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Specificity of Interval-Training
By: W. W. Heusner
Human Energy Research Laboratory
Michigan State University
East Lansing, Michigan
Table of Contents
List of Tables - 111
List of Illustrations - iv
I.  The History of interval-training
II.  The Scientific Bases Underlying interval-training
III.  The Applications of the Scientific Principles Underlying interval-training To the Energy Metabolism Variables
IV.  The Implementation of an interval-training Program
V.  Specificity of interval-training is a Valid Concept
List of Tables
1.  Hypothetical levels of venous blood ph and lactic acid in an untrained subject.  Subject and in a highly trained athlete before and after exhaustive work
2.  Typical energy metabolism values for untrained subjects and highly trained athletes during severe exercise
3.  Hypothetical energy metabolism of an athlete (average oxygen intake capacity of 4.1 per min. and oxygen debt tolerance of 16.1) during exhaustive work periods of one and sixteen minutes duration
4.  Classification of work loads by levels of energy metabolism
5.  Total oxygen intake and work efficiency during continuous and interval work
6.  Schematic design of a progressive interval-training program for swimming or track using only one distance and large steps
7.  Changes in the energy metabolism of two athletes produced by two and one-half years of specific interval-training
List of Illustrations
1.  Hypothetical stroke volume variations in an untrained subject and in a highly trained athlete before, during, and after severe exercise
2.  Relationship between lactic acid and oxygen debt during exercise
3.  Hypothetical energy metabolism of a subject (oxygen intake capacity of 5.1 per min. and oxygen debt tolerance of approximately 16.1) before, during, and after a task demanding a total oxygen utilization of 10.1 per minute
4.  Hypothetical energy expenditure required to perform a specific motor skill while learning first a less efficient and later a more efficient technique
5.  Reduction of the cardiovascular stress (induced by tending industrial furnaces) during the afternoon shift obtained by proper adjustment of work-rest periods
6.  Hypothetical energy metabolism of a subject (oxygen intake capacity of 5.1 per min. and oxygen debt tolerance of 14.1) during an interval-training workout
Specificity of Interval-Training
By: W.W. Heusner
Michigan State University
I.  The history of interval-training explains its modern nature.
Various forms of interval-training have been used on trial and error basis for years. Generally though, prior to the middle 1950’s, interval-training constituted a relatively small part of the total workout of athletes in this country.  These Trial and error excursions into interval-training have been called “wind sprints,” “alternates,” “locomotives.” Etc.  Usually there was little formal structure involved. Work and rest intervals were not standardized.  Surprisingly, modern interval-training did not evolve from these crude beginnings.  Its origin is somewhat unique.
During the early 1920’s, a form of fartlek (speed play) running developed in the Scandinavian countries.  The great Finnish distance man, Pikhala, was an early proponent of the fartlek system (125).
In this type of training, the athlete runs for varying periods of time at several different speeds according to his own subjective feelings at the moments.  No attempt is made to standardize the distance or the lengths of time to be run at each speed or even to use a uniform order of alternation of paces.  The total distance to be covered in a given workout is not predetermined.
The athlete starts out with only a rough idea of the total time which he intends to spend in a given training period.  Over a continuous run of hour or two, he will run at each pace a large number of times.  Obviously, to be of maximum use, fartlek training requires great dedication on the part of the participant.  The Scandinavian people made great use of fartlek running in preparation for the distance races in which they excelled.  It is still used successfully by a number of modern marathon runners.
Gradually the concept of interval-training grew out of fartlek running.  Both the distance and the speed of the slow and fast intervals were standardized.  The criterion measure of the total time to be spent in a workout was replaces by the total distance to be covered or by the number of fast intervals to be completed.  The element of speed play was eliminated in the new interval-training since the athlete was forced into a predetermined pace at a predetermined time.
Many European track coaches were using interval-training extensively just prior to World War II.  Although the war disrupted most athletic competition, it stimulated research into the area of work tolerance.  Widespread recognition of the scientific competition, it stimulated research in to the area of work tolerance.
Emil Zatopek, the great Czechoslovakian distance man of the era just after World War II and the winner of three gold medals in the 1952 Olympic Games, was one of the most successful of the early proponents of interval-training (96).  Zatopek’s workouts were definitely of an interval nature, but they would be considered old-fashioned by today’s standards.  The quantity of his work was very high.  However, the quality was quite low.
For ten days in succession prior to the 1948 Olympic Games in London, Zatopek took a workout consisting of 60 times 400 meters run at 60 to 75 seconds each, alternated with 60 times 200 meters jog about 60 seconds each (160, p.87).  Before the 1952 Olympic Games in Helsinki, Zatopek’s training was 20 times 200 meters run at about 34 seconds each, 40 times 400 meters at 75 to 90 seconds each, and again 20 times 200 meters.  Each fast interval was followed by a jog of 200 meters at about 60 seconds (160, p.87).
At present, there six basic variables in interval-training. Items which can be controlled are:
1.  The length of time of the work interval.
2.  The work rate during the work interval.
3.  The length of time of the rest interval, which can be measured by timing:
a.  The rest interval itself.      b.  The start interval, which is the length of time between the starts of successive work intervals. 4.  The work rate during the rest interval.
5.  The number of repetitions of the work interval to be completed during each workout.
6.  The periodicity of workouts.
a.  Diurnal of circadian variations undoubtedly play a large role in the level of adaptation of the individual (80, 81).
b.  The optimum spacing of days of workout may also depend upon both the individual athlete and his chosen event.
The scientific bases underlying interval-training gradually are becoming more apparent.  Although some of the data appear to be contradictory and many questions are unanswered as yet, there is much evidence to quantitatively support and qualitatively design interval-training programs. The overload principle accounts for the general phenomenon of adaptation to stress.  That is, whenever a stress is imposed upon the living body causing strain, the body will attempt to compensate or adapt so that future applications of the same stress will produce less strain.  The process of progressively overloading or stressing one or more aspects of the living body is called “training” or “conditioning.”  Even though in some cases the mechanisms of change are unexplained, many specific effects or training are well known, including:
1.  Increased muscular strength
2.  Increased capillarization of muscle
3.  Improved neuromuscular coordination
4.  Increased cardiac output
5.  Reduced pulse rates for any given work load
6.  More effective oxygen utilization and energy expenditure mechanisms.
Quality or intensity of training is somewhat more critical than quantity or duration (66).  Assume an untrained athlete has been given a competent (valid and reliable) test of work capacity.  If the athlete is then subjected to a well-designed, standard training program for some time, his work capacity, as periodically measured by the test, will increase until a plateau of performance is reached.  If at this time the quantity of training is increased while the work rate is held constant, little improvement of work capacity can be expected.  On the other hand, if the work rate or quality is increased while the duration is held constant, the athlete will again increase his work capacity up to a new plateau.  Improvement from plateau to plateau thereafter will depend primarily upon achieving adaptation to graduated increases in intensity of work.
During interval-training, much of the actual process of adaptation of the heart occurs during the rest interval.  Strenuous exertion causes a marked initial increase in stroke volume.  This increase will be greater in the trained man than in the untrained. (See Figure 1)  With approaching exhaustion, the stroke volume begins to fall off in site of maximum demands for blood circulation.  This phenomenon is probably due to:
1.  Elevation of the heart rate to the point that inadequate diastolic filling occurs, and/or
2.  Elevation of the peripheral resistance at various places in the vascular system.
At the cessation of exercise there will be another increase in stroke volume to a level even higher than that achieved initially (125).  This second increase will be maintained for approximately thirty seconds.  Most likely, the post-exercise rise is a result of the relatively complete removal of those factors which limited stroke volume with approaching exhaustion.  The post exercise increase in stroke volume serves as a direct stimulus for adaptation of the heart muscle to the stress of severe exercise.  Obviously, the rest interval cannot be considered to be a true “recovery period” when all of the various component parts of the body are considered.
The energy metabolism of the highly trained athlete differs greatly from that of the untrained subject (86).  Oxygen debt is the ultimate limiting factor in exercise carried to exhaustion.  Oxygen debt represents the expenditure during strenuous exercise of chemical energy previously stored in the body.
Figure 1.  Hypothetical Stroke Volume variations in an untrained subject and in a highly trained athlete before, during and after severe exercise.
If a man swims 50 yards without breathing. He must use stored energy.  This expenditure of stored energy or oxygen debt is classified in two ways (133, pp. 499-503) (See figure 2).  Alactacid debt amounts to only the first three liters or so of oxygen and represents energy obtained from:
,br> 1.  A small amount of oxygen stored in venous blood and in muscle myoglobin, and
2.  High energy phosphate bonds in adenosine triphosphate, phosphocreatine, etc.  Lactacid debt is that above approximately three liters and represents the energy obtained from the breakdown of pyruvate into lactic acid.
Recovery after exercise is a process of replenishing the depleted energy stores in the body.  For recovery to take place, an elevated intake of oxygen is necessary, thence the name “oxygen debt” (90).  The obvious proof of the existence of the oxygen debt mechanism is the heavy breathing which occurs in recovery after exercise.  Recovery from oxygen debt takes place at two different rates (133, pp. 499-500).  Alactacid debt is paid off rapidly—usually during the first minute or two of recovery.  Lactacid debt is paid off more slowly, requiring at least thirty times as long a period as that necessary for the repayment of the alactacid debt.
Although an athlete may recover from exhaustive work well enough to compete successfully a second time in less than an hour, complete recovery probably requires two hours or more (133, p. 502).  An increased oxygen debt tolerance can be conditioned through proper training (86; 135; 152, p. 151).  Oxygen debt is indirectly reflected by the level of venous blood pH as well as by circulating lactic acid.
Figure 2.  Relationship between blood lactic acid and oxygen debt during exercise.
From the data of Margaria, R; Edwards, H. T; and Dill, D. B. The possible mechanisms of contracting and paying the oxygen debt and the role of lactic acid in muscular contraction. Am J. Physiol. 106: 689 , 1933.  Venous blood pH is a measure of the hydrogen ion concentration (i.e. the relative acidity or alkalinity of the blood).  A value of 7.0 is neutral. Above 7.0 alkalinity increases; below 7.0 acidity increases.  The average resting value for venous blood is 7.4. That is, human blood is normally slightly alkaline.  As acid metabolites of heavy work (e.g. lactic acid) build up, blood pH is gradually lowered.
The relationship between oxygen debt and circulating lactic acid has been shown previously in Figure 2.  Table 1 illustrates hypothetical blood pH and lactic acid shifts during exhaustive exercise in an untrained subject and in a highly trained athlete.  It can be seen that the athlete is able to tolerate considerably greater changes due to exercise.  Typical oxygen debt tolerance values for untrained subjects and highly trained athletes are given in Table 2.  Again, it can be seen that training produces greater tolerance.
The rate at which a given subject will accumulate oxygen debt will depend entirely upon the intensity of work which he performs.  The oxygen debt tolerance level may be reached very quickly (e.g. in 45 seconds or less) in sprints to exhaustion.  The oxygen debt tolerance level may be reached quite slowly in middle distance events which are carried to exhaustion.  Regardless of the rate of build-up, when a subject reaches his minimum oxygen debt tolerance, exhaustion occurs and work must be terminated or at least sharply reduced.
TABLE 1 – Hypothetical levels of venous blood pH and lactic acid in an untrained subject and a highly trained athlete before and after exhaustive exercise.
From the data of Heusner, W.W.; and Barnauer, E.M. Relationship between level of physical condition and pH of antecubital venous blood. J. Appl. Physiol. 9: 171, 1956.  From the data of Margaria, R; Edwards, H. T; and Dill, D. B. The possible mechanisms of contracting and paying the oxygen debt and the role of lactic acid in muscular contraction. Am J. Physiol. 106: 689, 1933.
Table 2 – Typical energy metabolism values for untrained subjects and highly trained athletes during severe exercise.  From the data of Robinson, S.; and Harrison, P.M. The effects of training and of gelatin upon certain factors which limit muscular work. AM. J. Physiol. 133: 161, 1941.  From the data of Taylor. H.L.; Buskirk, E.; and Henschel, A. Maximal oxygen intake as an objective measure of cardiorespiratory performance. J. Appl. Physiol. 8: 73, 1955.
The rate of oxygen intake also limits athletic achievement.  Oxygen intake is the oxygen which is absorbed by the body through the lungs during respiration.  If a man swims 50 yards breathing every stroke, at least some of the oxygen which he needs to swim this distance will be provided by his oxygen intake.  An increased oxygen intake capacity can be conditioned through proper training.  Typical maximum oxygen intake capacity values for untrained subjects and highly trained athletes are given in Table 2. It can be seen that training produces greater capacity.
Several factors which are influenced by training help to determine the rate of oxygen.  During exercise, the trained athlete can pass a greater volume of blood through his lungs in any given unit of time that can the untrained subject (152, pp. 145-146).  The blood of the properly trained athlete will have a higher concentration of hemoglobin than will the untrained subject (3, 34, 41, 104, 105).  The total quantity of tissue active during severe exercise will be greater in the athlete than in the untrained subject (153).  The athlete will have more capillaries per unit volume of active muscle tissue than will the untrained subject (128; 152, pp. 146-147).
The total oxygen utilization of a given subject in a particular activity is the sum of that subject’s oxygen intake plus his oxygen debt.  The oxygen requirement of a specified task for a given individual is that subject’s oxygen utilization while performing the task.  Exercise may be classified grossly according to level of oxygen requirement.  Aerobic work for a given subject is that which is of a low enough intensity so that the subject’s oxygen intake is equal to the requirement of the work for him; and as a consequence, no significant oxygen debt develops.
Anaerobic work for an individual is that which is of a high enough intensity so that the oxygen requirement of the task for the subject exceeds his oxygen intake, and thus an oxygen debt develops.  I has been shown that anaerobic work is considerably less efficient than aerobic work (4, 49).
Aparticular skill will have a lower oxygen requirement for an athlete trained in that skill for an untrained subject, at any given level of work intensity.  The skilled athlete will recruit and isolate effective musculature, thus making his performance relatively efficient.  The unskilled subject will use extra, uncoordinated musculature in performance the skill in and inefficient manner.
I exhaustive exercise, due to his oxygen debt and oxygen intake reserves, a trained athlete can increase his resting oxygen utilization by many times more than can the untrained subject. (See table 2.)  Although the technique of measuring oxygen debt, intake, and utilization is somewhat complicated, the concept is very simple.  An illustration of this concept will help to clarify the relationships which exist between these three metabolic variables. (See Figure 3.)
A subject’s resting rate of oxygen intake is determined prior to exercise.  Resting oxygen intake is represented by the solid shaded area along the base line in Figure 3.  Oxygen intake is measured by collecting the subject’s expired air and comparing it with normal room air for the oxygen concentration. The oxygen extracted by the subject per minute represents his rate of oxygen intake.
The subject then performs some given activity.  In this example, the oxygen requirement of the activity is 10 liters per minute.  The duration of exercise is 3 minutes.
During the exercise, the subject’s oxygen intake will rise rapidly to an elevated level (e.g. 5 liters per minute and then fall off as exhaustion approaches).  In figure 3, the exercise oxygen intake above that normally used at rest (net exercise oxygen intake) is denoted by the dotted area.  The decline in oxygen intake near the end of exhaustive work may be due to inefficient, overly-rapid, shallow heart and respiratory actions.
The oxygen debt which is incurred during the exercise is the difference between the subject’s oxygen utilization and his oxygen intake.  Oxygen debt is denoted by the right-to-left diagonally shaded area in Figure 3.  Oxygen debt cannot be measured directly.
After the cessation of exhaustive exercise, the subject assumes a resting position, while oxygen intake is being measured.  The recovery oxygen intake above that normally used at rest (net recovery oxygen intake) is a measure of the oxygen debt which is being repaid.  The left-to-right diagonally shaded area in Figure 3 represents the net recovery oxygen intake or the oxygen debt being paid off during recovery.  At the start of recovery, the subject’s oxygen intake will rise briefly, reflecting the increased stroke volume shown in Figure 1, and then return to the resting level logarithmically.  In some subjects, severe exercise may elevate the resting oxygen intake slightly for a period of several hours (not shown in Figure 3) (62,114).  This metabolic disturbance complicates the determination of oxygen debt since it is probably incorrect to include the increase in the resting requirement as a part of oxygen debt.  Most oxygen debt determinations how are based upon a timed recovery interval to minimize the effect of this resting disturbance.
Specificity is a fundamental principle of training.  As early as 1945 Brouha (37) studied the concept of specificity of training between sports.  Brouha was fascinated by the simple observation that an athlete who is highly trained in one sport will require considerable time to adapt to and reach maximum efficiency in another sport—even when equally skilled in both activities.  he conducted an interesting experiment.  Two groups of highly trained athletes (runners and carsmen) were tested.  Two standardized tests (a treadmill run and a rowing tank stroke test) were administered to each group.  The two tests were of nearly equal work intensity.  Both tests were submaximal for all subjects.  The cardiovascular responses of the two groups were nearly identical to both tests.  However, the track athletes accumulated lactic acid more quickly and reached subjective exhaustion sooner when rowing than when running.  The oarsmen responded less favorably to running than to rowing.
Brouha drew two conclusions.  Training has a specific action in relation to lactic acid production during heavy muscular work.  The specific training process occurs partly, at least, in the muscles themselves.
It would appear that the principle of specificity of training has far greater application that Brouha suspected in 1945.  Specificity applies not only between sports but also between events within sports.  Apparently, specificity applies even between precise skills, paces, etc. within events.
It has been shown that a training program which will increase the strength of the elbow flexor muscles with the elbow at the waist will have no effect upon the strength of those muscles when the arm is overhead (120).  When even a relatively minor adjustment is made in the neuromuscular coordination of a highly trained athlete, the result will be a temporary loss of efficiency with poorer performances until the subject relearns the skill (31, 70, 74, 91, 119, and 143). (See figure 4.)  the energy cost of performing any given motor skill will gradually be reduced as the subject learns to recruit and isolate efficient motor units.  After a plateau of energy expenditure is reaches, no further amount of practice will lower the energy cost of performing the skill so long as the technique remains unchanged. If the subject then attempts to substitute a new and more efficient technique, the energy cost of performing will be increased again and the learning process must be repeated. Eventually a second and lower (better) plateau of energy cost will be achieved.  Many an athlete who has been trained for a distance event has reported experiencing relative comfort and ease of movement when performing at a specific pace, but great discomfort when working at a pace either slightly slower or faster than that for which he has been specifically trained.
The applications of the scientific principles underlying interval-training are reflected in the energy metabolism variables.  It is impossible to assign quantitative values to the relative contributions of oxygen debt and oxygen intake to specific events for all competitors.
In general, the duration and rate of work are the determining factors.  A high debt tolerance is most important in those events having a short duration and a high rate of work.  (the greatest oxygen debts have been measured in athletes trained to compete in exhaustive events of 30 seconds to 2 minutes duration.)  A high oxygen intake capacity is most important in those events having a relatively long duration and a moderate rate of work. (the greatest oxygen intakes have been measured in athletes trained to compete in exhaustive events of 4 to 20 minutes duration.)
Figure 4 - Hypothetical energy expenditure required to perform a specific motor skill while learning first a less efficient and later a more efficient technique.
Table 3 shows the energy metabolism of a hypothetical athlete who has an average oxygen intake capacity of 4 liters per minute and an oxygen debt tolerance of 16 liters.
If this athlete competes in an event in which he reaches exhaustion after 1 minute, he will utilize a total of 20 liters of oxygen.  Oxygen debt will supply 16 liters of 80 per cent of the utilization at a rate of 16 liters per minute.  Oxygen intake will supply 4 liters or 20 per cent of the total utilization.  If this athlete compares in an event in which he reaches exhaustion after 16 minute, he will utilize a total of 80 liters of oxygen.  Oxygen debt will supply 16 liters or 20 per cent of the total utilization at a rate of 1 liter per minute.  Oxygen intake will supply 64 liters or 80 per cent of the total utilization.
The duration of exhaustive work at which the oxygen debt and the oxygen intake would make equal contributions would depend upon the individual athlete.  The factors which would determine this duration of work would be the current levels of the subject’s:
1.  Oxygen debt tolerance
2.  Oxygen intake capacity
3.  Oxygen utilization or neuromuscular coordination and skill.
In general, oxygen intake and oxygen debt will contribute approximately equally to the performance of exhaustive events of about 3 to 4 minute duration.  Work loads may be classified for a given individual according to the relative contributions of oxygen intake oxygen debt. (See table 4.)  Note that the basic difference between a power event and a sprint type of endurance event is that in a power event the subject never reaches his maximum oxygen debt tolerance level, whereas in a sprint event he does.  There should be no drop-off of performance level from the start to the finish of a power event.
Table 3 – Hypothetical energy metabolism of an athlete (average oxygen intake capacity of 4 1. per minute and oxygen debt tolerance of 16 1.) during exhaustive work periods of one and sixteen minutes duration.
Table 4 – Classification of work loads by levels of energy metabolism.
There will be some drop-off of performance level during sprint events since the subject does reach exhaustion.  Note that basic difference between a sprint event and a middle distance event is that in a sprint event oxygen debt is more important than oxygen intake, whereas in a middle distance event the opposite is true.  Note that the basic difference between a middle distance event and a distance event is that the distance event is primarily aerobic in nature, whereas the middle distance event is anaerobic.
Oxygen debt tolerance, oxygen intake capacity, and oxygen utilization each may be conditioned specifically through appropriate training techniques (86).  According to the overload principle, the human body develops functional ability only when stimulated to do so.  Therefore, oxygen debt tolerance will be increased whenever an athlete forces himself up to his existing limit.  The more nearly and the more frequently that limit is reached, the faster will be the development of oxygen debt tolerance and the higher will be his eventual limit.
Oxygen intake will be increased whenever an athlete subjects himself to situations in which his existing oxygen intake mechanism is taxed to the limit.’  That is, the athlete must train anaerobically.  The more time an athlete spends in this type of work, the greater will be his eventual oxygen intake capacity.
Oxygen utilization will be minimized whenever an athlete improves his skill and efficiency in performing a given activity.  Since training is specific, skill and efficiency will be improved most rapidly if the pace, intensity of work, etc. simulates competitive conditions as nearly as possible.  Fatigue has been shown to impair motor learning (109, 116). Therefore, training should be interrupted at the onset of fatigue whenever improvement of efficiency is the primary goal.
Interval-training, because of its inherent nature, lends itself ideally to the specific improvement of the energy metabolism variables.  Christensen, Hedman, and Saltin (47) conducted an experiment in which they contrasted continuous running with interval running having 5, 10, and 15 seconds alternate work and rest periods. Their conclusions were as follows:
1.  Two physically trained subjects can run continuously for 3 respectively 4 min on the treadmill at a speed of 20 km/h, reaching maximal values for oxygen uptake and for blood lactic acid.  At the end of this time when they have run a total distance of 1 and 1.3 km respectively they will be totally exhausted and will need a comparatively long time for recovery.
2.  Running at the same speed but intermittent with short spells of activity and rest, the character of work will change entirely; despite a marked decrease in oxygen uptake during the actual work periods, the work can be performed without or with only a comparatively slight increase in blood lactic acid concentration, indicating aerobic work conditions.
3.  The trained subjects can stand an effective work time of 15 respectively 20 min within the experimental time of 30 min and run a total distance of 5 respectively 6.67 km without being totally exhausted.
Brouha (35) studied the heart rate reactions of firemen tending industrial furnaces. (see figure 5.)  The stress imposed upon these men was not only that of strenuous exercise but also that of severe heat.  Brouha found that proper adjustment of the work and rest periods resulted in considerably reduced cardiovascular stress, especially during the afternoon shift.  It is interesting to note that the results of Brouha’s and Christenson’s investigations are in agreement.  That is, work performed in intervals does have entirely different characteristics than that performed continuously.
Astrand and her associates (13) conducted an experiment which more nearly duplicates the typical interval-training program. (see table 5.)  One well-trained subject performed a standard amount of work on a bicycle ergometer during a 1 hour period on several different occasions using various work patterns.  When the work was performed continuously, the rate of work was low enough so that there was little stress placed upon the subject.  All interval work has conducted with a work to rest ratio of 1 to 1.  Therefore, in order to complete the same total amount of work in a 1 hour period, the interval work was performed at twice the intensity of the continuous work.
Table 5 – Total oxygen intake and work efficiency during continuous and interval work
From the data of Astrand, I., and others Intermittent muscular work. Acta Physiol. Scand. 48 448. 1960.  When the work and rest intervals were set at 30 seconds each, the subject was under little stress and developed a low concentration of lactic acid in the blood.  However, when the work and rest intervals were extended to 3 minutes each, the stress was quite high as indicated by the large accumulations of blood lactic acid.  The fact that interval-training is superior to continuous training is an obvious conclusion which can be drawn from Astrand’s work, as well as from the investigations of Christensen and Brouha.  Using continuous training, an athlete can subject himself to either:
1.  A high quantity of low quality work, or
2.  A low quantity of high quality work.
Using interval-training, an athlete can perform a relatively high quantity of high quality work which is specifically designed to simulate the pace, intensity of work, etc. of actual competition.  A specific interval-training stimulus will produce a specific energy metabolism adaptation.  Frequent intervals of high-intensity anaerobic work of 1 to 15 minutes in duration, with breathing forced and proportional rest intervals between, will produce a considerable improvement in oxygen intake capacity.  Some improvement will be noted also in oxygen intake capacity.  Changes in oxygen utilization will be produced very slowly, at best, by this type of training.
Less frequent intervals of moderate-intensity anaerobic work of 1 to 15 minutes in duration, with breathing forced and proportional rest intervals between, will produce a considerable improvement in oxygen intake capacity.  Some increase in oxygen debt tolerance can be expected also.  Oxygen utilization should be lowered slightly by this regimen.
Infrequent intervals of low-intensity aerobic work of over 10 minutes duration will reduce oxygen utilization.  But, since the work rate does not simulate competition, there is a question as to the value of this type of training.  There will be little change in intake capacity or in debt tolerance as a result of aerobic work.
Figure 6 illustrates the hypothetical energy metabolism of an athlete during an interval-training workout.  Note that the first several work intervals should be performed at a high enough work rate to bring the subject up to his oxygen debt tolerance quickly.
During each rest interval the subject will pay off some, but not all of his accumulated oxygen debt.  He should remain mildly active during this period to avoid blacking out from blood pooling in his lower extremities.
During each work interval after the first few, the subject should reach, or nearly reach, his oxygen debt tolerance level.  The rate of oxygen debt accumulation is accelerated each time the tolerance level is approached.  Work becomes progressively less efficient as oxygen debt builds up.  Oxygen intake declines with approaching exhaustion.  The implication for pacing middle distance and longer sprint events is clear: the accumulation of a significant level of oxygen debt should be put off until as late in the event as is possible.  Of course, by the finish of the event, the athlete must reach his oxygen debt tolerance level if he is to make a maximum exhaustive effort.  It would appear that the most economical pace would be a uniform one throughout most of the event, with a sprint at the end.
,br> The implementation of an interval-training program for a given athlete preparing himself for a particular sports event is merely a matter of application of the concepts already discussed.
A schematic design of a progressive interval-training program for the 400-yard freestyle in swimming or the 1-mile run in track is given in table 6.  It should be assumed that the athlete for whom Table 6 was devised was capable of completing his race in 4:45 at the start of the training program; therefore, an initial objective of 4:40 was reasonable.  The initial speed goal time was set automatically.
Table 6 - Schematic design of a progressive interval-training program for swimming or track using one distance and large steps.  For purpose of this example, a single work interval of one-fourth of the tentative objective time was selected.  Therefore, in accord with the concept of specificity of training in regard to pace, the athlete would be expected to complete one-fourth of the total race distance during the work interval.
The initial rest interval was selected empirically.  It has been observed that a work to rest ration of about 1 to 1 will enable the athlete to compete eight or ten repetitions of the work interval at the specific rate of work desired when:
1.  a new but reasonable speed goal is introduced, and
2.  The work interval is chosen as one-fourth of the tentative objective time.  However, it might be that a considerably longer rest interval would be required at the beginning of training.  Subjectively, it would appear that the rest interval should be increased if the athlete is unable to complete at least six repetitions of the work interval at the specific rate of work desired.
3.  The start interval was determined by adding the length of the rest interval to the length of the work interval.
4.  The number of work periods also was determined arbitrarily.  Each coach and athlete working together must arrive at an optimal number for themselves.  There is no scientific information available at present in regard to this matter.  The writer selected the initial goal number of sixteen only because he has had favorable experience with it when using a work interval of one-fourth of the objective time.
Table 6 represents only the schematic design of an interval-training program.  Table 6 should not and probably could not be followed verbatim.  Undoubtedly several years of training is represented if improvement from 4:45 to 4:00 is achieved.  Only one work interval is given. Obviously, to relieve boredom several intervals would be used simultaneously and alternated from workout to workout.  In the writer’s opinion, the graduations shown between steps are definitely too coarse.  Objective times should be lowered by no more that 4 to 6 seconds per step in events of this duration.  The rest interval should be shortened by 5 to 10 seconds per step.  Similar schematic designs can be devised for specific events in most other sports.
Specificity of interval-training has been shown to be a valid concept.
Most present day track and swimming coaches attribute the recent assault on the record books to the widespread adoption of interval-training.  Of course, scientific coaching can never take the place of initiative and so called natural talent on the part of the athlete himself.  Scientific coaching can help the athlete make the most of his talent.
Table 7 shows the actual changes which took place in the energy metabolism of two athletes during a two and one-half year specific interval-training program.  Table 7 - Changes in the energy metabolism of two athletes produced by two and one-half years of specific interval-training.
The distance man was trained with the specific intention of increasing his maximum oxygen intake capacity.  His oxygen intake capacity improved a remarkable 1.8 liters per minute.  He also made a moderate improvement of 5.1 liters in maximum oxygen debt tolerance.
The sprinter was trained with the specific intention of increasing his maximum debt tolerance.  His oxygen debt tolerance rose a striking 9.3 liters.  His oxygen intake capacity improved a moderate 1.0 liters per minute.
01.  How could the fartlek principle be applied to modern dance?
02.  Explain the changes which would have to be made to modernize the workouts which Zatopek took prior to the 1948 and 1952 Olympic Games.
03.  What coaching advantages can you see for using a start interval instead of a rest interval?
04.  By convention, track athletes have spent the rest interval jogging whereas swimmers have remained relatively inactive in the water.  Explain the physiology involved in both cases.
05.  Assume you are coaching a sport in which competition is usually held during the late afternoon of early evening hours.  Your team becomes involved in come championships in which the qualifying rounds are held during the morning.  What adjustments would you attempt to make?  Why?  How?
06.  Many coaches have observed that athletic improvement occurs in spurts with plateaus of performance in between.  Explain this phenomenon in view of current coaching practices.
07.  What objection, if any, would there be to an interval-training program involving alternate work and rest periods of 10 seconds each.
08.  Assume arterial blood is always oxygen saturated.  (This is not true.)  Would breathing a high concentration of oxygen just prior to running a mile improve performance?
09.  Speculate as to why aerobic work is more efficient than anaerobic work.
10.  When measuring oxygen debts, the use of a timed recovery period will minimize the effect of any prolonged metabolic disturbance in resting oxygen intake.  However, this technique introduces another serious difficulty when athletes trained for different types of events are compared.  Speculate as to the nature of this difficulty.
11.  Many contestants in swimming and in the field events in track use weight training programs as an aid to preparation for competition.  Can such practices be justified in view of the principle of specificity of training?  How?
12.  Assume the improbable situation of a subject who has been perfectly and maximally conditioned in regard to his skill of performing in and his cardiovascular-respiratory response to a specific strenuous activity.  That is, no further changes are possible in oxygen utilization, oxygen debt tolerance, oxygen intake, or carbon dioxide and training could improve even this subject’s performance.  Hypothesize as to which gross organic systems might be involved if such speculations are true.
13.  It is known that fatigue hinders motor learning.  Speculate as to how knowledge or this fact by a coach could influence the lower (dotted) line in Figure 4.
14.  At what duration of exhaustive work would the oxygen debt and the oxygen intake make equal contributions to the performance of the subject in Table 3?  What would be the effect of raising his maximum oxygen debt tolerance to 20 liters?  What would be the effect of raising his average working oxygen intake capacity to 5.25 liters per minute?
15.  A power event has been described as one in which the maximum oxygen debt tolerance is not reached.  Therefore, cardiovascular-respiratory endurance is not a factor.  Certainly, events lasting 15 seconds or less would be classified in the power category.  Nevertheless, relatively untrained subjects often have the experience of “dying” or unavoidably slowing down before the end of a 100-yard sprint in track.  Explain this occurrence.
16.  Assume that the subject used by Astrand to collect the data given in Table 5 was a cyclist who was high trained for middle distance events.  What values could be expected if Astrand were to conduct the same experiment again using a highly trained sprint cyclist as a subject.
17.  Devise a workout (not necessary interval in nature) which is specifically designed to lower an athlete’s oxygen utilization while running the 400-meter hurdles.
18.  Explain the slight increase in oxygen intake at the start of the sixth minute in figure 6.  Why is there no similar increase at the start of the second minute?
19.  Using distance as the ordinate and time as the absicissa, plot the following three 1-mile ruins.
20.  Establish a schematic design for a progressive interval-training program for wrestling.  Do the same for basketball, tennis, and hockey.
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