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1 Copenhagen Muscle Research Centre, Institute of Exercise and Sport Sciences, August Krogh Institute, University of Copenhagen, DK 2100 Copenhagen, Denmark; 2 Neuromuscular Biology Group, Department of Exercise and Sport Science, Manchester Metropolitan University, Alsager ST7 2HL, United Kingdom; and 3 Department of Neurophysiology, Institute of Medical Physiology, Panum Institute, University of Copenhagen, DK 2100 Copenhagen, Denmark
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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A novel approach has been developed for the quantification of total mechanical power output produced by an isolated, well-defined muscle group during dynamic exercise in humans at different contraction frequencies. The calculation of total power output comprises the external power delivered to the ergometer (i.e., the external power output setting of the ergometer) and the "internal" power generated to overcome inertial and gravitational forces related to movement of the lower limb. Total power output was determined at contraction frequencies of 60 and 100 rpm. At 60 rpm, the internal power was 18 ± 1 W (range: 16-19 W) at external power outputs that ranged between 0 and 50 W. This was less (P < 0.05) than the internal power of 33 ± 2 W (27-38 W) at 100 rpm at 0-50 W. Moreover, at 100 rpm, internal power was lower (P < 0.05) at the higher external power outputs. Pulmonary oxygen uptake was observed to be greater (P < 0.05) at 100 than at 60 rpm at comparable total power outputs, suggesting that mechanical efficiency is lower at 100 rpm. Thus a method was developed that allowed accurate determination of the total power output during exercise generated by an isolated muscle group at different contraction frequencies.
quadriceps; contraction velocity; muscle moment; internal power; internal work; mechanical efficiency
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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DIFFERENT
APPROACHES HAVE been used for the quantification of energy
turnover and mechanical efficiency during exercise in humans. During
whole body exercise, such as cycling, pulmonary oxygen uptake
(
O2) has been measured and used as an
estimation of energy turnover. This has then been related to the
mechanical output during exercise, which, in cycling, is usually
determined as the external power delivered to the ergometer (e.g., Ref.
7). This approach, however, clearly underestimates the work
performed because the power generated to overcome the inertial and
gravitational forces opposing the movement of the limb, the so-called
"internal" work, should also be taken into consideration
(15). For example, values for mechanical efficiency during
cycle exercise, defined as the ratio between external mechanical work
and net energy expenditure, ranged between 18 and 22% at a range of
work rates (9). When both external and internal mechanical
work were determined, efficiency varied between 21 and 30%
(9).
Measurements of pulmonary O2 during
whole body exercise do not, however, provide accurate information about
the energy turnover and mechanical efficiency of the working muscles,
because tissues other than the skeletal muscle contribute to energy
turnover. Another problem in determining the energy turnover of the
working skeletal muscles is that it is difficult to quantify the mass of the muscles involved in the exercise and to determine to what extent
these muscles are active (12). To determine the energy turnover of the active muscles during exercise in humans, an isolated muscle model needs to be used. One such model is single-leg, dynamic knee-extension exercise (2), which allows a functionally
significant muscle group (i.e., quadriceps) to be investigated in a
well-defined and reproducible movement and in which the active muscle
mass can be readily estimated. Energy turnover can be determined from measurements of thigh blood flow, femoral venous and arterial blood gas
and metabolites, as well as changes in metabolites in the contracting
quadriceps muscle (3, 4).
The external power delivered to the ergometer during knee-extensor exercise can be determined with high precision, and estimates of mechanical efficiency have been made (~24%) (3). However, in these calculations, the internal work was not taken into account. This needs to be carefully evaluated if this exercise model is to be used to determine the true mechanical efficiency of contracting muscle in vivo. This is even more obvious if measurements are to be made during exercise at different contraction frequencies. During cycle exercise, for example, it has been shown that internal work is greater as contraction frequency increases (13, 14). There is, however, no clear picture as to how contraction frequency affects mechanical efficiency. Some investigations report a decline in mechanical efficiency (7), whereas others report an increase (5) as contraction frequency increases.
Therefore, one of the aims of this study was to develop a method to
quantify the total mechanical power output produced during exercise of
an isolated muscle group in humans, i.e., during dynamic, single-leg
knee-extensor exercise. It was proposed that external power alone would
markedly underestimate the actual total power developed. Another aim of
the study was to evaluate whether there was any difference in energy
turnover during exercise at the same total power output at two
different contraction frequencies (60 and 100 rpm). The study
demonstrated that the method can be used to determine the internal and
total power developed by an isolated muscle group during exercise at
different contraction frequencies. It was also shown, based on
pulmonary O2, that the energy cost of
exercise is higher at 100 compared with that at 60 rpm at the same
total power output, suggesting that mechanical efficiency is lower at
the higher contraction frequency.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects. Twelve healthy men volunteered to participate in the investigation. All subjects were physically active, but none was specifically trained, and all gave written, informed consent. Mean (±SE) age, height, and body mass were 24 ± 1 yr, 180.4 ± 1.8 cm, and 75.4 ± 2.4 kg, respectively. The study was approved by the local Ethics Committee.
Exercise model.
Single-leg, dynamic knee-extension exercise (2) was
performed in a supine position, with the subjects secured to the seat with straps across the torso, hips, and both thighs (Fig.
1). The subjects were instructed to
contract the knee-extensor muscles during extension and to completely
relax all muscles during passive flexion. During habituation,
consisting of at least three, 30-min sessions, the subjects were fully
familiarized to the experimental procedures.
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Total power determination. The determination of total muscle power required continuous measurements of the force exerted between the moving lower limb and the pedal arm of the ergometer, which was recorded by a strain gauge incorporated in the rod attachment of the ergometer (Fig. 1). It also required continuous measurement of knee joint angle and the angle between the lower limb and the connecting bar attached to the pedal arm of the ergometer. Therefore, a flexible electrogoniometer (type XM180, Penny and Giles Biometrics, Gwent, UK) was placed across the lateral aspect of the knee joint and attached proximally to the strapping that firmly secured the thigh and distally to just below the lateral epicondyle of the tibia (Fig. 1). A second electrogoniometer was positioned between the lower leg (i.e., the boot attachment) and the connecting bar of the ergometer (Fig. 1). During exercise at both 60 and 100 rpm, the angular displacement of the knee ranged between 90 and 170° at flexion and extension. In situ calibration of the electrogoniometer at the knee was performed immediately before and after each exercise bout by moving the lower limb between a fully extended and a 90° flexed position. During experiments, the strain gauge and electrogoniometer signals were recorded onto the hard drive of a computer (Apple Macintosh Performa 5200) using an A-D board with a 400-Hz analog-to-digital sampling rate (MacLab 8s data acquisition system, Chart v3.3 software, ADInstruments, Sydney, Australia). During the subsequent analysis, electrogoniometer signals were smoothed by means of a digital, fourth-order, zero-lag Butterworth low-pass filter (16), using a cut-off frequency of 3 Hz.
Typical changes in joint angle and strain gauge output from one representative subject during exercise at an external power output of 30 W at either contraction frequency are illustrated in Fig. 2. At 60 rpm, from the beginning of extension, there is a rapid increase in strain gauge force that peaks at approximately midextension. As extension continues, the force declines but then begins to rise slightly at the transition between extension and flexion. As flexion continues, there is a slight negative force. At 100 rpm, from the beginning of extension, there is an increase in force that initially peaks at midextension. As flexion begins, there is a sharp and rapid increase in force as the momentum of the flywheel causes the connecting rod to reverse direction, thus overcoming the kinetic energy of the leg generated during the fast extension phase. There is a rapid decline in force during the flexion phase, with a large negative peak before the extension phase begins again.
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(1) |
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(2) |
pull was the angle
between the connecting bar and the lower limb, as measured by the
electrogoniometer (Fig. 1).
Mgravity was calculated as
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(3) |
g was the angle of the lower limb relative to vertical,
as measured by the electrogoniometer (Fig. 1).
Minertia was calculated as
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(4) |
was the
angular acceleration of the lower limb (in rad/s2), as
obtained by double differentiation of the signal measured by the
electrogoniometer (Fig. 1).
Figure 3 illustrates the contribution
from each moment to the determination of Mtotal and the
calculation of instantaneous muscle power output during a typical
single flexion-extension cycle for an external power output of 30 W at
60 and 100 rpm. The magnitude of Mergometer during the leg
extension phase was slightly lower at 100 compared with at 60 rpm.
Because the determinants of Mgravity do not change between
the conditions examined (see Eq. 4) the magnitude of
Mgravity was the same at 60 and 100 rpm. As expected from
the differences in acceleration, Minertia was greater at
100 compared with at 60 rpm. Thus the greater Mtotal at 100 rpm was primarily a result of the higher Minertia. The instantaneous angular velocity was obviously higher at 100 rpm. Thus
the product of Mtotal and angular velocity, resulting in the instantaneous muscle power output, had a greater magnitude during
the extension phase at 100 compared with at 60 rpm.
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Experimental protocol. In the first part of the study, subjects performed, in randomized order, exercise at external power outputs (i.e., ergometer power settings) of 0, 10, 30, 40, and 50 W at contraction frequencies of 60 and 100 rpm. Each exercise bout was performed for 5 min, with at least 15 min of passive rest separating the bouts. Total power output was calculated for each exercise bout as described in Total power determination.
In the second part of the study, an attempt was made to match the total power output during exercise at the two contraction frequencies. To do this, the data for each individual subject from the first part of the study were used to establish a linear relationship between total and external power outputs for each contraction frequency. Initially, the external power for exercise at 60 rpm was set at 30 W, and the total power output for this was predicted using the regression equation for 60 rpm. To match this total power output, the external power output at 100 rpm was predicted from the regression equation between total and external power at 100 rpm. With these two external power outputs, exercise was performed for 5 min, again in a randomized order, with a 30-min rest period between each bout. Total power output was again calculated for each exercise bout, as described in Total power determination.Variability and reliability. The average coefficient of variation for the five measurements of total power output analyzed during each exercise bout was 8.3%. Furthermore, there were no significant differences in total power output between the five sections measured at each external power output. To enable the determination of test-retest reliability of the power output measurements, 10 subjects performed the experimental protocol on two separate occasions. The coefficient of variation in total power output between the repeated trials was 7.2%.
Expired air analysis.
During each exercise period, pulmonary gas exchange was measured at
rest during a 3-min period before exercise, throughout exercise, and
for 3 min after the end of exercise. Expired air was continuously
measured for the percent O2 and CO2 content and volume using a breath-by-breath gas analysis system (Medgraphics CPX/D
Metabolic Cart). This was calibrated with gases of known concentration
and a 3-liter syringe before each testing session. Breath-by-breath
oxygen uptake (O2) data was computed and
then averaged over each minute of sampling for each trial.
Statistical analyses. All data were analyzed with either a one- or two-way ANOVA where appropriate. When a significant effect was observed, the differences were located with a post hoc Tukey's test. Linear regression and Pearson correlation were used where necessary. Significance was accepted at P < 0.05. All data are presented as means ± SE.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Relationship between external and total power output.
Total power output increased as external power increased from 0 to 50 W
and was significantly (P < 0.05) greater at 100 compared with 60 rpm at all external power outputs (Fig.
4). At 60 rpm, the mean internal power
output (calculated as the difference between total and external power)
was 18 ± 1 W (range: 16-19 W) and did not change as external
power output increased (Table 1). At 100 rpm, the mean internal power output was 33 ± 2 W (27-38 W),
with internal power being lower (P < 0.05) at an
external power of 30, 40, and 50 W compared with 0 and 10 W (Table 1).
No correlation was observed between lower limb dimensions (i.e., limb
length, estimated mass, and moment of inertia) and the level of
internal power output at any of the external workloads.
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Matched total power at 60 and 100 rpm.
In the second part of the investigation, we sought to match the total
power produced during exercise at 60 and 100 rpm. For an external power
output of 30 W at 60 rpm, the regression analysis resulted in a target
total power output of 45 ± 2 W (38-54 W) (Table
2). To match this total power output
during exercise at 100 rpm, an external power output of 14 ± 2 W
(7-26 W) was predicted to be required (Table 2). When exercise was
performed at each respective external power output for either 60 or 100 rpm, there was no difference in the measured total power output between
the two contraction frequencies (Table 2). In addition, there was no
difference between the target total power output and the measured total
power output during these exercise bouts (Table 2).
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Pulmonary O2.
At 100 rpm, pulmonary O2 was ~0.3
l/min higher (P < 0.05) at all external power outputs
than at 60 rpm (Fig. 5A). Even
when O2 was expressed in relation to
total power output, the oxygen cost at 100 rpm was greater than at 60 rpm at all power outputs (Fig. 5B). In the second part of
the study, when exercise was performed at the same total power output,
pulmonary O2 was ~0.1 l/min higher
(P < 0.05) at 100 compared with at 60 rpm.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The single-leg knee-extensor exercise model provides a unique
opportunity to study the physiological mechanisms relating to human
muscle metabolism and peripheral cardiovascular control in vivo
(2-4). In the present study, a method was developed
to quantify the total mechanical power output produced during dynamic exercise at different contraction frequencies using the knee-extensor model. It was demonstrated that internal power contributes
significantly to the total power output. At a contraction frequency of
60 rpm, the internal power is the same, regardless of the external
workload applied; whereas, at 100 rpm, internal power output is highest at low external power outputs. It was also observed that pulmonary O2 is higher at a contraction frequency
of 100 than at 60 rpm at comparable power outputs, suggesting that
mechanical efficiency is lower at the high contraction frequency.
Measurements of total power output at two different contraction frequencies revealed significantly greater levels of internal power at the higher contraction frequency, which has been previously observed during walking and running (11) as well as cycling (6, 8, 9, 13, 14). For example, internal power outputs of 20 and 62 W (16-26 and 50-72 W, respectively) were calculated for the entire leg during cycle exercise at pedal frequencies of 60 and 90 rpm, respectively, at external power outputs of 0, 60, 120, and 180 W (13). Furthermore, in the present study, the internal power at 60 rpm was independent of the external power output. In contrast, at 100 rpm, the internal power was lower at higher external power outputs (Table 1). Thus, at both contraction frequencies, the relative proportion of internal power to total power was lowered as total power increased, and there was an apparent convergence in total power output between 60 and 100 rpm as external power increased (Fig. 4). During cycle exercise at pedal frequencies of 30, 60, and 90 rpm, it was reported that internal power output remained constant as external power increased (13). However, there was considerable variation in internal power such that, during exercise at 90 rpm, the internal power in one subject was 84, 55, 66, and 68 W at external power outputs of 0, 60, 120, and 180 W, respectively (13).
In the second part of the study, the total power produced during
exercise at the two contraction frequencies was matched. This was done
by predicting the external power necessary at 100 rpm to produce the
same total power as that at 60 rpm. The data in Table 2 show that this
could be achieved with reasonable accuracy. At the same time, however,
considerable intersubject variation was observed in the magnitude of
internal power output at 100 rpm. This resulted in a large variation in
the external power required to match the total power produced at the
slow contraction frequency; the reasons for this are unclear. Internal
power is affected by the anatomic dimensions of the lower limb as well as the time history of angular acceleration and velocity. However, there was no relationship between lower limb length or estimated limb
mass and the level of internal power produced. It is likely that other
factors, such as intersubject differences in limb acceleration and
velocity, must have influenced the internal power output. Nevertheless,
the total power output was the same at 60 and 100 rpm and allowed a
direct comparison of the energy turnover during exercise at these two
contraction frequencies. Pulmonary O2 was greater at 100 than at 60 rpm, suggesting that muscle mechanical efficiency was lower at 100 rpm. It should be recognized that pulmonary
O2 does not necessarily represent the
energy turnover by the contracting muscles. The method developed in the
present study can, however, be used to relate the work performed at
different contraction frequencies to muscle
O2, which is accurately determined as
the product of thigh blood flow and femoral arterial and venous blood
O2 difference. Thus true muscle mechanical efficiency can be determined in vivo.
In summary, a novel method has been developed in which the total mechanical power output can be quantified during dynamic exercise of a single, well-defined muscle group in humans in vivo. It is concluded that the internal power is independent of external power at a low contraction frequency, whereas, at a high contraction frequency, internal power decreases as the external workload increases. The model presented allows total power output to be matched, which will enable an accurate evaluation of the physiological responses during exercise at different contraction frequencies. The present data indicate that the muscle energy cost is higher at a frequency of 100 rpm compared with at a frequency of 60 rpm.
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ACKNOWLEDGEMENTS |
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This work was supported by the Danish National Research Foundation (Grant 504-14). In addition, support was obtained from Idrættens Forskningsråd and Team Danmark. R. A. Ferguson was supported, in part, by a Socrates Erasmus Scheme Mobility Grant.
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FOOTNOTES |
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Address for reprint requests and other correspondence: J. Bangsbo, August Krogh Institute, Universitetsparken 13, DK 2100 Copenhagen, Denmark (E-mail: JBangsbo{at}aki.ku.dk).
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 22 December 1999; accepted in final form 21 June 2000.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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