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Introduction
!
Running economy is strongly related to distance
running performance [4,19], and it is typically
determined by measuring the steady-state oxy-
gen consumption at the submaximal running
speed. Taking body mass into consideration, run-
ners with good running economy use less energy
and therefore less oxygen than runners with poor
running economy at the same speed. Researchers
have reported a 20– 30% range in the oxygen
consumption for a given submaximal running
speed among trained distance runners [4, 6,18].
Interindividual variability in running economy
has been explained by various physiological, bio-
mechanical, environmental, anthropometrical
and psychological factors. Biomechanists have
identified that running economy is affected by
the net vertical impulse of the ground reaction
force [9], stride length [3], change in speed dur-
ing ground contact phase [11] and vertical stiff-
ness of a leg spring [5, 9].
Running speed is the product of stride rate and
stride length. Although both stride rate and stride
length are increased with increasing running
speed, stride length is responsible for increasing
speed up to 90% of an individual maximum speed
and, thereafter, the speed is only increased by in-
creasing stride rate [15,16,23]. Cavanagh and
Williams [3] found the most economical stride
length of a group of runners was close to that
which was freely chosen. Ground contact phase
is the only phase during a running cycle in which
a runner can produce force and influence stride
length and running speed. Functional and me-
chanical requirements during stance are re-
flected in the characteristics of the ground reac-
tion force. It has been shown that vertical and
horizontal components of ground reaction forces
increase with increasing running speed [14,16,
23]. Weyand et al. [23] concluded that runners
reach faster top speeds by applying greater sup-
port forces to the ground not by more rapid leg
movements. The critical point in maximal sprint
running is the change in running speed during
the ground contact phase. The horizontal break-
ing force and braking time as well as the horizon-
tal distance between the first contact point and
the center of gravity of the body at touchdown
should be very small to avoid loss of speed during
the braking phase of ground contact [17].
Abstract
!
The main purpose of the present study was to in-
vestigate the relationships between running me-
chanics, top running speed and economy in
young endurance athletes. Twenty five endur-
ance athletes (age 19.8 ± 1.1 years, stature
1.82 ± 0.07 m and body mass 69.4 ± 7.5 kg) per-
formed two separate tests on an indoor track. The
first test was 8 × 30 m with increasing speed, and
the second test was incremental 5 – 6 × 1000 m.
In the first test, ground reaction forces and stride
characteristics were measured from each run-
ning speed. In the second test, running economy
at the speed of 3.89 m
•
s
-1
and maximal oxygen
uptake were determined. Ground contact time
was the only factor which correlated significantly
with both running economy (r = 0.49, p < 0.05)
and maximal running speed (r = – 0.52, p < 0.01).
Furthermore, maximal running speed was corre-
lated significantly with the mass-specific hori-
zontal force (r = 0.56, p < 0.01) but not with the
vertical effective force. It is concluded that the
short contact times required in economical and
high speed running suggests that fast force pro-
duction is important for both economical run-
ning and high top running speed in distance run-
ners.
Factors Related to Top Running Speed and Economy
Authors A. Nummela
1
, T. Keränen
1
, L. O. Mikkelsson
2
Affiliations
1
Research Institute for Olympic Sports, Jyväskylä, Finland
2
Pajulahti Sports Institute, Nastola, Finland
Key words
l
"
running economy
l
"
distance running
l
"
ground reaction force
l
"
stride length
l
"
ground contact time
accepted after revision
July 3, 2006
Bibliography
DOI 10.1055/s-2007-964896
Published online June 1, 2007
Int J Spor ts Med 2007; 28:
655 –661 © Georg Thieme
Verlag KG Stuttgart
•
New York
•
ISSN 0172-4622
Correspondence
Dr. Ari Nummela
Research Institute for Olympic
Sports
Rautpohjankatu 6
40700 Jyväsk ylä
Finland
ari.nummela@kihu.fi
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The amount of energy used to run a constant distance is nearly
the same whether it is run at top speed or at leisure pace. The
results of Kram and Taylor [13] suggest that, primarily, the cost
of supporting the body weight and the time course generating
this force determines the cost of running. Therefore, a long
ground contact phase and great deceleration of horizontal speed
during a braking phase of the ground contact could be consid-
ered wasteful in terms of the metabolic energy requirements. A
successful endurance runner is characterized by less vertical os-
cillation [7], longer strides [3], shorter ground contact times
[21], less change in speed during the ground contact [11], and
lower first peak in the vertical component of the ground reaction
force, associated with a tendency to have smaller anteroposte-
rior peak forces [25]. In a well controlled study, Heise and Martin
[9] showed that less economical runners exhibited greater total
and net vertical impulse, indicating wasteful vertical motion.
It is a well known fact that elite sprint runners have a higher top
running speed and a lower running economy than elite distance
runners. Although there is a discrepancy between the high neu-
romuscular capacity to produce force and running economy,
some previous studies have shown that the explosive type
strength training can be used to improve both maximal power
output and running economy [10, 20]. Therefore, the main pur-
pose of the present study was to investigate the factors of run-
ning stride, which were related to both top running speed and
running economy in young endurance athletes.
Methods
!
Subjects
Twenty-five young male endurance athletes (ten distance run-
ners, eight orienteers and seven triathletes) volunteered as sub-
jects for the present study. All the athletes belonged to the na-
tional junior team and their mean ± SD age, height and body
mass were: 19.8 ± 1.1 years, 1.82 ± 0.07 m and 69.4 ± 7.5 kg, re-
spectively. All the subjects were fully informed of the procedures
and possible risks of the experiments, and written informed con-
sent was obtained from each subject. The informed consent was
in accordance with the guidelines of the Ethical Committee of
the University of Jyväskylä. The tests of the present study were
part of the athletes normal exercise testing and they complied
with current Finnish laws regarding the testing of human sub-
jects.
Procedure and measurements
In order to measure ground reaction forces and stride character-
istics at different running speeds and determine maximal sprint-
ing speed, running economy, distance running performance and
maximal oxygen uptake, the athletes ran two separate running
tests on a 200-m indoor track. The first test was 8 × 30 m with in-
creasing speed, and the second test was an incremental 5 –
6 ×1000 m.
In the first test (8 × 30 m), the athletes were able to accelerate
50 m to ensure a normal and steady running gait throughout
the 30-m measurement section. The speed of the first run was
5.0 m
•
s
–1
and, thereafter, the speed was increased by 0.40 m
•
s
–1
after each run until the sixth run (7.0 m
•
s
–1
). The running speed
was regulated by small lights placed on the next lane at intervals
of 4.0 m (Naakka Ltd., Lappeenranta, Finland). The athletes were
instructed to adjust their speed to coincide with the lights,
which were switched on and off along the track. The athletes
were asked to run the last two runs at maximal speed. Each run-
ning bout was separated by a two-minute recovery period dur-
ing which the runners returned to the start of the sprint course.
In order to measure running speed, stride rate and stride length,
a photocell contact mat [22] and two photocell gates were
placed on the sprint course (Ivar Ltd., Tallin, Estonia). A special
9-m long force platform system was placed in the middle of the
30-m measurement section. The system consisted of five two di-
mensional (2D) and three 3D force platforms (0.9 × 1.0 m each,
TR Test Ltd., Jyväskylä, Finland, natural frequency in the vertical
direction 170 Hz) and one Kistler 3D force platform (0.9 × 0.9 m,
400 Hz, Honeycomb, Kistler, Switzerland) connected in series
and covered with a tartan mat. Each force platform registered
both vertical and horizontal components of the ground reaction
forces. Vertical and horizontal ground reaction forces were re-
corded by a microcomputer using an AT Codas A/D converter
card (Dataq Instruments, Inc., Akron, OH, USA) with a sampling
frequency of 1000 Hz.
The incremental 5 –6 × 1000-m test was performed after 15 min
recovery from the first test. The initial speed was 2.78 m
•
s
–1
for
the orienteers and triathletes and 3.33 m
•
s
–1
for the distance
runners. Thereafter, the speed was 3.89, 4.44, 5.00, and 5.56 m
•
s
–1
. The running speed was regulated by small lights embedded
on the inside of the 200-m indoor track at intervals of 4.0 m
(Naakka Ltd.). The athletes were instructed to adjust their speed
to coincide with the lights, which were turned on and off inside
the track. The athletes were asked to run the last 1000 m at max-
imal effort. The athletes performed five or six runs depending on
their maximal aerobic capacity. Oxygen consumption and heart
rate (Oxycon Mobile, Viasys Healthcare GmbH, Hoechberg, Ger-
many) were measured continuously during the whole test.
Running economy was measured as steady-state oxygen uptake
of submaximal running at the speed of 3.89 m
•
s
–1
, and maximal
oxygen uptake (V
˙
O
2max
) was defined as the highest oxygen
consumption during the test over a 60-second period. Running
economy and maximal oxygen uptake were expressed as
ml
•
kg
–0.75
•
min
–1
, since submaximal V
˙
O
2
and V
˙
O
2max
measures
during running may be better related to the 0.75 power of body
mass than [body
•
mass]
–1
[1].
Data analysis
Three to five contact phases of each subject at all running speeds
were selected for analysis. The horizontal force-time curve was
used to separate the ground reaction forces into the braking
and propulsion phases [16]. The integrals of both force-time
curves were calculated and divided by the respective time peri-
od to obtain the average force for the whole contact phase and
for horizontal braking and propulsion phases separately. The ef-
fective force (F
yeff
) which exceeds the body mass was determined
from (F
y
–F
BW
)
•
F
BW
–1
, where F
BW
= body weight.
Effective impulse values were determined from the effective
force applied to the running surface and ground contact times:
F
y
I
eff
=F
yeff
•
CT, where CT = ground contact time. CT was deter-
mined from the time the applied vertical force exceeded 0 N on
the force platform. Horizontal impulses (F
x
I) were calculated as
absolute impulses because horizontal force displays negative
values during breaking phase and positive values during propul-
sion phase: F
x
I=(F
x
•
F
BW
–1
)
•
CT.
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Statistics
The relationship between running speed, top running speed,
running economy and ground reaction force characteristics were
investigated using a standard Pearson product moment correla-
tion and linear regression analyses. Values are expressed as
mean ± standard deviation. All the statistical analyses were
undertaken using SPSSWIN 13.0 (SPSS, Inc., Chicago, IL, USA).
Results
!
Running economy and V
˙
O
2max
Although the subjects were all endurance athletes and belonged
to the national junior team, their V
˙
O
2max
and maximal 1000-m
speed was quite heterogeneous ranging from 152 to 240 ml
•
kg
–0.75
•
min
–1
(184±18ml
•
kg
–0.75
•
min
–1
or 63.9 ± 5.7 ml
•
kg
–1
•
min
–1
) and from 4.31 to 6.27 m
•
s
–1
(5.32 ± 0.55 m
•
s
–1
), respec-
tively. Running economy or oxygen uptake at the speed of
3.89 m
•
s
–1
ranged from 121 to 161 ml
•
kg
–0.75
•
min
–1
(144 ±
10 ml
•
kg
–0.75
•
min
–1
or 49.9 ± 3.3 ml
•
kg
–1
•
min
–1
), which were
typical of those which have been reported previously for similar
speeds [9,21]. The difference between the most and least eco-
nomical runners showed that the athletes were much more ho-
mogeneous in terms of running economy than they were in
V
˙
O
2max
.
Running mechanics as a function of speed
In pooled data across the speed range, speed increases were
achieved by increasing both stride lengths and stride frequen-
cies at the speeds below 7 m
•
s
–1
, and at the speeds above
7m
•
s
–1
, stride frequency was solely responsible for speed in-
crease (l
"
Fig. 1A). The increases in stride frequencies resulted
from the reductions in ground contact time at speeds below
6m
•
s
–1
, although stride frequency depends on both contact time
and flight time (l
"
Fig. 1B). As subjects approach their top speed,
both contact time and flight time decreased. These flight time
reductions coincided with the decreases in effective vertical im-
pulse and horizontal impulse (l
"
Fig. 1C, D). In pooled data, the
increase in stride length was related to the increase in effective
vertical force (r = 0.58, p < 0.001) and horizontal propulsion force
(r = 0.73, p < 0.001, l
"
Fig. 2) but not with the vertical effective
impulse or horizontal impulse.
Maximal running speed
In the present athletes, maximal 30-m running speed ranged
from 7.70 to 9.40 m
•
s
–1
(8.39 ± 0.45 m
•
s
–1
). Maximal running
speed of the athletes was significantly correlated with the
ground contact times (r = – 0.52, p < 0.01, l
"
Fig. 3A) and braking
phase time (r = – 0.64, p < 0.001) but not with the propulsion
phase time. The mass-specific vertical force was not related to
the maximal running speed, but a significant correlation was ob-
Fig. 1A and B Running mechanics as a function of
speed for the data of 25 endurance athletes with
eight different speeds. Each dot represents the
average value of a certain speed of an athlete. Lin-
ear or second order polynomial trend line is also
added in the figures. A Increase in stride length
(dots) and stride frequency (open circles) with in-
creasing speed. B Decrease in ground contact time
(dots) and flight time (open circles) with increasing
speed.
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Fig. 1C and D Running mechanics as a function of
speed for the data of 25 endurance athletes with
eight different speeds. Each dot represents the
average value of a certain speed of an athlete. Lin-
ear or second order polynomial trend line is also
added in the figures. C Increase in vertical effective
force (dots) and decrease in vertical effective im-
pulse (open circles) with increasing speed. D In-
crease in mass-specific horizontal force (dots) and
horizontal impulse (open circles) with increasing
running speed.
Fig. 2 Vertical effective force (open circles) and
mass-specific horizontal force (dots) during a pro-
pulsion phase as a function of stride length.
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served between the maximal running speed and the mass-spe-
cific horizontal forces of the whole ground contact (r = 0.56,
p < 0.01) and propulsion phase (r = 0.66, p < 0.001, l
"
Fig. 3B) but
not braking phase.
Running economy
In order to investigate the relationships between running econo-
my and ground reaction force and stride characteristics, the val-
ues at the running speeds of 5.4, 5.8, 6.2, and 6.6 m
•
s
–1
were used
in the analyses. No correlation was observed between ground re-
action force characteristics and running economy. The only sig-
nificant correlations were observed between running economy
and ground contact time at the running speeds of 5.8 m
•
s
–1
(r = 0.49, p < 0.05), 6.2 m
•
s
–1
(r = 0.44, p < 0.05) and 6.6 m
•
s
–1
(r = 0.41, p < 0.05). Although there were only few significant cor-
relations between running economy and ground reaction force
and stride characteristics, biomechanical factors might explain
individual differences in running economy as shown in l
"
Table 1.
Discussion
!
The present study was designed to investigate the relationships
between force production and running economy and running
speed in endurance athletes. Previous studies have shown a dis-
crepancy between running economy and top running speed,
Table 1 Ground contact time, vertical effective impulse and relative stride length (stride length
•
stature
–1
) of the two most economical runners (RE121
ml
•
kg
–0.75
•
min
–1
and RE123 ml
•
kg
–0.75
•
min
–1
), the two most uneconomical runners (RE158
•
kg
–0.75
•
min
–1
and RE161 ml
•
kg
–0.75
•
min
–1
) and average value of
the whole group. The ground reaction force and stride characteristics are average values from the velocity of 5.8 m
•
s
–1
RE121 RE123 Mean ± SD RE158 RE161
CT (ms) 165 150 175 ± 13 190 170
F
y
I
eff
(s) 0.144 0.151 0.154 ± 0.014 0.151 0.160
Rel SL 1.03 1.04 1 .03 ± 0.05 1.03 1.07
CT: ground contact time; F
y
I
eff
: vertical effective impulse; Rel SL: relative stride length; RE: running economy
Fig. 3A and B Ground contact time (A) and mass-
specific horizontal force during a propulsion phase
(B) as a function of maximal running speed for 25
endurance athletes.
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since both can be improved by strength training [20] but the
fastest runners apply greater support forces to the ground [23]
and less economical runners exhibit greater ground reaction im-
pulses [9]. In the present study, ground reaction force character-
istics were related to running speed but not to running economy.
The ground contact time was the only stride variable, which was
related to both running speed and running economy.
It is well known that ground contact time decreases linearly
with increasing running speed [15] and this was also observed
in the present study (l
"
Fig. 1B). The relationship between run-
ning economy and short contact times has also been observed
in a previous study [21]. Furthermore, Williams [24] has found
that during submaximal running, ground contact times differ
between a rearfoot and a midfoot striker. Short contact time
seems to be beneficial for both running economy and maximal
running speed. This seems to be logical since the critical point
in maximal sprint running and economical running is the speed
lost during the breaking phase [17]. The horizontal braking force
and braking time as well as the horizontal distance between the
first contact point and the center of gravity of the body at touch-
down should be very small to avoid loss of speed during the
braking phase of ground contact. Kyröläinen et al. [14] suggested
that increasing the pre-landing and braking activity of the leg
extensor muscles might prevent unnecessary yielding of the
runner during the braking phase, helping them tolerate higher
impact loads. A short and rapid stretch with a short coupling
time and a high force at the end of pre-stretch creates a good
precondition for utilizing elasticity [2,12]. Pre-activity increases
the sensitivity of the muscle spindle via enhanced alpha-gamma
coactivation potentiating stretch reflexes, and enhancing mus-
culo-tendon stiffness, with a resulting improvement in running
economy.
In a well controlled study, Heise and Martin [9] observed that
less economical runners exhibited greater total and net vertical
impulse, indicating wasteful vertical motion. The influence of
the total vertical impulse was 38% of the interindividual varia-
bility in running economy. In the present study, no significant
correlation was observed between any ground reaction force
characteristics and running economy. One reason for the differ-
ence in the results might be that in the study of the Heise and
Martin [9] the ground reaction forces were measured from the
same running speed (3.35 m
•
s
–1
) as the running economy. In
the present study, we assumed that a runner who is economical
at a given running speed will be economical at other speeds as
well and therefore ground reaction forces were measured from
the running speeds which were close to the athletes’ speed dur-
ing a track running competition. A number of physiological and
biomechanical factors appear to influence running economy in
endurance athletes and therefore the interactions between me-
chanical and metabolic variables appear to be very complex [14,
25]. This is one reason why in some studies a relationship exists
between ground reaction force characteristics and running
economy [9] and in some other studies not (present study, [14]).
Based on the findings of the previous studies, we could hypothe-
size that short ground contact time [21], low vertical effective
impulse [9] and optimal or self-selected stride length [3] at a
given speed are associated with high running economy.
Although we did observe a significant correlation only between
contact time and running economy, vertical impulse and stride
length might also explain individual differences in running
economy. Excellent running economy of the two most economi-
cal runners in the present study can be explained, at least partly,
by short ground contact times (l
"
Table 1). Furthermore, the high
running economy of the RE121 (l
"
Table 1) could also be ex-
plained by the low vertical effective force. On the other hand,
the same biomechanical factor did not explain the poor running
economy of the two least economical runners in the present
study (l
"
Table 1). One possible reason for the poor running
economy for the RE158 (l
"
Table 1) is long ground contact times,
whereas high vertical effective impulse and long relative stride
length might explain, at least partly, why the RE161 (l
"
Table 1)
used more oxygen than the other runners in the present study.
These individual examples showed the complexity of the run-
ning economy, and that none single biomechanical factor can
fully explain the differences in running economy between the in-
dividual runners.
Stride length, stride frequency, ground contact time and flight
time as a function of running speed in the present study
(l
"
Fig. 1A, B) are well in line with previous findings [15,16, 23].
The results of the present study also showed that the increase
in stride length resulted from increasing both vertical and hori-
zontal ground reaction forces (l
"
Fig. 2). These relationships
seem obvious, since ground contact is the only phase during the
running cycle in which a runner can produce force and influence
stride length and running speed. Vertical effective force in-
creased with the increasing speed until the speed of 7 m
•
s
–1
,
thereafter the speed was increased without further increase in
vertical effective force (l
"
Fig. 1C) and no correlation was ob-
Fig. 4 The relationships between running econo-
my or oxygen uptake (3.89 m
•
s
–1
) and ground
contact time at the speeds of 5.8 m
•
s
–1
(dots) and
6.6 m
•
s
–1
(open circles).
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served between the vertical effective force and maximal running
speed in present athletes. This is contradictory to the results of
Weyand et al. [23]. They concluded that runners reach faster
top speeds by applying greater vertical support forces to the
ground. In the present study, horizontal force increased linearly
with the running speed (l
"
Fig. 1D), and a significant relationship
was observed between maximal running speed and horizontal
force (l
"
Fig. 3B). The results of the present study suggest that
maximal running speed is more dependent on horizontal than
vertical force. This seems to be logical since one can not increase
horizontal speed by increasing vertical force, but acceleration
and deceleration of running speed is produced mainly by chang-
ing horizontal force.
In conclusion, only ground contact times exhibited statistically
significant correlations with both running economy and maxi-
mal running speed. This suggests that the short braking phase
and use of elastic energy are important factors both in econom-
ical and high speed running in the group of young well-trained
endurance athletes. Although any other biomechanical factors
did not correlate with running economy, there are numerous
biomechanical factors which could be used to partially explain
differences in running economy between two runners. Another
important finding of the present study was that horizontal
ground reaction force was linearly increased with running speed
and was correlated with the maximal running speed. This sug-
gests that the horizontal component of ground reaction force is
more important in attaining high top running speed in distance
runners than the vertical component of it, since similar linear re-
lationships were not observed between vertical effective force
and running speed, and no significant correlation was observed
between vertical force and maximal running speed.
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