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.