Muscular physiology in exercise

Understanding the physiology of muscles in exercise is very important. The most important determinant of success in sports depends upon three qualities of muscles- Strength, Power & Endurance.

The strength of a muscle is determined mainly by its size, & resistance training which it received (which also increase a muscle size) & to some extent on hereditary/genetic & other factors. Males has a relative greater muscle mass in compare to females, thanks to the male hormones, testosterone which has a powerful anabolic effect & causing greatly increased deposition of protein everywhere in the body, specially in the muscles. Female hormone, estrogen on the other hand causes mainly deposition of fat specially in the breasts, hips & subcutaneous tissue.

This does not mean that females will always lag behind males in every sports, in some events like swimming in cool water for long distance, women have at times held records, which may be due to the availability of extra fat, providing heat insulation, buoyancy & the extra long-term energy.

The contractile & holding strength of a muscle depends upon its cross sectional area- maximal contractile force: 3-4 kg/cm sq. of cross sectional area & maximal holding force: 4.2-5.6 kg/ cm sq. of cross sectional area (1kg force means approx.10newton force). This amount of force is large enough to easily damage tendons, joints, ligaments & the muscle itself. As the holding strength is about 40% more than the contractile strength, a forceful stretching of maximally contracted muscle is one of the surest ways to create the highest degree of muscle soreness.

With proper training muscles get hypertrophied (an addition of 30 to 60 %), due to mainly increase in diameter of muscle fibers, & to some extent a little increase in the number of fibers. Not only the muscles enlarged but also their capabilities of both anaerobic metabolic system & aerobic metabolic system (specially increase in the maximum oxidation rate & increase in the efficiency of the oxidative metabolic system as much as 45%) are greatly increased.

Muscle training is equally important for muscle development & muscle strength. It has been established now that muscles that function under no load, even if they are exercised for hours on end, increase little in strength. This shows the importance of resistance muscle training programs. Also it has been proved that for rapid development of strength in muscles they have to contract at more than 50% maximal force of contraction, even if the contractions are performed only a few times each day.

This leads to a useful conclusion- six nearly maximal muscle contractions performed in three sets 3 days a week give approximately optimal increase in muscle strength, without producing chronic muscle fatigue.

Another point to note is that muscle strength increases about 30% during the first 6 to 8 weeks of resistive training program but almost plateaus after that time. Muscle training is very useful in old age when most of the muscles get atrophied due to sedentary life style, in whose case often more than 100% muscle strength can be achieved.

Another point to discuss is regarding power of muscle contraction which is very different from muscle strength. It is the measure of total amount of work that the muscle performs in a unit period of time, measured in Kg-meters per minute. A person has the capability of extreme power surges for short period of time (a maximum of 7000kg-m/min in the first 8 to 10 seconds), whereas for long-term endurance events, the power output of the muscles is only one fourth (about 1700kg-m/min in the next 30 min, 1.167 min after starting the exercise) as great as the initial power surge.

But the athletic performance is not four times (actually about 1.75 times greater) during the initial power surge as it is for the next 30 min, this is due to the fact that the efficiency for the translation of muscle power output into athletic performance is often much less during rapid activity than during less rapid but sustained activity.

The ability of muscles to deliver extreme amounts of power for a few seconds to a min or so depends upon the percentage of ‘fast-twitch muscle fibers’ present in them, which are specially designed for that purpose. For example, the higher preponderance of fast-twitch fibers in gastrocnemius muscle gives it the capability of forceful & rapid contraction of the type used in jumping.

On the other hand, ‘slow-twitch muscle fibers’ are designed to provide endurance, delivering strength of contraction over many minutes to hours; the greater percentage whose presence in soleus muscle makes it suitable for prolonged lower leg activities.

Genetics inheritance determines the varying percentages of fast-twitch & slow-twitch muscle fibers in the muscles of an individual, & hence to some extent the athletic capabilities of different individuals. This also explains the fact that some types of sports are most suited to some persons, for example some people are born to be marathoners, others jumpers or gymnasts. Unfortunately, athletic training has not been shown to change the relative proportions of the two types of muscle fibers.

Endurance is another measure of muscle performance. Stamina of a healthy player is largely determined by the endurance of his or her muscles. Endurance can be roughly measured by the time a player can sustain the sporting event until complete exhaustion. Endurance depends upon the amount of glycogen that has been stored in the muscle before the period of exercise.

High carbohydrate diet stores maximum amount of glycogen (40g/kg of muscle) in the muscle than other types of diet (high fat diet in contrast stores only 6g/kg of glycogen, being the least). Also high carbohydrate diet makes full recovery of exhausted muscle glycogen in about 2 days (48hours), in contrast players on high fat, high protein diet or on no food al all show very little recovery even after as long as 5 days.

These lead us to an important conclusion-Players should have a high-carbohydrate diet before a grueling sportive event, also he/she should not participate in another exhaustive exercise or sportive event during 48 hours preceding the previous event.

Muscles require energy constantly not only for contraction (a muscle can shorten up to a maximum of 30 % of its total length during contraction) but also for relaxation, in the form of ATP (Adenosine triphosphate). ATP is supplied to muscles by-stored ATP in muscle itself, Phosphocreatine-creatine system, which can reconstitute the stored muscle ATP (together called phosphagen system); Glycogen-lactic acid system, having the ability to reconstitute phosphagen system & the last, Aerobic System which can provide enough energy to reconstitute all the above system.

The phosphagen system can provide upto 4 moles of ATP per minute & can provide maximal muscle power for 8 to 10 seconds ;i,e; used by the muscle for power surges of a few seconds. Glycogen-lactic acid system generates 2.5 moles of ATP per minute & can provide 1.3 to 1.6 minutes of maximal muscle activity but the muscle power is somewhat reduced.

The aerobic system (concerns with oxidation of foodstuffs-glucose, fatty acids & amino acids-to provide energy) on the other hand generates only 1 mole of ATP per minute, but can provide maximal muscle activity for unlimited time (as long as the nutrients last), & hence is required for prolonged athletic activity.

Most of the combat sports sparring including boxing utilize both Glycogen-lactic & aerobic systems, whereas the performing competition like for kata in karate or poomse in Taekwondo mainly uses Phosphagen & glycogen-lactic acid systems.

The stored glycogen is metabolized in two steps-1st step with no oxygen requiring & generating energy (4 ATP for each glycogen) & 2 pyruvic acid molecules for each glycogen-this is anaerobic glycolysis; 2nd step with with the oxidation of pyruvic acids in the mitochondria of muscle fibers & release of large sum of energy.When there is insufficient oxygen in this 2nd step, most of the pyruvic acid of the 1st step is converted in to lactic acid (this constitutes the Glycogen-Lactic acid system).

Also when large amount of ATP are needed for relatively short to moderate periods of muscle contraction, this anaerobic glycolysis is used as it provides ATP 2.5 times faster than the oxidative mechanism of the mitochondria (as we know for rapid muscle contraction Phosphagen system is used which can provide ATP twice as fast as the Glycogen-lactic acid system). The use of this anaerobic system means generation of large amount of lactic acid which can cause extreme fatigue.

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 ter 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.

Cardiovascular Physiology in Exercise

Exercise causes activation of sympathetic system & secretion of large amounts of epinephrine & norepinephrin from the medullae of the two adrenal glands, along with release of stress hormones. Theses lead to vasoconstrictor response of the peripheral circulatory system-both arterioles (increase blood pressure) & capacitative areas of circulation including veins (increase the venous return to heart), & also the heart itself is stimulated (heart rate increases, the pumping strength increases etc.).

Due to poor vasoconstrictor innervations the coronary (instead during maximal exercise, coronal blood flow increases by 5 times with 100% coefficient of oxygen utilization) & cerebral systems (no change in cerebral blood flow during any grade of exercise) are spared.

In exercising muscles, however, there is vasodilatation of arterioles due to local chemical changes .Reduction in oxygen is the most important cause- the arterioles walls cannot maintain contraction in absence of oxygen & release of many vasodilators. These are adenosine (most important), potassium ions, ATP, lactic acid & carbon dioxide etc. They cause about 12.5 increases in muscles blood flow during strenuous exercise.

The moderate rise in arterial blood pressure (usually about 30%) & many other factors (like opening up of dormant capillaries which delivers 2-3times more oxygen & nutrients to exercising muscles) occurring during exercise cause another 12.5 increase in muscles blood flow. But during the muscles contraction phase of exercise, blood flow decreases as intramuscular blood vessels are compressed. This explains the rapid muscle fatigue develops during strong tonic muscle contractions due to lack of enough oxygen & nutrients.

Muscle work output, oxygen consumption & cardiac output during exercise are linearly related to one other; i.e; the muscle work output increases oxygen consumption, which in turn dilates the muscle blood vessels, thus increasing the venous return & cardiac output. Physical training can increases the maximal cardiac output in well trained athlete about 1.3-2 times that of an untrained person during exercises, & almost 5.4 times the cardiac output at rest. This is due to the effect of training & exercise on the heart muscles.

In a trained persons heart chambers may enlarge up to 40 %, also the heart mass up to 40% or more. The heart-pumping effectiveness of each heart beat (stroke volume- the amount of blood pumped by each ventricle of the heart per beat) is also 40-50% greater in the highly trained athlete than in a normal person.

Important point to be noted is that heart enlargement & increase in pumping capacity occur entirely in endurance types, not in the sprint types of physical training. Another important point is that though the heart of an athlete is considerable larger than that of a normal person, resting cardiac output (the amount of blood pumped out by each ventricle into the circulation per minute) is almost the same. This is possible only when there is corresponding decrease in the heart rate of trained athlete at rest (as Cardiac output = Stroke volume multiply heart rate).

Stroke volume can be increased twice the normal value during exercise. The stroke volume normally reaches its maximum by the time the cardiac output has increase only halfway to its maximum (Cardiac output can be increased to 5-6 times-approx.25-36 liters/min- during maximum exercise).For further increase in cardiac output, the heart rate must increase correspondingly (heart rate increase linearly with increase in physical effort.

Heart rate during exercise can be used to roughly grade exercise: Grade I/light or mild exercise-below 100/min; Grade II/moderate-100to125/min; Grade III/heavy-125to150/min; Grade IV/very heavy or severe-above 150/min heart rate. Maximum heart rate attainable during exercise is 220 minus age (in years), therefore higher values of maximum cardiac output can be achieved in younger athletes compared to older ones). Hence the increase in heart rate accounts by far for a greater proportion of the increase in cardiac output than does the increase in stroke volume during strenuous exercise.

During maximal exercise, both the heart rate & the stroke volume are increased to about 95% of their maximal levels, this means cardiac output is increased to 90 % of the maximum that a person can attend. On the other hand, as discussed above, pulmonary ventilation is increased to about 65% of the maximum.

This leads to the conclusion that cardiovascular system is the one, rather respiratory system, that limits Vo2max., as oxygen utilization by the body can never be more than the rate at which the cardiovascular system can transport oxygen to the tissues.

Hence athletic performance mainly depends upon the performance capability of his/her heart. And training, as we have discussed above, increases the performance capability of one heart (for example ability to achieve 40% greater cardiac output in trained athlete over the average untrained male) to an optimal level.

Due to generalized vasoconstriction as discussed earlier during exercise many visceral blood flow decreases, specially during severe exercise, which may damage the system. Athletic pseudonephritis is a condition characterizes presence of proteins, cells & other abnormal substances in urine due to prolonged, heavy exercise.

Body thermal physiology in exercise

The maximal efficiency for conversion of nutrient energy into muscle work is only about 20-25%, remaining 80-75% is lost as heat energy. Also almost all the energy (20-25%) that goes into creating muscle work still becomes body heat because all but a small portion of this energy is used to overcome viscous resistance to the movement of the muscles & joints, & also to overcome the friction of the blood flowing through the blood vessels etc.

The amount of heat liberated in the body is almost proportional to the oxygen consumption, & as we know that in a well trained athlete, the amount of oxygen consumption can increase almost up to 20 times, the heat generated is tremendous performing endurance sportive events. Also during severe exercise the blood flow in skin is also decreased due to severe vasoconstriction. This coupled with hot & humid weather or excess clothing the body temperature can easily rise to 41-42 degree Celsius, in spite of the sweating mechanism.

The stoppage of the exercise itself at this level, does not easily decline the temperature itself, due to failure of temperature-regulating system & liberation of still more heat caused by the very high body temperature doubling of all the intracellular chemical reactions.

This much elevated temperature becomes destructive to body tissues & cells, specially the brain cells- leading to multiples symptoms: extreme weakness, headache, exhaustion, dizziness, nausea, profuse sweating, confusion, staggering gait, collapse & unconsciousness. This whole complex is called heatstoke & if not treated immediately can lead to death.

At this stage body temperature should be tried to reduce as rapidly as possible. Remove all the clothing, spay cold water on all the surfaces of the body or continually sponge the body with cold water & blow air over the body with a fan. Instead the body can be immersed totally in water containing a mush of crushed ice.

Physiology of Body fluids & salts in exercise

Normally the daily output of water is about 2.3 liters & we should ingest fluid more than 2.1 liters per day (700ml, approx. 2.8glasses of water, as water in cooking; & more than 1.4 liters as the fluid by mouth as water & beverages, approx.> 5.6glasses of water). In prolonged & heavy exercise, the daily output of water increases more than 2.87 times, & therefore the fluid ingested should be more than 3.05 times the normal value (more than 5.7 liters of fluid by mouth as water & beverages, approx. > 22.8glasses of water).

During heavy exercise or in very hot weather, water loss in sweat may increase to 1-2 liters per hour or almost 50 times normal value per day. Similarly, in prolonged & heavy exercise, insensible water lost through lungs increases almost 2 times that of the normal value per day. All of this will rapidly deplete the body fluids unless adequately taken.

More than 70% of body weight (some say-60% of body weight is total body water) is because of the water it has. Loss of water upto 10% of the total body water, makes a person feel extremely tired & fatigued. A more than 20% loss may result in death. As much as 2.268 to 4.538kg weight loss has been recorded in athletes in a period of one hour during endurance athletic events under hot & humid conditions.

This mainly results from loss of sweat. Loss of enough sweat to decrease body weight only 3% can significantly diminish a person’s performance, & a 5 to 10% rapid decrease in weight can often lead to serious muscle cramps, nausea etc. Hence it is essential to replace fluids as it is lost.

Sweat contains large amount of sodium chloride, some urea & lactic acid. So, considerable amount of salt is lost through sweating. The sweat glands, however, become acclimatized when an athlete becomes acclimatized to the heat by progressive increase in athletic exposure over a period of 1 to 2 weeks rather than performing maximal athletic feats on the first day (acclimatization to heat: the various physiological readjustments & compensatory mechanisms in the body that reduces the bad effects of prolonged heat exposure).

This acclimatization of sweat glands results mainly due to increase secretion of aldosterone by the adrenal cortex. The aldosterone increases the reabsorption of sodium chloride from the sweat & urine, & loss of potassium through sweat & urine.

This leads to an important conclusion-(a) Salt supplementation should be there for all athletes performing exercise on hot & humid days, but once the athletes are acclimatized, only rarely do the salt supplementation need to be considered during the sporting events. Hence there may at many a times be over dose of this salt supplementation which is equally harmful. (b) Some of the supplemental fluids of athletics should also contain proportioned amounts of potassium along with sodium, usually in the form of fruits juices & coconut water etc.

Basic Sport Pharmacology

(a)Caffeine (found in coffee) is said to increase athletic performance, but reliable data are lacking.

(b)Androgens/male sex hormones or any other anabolic steroids. They can increase muscle strength & increase athletic performance. These are banned drugs in sports. They causes hypertension, promote heart attacks & strokes. External male sex hormone preparation decreases testicular function, may lead to infertility & decreases the secretion of the person own hormone. In females, musculinization can take place-hirsutism, bass voice, ruddy skin, cessation of menses, infertility etc.

(c)Others drugs such as amphetamines, cocaine (a narcotic drug) etc have been said to increase one’s athletic performance, mainly due to psychic stimulant. The overuse can lead to deterioration of performance. Such drugs may interact with the epinephrine & norepinephrine (catecholamines) released during exercise, & may cause over excitability of the heart, leading to ventricular fibrillation which may lead to the death of athletes within seconds.

(d)Nicotine, present in tobacco smoking, is widely regarded as one of the biggest enemy of an athlete. It causes constriction of the terminals bronchioles of the lungs, increasing the airflow into & out of them. Nicotine paralyzes the cilia on the surfaces of the respiratory epithelial cells that normally beat continuously to remove excess fluids & foreign particles from the respiratory passageways, resulting in much accumulation of debris in the air-passages adding further difficulty of breathing. The irritating effects of the smoke itself cause increase fluid secretion into the bronchial tree, as well as swelling of the epithelial linings. All of these mean that mean that even a light smoker will feel respiratory strain during maximal exercise, reducing his/her performance significantly.

(e)Alcohol is a neuronal depressant. Any measurable concentration of alcohol produces a measurable slowing of reflexes- performance is impaired, fine discrimination & precise movements are obliterated & errors increases. Chronic alcoholism damages brain neurons.

In small doses, BP is not affected much, but moderate doses may cause mild rise in BP due to increase muscular activity & sympathetic stimulation & also causing tachycardia. This may interact with the exercise induced rise in catecholamines & may produce dangerous results.

In large doses, alcohol causes direct myocardial & vasomotor center depressant with fall in BP. Chronic alcoholism causes hypertension, cardiomyopathy & cardiac arrhythmias, & megaloblastic anaemia etc. Alcohol is a depressant on respiratory centre, but may transiently stimulate respiration reflexly due to its mucosal irritating nature. Chronic alcoholism also causes muscles weakness & myopathy, & its high doses depress temperature regulating center & deplete hepatic glycogen causing hypoglyceamia.All these mean alcohol should be avoided, specially by competiting athletes & sport persons.

Prolongation of life is possible through sport & fitness

Mortality is shown to be 3 times less among people who maintain appropriate body fitness, using judicious regimes of exercise & weight control, specially between the ages of 50 & 70. These may be due to the cardiovascular protective effect of body fitness: maintenance of moderately lowered blood pressure, & reduced blood cholesterol & decrease ratio of LDL (enemy lipoprotein) to HDL (protective or friendly lipoprotein).

Another important point is that athletically fit person has more bodily reserves to call on when he or she does become sick. For example an athletically fit old man has twice as much as respiratory reserve (ability to increase oxygen delivery to the tissues in times of need)as a non fit old man, & 50% greater cardiac reserve (ability to increase cardiac output in times of need) than in a non fit old man.

Hence, understanding exercise physiology & the scientific data thus collected has been used to improve the performance of athletes, sportsmen & military personnel.

The knowledge of this field also helps a physician in many ways- aiding the diagnosis of heart & lung diseases; pre- & post-operated assessment of cardio-thoracic surgical patients; rehabilitation of cardiac invalids; & screening workers for disability compensation etc.

For players & coaches also, the basic understanding of sport & exercise physiology will definitely help increase their efficiency & performance.

Dr.HanjabamCha Barun,

Martial artist & Thang Ta practitioner, NS NIS, Patiala, Punjab, India, sports medicine student,

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