid; they also use to a much extent proteins in the form of amino acids. Most of the energy of muscles is derived from carbohydrates during the 1st few seconds or minutes of the exercise, but at the time of exhaustion, as much as 60 to 85 % of the energy is derived from fats.

Also, the glycogen stores of the muscles become totally depleted in those endurance sportive events that last longer than 4 to 5 hours, & hence fat supplies more than 50 % of the required energy after about the 1st 3 to 4 hours of a long term endurance events.

Respiratory physiology in exercise

Respiratory ability is very important for the maximal performance in endurance sportive events.

The oxygen consumption & total pulmonary ventilation has a linear relationship with levels of exercise. Both can increase, in a well trained person, in maximal intensity of exercise to 20 times the value at resting state. Physical training is important as oxygen consumption under maximal conditions in a well trained person can be increased to 1.4-2 times the highest for an untrained average person.

The intense ventilation during exercise results mainly from neurogenic signals transmitted directly into the brain stem respiratory center as a collateral impulse when the brain is transmitting motor impulse to the exercising muscles (much like the stimulation of the vasomotor center in the brain stem during exercise causes a simultaneous rise in blood pressure),& partly form sensory signals from contracting muscles & moving joints.

This neurogenic anticipatory stimulation of the respiration at the onset of exercise, even before it is needed, is used to supply the extra oxygen required for the exercise & also to blows off extra carbon dioxide. The chemical factor & the thermal factor do the required fine adjustment in the respiration initiated by the neurogenic factor to keep the oxygen, carbon dioxide, pH of body fluids as normal as possible.

This neurogenic factor is now shown be partly a learned response; i.e; with repeated periods of exercise, the brain becomes progressively more able to provide the proper signal to keep the Pco2 in body fluids (direct function of amount of carbon dioxide) etc at its normal levels.

This again shows the importance of physical & breathing training. One important thing is that the maximal breathing capacity (voluntary) is about 50% greater than the actual pulmonary ventilation during maximal exercise. This means that respiratory system is not normally the limiting factor in delivery of oxygen to the exercising muscles.

The rate of oxygen utilization under maximal aerobic metabolism (Vo2max) increases only 10 %after short term athletic training, but may increases more after years of training.Vo2max on the other hand is largely determined genetically-people with grater chest sizes in relation to body size, & stronger respiratory muscles have more Vo2max, & hence such person are more successful as marathoners etc.

The oxygen diffusing capacity of lungs increases above 3 times during maximal exercise. The maximum diffusing capacity of a trained person is almost 2 times that of a nonathlete during maximal exercise. This lays the importance of physical training, specially the endurance training. Genetics is also an important determining factor of maximal oxygen diffusing capacity.

To be continued ...