Incalza. Stride Parameters in Endurance Runners

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Introduction

ttempts to classify sports are count-less. In the areas of science, tech-nique and methodology, when we

want to emphasise the physiological compo-nents and conditional capacities, we general-ly place the disciplines in three large groups(SAIBENE et al., 1986). The first groupincludes activities in which a maximum levelof intensity is reached and exhausted in a fewseconds (intensive sports); the second groupcomprises intermediate activities, with mixedmetabolic implications; in the third group areactivities that are characterised by the repe-tition of effort for protracted lengths of time(endurance sports with a strong metaboliccomponent of aerobic origin).

The same school of thought places researchwith regards to the first group in the area ofthe study of biomechanics, neurophysiology,and endocrinology, while the others areassigned to the study of bioenergetics and themajor organic functions. However, if co-ordi-nation abilities are used as the criteria for dis-

© by IAAF

22:4; 41-60, 2007Stride parameters inendurance runners

By Piero Incalza

AU

TH

ORPiero Incalza is the co-ordinator

for technical scientific and educa-tional activities at the Italian ath-letic federation’s centre for studyand research. He is also a Ph.Dcandidate and a part-time lectur-er in functional evaluation andtraining methodology. He hascoached many international-levelathletes including 1996 New YorkCity Marathon winner GiacomoLeone (ITA).

AB

ST

RA

CT

A

It is commonly thought that ath-letes automatically adapt to a sim-ple, natural and cyclical action, suchas running, and that any increasesin performance level are almostentirely due to improvements of theconditional capacity and metabolicsystems. The importance ofmechanical and co-ordinativeattributes and the possibilities forconserving movement and re-utilis-ing elastic energy deriving from theeccentric contraction movementshave rarely been explored and havenever been a focal point of researchin endurance sports. This studyincluded 61 male and female long-distance runners, 31 of whom per-formed at a national/internationallevel and 30 of whom were amateurlevel competitors. In periodic testsconducted for functional evalua-tions, video recordings were madeof the athletes’ running movementsat different speeds. Several kine-matic parameters were analysed.The statistical results, behaviouralanalysis and individual adaptationsobtained are useful for elaboratingand proposing methodologicalelements in training for correctingand perfecting distance runningtechnique.

tinguishing the disciplines, it becomes moredifficult to isolate and define each specialty.

In the sports literature, endurance runningis classified among the cyclical aerobic spe-cialties with low co-ordination and technicalcontent. VERCHOSSHANSKJ (2001), for exam-ple, stated: “the composition of cyclicalmovements (that require the development ofendurance) seems to be simpler than that ofacyclic locomotion”.

The opinions and studies published so far,for example McARDLE et al. (1996), reinforcethe idea that each athlete automaticallyadapts to a simple, natural and repetitivemovement, like that of running, and thatimprovements in performance are entirelyrelated to the improvement of conditionalabilities and metabolic implications:

“An exercise that can be performed for aprolonged period of time is relatively simplefrom a mechanical standpoint, since the ges-ture is repeated from the beginning to theend; the movements – some of which werelearned in childhood - are completely auto-matic and racing strategy is relatively sim-ple… it’s pretty evident that in these situa-tions the nervous and psychological factorsare much less important than the physical,physiological and biomechanical ones”(SAIBENE et al., 1986).

Although various studies have shown thatmechanical efficiency is the component thatdifferentiates elite athletes (among them-selves) in endurance running, even thesestudies continue to hint that an individualathlete’s running technique cannot be effec-tively modified to produce a significant resultand that, at any rate, adaptations are conse-quences of bio-structural and metabolicmodifications that occur during training.

Furthermore, when biomechanical parame-ters were taken into consideration, the resultsand observations made by experts lead toconclusions that are diametrically opposed.FORTNER (2006), after a lengthy observationof the rhythmic behaviour of the first three

finishers at the 1999 New York Marathon(Kagwe, Chebe and Bayo), noted that the win-ner “adopted a longer stride (at the samespeed) as opposed to a greater frequency”.NOAKES (2001), citing other authors (MOR-GAN, 1995 and NELSON & GREGOR, 1976),states: ”through training, athletes lengthentheir stride and reduce frequency. Someresearchers maintain that this optimises run-ning economy because the increased stridelength is more economical than an increase infrequency”. On the other hand, McARDLE etal. (1996) claim that: “it generally costs moreto lengthen one’s stride than it does to short-en it” and VITTORI (1997) states: “There is nodoubt that the energetic cost required tomaintain the same speed is greater for a longstride than it is for a shorter stride at a high-er frequency… it costs more to create lengththan it does to create frequency”.

According to CAVANAGH & WILLIAMS(1982), athletes themselves automaticallychoose the best speed and any attempt atmodifying the stride length or stride frequen-cy “produces a negative effect on themechanical efficiency”.

Within the study conducted in an attemptto establish the best individual energetic costby way of a variation in stride frequency andstride length at a given speed there exists aconceptual error that excludes all resultsexcept those obtained: modifying a “sponta-neous” movement by way of any voluntaryintervention produces the effect of increasingenergy expenditure.

The question that we might pose at thispoint is: are we absolutely certain that anathlete is able, always and in any circum-stance, to match his/her motor potential tothe technical and mechanical parameters ofendurance running in an automatic andinstinctive manner? More precisely, can thework of the middle- and long-distance coach(on a level with the trainers of any other spe-cialty or discipline) be targeted towards mod-ifying the characteristics and technical,rhythmical and mechanical running abilitiesof an individual athlete?1

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Stride parameters in endurance runners

This article is not meant to be a “scientific”study in the strict sense, nor does it mean topoint to definite and certain conclusions.Instead, we have gathered, arranged andorganised field data, applying the work meth-ods usually employed by sports technicians.Accordingly, the instruments that have beenused are the same ones that we commonly use.

Using the parameters given by the exami-nation of stride length and stride frequency,we are investigating the possible develop-ment of a reference model. The ultimate aimis to find elements of analysis that help toprovide field technicians with an additionalinterpretation of the methodological man-agement of running technique, in order tooptimise training programmes, especially forelite athletes. With such, coaches will be bet-ter equipped to identify the characteristics oftheir athletes and to assign the exercises thatare most appropriate for improving perform-ance.

Running: Stride frequency andstride length

In physics, frequency is defined as the num-ber of times that a periodical phenomenonoccurs in a unit of time. The unit of measure– the Hertz (Hz) -- is equivalent to one cycleper second. In running, stride frequency timesstride length determines the speed of move-ment. An athlete who wants to improvehis/her personal record at any distance has nochoice: he/she must do something to improvethe product of the two elements.

Coaches, especially those of middle-dis-tance runners, generally record the time thattheir athlete takes to cover a given distance intraining, in other words, the product of lengthand frequency. But if we want to disassemblethe technical elements of running in order tostudy possible adjustments and corrections, itis a good idea to record both, given the fact

that it isn’t clear which strategy for improve-ment (increasing length or frequency) shouldbe preferred and whether this generallyapplies to all types of runners.

One of the first studies on the rhythmicbehaviour of endurance running was per-formed by a Danish researcher, BOJE (1944).He got an endurance runner to run on atreadmill at various speeds. He recorded a fre-quency of 2.66Hz at 9.3km/h (97cm stridelength) and 2.93Hz at 17.8km/h (168cm stridelength). A 91% increase in speed correspond-ed to an increase in stride length of 83%while the frequency increased by only 10%.

There are many ways to measure an ath-lete’s stride frequency during a race or intraining. You can count the number of stridesover a set time or, more simply, record thetime it takes to perform a certain number ofstrides. For example, you record the time ittakes for the same foot to touch the ground10 times. To determine the frequency, divide20 by the time registered for 10 strides. If youwant to calculate the strides per minute (Hz x60), divide 1200 by the recorded time. Forspeed trials that are measured from a startingposition, the calculation of the kinematicparameters of running is relatively more com-plex. For prolonged trials, on the other hand,the travelling speeds are much more uniform.It is for this reason that, when we know thespeed that corresponds to the distance atwhich the frequency was measured, we caneasily derive the average stride length by wayof an indirect calculation.

Stride frequency and stride length are theresults of a series of related forces that inter-act and create a succession of movements,which, in their entirety, we call running. Anuntrained eye that observes two people run-ning, even if they are at different perform-ance levels, will probably not be able to dis-tinguish the differences.

1 It is important to emphasize the fact that the individual modifications that training can produce are difficult to detect with the naked eye. The units ofmeasure are millimeters and hundredths of Hz. It must be remembered that, in order to cover the distance of a marathon, a top athlete takes from 25,000to 30,000 steps, an amateur may take 40,000 or more. It’s not difficult to imagine the repercussions that imperceptible technical variations might haveon the result.

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In reality, a person’s anthropometric andbio-structural characteristics, his/herstrength level, his/her available energy and –not least – his/her capability to perform thespecific action, make an individual’s way ofrunning unique. Training effects each of thecomponents cited and can have a big effecton raising the peak of sustainable maximumspeed as well as on the possibility of prolong-ing work sessions at a certain speed. Inendurance running, the latter is paramount:reducing the energy cost at a given intensityis crucial.

The adaptation process reaches highly tan-gible levels in the first cycle of organisedtraining. For example, take a group of youthswith little experience and scarce adaptationto prolonged exertion. Have them run a dis-tance between 1km and 3km as fast as theycan. In the three weeks following the test,let the subjects train regularly by runningaerobically. In the fourth week, have themrepeat the test they took on the first day. Allcoaches with experience in this field willagree that the resulting improvements (withrare exceptions) will be striking. Theimprovements registered for subjects whohad relatively low starting values will be evenmore remarkable.

In such a short space of time (15-20 days)what are the transformations that act uponthe group? Did their heart capacity increase?Did their capillaries widen? Did their musclesdevelop? Did their mitochondria double innumber? Did the enzymes speed up theiractivity?

Maybe, to a certain extent. However, nocombination of these is enough to explainaverage increases in speed of 10-20% over a2km distance.

It is more probable that the rate of increasein performance is due to two factors:1) A better distribution of effort (strategy):

For inexpert athletes (and not only!), it isdifficult to choose the optimal speed. Anoverly prudent approach and (worse yet)an excessively fast start can weigh heavi-

ly on the final result. Learning to bettercalibrate one’s speed based on theresources available is an objective thatcan be reached even in a short time;

2) An increase in mechanical output: Inshort, it is the start of learning processaimed at perfecting a running style thanksto which the athlete will be able cover thesame distance at ever faster speeds.Alternatively, he/she will be runninglonger distances while keeping the sameaverage speed. Useless movements andthe abnormal contractions of some mus-cular groups diminish, the kinetic chainswill be activated more effectively in thedirection of movement, breathing willbecome deeper and the arms will be syn-chronised with the movements of thelower limbs.

In other words, one is able to use theresources he/she already possesses in a moreeffective manner towards the accomplish-ment of the task at hand.

Mechanical output can be defined as theratio between useful energy (motion) and theeffort spent in transforming energy. Thehuman system is regulated in such a way thatall of its sub-systems adapt themselves inorder to function in the most effective man-ner. It remains to be seen whether the abilityto run (especially at submaximal speeds) per-tains to the instinctive adaptation/perfectingprocess (that is, if running must be consideredan activity requiring a low degree of co-ordi-nation) or whether it can be coached andimproved through a teaching/learningprocess.

Mechanical output in running may varyfrom person to person and at different speeds,from 25 to 40-45%2, a huge difference. Thetwo main elements that affect the output inrunning are:a) Preservation of the quantity of motion;b) Re-utilisation of elastic energy.3

Objective a) is pursued by reducing brak-ing, or negative work, to a minimum (fromthe beginning of the phase in which the

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foot makes contact with the ground untilthe passage of the centre of mass beyondthe projection of the point at which theforce was applied. Objective b) comes aboutduring positive work following the exploita-tion of energy accumulated in the deforma-tion of elastic elements during the samerunning phase.

The paradox of mechanical output lies inthe need to reduce negative work to a mini-mum in order to preserve the quantity ofmotion (first principle of dynamics) and, atthe same time, increase the “loading of thesprings” that supply free energy in the posi-tive phase.

This reflection alone would be sufficient tosubstantiate the conviction that it isextremely difficult (if not impossible) to try tostandardise all of the possible variables andformulate an unequivocal reference model.Deductions, reasoning and possible hypothe-ses lead to an individual analysis of the sub-ject and not so much to the comparison ofthe behaviour of various individuals. It is pos-sible to come to the same result by “mixingthe ingredients” in a different way. Neverthe-less, the comparative study of different indi-viduals’ behaviour and the statistical analysis

of the data can provide us with ideas forpointing out reference models and parame-ters to apply during individualised evalua-tions.

Methods

The participants in this study were 61 ath-letes of various qualifications that were thendivided into four groups based on their per-formance level (Table 1). The data (500 entriesin all) were gathered during the period ofMarch 2005 to September 2006. The tablespecifies average and standard deviation forage, VO2max and height. The specifics of thesample include:• Elite Male Runners (n=21): middle-dis-

tance, long-distance and marathon run-ners from the Italian national team, allhave an MAS (maximum aerobic speed)higher than 20km/h, they train on a dailybasis (generally two sessions per day) withan average volume of 120-220km/week

• Elite Women Runners (n=10): middle-dis-tance, long distance and marathon run-ners from the Italian national team, allhave an MAS higher than 18km/h, theytrain on a daily basis (generally two ses-sions per day) with an averaging volumeof 100-200km/week

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2 Amount of chemical energy transformed into mechanical energy. The rest (80-95%) is dispersed in the form of heat. In research methodology the netoutput is also identified, that is, it subtracts the amount of energy necessary to maintain the person in an erect position. See CAVAGNA (1988).

3 Although the factors that make up for the reduction or increase in output are different, I preferred to focus on these two dynamics upon which almostall of the other components converge.

Elite male

Elite women

Non-elitemale

Non-elitewomen �

Partcipants n. 21 10 22 8 61

Age (average)SD (years)

28.45.4

27.16.0

45.05.6

40.910.0

35.910.3

MAS (km/h) averageSD (km/h)

21.61.0

19.10.9

15.21.5

13.11.6

17.73.4

Height (cm) averageSD (cm)

176.25.5

166.36.6

177.46.0

169.69.1

174.37.6

Entries n. 200 160 100 40 500

Table 1:

• Non-Elite Male Runners (n=22): runningis a hobby and they train from three to sixtimes a week with an average volume of40-80km/week, all have an MAS lowerthan 17km/h

• Non-Elite Women Runners (n=8): runningis a hobby and they train from three to sixtimes a week with an average volume of30-60km/week, they all have an MASlower than 15km/h

The surveys on average stride frequency/stride length at various running speeds weremade during routine evaluation tests on stan-dard athletic tracks during team meetings ororganised gatherings.

Two types of evaluation test were used:1. Continuous speed progression following

the Conconi protocol (heart rate progres-sion as a function of running speed).

2. 5-6 incremental fractions of 2km, at con-stant speed within each single step follow-ing the Fiorella-Gigliotti protocol (lactic acidprogression as a function of running speed).

Only the first test was used with the non-elite runners. Around 40% of the total surveywas also filmed with a Panasonic FZ30 30frame/s digital camera.

The measurement of the stride frequency/stride length was performed on the straightpart of the track, recording the time it tookto take 20 steps (frequency): every 200m inthe first test and every 400m in the secondtest. Afterwards, the stride frequency val-ues were linked to the time it took the ath-letes to cover the corresponding 100m dis-tance.

Comparison of stride length andstride frequency

Men

Figure 1 represents the parameters of stridelength (in red) and stride frequency (in blue)at various running speeds relative to thegroup of top male athletes.

Around 200 surveys were performed onthe 21 elite middle and long distance ath-letes. The correlation rate is markedly highfor the stride length parameter(R=0.878/P=0.000)4 as well as for that ofstride frequency (R=0.7333/P=0.000) inrelation to the running speed. Figure 2shows the same co-ordinates referred to thegroup of non-elite runners with around 100surveys on 22 athletes.

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4 R = correlation coefficient between two variables, P = correlation significance index

Figure 1: Stride length and stride frequency at different velocities in elite male runners

The consistency of the length parameter isevident in the group of elite athletes(R=0.924/P=0.000) while the frequency(R=0.345/P=0.001) is not. In both cases, theparameters are highly significant.

If we use the same graph, superimposingthe progress of the two reference groups andseparating stride length (Figure 3) and fre-quency (Figure 4), the earlier observationsappear even more evident.

The comparison of stride length in the tworeference groups emphasises a fundamentaland significant correlation with alterations invelocity. This fact leads us to suppose thatthe stride length is a relatively “stable”parameter with a restricted range of variabil-ity. On the other hand, if we compare the datareferring to turnover rate (Figure 4), we cansee a clear distinction between the behaviourof the two groups: the dots on the chartrelating to the elite runners remain correlated

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Leng

th m

.

Figure 3: Stride length at different velocities in elite and non-elite male runners

Figure 2: Stride length and stride frequency at different velocities in non-elite male runners

while those of the non-elites show a markedscattering. This evidence brings us to presumethat the frequency of movement is the dis-criminating element in the technical evolu-tion of the mechanics of endurance running.This adaptation, however, remains linked tothat of stride length, which is the parameterthat varies most in relation to speed variation.

Women

The same analysis procedures were used tostudy the behaviour of the two female groups.

Bear in mind, however, that the data gatheredin this study was lower in number than thatavailable for the two male groups. It is rea-sonable nevertheless to assert that the statis-tical data that emerged from the two femalegroups substantially confirms what emergedfrom the two male groups:a) stride length follows a stable and homog-

enous correlation at all levels of perform-ance;

b) stride frequency shows levels of correla-tion that are proportionate to perform-ance level.

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Figure 4: Stride frequency at different velocities in elite and non-elite male runners

Figure 5: Stride length and stride frequency at different velocities in elite female runners

To provide more detail, 160 surveys of stridefrequency/stride length were made on 20 ath-letes from the Italian middle- and long-dis-tance teams (or, at any rate, top national run-ners) (Figure 5). The correlation of the stridelength in relation to the various speeds ispractically identical to that of the male group(R=0.879 vs. 0.878); that of frequency is less(R=0.582 vs. 0.733). This deviation could bepartly ascribed to different internationalqualification between the male group (whichwe could define as excellent) and the femalegroup. This fact would confirm the hypothe-

sis that the higher the athlete’s performancelevel, the greater is the degree of correlationbetween speed variation and turnover. Theresulting data is highly significant for bothparameters.

The non-elite female group was the small-est, composed of only eight athletes, and 40surveys on frequency and stride length at dif-ferent running speeds (7<15km/h) weremade. As in the other group, this one showeda high correlation in the stride length param-eter (R=0.878 – P=000) and a lower one

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Figure 6: Stride length and stride frequency at different velocities in non-elite female runners

Figure 7: Stride length at different velocities in elite and non-elite female runners

(R=0.429 – P=0.029) (Figure 6) in the stridefrequency parameter.

Figures 7 and 8 compare the data of thetwo female groups, separating stride length(Figure 7) from stride frequency (Figure 8).Compared to the same data from the malegroups, we note a closer relationship inturnover behaviour. In this case, too, we canpropose the hypothesis of a less marked dif-ference in the qualification of the femalegroup with respect to the male group.

All Groups

Figures 9 and 10 show the scattergramreferring to all four groups in their respectiveexpressions of frequency and stride length.Whilst we must not overlook the overall cor-relation of frequency (R=0.681, P=0.000), wehave to emphasise the significance of thestride length data (R=0.939, P=0.000) gath-ered from the test groups. This evidencewould support, among other things, thehypothesis that considers the subject’s height

to be of little relevance in stride length varia-tion when related to speed of movement5.

Figure 11 shows the values of the twoparameters (stride frequency/length) in anattempt to find an internal correlation profile.The meaning that we can give to this correla-tion is that less-trained (or less efficient) ath-letes tend to uphold only one of the twoparameters, fundamentally because of a lim-ited (or imbalanced) gradient of muscularstrength (the correlation among the dots isnegative). On the contrary, the data on highlyefficient athletes shows a progressive line,which leads us to presume a greater controland a greater balance6 in the use of strengthbetween the negative and positive phases ofthe running stride.

Correlation rates in the various groups andtheir significance:Non-elite Males: R= -0,034 P= 0.733Non-elite Females: R= -0.017 P= 0.936Elite Males: R= 0.320 P= 0.000Elite Females: R= 0.128 P= 0.100

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5 Some studies had hypothesized an important height incidence (more specifically on the length of the inferior limbs) on the stride length (FORTNER, 2006).In the context of the aerobic speeds this element would statistically seem not very important since both the men and women (average height differenceof 10cm) have shown a similar behaviour in the variation of stride length at equal speed.

6 VITTORI (1997) summarises the activity of the muscular groups that are involved in the frequency and length of a step. In the first case (frequency) theforce of the anti-gravitational muscles that improves the stiffness and decreases the “collapsing” of the knee at the moment of the foot’s impact with theground. Stride length, on the other hand, is regulated by the strength of the flexor muscles. As we’ll see further on (note 8), we propose the hypothe-sis that, at least in the case of long distance runners, the anti-gravitational muscles are also involved in the regulation of stride length.

Figure 8: Stride frequency at different velocities in elite and non-elite female runners

The statistical analysis emphasises the sig-nificance of the data as a whole and for theelite male group. The correlation of the othergroups, on the other hand, is less significant.

Individual behaviour: Analysis andobservations

The underlying theme of this study springsfrom the conviction that, at the basis of ani-mal movement (in our case, human), there are

electro-mechanical characteristics and thatenergetic metabolism is “in tow”. Thesemechanisms act in such a way as to satisfyand support what is requested of them in thebest possible way. In other words, it’s not theanaerobic metabolism’s activity that bringsabout the increase in muscular force but, onthe contrary, it is the recruitment of a greaternumber of motor units that draws more ener-gy in order to keep the movement “alive”. Inorder to understand the range of motor

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Figure 10: Scattergram of stride length at different velocities for all groups

Figure 9: Scattergram of stride frequency at different velocities for all groups

expressions, it is necessary to examine andmeasure the physiological parameters, keep-ing in mind that minimal variations in inten-sity can produce enormous differences in theduration of the effort.

A second concept that we should clarifyrefers to the condition by which all of thedeductions brought forth from the analysis ofthe statistical data allow us to formulate ageneral idea and trace lines of tendency forthe proposal of a reference model. They donot provide us, in any case, with mathemati-cal formulas that can be applied to every sin-gle athlete.

In this, a critical examination by the coach(or training expert) is still essential in order toprovide guidelines useful for perfecting thetechnical action.

“All humans who are able to run possess arange of velocity in which their mechanicaloutput reaches a peak” (CAVAGNA, 2006). Arunner becomes a specialist in a particulardistance when his innate characteristics andthose that he has acquired (through training)bring him to a technical level that, within a

group of his peers, gives him an optimal com-bination between average speed and distancecovered. Obviously, the limit is represented bymaximum mechanical power. Output, howev-er, distinguishes athletes within the samepotential range7. In general, elite athleteshave a greater range of velocity in which theyare efficient. The modulation of the propul-sive impulses (kinesthetic differentiation andthe proper succession of stimulus frequenciesof the locomotive plates) combined with arhythm that is appropriate for the variation inspeed, yield optimal use of total metabolicenergy as well as of the energy that comesfrom the elastic response of the structures ofthe locomotive apparatus.

The following are examples of how it ispossible to gather elements for evaluation bymonitoring the kinematic parameters of run-ning in an isolated fashion or together withinformation drawn from metabolic indica-tors.

Stride length and stride frequency obtainedfrom the same athlete in various situations

Figures 12 and 13 superimpose the datacollected on a marathon runner during a test

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7 A study performed by MORGAN & CRAIB (1992) on runners of the same aerobic potential over 10,000m revealed that, statistically, at least 64% of thevariation in performance is due to running economy.

Figure 11

of continuous progression from 13 to 23km/h(purple) and at increasing work loads (blue)represented by six steps of 2km (increase inspeed of 0.2km/h from 19 to 22km/h). Thelength of the stride as well as the stride fre-quency at equal speed remained stable inboth conditions (the dots correspond perfect-ly with each other).

The data of a second athlete of equivalenttechnical level in the same conditions isshown in Figures 14 and 15. There is amarked technical variation between the test

in progression and that at constant speed.The latter shows that the frequency had asharper increase than the stride length incomparison with the test of continuous pro-gression. This analysis refers to the stabilisa-tion of running technique and rhythmic abili-ties in various work situations.

Stride Length and Heart Rate

In Figures 16 to 20, the relationshipbetween running speed, stride length andheart rate variation is shown. All studied

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Figures 12 and 13: Changes in stride length and frequency with increasing velocity (purple) and work load (blue)for Athlete A

Figure 12

groups are included and for elite runners wealso specified the type of race (middle-dis-tance or marathon). Almost all subjects in thestudy showed a very similar behavior. Stridelength as a function of speed follows thesame path as heart rate:a) linear relationship up until the anaerobic

threshold;b) curve deflection at higher speeds.

In other words, above a certain level, anincrease in speed can be attributed more toan increase in stride frequency than to stridelength8. This seems to show that correlationbetween running speed and stride length –high in all groups of runner – is, in fact, evenhigher if we only take the aerobic intensityarea into account.

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8 One of the hypotheses that could explain this behavior is that long-distance runners – because of the great amount of medium/low intensity work) traintheir flexor muscles little while their antigravity muscles are always under tension. At higher speeds they try to compensate this “mechanical limitation”by increasing stride frequency. This obviously results in a higher energy cost. In short we could say – thus contradicting VITTORI’s statement (note 6) onlyin part – that in prolonged running, athletes use the strength of their antigravity muscles even to control stride length, when speed is relatively low.

Figure 14

Figures 14 and 15: Changes in stride length and frequency with increasing velocity (purple) and work load (blue)for Athlete B

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Figure 16: Stride length and heart rate in elite male middle-distance runners

Figure 17: Stride length and heart rate in elite male marathon runners

Figure 18: Stride length and heart rate in elite female middle-distance runners

Stride Frequency and Heart Rate

Figures 21 and 22 show the behaviour of amale elite middle-distance (under 23) in refer-ence to stride length and frequency. It is clearthat, up to 19km/h, the athlete’s stride frequencyis more or less constant (under 3Hz), whichmeans that an increase in speed is almost entire-ly a function of stride length. Between 19 and21.5km/h, stride frequency changes, adaptingitself to the change in speed. At the same time,heart rate changes pattern: while it seems toshow a deflection (as if it had passed the anaer-

obic threshold), what actually happens is that itfollows a new straight line to the right of the firstone and is therefore more efficient (red arrow).

Figure 23 shows the example of an athletefrom the male elite short middle-distancegroup (800m and 1500m). Unlike most of hiscolleagues, he originally specialised in longerdistances and unlike non-elite runners, thecorrelation between his stride length and run-ning speed remains high (0.994 is the overallcorrelation) even for speeds that are higherthan anaerobic threshold and VO2max speed.

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Figure 19: Stride length and heart rate in non-elite female distance runners

Figure 20: Stride length and heart rate in non-elite male distance runners

This example is consistent with the hypothe-sis (note number 8) that athletes who aretrained to reach and maintain higher speedsare more capable of activating their flexormuscles, thus increasing stride length even attypically anaerobic speeds.

Methodological implications, tech-niques and field practice

Far from offering solutions for every cir-cumstance, our results show the need, in anycase, for the coach or technical specialist of

endurance runners to pay special attention tothe development of the motor potential of theathlete as far as co-ordination, technique andtactics are concerned. Action to improve thebiomechanical parameters of stride frequen-cy/stride length can be:a) Direct: Through input during the global

execution of motion (at all rates of speedand, particularly, close to race speed);

b) Indirect: Through all the training meanslikely to improve performance: develop-ment and balancing of speed, intra andinter-muscular synchronisation, supple-

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Figure 21: Stride length and heart rate in an elite male U23 middle-distance runner

Figure 22: Stride frequency and heart rate in an elite male U23 middle-distance runner

ness/muscle relaxation, sense of rhythm,kinesthetic perception and differentiation.

In any case, the longer-lasting and deepermodifications and adaptations occur as aneffect of extensive exercising related asclosely as possible to the specific action. Afew studies9 and, most of all, direct experi-ence of the methods used by top eliteendurance runners, show it is more produc-tive to give the athlete feedback during spe-cific exercise. With these premises, exercisingspeed and stride10 for a few dozen metresmust be considered instructive and/or com-plementary but not specific.

Conclusion

Running is the fastest means of unaidedhuman locomotion. Machines of all kinds havecut the need for running in normal endeavoursto zero, thereby confining it to the exclusiveworld of sports and fitness. The growing phe-nomenon of more and more sedentary peoplebecoming “marathoners” is in a sense Nature’scall after being overpowered and abused. Fur-thermore, mankind is interested in discoveringits limits through records.

The world of research can address both ofthese expectations.

The analysis of two elementary parametersperformed with unsophisticated instruments(compared with those in a laboratory) led tofew certainties and many doubts on a seriesof issues that can be addressed when dealingwith prolonged running. Hopefully, therefore,further and more detailed studies will follow,with the goal of confirming or refuting thehypotheses we put forth in this paper.

Acknowledgements

I would like to thank the athletes, technicians,and doctors of the Italian national middle-dis-tance and marathon team with whom I haveworked in the past ten years; all of the otherenthusiasts and running buffs who were willingto be “measured” for this study and, not last,Prof. Alberto Madella for “guiding” me throughevery phase of the drafting of this document.

Please send all correspondence to:Piero [email protected]

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9 CONLEY proved that a long period of training at prolonged running leads to an improvement in running economy. On the contrary, short training boutsdirected at fine-tuning specific aspects of running have negligible effects on performance (KRAHENBUHL & PANGRASI, 1983).

10 A school of thought maintains that brief exercises on technique have a direct effect on prolonged running and must therefore be considered a specifictraining means.

Figure 23: Stride length and heart rate in an elite male middle-distance runner

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Incalza. Stride Parameters in Endurance Runners

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