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1 United States Army Research Institute for Environmental Medicine, Natick 01760; 2 FitSense Technology Incorporated, Wellesley 02481; and 3 Concord Field Station, Museum of Comparative Zoology, Harvard University, Bedford, Massachusetts 01730
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Seeking to develop a
simple ambulatory test of maximal aerobic power
(
O2 max), we hypothesized that the
ratio of inverse foot-ground contact time (1/tc)
to heart rate (HR) during steady-speed running would accurately predict
O2 max. Given the direct relationship
between 1/tc and mass-specific O2 uptake during running, the ratio
1/tc · HR should reflect mass-specific O2 pulse and, in turn, aerobic power. We divided 36 volunteers into matched experimental and validation groups.
O2 max was determined by a treadmill
test to volitional fatigue. Ambulatory monitors on the shoe and chest
recorded foot-ground contact time (tc) and
steady-state HR, respectively, at a series of submaximal running
speeds. In the experimental group, aerobic fitness index (1/tc · HR) was nearly constant across
running speed and correlated with
O2 max (r = 0.90). The
regression equation derived from data from the experimental group
predicted O2 max from the
1/tc · HR values in the validation group
within 8.3% and 4.7 ml
O2 · kg
1 · min1
(r = 0.84) of measured values. We conclude that
simultaneous measurements of foot-ground constant times and heart rates
during level running at a freely chosen constant speed can provide
accurate estimates of maximal aerobic power.
aerobic fitness index; oxygen pulse; cost coefficient; locomotion; running mechanics
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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AN INDIVIDUAL'S MAXIMAL
RATE of O2 uptake
(O2 max) sets the upper limit for
sustained physical activity and is, therefore, the standard measure of
aerobic fitness. The extensive laboratory measurements of
O2 max over the last half-century have provided an empirical foundation from which numerous population norms
have been developed (2, 25). The wide dissemination of
this information has increased public awareness of aerobic fitness and
has helped establish the fitness benefits of regular running and
walking (5). Despite compelling physiological importance and considerable attention (3, 16, 27, 35), a field method for the assessment of aerobic fitness that can be easily incorporated into daily exercise routines is not available. The development of a
simple and reliable means of assessing aerobic fitness during running
or walking would provide a valuable public fitness and health tool.
The field tests available to estimate aerobic power outside the laboratory setting cannot be easily incorporated into daily routines. Many measure maximal performance during runs or walks of a specified time or distance (7, 10, 11, 15, 17, 33) and, therefore, require high levels of exertion. These tests provide aerobic power estimates of modest accuracy and can be compromised by insufficient motivation or uneven pacing. The more recently developed 20-m shuttle run test (1, 20, 22, 28, 30, 34) appears to provide more accurate estimates but also requires maximal exertion. Other tests, such as the Åstrand-Ryhming ergometer test and the Harvard bench step test, do not require all-out efforts but are, nonetheless, taxing and require strict adherence to specific protocols (3, 9, 21, 23, 24, 29, 35). Exertion and the requirements for specific procedures that fall outside the realm of normal daily activity limit the practicality of these tests as tools for ongoing personal assessment.
Ambulatory foot-ground contact monitors (12) used
simultaneously with conventional heart rate (HR) monitors may allow
aerobic power to be assessed from nothing more than several minutes of running at a freely chosen speed. Simply inverting the time of foot-ground contact (1/tc) obtained from the
monitors at any running speed provides a close approximation of
mass-specific rates of O2 uptake
(O2/Wb, where
Wb is body weight) (12, 19).
Although the relationship between 1/tc and
O2/Wb varies
little with aerobic fitness level, the HR required to support a given
O2/Wb is
inversely related to the aerobic power of the individual
(32). Regardless of the level of aerobic fitness, HR,
O2/Wb, and rates
of ground force application increase linearly with running speed. The
linear and parallel increases in HR and 1/tc
with increases in running speed suggest that the ratio of these two
variables may be independent of speed. This outcome would potentially
allow aerobic fitness to be estimated from the ratio of
1/tc to HR
(1/tc · HR) at whatever steady running
speed a person chooses.
Here, we hypothesized that the ratio of 1/tc to heart rate would be proportional to the mass-specific energy provided per heartbeat and, therefore, provide an aerobic fitness index (AFI) that could be easily obtained in the field using existing technology. We specifically predicted that the ratio of 1/tc to heart rate during level running would enable us to predict maximal aerobic power to within 10% of measured values.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects
Thirty-six subjects (18 men and 18 women) between 18 and 37 yr of age volunteered and provided written informed consent in accordance with the guidelines of Harvard University before participating. All the subjects were healthy and engaged in some form of regular physical activity. The least active subjects performed a minimum of ~20 min of aerobic exercise twice a week. The most active subjects were competitive distance runners who ran for >1 h/day and typically ran at speeds at or above that elicitingO2 max two to three times per week.
Physical characteristics by gender and group appear in Table
1.
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Experimental Design
We used a cross-validation approach to test the hypothesis that aerobic power could be predicted to within an average of 10% from the ratio of 1/tc to HR during steady-speed running. We recruited individuals varying from low-average to high levels of aerobic fitness to obtain a range of fitness levels similar to that in the general and military populations that would be potentially served by a new index. After directly measuring the aerobic power of 18 male and 18 female volunteers of various fitness levels, we ranked subjects within each gender on the basis of their maximal aerobic power. Subjects were then paired in a sequential fashion on the basis of these rankings. Within each gender and with respect to aerobic power, this procedure paired the subjects with the first- and second-greatest values, the third- and fourth-greatest values, etc. From each of nine pairs of men and nine pairs of women, one subject was assigned to an experimental group and the other to a validation group in a random fashion.Once experimental and validation groups were established, a best-fit
regression relationship between the ratio of
1/tc to HR during steady-speed submaximal
running and O2 max was formulated for
the subjects in the experimental group. Subsequently, predicted
O2 max values for the 18 subjects in
the validation group were calculated using their
1/tc · HR values. We then compared the
predicted and actual O2 max values for
the 18 subjects in the validation group to assess whether the predicted
values were within an average of 10% of the measured values as hypothesized.
Treadmill Protocol
All subjects underwent a progressive-speed, discontinuous, horizontal treadmill test to volitional fatigue. Each bout of running lasted 5.5 min; rest intervals between bouts lasted 3-5 min. Tests were initiated at 2.4-2.7 m/s, with subsequent speed increments being determined by the level of fitness each subject reported before the test. Speeds were selected conservatively so that a minimum of four speeds would be completed before each subject reached the speed elicitingO2 max. Tests were terminated when the belt speed prevented the subject from completing the full
5.5-min bout while putting forth a maximal effort.
Measurements
Rates of oxygen uptake (O2,
ml · kg1 · min1).
Steady-state O2 values
(ml · kg1 · min1) were
determined in accordance with Consolazio et al. (6) using
a Douglas bag method. Each subject wore nose clips and headgear
equipped with a mouthpiece and one-way valve. One side of the valve was
open to room air; the other directed gas via corrugated tubing into one
of two valved latex balloons arranged in series on a rack next to the
treadmill. Air was collected during the last 2 min of each 5.5-min
exercise bout to ensure steady-state O2.
A 400-ml aliquot of the expired air in each bag was then analyzed for
O2 (model SA 3, Ametek, Pittsburgh, PA) and CO2
(model CD-3A, Ametek) fractions after calibration of the analyzer with
a gas of known concentrations. Gas volumes were determined by pushing
the collected gas through a Parkinson-Cowan dry gas meter with
simultaneous temperature reading. O2
values (STPD) were determined from O2 and
CO2 fractions and the expired volumes.
Maximal aerobic power (ml
O2 · kg1 · min1).
O2 max
(ml · kg1 · min1) was the
highest single-minute O2 measured during
the progressive, discontinuous treadmill test with an accompanying
criterion of a minimum respiratory exchange ratio of 1.10.
Foot-ground contact times (s).
In this study, a patented foot pod device (model 6122340, FitSense
Technology, Wellesley, MA), in which the authors affiliated with
FitSense hold a proprietary interest, was used to measure tc (s). The plastic pods contained
accelerometers (0-10 g; model ADXL-210, Analog Devices, Norwood,
MA), radio transmitters, and microprocessors (Fig.
1) that analyzed the vertical waveforms generated during the stride to identify periods of foot-ground contact
to within ±2 ms. The pods were mounted on the top of each subject's
shoe and secured with the shoe's laces before the start of the
treadmill test. For each step, tc values were
telemetered to a receiver mounted on the railing of the treadmill.
Accelerometric tc values at each speed were
averaged from 
20 consecutive steps of the same foot at some point
later than 30 s in the bout. Foot pod tc
values were identified by the microprocessor from the time elapsing
between foot-down and foot-up for each step. The identification of the
time of foot strike and toe off from the waveform output of the
accelerometer is depicted for a representative trace in Fig.
2.
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Heart rates (beats/min). HR (beats/min) was measured using HR monitors (Polar Electrode Oy, Kempele, Finland), which telemetered a running 5-s average from a bipolar electrode unit fastened to the subject's chest to a wristwatch display mounted on the treadmill rail. Values were recorded at 3.75, 4.75, and 5.25 min of each bout and averaged to obtain a final value for each speed. The highest value recorded was considered the subject's maximum HR.
Aerobic fitness index (1/beat). A single average value for 1/tc · HR was determined for each subject at each speed using the accelerometric tc and the average of the three HR values recorded during each bout.
Statistical Analyses
Means for age, mass, height,O2 max, maximal HR, and the aerobic
fitness index for the experimental and validation groups were compared
using the Student's t-test for paired samples (P < 0.05). The aerobic fitness index with respect to
speed was tested for slopes significantly different from zero using
simple linear regression (P < 0.05). Linear
least-squares regression lines were formed to assess the relationship
between O2 max and the AFI for the
experimental group and between actual and predicted aerobic power for
the validation group. Values are means ± SE.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Force Plate vs. Accelerometric Foot-ground Contact Times
The tc values identified by the accelerometers were highly correlated with those measured using the treadmill-mounted force plate (Fig. 3). On average, accelerometric tc values were 14.6 ± 0.5% longer than those measured from the force plate. Longer contact times from the accelerometer resulted primarily from the interval during the latter portion of the contact period when no force is exerted on the plate but the foot has not yet been accelerated off the running surface to begin the swing phase. The ratio of accelerometric to force plate tc values increased slightly with running speed. From the slowest to the fastest speed administered to each subject, the average increase in this ratio was +5.9%. These increases tended to be greater in the subjects tested over a greater range of speeds.
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Inverse Foot-ground Contact Times vs. Mass-specific Rates of Oxygen Uptake
In the 36 subjects tested, 1/tc accounted for an average of 98.5% of the within-subject variance measured inO2/Wb as a function of running speed in the 36 subjects tested. The slope and
intercept representing the average for all subjects appears in Fig.
4, as do the regression relationships for
three individual subjects: one extremely low, one average, and one
extremely high, with respect to O2 at a
given 1/tc during running. Although 1/tc values accounted for virtually all the
variance in O2 across speed for
individual subjects, there was appreciable between-subject variation, as illustrated by the different values for the three individual subjects in Fig. 4. Consequently, the proportion of variance
in O2/Wb
accounted for by 1/tc among the entire group of
36 was considerably less than that accounted for across speed in
individual subjects (61 vs. 98%).
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Aerobic Fitness Index
HR and 1/tc increased linearly with increases in running speed for all 36 subjects. These relationships are illustrated in Fig. 5 for two subjects: one with high aerobic power and another with lesser aerobic power. The linear and parallel increases in HR and 1/tc resulted in values of 1/tc · HR (i.e., our proposed aerobic fitness index) that were independent of running speed. The slope of the relationship between running speed and 1/tc · HR was not different from zero (P < 0.05) in 31 of the 36 subjects tested; the average percent change from the slowest to the highest speed completed was +5.8%.
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Mean values for the aerobic fitness index were 17.9 and 20.0% greater
for men than for women in the experimental and validation groups,
respectively, but were not different between experimental and
validation groups for either gender (Table
2).
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Maximal Aerobic Power
O2 max (Table 2) did not differ
between the experimental and validation groups, with similar ranges of
37.4-72.6 and 40.8-74.8
ml · kg1 · min1,
respectively. Mean values for aerobic power were 24.5 and 29.1% lower,
and maximum HR values were 11 and 5 beats/min greater for women than
for men in the experimental and validation groups, respectively. Mean
respiratory exchange ratios at O2 max were 1.15 and 1.16 for the experimental and validation groups, respectively.
Aerobic Fitness Index as a Predictor of
O2 max
O2 max (Fig.
6). When average values for the aerobic fitness index measured for the 18 subjects in the validation group were
calculated using the regression equation formulated on the experimental
group, the predicted values for O2 max
were within an average of 8.3% (4.7 ± 0.8 ml · kg1 · min1, range
0.2-11.6
ml · kg1 · min1) of the
actual values. Predicted values accounted for 70% of the variance in
measured values (Fig. 7). Predicting
aerobic power from the aerobic fitness index values at all speeds,
rather than a single average value, had almost no effect on the
accuracy of the predictions. In the latter case, the proportion of
variance in O2 max accounted for was
74.2 and 66.5% for the experimental and validation group subjects,
respectively. Finally, predicting O2 max from an aerobic fitness index
determined using force plate, rather than accelerometric,
tc values had virtually no effect on the
accuracy of the predictions provided (r2
difference < 0.02).
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To evaluate for the possible influence of gender on the relationship
between our AFI and O2 max, regression
equations were formulated for the 18 male and 18 female subjects
separately. The resulting best-fit regression relationships
(O2 max = 11.1 + 34.4 · AFI and 10.3 + 30.9 · AFI for men and
women, respectively) indicated that, in the range of gender overlap for
aerobic power (47.5 to 63.7 ml · kg1 · min1) for any
given value of the AFI, O2 max was
9.8% greater in male than in female subjects. When the AFI and gender
were used to predict aerobic power using multiple regression, the
proportion of variance accounted for increased from 74.0 to 79.8%
(O2 max = 19.4 + 32.45 · AFI 
5.5 · gender, where a value of 1 was assigned to males and 2 to females, P = 0.02).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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As anticipated, we succeeded in predicting
O2 max from
1/tc · HR during running to within 10%
of actual values. The values for O2 max
predicted from our aerobic fitness index matched the values measured in
the 18 validation group subjects to within an average of 8.3%. Of
equal importance to the accuracy of these estimates is the ease with
which they can be attained. The virtual independence of the aerobic
fitness index from running speed allows estimates to be acquired from
only a few minutes of running at whatever constant speed a person
chooses. This new technique offers the public a practical means of
assessing aerobic fitness during day-to-day living.
Physiological Basis of 1/tc · HR as an Index of Aerobic Fitness
Our strategy for establishing an ambulatory index of aerobic fitness combines a novel approach with one that is nearly a half-century old (3). The earliest field tests ofO2 max and many in use today are based
on the inverse relationship between
O2 max and HR at some known sustainable
mechanical work rate. These tests provide reasonable estimates
of O2 max as long as physical activity incurring a known O2, and
therefore cardiovascular demand, can be implemented in the testing. In
practice, this can be achieved by having subjects perform mechanical
work at prescribed rates, either against the pedals of a cycle
ergometer or against gravity during bench stepping. Here, in the
interest of developing an ambulatory assessment tool, we combined the
old idea of using steady-state HRs with a promising approach to
estimating O2/Wb during locomotion (12, 13, 19).
The immediate impetus for our attempt to develop an ambulatory index
was provided by the close relationship between
1/tc and the mass-specific metabolic rates of
runners originally reported by Kram and Taylor (19). These
authors presented this relationship as follows:
metab/Wb = C · 1/tc, where C
represents the amount of energy expended per unit of force applied to
the ground to support the body's weight. Although Kram and Taylor
reported that the value of C was nearly invariant among
different species of quadrupedal runners over a 10-fold range of
running speeds, we did not know a priori how much C might
vary among different human runners. A similarly invariant relationship
among human runners would have enabled us to express the fitness index
as the amount of O2 provided per heartbeat or as a
mass-specific O2 pulse. However, appreciable variability in
C among different runners and across running speeds
prevented this.
Nonetheless, some more concrete link to the physiological basis of this
new index seems warranted for the purposes of basic understanding and
appropriate use of this new assessment tool. The units we report
(1/beat) can be converted to an approximate mass-specific
O2 pulse by multiplying by 0.18 ml O2/kg, the
average cost coefficient measured for all the runners in this study.
This calculation provides a reasonable approximation of the volume of
O2 consumed per kilogram of body weight per beat of the
heart during running (thus 0.18 · AFI = ml
O2 · kg1 · beat1),
a variable more intuitively related to the maximal aerobic power of the runner.
Independence of the Aerobic Fitness Index From Running Speed
The independence of individual 1/tc · HR values from running speed, which enhances the practicality of our new assessment technique, was not a foregone conclusion at the outset of this study. This result was unlikely if either the amount of O2 consumed per heartbeat or the energy cost of applying ground force (C) changed appreciably as a function of running speed. Although we did find values of 1/tc · HR to be consistently independent of speed, this occurred, despite significant speed-induced increases in both of the aforementioned variables. However, because the increases in mass-specific O2 pulses and C were largely parallel, values for the aerobic fitness index were statistically unchanged across speed in 31 of our 36 subjects. Although a causal explanation for these results is more appealing than a noncausal one, such an explanation would not be correct. The stance limb mechanics responsible for the increases in the energy cost of applying ground force at higher speeds (4, 31) are not directly linked to the cardiovascular changes responsible for the increases in mass-specific O2 pulses.Utility of the Aerobic Fitness Index as a Field Test of Maximal Aerobic Power
The accuracy of theO2 max
predictions provided by the aerobic fitness index are as good as or
better than those reported for other predictive tests. Although some
individual studies have reported marginally higher correlations from
running (7, 26) or other tests (3, 23), the
predictions generally reported in the literature for these tests
(25, 35) are equally or less accurate than those we report
here. As with many of the existing tests, the greatest source of error
in the predictions resulted from individual variation from estimated
mass-specific rates of oxygen uptake, which in this case were estimated
from rates of ground force application (1/tc).
For any given subject running at any speed,
O2/Wb predicted
from 1/tc differed from the measured value by an
average of 7.7%, similar to the error reported for subjects performing
mechanical work at the same rates (25).
A sense of the error introduced into the predictions of
O2 max as a result of the individual
variability in the energetic cost of applying ground force
(C) is provided by the relationship between the rates of
ground force application (1/tc) and
O2/Wb illustrated
in Fig. 4. The aerobic fitness index consistently underpredicted
O2 max values for subjects that had
relatively low
O2/Wb for any
given value for 1/tc, and vice versa.
Predictions for those subjects whose
O2/Wb values were
close to the group mean with respect their rates of ground force
application were the most accurate.
The influence of factors other than individual variability in
C that might have weakened the relationship between
O2 max and the aerobic fitness index
was small. Adding maximal HR or gender to the aerobic fitness index as
copredictors of O2 max with the use of
a multiple-regression analysis increased the proportion of variance in
O2 max accounted for by 7 and 8%,
respectively. In contrast, when measured values for mass-specific
O2 pulses were used as a single predictor of
O2 max, rather than 1/tc · HR, the proportion of variance
accounted for in the measured maximums for aerobic power for the
subjects in our validation group increased by 21%, from 69 to 90%.
Although variability in maximal HR and other factors may have a greater
effect on predictions of aerobic power from our ambulatory index in
more heterogenous populations, the influence of these factors among a
population of healthy adults in the age range tested here was modest.
We anticipate that the accuracy of predictions of
O2 max provided by the aerobic fitness
index during treadmill running in health club and other settings should
be similar to those reported here. However, we cannot know how accurate
predictions from overground running on a level surface will be. Running
at volitional speeds in the field, rather than controlled speeds on a
treadmill, may weaken the predictive ability of our index. Further work
to determine the accuracy of values obtained during overground running
is warranted, as is an assessment of how well our index will track
individual changes in aerobic fitness over time. The close relationship
between training-induced changes in
O2 max and the HR elicited by any given
submaximal exercise intensity (8, 14) suggests that
aerobic fitness index estimates of changes in
O2 max over time may be more
accurate than estimates of absolute values.
Ease of use is an appealing aspect of this new technique for assessing
aerobic fitness. Given the ambulatory monitors necessary and a level
running surface, any person fit enough to jog can gain an estimate of
aerobic fitness with modest exertion in a matter of minutes. The
elimination of cumbersome procedures and the high levels of exertion
required by existing field tests offers an assessment technique that is
more practical for widespread public use and equally accurate (Table
3). The simplicity of the procedure and
the minimally obtrusive nature of the monitors required should,
ultimately, place the capability for the personal assessment of aerobic
fitness within the reach of most of the individuals in the developed
world. Convenience makes our new index a potentially powerful tool for
the modification of sedentary behavior and the improvement of aerobic
fitness and health.
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We conclude that simultaneous measurements of foot-ground constant times and heart rates during level running at a freely chosen steady speed can be used to estimate maximal aerobic power.
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ACKNOWLEDGEMENTS |
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We thank our subjects for their rigorous efforts, Seth Wright for technical support, and Andrew Biewener for support and use of the facilities at the Concord Field Station.
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FOOTNOTES |
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This work was supported by a National Research Council Senior Fellowship to P. G. Weyand.
Address for reprint requests and other correspondence: P. G. Weyand, USARIEM, 42 Kansas St., Natick, MA 01760.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 21 November 2000; accepted in final form 5 March 2001.
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DISCUSSION
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