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First published online May 8, 2007
Journal of Experimental Biology 210, 1825-1833 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.002162
Human locomotion on ice: the evolution of ice-skating energetics through history
Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University Cheshire, Hassall Road, Alsager, Stoke-on-Trent, ST7 2HL, UK
* Author for correspondence at present address: Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford, OX1 3PT, UK (e-mail: federico.formenti{at}physiol.ox.ac.uk)
Accepted 27 February 2007
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Summary |
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This paper provides evidence for the fact that the metabolic cost of locomotion on ice decreased dramatically through history, the metabolic cost of modern ice-skating being only 25% of that associated with the use of bone-skates. Moreover, for the same metabolic power, nowadays skaters can achieve speeds four times higher than their ancestors could. In the range of speeds considered, the cost of travelling on ice was speed independent for each skate model, as for running. This latter finding, combined with the accepted relationship between time of exhaustion and the sustainable fraction of metabolic power, gives the opportunity to estimate the maximum skating speed according to the distance travelled.
Ice skates were probably the first human powered locomotion tools to take the maximum advantage from the biomechanical properties of the muscular system: even when travelling at relatively high speeds, the skating movement pattern required muscles to shorten slowly so that they could also develop a considerable amount of force.
Key words: energy cost of locomotion, bioenergetics, biomechanics, ice skate
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Introduction |
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Within this framework, our study focuses on the development of human locomotion on ice. Particularly, we tried to understand how much the metabolic cost of transport changed through history and what the longest distances people could travel were. We hypothesized that changes brought to subsequent models of ice skates decreased the cost of transport, and we show here to what extent this happened.
Ice skates, nowadays mainly used for sport, art or leisure, first developed
as a means of locomotion at least 3000 years ago
(Luik, 2000
). Ancient
Scandinavian sagas suggest that the first ice skates were small wooden plates,
but no archaeological finding can support this perspective at present. On the
other hand, many findings show evidence of animal bones used as ice skates in
large European areas and in some of the Russian northern regions since the
Bronze Age (Brown, 1959;
Muhonen, 2005). Originally,
bone skates were used while pulling sledges transporting goods and to go
fishing in times and regions where freezing winters did not allow fishing from
the shore, as was possible during the mild season. It seems that the oldest
remains were found in Scandinavian countries, but ice-skating mainly developed
in The Netherlands, where it has been the most popular means of winter
transport for centuries. The presence of heavier and lighter snowfalls could
have determined the development of cross-country skiing and ice-skating,
respectively, in Scandinavia and in The Netherlands. Dutch people were the
very early pioneers since, as far back as the 13th century, they could
maintain communication by skating for miles from village to village along
frozen rivers and canals (Heathcote et
al., 1892), keeping the ice free from snow and filling up cracks.
From its very conception, skating on ice was a form of human-powered
locomotion that was simple and effective, very cheap (and thus accessible to a
large part of the population) and allowed people to reach more distant
destinations than they could do by walking or running. In fact, unless more
expensive means of transport such as horses or (later) trains were used, ice
skates were probably the most convenient locomotion tool until bicycles were
built (Minetti et al., 2001),
the latter probably not being very safe on slippery roads in winter. It is
interesting from this point of view to observe that the countries where
ice-skating began are those where bicycles are most used now, possibly because
of the flatness of the territory, combined with the widespread presence of
waterways (CIA, 2005).
Despite the fact that research has widely explored the bioenergetics of
modern ice-skating (di Prampero et al.,
1976), there is no evidence to support how the metabolic cost
associated to its development changed over time. Previous studies showed how
humans created different passive tools and invented strategies to enhance
their performance through history. The evolution of these tools, for example
bicycles (Minetti et al.,
2001), wheelchairs
(Ardigò et al., 2005)
and skis (Formenti et al.,
2005), clearly shows that the achievement of high speeds, for the
same metabolic effort, has always been a primary target. In the current paper,
we point out which limiting factors humans empirically understood to be the
most important in determining their travelling performance on ice, since their
first attempt. Moreover, an estimation of the highest sustainable ice-skating
speed over a range of given distances is proposed. Presenting many
similarities with cross-country skiing, ice-skating performance evolved both
in terms of speed and metabolic cost.
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). This type of movement is of peculiar importance since with
conventional and historical skates, ankle plantarflexions should be avoided
(Houdijk et al., 2000b).
Participants were informed about the aims, procedures and details of the study
and signed an informed consent form to take part in the experiments. Ethics
approval was given by the Dept of Exercise and Sport Science Ethics Committee
(Manchester Metropolitan University) before beginning the experiments. On the
first day, the opportunity to try the different skates was given and, at the
end of this familiarization session, participants declared that they felt
comfortable with all the five models needed for the tests.
The skates
Five pairs of ice skates were chosen as representative of the main steps in
technological development and possibly as good candidates to show how
metabolic cost changed between subsequent models. Archaeological specimens of
the two oldest models could not be used, so a pair of horse metatarsal bones
was prepared by the authors, and an accurate replica of the 5th century model
was made by the Department of Design and Technology (Manchester Metropolitan
University, Cheshire, UK). The two oldest pairs of skates were normally
associated with leather bindings. Nevertheless, for health and safety reasons,
we made sure that the skates were securely fastened to the participants' feet,
assuming that safety has always been a priority. Consistent with this
principle, participants wore walking boots during the tests on the historical
skates, while ice-skating boots were used when skating with the modern models.
Fig. 1 shows the skate models
employed for the experiments; technical information about them can be found in
Table 1. The following
paragraphs describe the ice skates in more detail.
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The bone skates
Apart from ancient Scandinavian sagas telling about small wooden plates
used as a means of travelling on ice and hard snow, the oldest detailed
written references to ice skates can be found in Heathcote et al.
(Heathcote et al., 1892) and
Munro (Munro, 1893).
Apparently, animal bones were the first step in the development of ice skates.
Most of the 211 findings considered by Jacobi
(Jacobi, 1976) are horse bones
(66%), while the remaining 34% come from cow bones. Amongst the horse bones,
metatarsi represented 45% of the findings, while about 39% were metacarpi and
16% radii; similar proportions are found among cow bones. It seems that the
bones were chosen to match the size of the skater's feet.
Numerous bone skates show holes pierced horizontally, perpendicular to the main axis of the bone, through which leather straps could pass and fasten the foot to the bone skate. Following this information, authors personally prepared the horse metatarsal bones needed for the experiments, taking particular care not to warm up the bone during the piercing process and thus alter the mechanical properties of the bone structure. It was noticed that the hole at the back was found approximately under the ankle, while the one at the front was always pierced very close to the front end of the bone, in the condyle. By means of PQCT (peripheral quantitative computed tomography) scans, authors observed that the trabecular area represented 45% of the sections at the front and at the back of the bones. While the cortical density was similar in the two sections considered, the front section showed a much higher trabecular density (+70%) than the section at the back. This finding shows that piercing the hole very close to the condyle was probably more difficult than doing it slightly further at the back, but it seems reasonable to expect that this particular section was chosen for its higher strength and resistance to usage.
Bones did not have an edge that allowed the typical skating movement pattern, so the forward propulsion was given by the upper limbs: a stick was pushed backwards between the legs while the legs were kept almost straight.
Finally, it is surprising how bone skates were still in use in the 18th
century "in Iceland, Gotland and in parts of Hungary and
Germany", as reported by Roes
(Roes, 1963).
The first wooden skates with a blade
It was not until approximately the 13th century that a few skates began to
be made in wood with a metal blade fixed on the underside, and by the 14th
century, these were generally the most used ice skates in The Netherlands:
wood was easy to work and metal lasted for a long time. The model used in the
present study was the oldest one that authors could find a detailed
description of (Goubitz,
2000); a replica is kept at the National Service for
Archaeological Heritage in Amersfoort (The Netherlands). Surprisingly, the
dynamic coefficient of friction (µ) measured for these skates was higher
than that recorded for the bone skates
(Table 1). Nevertheless, the
use of these materials allowed the skater to propel him/herself with their
more-powerful lower limbs. Ice-skating in its modern form began with a model
similar to this.
Skates used from the 15th to 18th centuries
Although still made from the same materials, ice skates used between the
15th and the 18th centuries were much lighter, by about 30%, than their
predecessors. Authors chose to include this model among those considered for
the study because of its reduced mass, which could reduce the cost of skating
by limiting the mechanical internal work. On the other hand, if compared with
more recent models, the blades of these skates were still quite short, a
factor that could make it more difficult to control balance. Nevertheless, it
was with models similar to this that a massive production of skates began in
this epoch, particularly in The Netherlands, where ice-skating rapidly spread,
becoming the most popular means of locomotion and transport in winter. Even
before this time, the shallow waters surrounding The Netherlands froze easily,
but the greatest incentive for a further increase in ice-skating popularity in
the 15th century is due to the building of windmills used to remove excess
water from the low-lying districts. The largest network of canals in the world
was set up and, associated with the numerous freezing winters recorded during
the Little Ice Age, this probably determined the success of ice-skating. This
popularity is confirmed and was appreciated by the Dutch and English artists
of that time (an example is reported in
Fig. 1): hundreds of
ice-skating scenes on canals, rivers and lakes were recorded on painters'
drawings, now on show at the most prestigious museums and art galleries in The
Netherlands and in England.
The 18th century ice skates
Longer blades could possibly allow an easier balance control during
ice-skating and could consequently be associated with a benefit in terms of
economy. At the same time, from a mechanical perspective, only the edge of the
blade is in contact with the ice for most of the time while skating,
particularly during the pushing phases. About three centuries ago, skaters
empirically understood that, at a given temperature and for a given force, the
pressure exerted by the blade on the ice would be lower if the contact area
between ice and blade could be larger. As a direct consequence, at each
stride, the blade would not be pressed as much into the ice, and a lower
resistance to progression would be encountered. The implications of such
geometrical variation brought the authors to include this skate model in the
study.
Modern ice skates
It is only since the 19th century that purpose-made boots were screwed onto
the metal frame of the skates, a change that increased the control of the
skates further, allowing for easier and safer travelling. This factor
potentially allows the skater to take fewer strides over a given distance.
Technology, materials and skills in skate-making allowed the development of
lighter skates, with longer and thinner blades, which showed a very small
dynamic coefficient of friction
(Kobayashi, 1973;
de Koning et al., 1992). The
combination of the two above-mentioned elements poses a good basis for
hypothesising a decrease in the internal work required to sustain a given
speed: lower limbs could move at slower speeds and had a lighter mass to lift
at each stride. By moving more slowly, muscles are recruited for more
efficient contractions, lying closer to the maximum power peak in the
speed–power curve. On the other hand, the bent position of the knee and
the high knee joint torque characterizing the long gliding phases
(Houdijk et al., 2000b)
restrict muscle blood flow, especially in the vastus lateralis muscle
(Foster et al., 1999).
Finally, at each given speed and depending on the value of the knee angle,
there should be an optimum stride frequency/length, possibly determined by the
need to balance, by properties such as force–length and
power–velocity relationships typical of the skeletal muscle as well as a
specific ratio between limiting the internal work while maintaining a
sufficient muscle blood flow.
In modern long-distance competitions, a recently developed model is used:
originally patented in 1891, it was not used in competitions until the
mid-1990s. Scientists initially named it `slapskate'
(van Ingen Schenau et al.,
1996), which was later changed to `klapskate'
(van Ingen Schenau, 1998). It
differs from the model considered in the present study because of a hinge
located on the frame of the skate, between boot and blade. Since klapskates
have been used, performance has increased, resulting in 3–5% higher
speeds. Several previous studies (van
Ingen Schenau et al., 1996;
van Ingen Schenau, 1998;
de Koning et al., 2000;
Houdijk et al., 2000b;
Houdijk et al., 2000a;
Houdijk et al., 2001) have
explored the mechanical determinants of the increased performance and its
effects on physiological variables such as oxygen uptake and heart rate.
Despite no difference being observed in heart rate and in the resistance
opposed by air friction (van Ingen
Schenau, 1982), an increased power output was calculated
(
10%). Finally, for speeds not statistically different, oxygen uptake was
lower when using klapskates than when skating on conventional skates
(de Koning et al., 2000).
Because these extensive measurements are reported in detail in the
above-mentioned literature, we have chosen not to replicate them here.
Experimental procedure
Experiments were performed in an indoor ice rink in Bormio (Italy). The ice
was properly prepared before the beginning of each recording session by means
of traditional ice-rink machines, which polish the surface of the ice, leaving
an extremely thin layer of water on the top of it. Temperature on the ice
surface was –4.5°C and relative humidity was 80%. Each participant
was asked to skate with each of the five skate models at two different,
constant speeds: the first speed was defined as sustainable for 8 h (low
speed; L), simulating quite a long journey, while the second as sustainable
for 4 h (high speed; H). Bone skates could only be tested at the low speed
because participants declared that they would not feel safe at a higher speed.
Two faster speeds were tested for modern ice skates; participants were asked
to travel at speeds corresponding to approximately 60% and 75% of their
maximum theoretical heart rate, continuously monitored by means of a heart
rate monitor transmitting real-time data to a portable PC. Time needed to
cover each lap was recorded; mean values and their standard deviation for the
last three minutes of activity were calculated.
Bioenergetic measurements
Skate models were randomly tested for 7 min each at every speed, and the
metabolic energy associated with their use was calculated over the last three
minutes of each trial. Participants were equipped with a portable telemetric
metabograph sampling on a breath-by-breath basis (Cosmed K4
b2; Rome, Italy)
(Hausswirth et al., 1997). The
K4 system includes a portable unit worn by the participant and a base station
for recording the data. The portable unit weighs 1.5 kg and consists of a
silicon mask with a flow-rate turbine fixed on the participant's face, a
processing unit containing O2 and CO2 analyzers placed
on the participant's chest and a transmitter/battery pack placed on the back
of the participant. Before each recording session, the turbine was calibrated
with a 3-l syringe, and a two-point calibration of the O2 and
CO2 analyzers was carried out using ambient air and a standard
calibration gas mixture (5% CO2, 16% O2, 79%
N2). Data considered for further calculations were oxygen uptake
(
O2; l
min–1), carbon dioxide output
(CO2; l
min–1), respiratory quotient (RQ) and heart rate
(fH; beats min–1). Oxygen uptake at rest
was measured while participants were standing quietly on the ice rink before
each experimental session and was used to calculate the net oxygen consumption
for skating with each set of ice skates. Metabolic energy was converted into
equivalent units (J) according to the measured respiratory quotient
coefficient (di Prampero,
1986). The energy cost of skating (J kg–1
m–1) was obtained from the ratio between steady-state net
oxygen uptake (J kg–1 s–1) and mean speed (m
s–1).
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).
Skate friction measurements
Skate friction was calculated on the basis of videos recorded by means of a
digital camera (25 Hz). One participant took part in these tests; he was
initially pushed by the experimenter and then glided while standing still,
with his trunk slightly flexed. Fifteen black markers were placed in a
straight line at 2 m intervals on the ice surface, and the time when the
skates crossed each marker was recorded. Deceleration (a; m
s–2) was calculated as the mean value of speed decay. This
procedure was repeated five times for each skate model, and the mean value of
deceleration was used to calculate the coefficient of dynamic friction (µ),
according to µ=a g–1, where
g is the acceleration of gravity (9.81 m
s–2).
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Stride frequency and length results
Differences between the two speeds considered
Student's paired t-statistic showed that the low speed and the
high speed, subjectively chosen by the participants, were significantly
different (L=3.05±1.40 m s–1; H=4.28± 1.77 m
s–1; P<0.001). Showing low values at both speeds,
mean stride frequencies (f) were also significantly different
(f at L=0.53±0.12 Hz; f at H=0.65±0.22 Hz;
P=0.03). The length of the strides (l) was also
significantly greater at the high speed (l at L=6.58±4.93 m;
l at H=7.83±5.32 m; P=0.03).
Differences between the skate models
Stride frequencies and lengths are summarized in
Table 2, which also gives the
level of significance for differences between skates models and, hence,
historical periods. For a given mass-specific metabolic power, faster speeds
are achieved on more recent skates by means of longer strides performed at low
frequency. When comparing variables relative to the skate models, a positive
relationship between stride length (l) and speed resulted from a
linear regression analysis (r2=0.85, intercept set=0,
N=23).
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Table 1 shows the coefficient of dynamic friction (µ) associated with the skates considered in the study. For these measurements, authors did not take air resistance into account, which might have played a partial role in the first part of our trials (when the highest speed recorded for the skater was almost 3 m s–1). It seems clear how the development of ice skates has aimed at reducing the resistance opposed by the ice, taking advantage of more suitable materials and shapes.
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Pandolf et al. measured the energy cost of locomotion on snow and reported
it as a function of footprint depth
(Pandolf et al., 1976)
(Fig. 2). A comparison of
Pandolf's results and those presented in the present study shows that, on
average, when travelling by a frozen lake or river, skating on bones was more
convenient than walking on snow, as long as the snow was more than six or
seven centimetres deep. It has also been shown that for a given speed and
gradient the cost of human walking is directly related to the transport of the
mass carried (Bastien et al.,
2005). It can probably be speculated that while travelling on
skates the cost of pulling a sledge with an extra given load would not be
linearly related to the mass of the load to be transported on the ice. The
widespread presence of water, and thus the numerous frozen lakes in winter,
together with the advantage given by pulling a load while skating rather than
carrying it by foot, seems to explain why humans first began skating on ice.
As soon as technology evolved, in approximately the 13th century, ice skates
also enabled humans to travel at speeds that could not be achieved by walking,
and at lower metabolic costs. Nowadays, ice-skating is about one-third cheaper
than running on firm terrain, and for a given metabolic power much higher
speeds can be sustained; only cycling on modern racing bicycles allows for
faster travelling.
On the basis of our results, it would be interesting to calculate
approximately how far humans could skate in the past and the maximum speed
that they could sustain over a given distance; in order to do this, two
fundamental assumptions need to be taken into account. It was shown here that
the metabolic cost of ice-skating is speed independent; the first assumption
to be considered is that this relationship remains unchanged for speeds
slightly outside of the investigated range and still relying on an aerobic
metabolism. This assumption is supported by evidence reported in a paper by de
Koning et al. (de Koning et al.,
2000): they studied speeds of approximately 10 and 12 m
s–1 and measured metabolic cost values similar to those
recorded in our experiments. Moreover, all the participants taking part in
this study subjectively chose an upright posture when travelling at the
slowest speeds while leaning slightly forward at high speeds. This is
determined by the need to keep the body centre of mass further ahead (leaning
forward) when higher accelerations are expected from the power generated by
the lower limbs, and at the same time it is clearly a consequence of the
mechanical power required to overcome the aerodynamic drag. Air does not
determine a strong resistance to the forward motion when travelling slowly but
becomes the most limiting factor for performance as speed increases
(di Prampero, 1985). This can
explain in part the independence of cost and speed in the present study and
could support the hypothesis that cost would not increase for slightly higher
speeds, when naturally a more aerodynamic position would further decrease the
proportion of power required to overcome air resistance. Unfortunately, at
present, literature does not exactly provide strong evidence from this
perspective because of the different methods and conditions in which the cost
of ice-skating has been measured (Houdijk
et al., 2000a; Nobes et al.,
2003). Measuring the partitioning of the energy needed to move and
to balance is a difficult task in studies about animal locomotion: when
travelling at slow speeds on skates (or by bike), a great portion of the
energy used is needed to balance; proportionally, balancing becomes more
economical at faster speeds. This could also contribute to the determination
of the cost of modern ice-skating recorded in the present experiments and may
support the assumption that it might keep on being unvaried at higher speeds
than those studied here.
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;
Formenti et al., 2005). A
prediction of the maximum sustainable speed as a function of the distance
travelled is shown in Fig. 3.
The validity of the model used is supported by the points representing the
highest average speeds achieved in recent competitions (the slowest records
lie almost parallel to the iso-cost lines). Differences between the prediction
and the actual records may be related to the different conditions in which the
competitions took place (particularly when the accelerative phase at the start
is a substantial contributor of the total cost), wind and temperature being
important determinants of ice-skating performance. It might be interesting to notice that, as mentioned above, quite strong winds must also have characterized the flat Dutch lands in past centuries. This is confirmed by the high number of windmills and by the peculiar technique of skating in a queue while holding a long stick, which has been recorded by numerous artists. Not only could the skaters at the back benefit from the aerodynamic protection offered by the skater at the front, but the skater at the front could also take advantage of the push coming from skaters behind. It is important to remember that speeds were generally not as fast as in modern competitions, so the posture adopted was almost upright in respect to the ground and even when travelling at low speeds, the queue strategy might reveal an efficient saving mechanism if going opposite to the wind direction. On the contrary, when going with the wind direction, skaters held the stick while skating side by side in a row, being partially pulled by the wind.
In terms of speed and energy demand, the advent of ice skates and skis
meant that, for centuries, travelling on ice was more convenient than moving
on snow or firm terrain: until bicycles were invented. The mechanical reasons
for this lie particularly in the different friction, the different progression
techniques and the different weights of the tools. At the same time, from a
biomechanical and physiological perspective, a crucial role was clearly played
by the practical implication of the force–speed relationship
characterizing muscular contraction
(Marsh, 1999). This can be
easily noticed especially when travelling fast: in contrast to walking,
running and classical cross-country skiing, the skating locomotion pattern
allows muscles to shorten slowly, at speeds probably close to those at which
they can develop their maximum mechanical power. In the same way that gear
ratios improved cycling performance by making the pedalling frequency
independent of speed, subsequent models of ice skates allowed faster speeds
for similar stride frequencies. Stride frequency is an important parameter
because, for a given movement pattern, and thus a muscular strain trajectory,
stride frequency is strictly related to the speed at which the propulsive
muscles shorten and consequently influences the power that the muscle can
develop while contracting. According to this principle, it seems clear that
when trying to achieve high power outputs, stride frequency needs to be
restrained to a limited range.
The use of klapskates employed nowadays implies a movement which, in terms of push-off mechanics, is slightly different from that of jumping (or running). In fact, no flexion of the metatarsal joint occurs: the ankle is extended, but the foot rotates as a single element around the hinge between the boot and the blade. By contrast, when we perform a standing jump (or when we run), just before our foot loses contact with the ground, it bends at the metatarsal–phalangeal joint level. The tendons in the arch of the foot, storing some elastic energy during the extension of the metatarsal joint, normally return a substantial portion of it in these structures' elastic recoil (Hick's windlass effect), possibly playing a role in determining performance. In the future, we think that it would be interesting to see the effect of a block that gives the opportunity to bend the foot at the level of the metatarsal–phalangeal joint in the last stage of the pushing phase.
Nowadays, the most interesting advances in human locomotion often result from sports competitions. From this perspective, the major limiting factor for future developments could be represented by the regulations governing sporting events, unless they are reviewed. Regulations changed in the past, allowing, for example, the use of mono-fins for swimming competitions. Performance on racing bicycles could benefit from a laying position and an aerodynamic shield; record times of cross-country skiing and ice-skating could improve if the skis and blades were kept lubricated while travelling. At present, these solutions are not allowed in official competitions. To a certain extent, rules still limit the evolution of performance in cycling, swimming, skiing and ice-skating in both a specific context and, generally speaking, the development of human-powered vehicles.
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Acknowledgments |
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Footnotes |
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Present address: Institute of Human Physiology I, Faculty of Medicine,
University of Milan, Via Mangiagalli, 32-20133 Milan, Italy 
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16 m s–1 before exhaustion, as indicated by the
intersection between the 99 J m–1 iso-cost curve and the 10
min iso-time line.