Metabolic costs of isometric force generation and maintenance
of human skeletal muscle
David W.
Russ1,
Mark A.
Elliott2,
Krista
Vandenborne3,
Glenn A.
Walter3, and
Stuart A.
Binder-Macleod1,4
Departments of 1 Biomechanics and Movement Science and
4 Physical Therapy, University of Delaware, Newark, Delaware
19716; and Departments of 2 Radiology and
3 Physiology, University of Pennsylvania, Philadelphia,
Pennsylvania 19014
 |
ABSTRACT |
During isometric
contractions, no true work is performed, so the force-time integral
(FTI) is often used to approximate isometric work. However, the
relationship between FTI and metabolic cost is not as linear. We tested
the hypothesis that this nonlinearity was due to the cost of attaining
a given force being greater than that of maintaining it. The ATP
consumed per contraction in the human medial gastrocnemius muscle
(n = 6) was determined by use of 31P-NMR
spectroscopy during eight different electrical stimulation protocols.
Each protocol consisted of 8 trains of a single frequency (20 or 80 Hz)
and duration (300, 600, 1,200, or 1,800 ms) performed under
ischemic conditions. The cost of force generation was
determined from the ATP turnover during the short-duration trains that
did not attain a steady force level. Estimates of the cost of force maintenance at each frequency were determined by subtracting the ATP
turnover during the shorter-duration trains from the turnover during
the long-duration trains. The force generation phase of an isometric
contraction was indeed more metabolically costly than the force
maintenance phase during both 20- and 80-Hz stimulation. Thus the mean
rate of ATP hydrolysis appeared to decline as contraction duration
increased. Interestingly, the metabolic costs of maintaining force
during 20-Hz and 80-Hz stimulation were comparable, although different
levels of force were produced.
skeletal muscle; metabolism; contractile cost; 31P-nuclear magnetic resonance
 |
INTRODUCTION |
TOTAL ATP
CONSUMPTION by skeletal muscle increases with increased work and
power (1, 12, 13, 33). In addition, ATP consumption in
response to single, isometric twitches is proportional to the area
under the force-time curve, or force-time integral (FTI)
(17). Because twitch FTI and work appear to relate to ATP
consumption in a similar manner, FTI is often used as an approximation of work (8, 11, 21) during isometric contractions that produce no true physical work. During electrically elicited isometric twitches, ATP consumption increases linearly over the range of frequencies below that at which summation of force occurs and the ATP
consumption per twitch remains constant (27).
During an isometric tetanus, however, it has been suggested that
metabolic cost is not a linear function of contraction duration, because the cost of achieving a level of force is greater than that of
maintaining it (14). If the cost of attaining a given force level is greater than that of maintaining it, then the net rate
of ATP hydrolysis should decrease as the duration of a tetanus increases. This effect has been observed in fast-twitch mouse extensor
digitorum longus (EDL) (11) and rat gastrocnemius muscles (35). Moreover, He et al. (20) found that the
rate of actin-myosin ATPase (AM- ATPase) activity was highest at the
onset of contraction and progressively decreased with subsequent
cross-bridge cycles in skinned, mammalian fibers. In addition, a number
of studies in human skeletal muscle have demonstrated that, during
intermittent isometric electrically elicited contractions, shorter
contractions produce greater total ATP utilization than longer
contractions when force, total contraction time, FTI, and number of
stimulation pulses are controlled (4, 8, 36). Newham et
al. (28) demonstrated, in human adductor pollicis muscles,
that 50 0.1-s contractions produced greater ATP consumption than a
single 5-s tetanus, despite the fact that the 0.1-s contractions
produced smaller peak forces. Ratkevi
ius et al.
(30), studying human triceps surae muscles, found that an
intermittent stimulation protocol consumed >60% as much ATP as a
continuous stimulation protocol, despite the fact that the intermittent
protocol produced only 20% of the total contraction time. Finally,
pilot work from our laboratory demonstrated that stimulation with
12-pulse 50-Hz trains produced more fatigue than stimulation with
12-pulse 10-Hz trains, even though the number of pulses and
contractions were equal and the 10-Hz trains produced higher FTIs
(32). None of these studies, however, specifically
addressed the different costs of force generation and force maintenance
in human muscle.
The purpose of this study was to determine the metabolic cost and
ATPase rates, by use of in vivo 31P-NMR spectroscopy,
associated with electrical stimulation of the human medial
gastrocnemius, with trains of different frequencies (20 and 80 Hz) and
durations (300, 600, 1,200, and 1,800 ms), and to use these values to
estimate the different costs of force generation and force maintenance.
We hypothesized that, for a given contraction duration, 80-Hz
stimulation would involve a greater metabolic cost than 20-Hz
stimulation, consistent with the greater force associated with the
higher frequency. We further hypothesized that, within a given
frequency, increasing duration would decrease the mean ATPase rate, as
ATP turnover would be greatest at the onset of contraction. As a
result, we expected to observe a relatively lower ATP turnover than
would be anticipated for a linear increase in ATPase rate with
increasing contraction duration.
 |
METHODS |
Subjects
Six healthy subjects (3 males), ranging from 25 to 42 yr of age
[mean 33.3 ± 5.54 (SE) yr], with no history of muscle or joint problems, participated in this study. All subjects were informed of the
purpose and procedures of the study and gave written, informed consent
to their participation. The experimental protocols were approved by the
Human Subjects Review Boards of the Universities of Delaware and Pennsylvania.
Experimental Procedures
Experimental setup.
Before any testing, one of the investigators (D. W. Russ) located the
motor point of the medial gastrocnemius muscle (MG) on each subject and
recorded its relationship to the following anatomic landmarks: the
posterior fibular head, the medial tibial plateau, and the popliteal
crease. These measurements were used to assist the investigators in
finding the motor point and placing the surface coil during subsequent
testing sessions. One carbonized rubber electrode (4.5 × 4 cm),
coated with a conductive gel and held in place with a paper-tape patch,
was placed directly over the MG motor point. A second electrode
(7.6 × 12.7 cm) was placed over the anterior surface of the knee
joint. This electrode placement produced comfortable contractions at
the same force level and improved the NMR signal-to-noise ratio
compared with the more traditional placement, with both electrodes over
the muscle (31). The subject lay supine, with the foot at
a 90° angle to the shank, in a custom-built nonmagnetic
plantar-/dorsi-flexion ergometer, locked in neutral position. A 3 × 7-cm oblong surface coil with a shallow penetration profile (see
Localization of 31P-NMR spectra), double-tuned
to both 1H and 31P, was placed over the upper
one-third of the MG. An inflatable blood pressure cuff was placed over
the middle of the subject's thigh. The subject was stabilized using
nonelastic nylon straps with Velcro closures across the foot, ankle,
shank, thigh, and trunk. The subject's plantarflexion maximum
volitional isometric contraction (MVIC) was assessed by using the mean
of three maximal-effort plantar flexions. Electrical stimulation of the
muscle was delivered using a Grass S48 stimulator with a Grass model
SIU8T stimulus isolation unit (Astro-Med, West Warwick, RI). All
stimulation pulses were 600 µs in duration. Stimulation intensity was
set such that a 1-s, 100-Hz train produced 50% of the subject's MVIC. Once the stimulation intensity was set, the force transducer was disconnected, as it was found that simultaneous operation of the transducer, the spectrometer, and the stimulator introduced an unacceptable amount of noise into the NMR spectra.
Testing session.
All experiments involved isometric testing of the MG in a 1-m-bore
2.0-Tesla superconducting magnet, interfaced with a custom-built spectrometer (39). An adaptation of the protocol
previously described by Blei et al. (5) was used to
measure the metabolic cost associated with each train. Changes in the
energy-rich phosphate content were measured with 31P-NMR
spectroscopy during electrically induced contractions in the absence of
oxygen. For this purpose, a blood pressure cuff was placed around the
thigh and inflated to 230 mmHg for 5 min before stimulation. It has
been demonstrated that a 5-min period of ischemia is sufficient
to eliminate the oxygen supplying the muscles (5).
31P-NMR spectroscopy was performed during eight
ischemic stimulation protocols. Each protocol consisted of 8 trains, delivered at a rate of 1 every 32 s. Only one train
frequency and duration was used per protocol (Table
1), and the order in which these
protocols were delivered was randomly determined for each subject. At
both frequencies, the bulk of force generation appeared to have been achieved by the 600-ms duration trains (Fig.
1). Two protocols were delivered per
experimental session, and the order in which the protocols were
delivered was randomly determined. Approximately 20 min of rest (5 min
of recovery data plus an additional 15 min of rest) were given between
protocols.

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Fig. 1.
Force responses from a typical subject to the different
trains used in the stimulation protocols. Vertical lines represent,
from left to right, the 300-, 600-, 1,200-, and
1,800-ms time points. Open symbols, 80-Hz trains; closed symbols, 20-Hz
trains.
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The session began with collecting NMR spectra for 5 min with the
subject at rest, after which circulation was occluded by inflating the
blood pressure cuff to 230 mmHg. Resting spectra were collected for an
additional 5 min during circulatory occlusion. At the end of these 5 min, the first stimulation protocol commenced. The blood pressure cuff
was released 30 s after the last train of the protocol was
delivered, and spectra were collected throughout 5 min of recovery. The
subject was allowed to rest for 15 min after the end of the protocol,
and then the process was repeated using a second ischemic protocol.
Data acquisition.
Phosphorous spectra were collected using an adiabatic 90° pulse with
a sweep width of 3 KHz and 1,024 complex data points. Pulse repetition
(TR) time was set at 4 s. Homogeneity of the magnetic field was
adjusted using the proton signal (full width at half-maximal height
30 Hz), and the spectral data were filtered with an exponential
filter corresponding to a line broadening of 5.1 Hz. These spectra were
collected into 8-sum bins, providing a temporal resolution of 32 s. This allowed us to collect spectra that included one stimulation
train per bin during exercise while improving our signal-to-noise ratio
through signal averaging. In addition, fully relaxed spectra (TR
time = 30 s) were collected to provide appropriate saturation
correction factors.
Localization of 31P-NMR spectra.
Because NMR spectra reflect metabolic changes that occur in the volume
of tissue "seen" by the coil, we performed two pilot experiments,
one designed to determine localization of the phosphorous signals and
the other designed to determine the activity of the muscle sampled
within the field of view (FOV) of our experimental setup.
We determined the thickness of the MG of the subjects in the present
study on the basis of transaxial surface-coil proton images of the
subjects's plantarflexor muscles acquired at the center and the
proximal and distal ends of the coil (FOV = 12.8 × 12.8 cm).
Two phantoms simulating the shape and size of the MG and soleus muscles
of the subject population were filled with 100 mM dibasic and 100 mM
monobasic sodium phosphate solutions, respectively (39).
31P spectra (TR time = 4 s) were collected from
the artificial "calf," and signal contamination was determined as
the percentage of the total integrated phosphate area accounted for by
the monobasic phosphate peak. Greater than 95% of the total
31P signal was from the gastrocnemius phantom (Fig.
2), indicating that the large majority of
the tissue sampled in the current experiment consisted of the MG.

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Fig. 2.
A: 1H image from phantom simulating a human
calf. Note that ~95% of signal intensity is recorded from the bag
simulating the medial gastrocnemius (MG). Sol, soleus. B:
31P spectrum recorded from the bag simulating the Sol,
alone. C: 31P spectrum recorded from the bag
simulating the MG, alone. D: 31P spectrum
recorded from the two-bag phantom used in A. Again, ~95%
of the signal came from the MG phantom bag.
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The second pilot study monitored signal changes in T2-weighted images
taken before and after stimulation to determine whether the MG was
active during the stimulation protocols. Changes in signal intensity in
such images have been demonstrated to reflect recent muscle activation
(23, 41). The imaging procedures were performed in a
1.5-Tesla clinical magnet (General Electric) with a birdcage extremity
coil. Subjects (n = 3) were placed supine, with their
leg in the coil. Multiple T2-weighted spin-echo images from the ankle
plantarflexor muscles were acquired with echo times of 30 and 60 ms,
TR = 2 s, 128 × 256 matrix, 18 cm FOV, and 7 mm slice
thickness. Images were acquired before and after stimulation with one
of the trains (80 Hz, 1,800 ms duration, stimulation intensity set to
50% MVIC) used in the present experiment. We chose this train because
we expected it to be the most metabolically demanding, and changes in
signal intensity seen in T2-weighted images have been related to
exercise intensity (23). This gave us the best chance of
observing a change, but use of the other lower-frequency
shorter-duration trains in the study should not affect the volume
of muscle activated, because the same stimulation intensity (50% MVIC)
was used for every protocol. The response of a typical subject
is presented in Fig. 3 and clearly
indicates that the electrode placement and stimulation intensity used
here activated the MG.

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Fig. 3.
A: prestimulation, T2-weighted image of the lower leg
from a typical subject. Mg, medial gastrocnemius; SOL, soleus; LG,
lateral gastrocnemius. B: poststimulation, T2-weighted image
of lower leg from the same subject. Note increased signal from MG,
evidenced by lighter shading of muscle, indicating that the entire
muscle was recruited during the stimulation protocol. C:
surface coil proton image of the calf of the same subject. Note that
the majority of the volume consists of the MG. Therefore, spectra
collected during ischemic stimulation protocol reflected
changes occurring predominantly in the MG.
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Data analysis.
Spectra were manually phased, and the areas of ATP, phosphocreatine
(PCr), phosphomonoester (PME), and Pi peaks were manually integrated by use of customized software (38). Resting
[PME] was below the noise threshold in the spectrum. The changes in [PME] reported for each protocol were therefore based on the
assumption that the resting [PME] was zero. Intracellular pH was
calculated from the chemical shift of Pi on the basis of
the equation, pH = 6.75 + log[(
3.27)/(5.69
)], where
is the chemical shift of the Pi peak in
parts per million (ppm) relative to PCr. Absolute concentrations of
phosphorous metabolites were calculated on the basis of a resting
[ATP] of 8.2 mM (18).
The ATPase rate during ischemic exercise was determined by the
equation
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(1)
|
where L represents anaerobic glycolysis. L can in turn be
calculated as follows
|
(2)
|
where
tot is the apparent buffer capacity of the
muscle in millimoles of acid added per unit change in pH (slykes) and
is determined initially from
[PCr]/
pH during ischemic
exercise.
represents the millimolar concentration of protons
released by PCr when coupled to Pi formation by functional
ATPases and is calculated as 1/[1+10(pH
6.75)].
tot, at any time point, is a function of
[Pi], glucose 6-phosphate (G-6-P),
[HCO
], the inherent nonbicarbonate buffer capacity
(
i), and pH. [G-6-P] was taken from the
area of the PME peak in the 31P spectra. The buffer
capacities associated with the Pi, G-6-P, and
CO2 can be calculated as follows
|
(4)
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(5)
|
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(6)
|
where S in Eq. 6 is the solubility
constant of CO2 in a closed system. These equations have
been used in previous work on intense voluntary exercise of human
skeletal muscle (24, 38). Once these different buffer
capacities have been determined,
i can then be
calculated by subtracting
Pi,
G-6-P, and
CO2 from
tot.
After the ATPase rate associated with each protocol was calculated, the
ATP consumed per train was determined by multiplying the mean ATPase
rate by the duration of the stimulation trains delivered during that
protocol. Then the relative costs (mM ATP consumed) of attaining and
maintaining force for both the 20- and 80-Hz protocols were determined
by subtracting the mean ATP consumed by the 600-ms protocols from the
mean ATP consumed by the 1,800-ms protocols at each frequency. In
addition, we subtracted the ATP turnover during the 600-ms protocols
from that during the 1,200-ms protocols, and the ATP turnover from the
1,200-ms protocols from that during the 1,800-ms protocols, to compare the metabolic costs of 600-ms blocks during the tetanic stimulation trains. Finally, the percentage of the total ATP turnover during each
protocol associated with the breakdown of PCr was calculated by
dividing the change in [PCr] by the total ATP consumption and multiplying by 100.
Statistical analysis.
Two-way repeated-measures ANOVAs were used to test for the effects of
frequency and duration (2 × 4) on the changes in [PCr], [Pi], [ATP], [PME], and pH produced by each protocol.
If significant main effects were detected, post hoc comparisons were
made using paired sample t-tests, corrected for multiple
comparisons by use of Holm's sequentially rejective Bonferroni test
(25). A priori, we chose to compare differences in the
changes in [PCr], [Pi], [ATP], [PME], and pH
between the 80- and 20-Hz protocols at each duration. Within each
frequency, we chose, a priori, to compare changes in these variables
resulting from a given protocol with changes from the protocol with
trains of the next highest duration. One-way repeated-measures ANOVAs
were performed on the ATP consumed per contraction within each protocol
to determine whether the cost per contraction changed over the course
of the eight contractions. If no significant effect was detected, the
mean ATP cost of the eight contractions was used for subsequent
comparisons. The mean ATP consumption and percentage of ATP turnover
associated with PCr breakdown were compared using a two-way
(frequency × duration) repeated-measures ANOVA. Post hoc
comparisons again were made using paired sample t-tests
modified for multiple comparisons with Holm's sequentially rejective
Bonferroni correction. Significance for all tests was set at
P
0.05.
 |
RESULTS |
Muscle Metabolites
There were no significant changes in ATP concentration throughout
any of the protocols tested (Figs. 4 and
5). The other metabolites examined
([PCr], [Pi], [PME], and pH) all exhibited marked
changes in response to the different stimulation protocols (see Figs. 4-7).
Generally, for a given frequency, increasing the duration of
stimulation increased the change in metabolites. Likewise, for a given
duration, increasing the frequency increased the change in metabolites
(Table 2).

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Fig. 4.
Typical 31P-NMR spectra taken during an
experimental protocol (80-Hz, 600-ms trains). Every other spectrum is
plotted, for clarity. Arrows, period of stimulation; PCr,
phosphocreatine.
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Fig. 5.
PCr and ATP concentrations (means ± SE) from 6 subjects during 80-Hz stimulation protocols (A) and 20-Hz
stimulation protocols (B). Despite declines in PCr, no
significant changes in ATP concentration occurred during any of the
protocols. Shaded area, duration of stimulation protocol.
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ATP Cost of Contraction
Increasing contraction duration at a given frequency increased the
ATP consumed per contraction, as did increasing the frequency for a
given duration (Fig. 8A). If
this cost per contraction was divided by the duration of that
contraction, however, the ATPase rate (mM/s) decreased with increasing
duration. At 20 Hz, the mean ATPase rates were 4.23 ± 0.62, 2.60 ± 0.25, 2.01 ± 0.19, and 1.94 ± 0.20 mM/s for
the 300-ms, 600-ms, 1,200-ms, and 1,800-ms protocols, respectively. At
80 Hz, the mean ATPase rates were 7.00 ± 0.86, 5.35 ± 0.54, 3.80 ± 0.29, and 3.05 ± 0.20 mM/s for the 300-ms, 600-ms,
1,200-ms, and 1,800-ms protocols, respectively. One-way
repeated-measures ANOVAs detected no significant effect of contraction
number on the ATP cost per contraction for any of the protocols. Thus
the ATPase rates reported were calculated from the mean of the eight
contractions in each protocol.

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Fig. 6.
Pi and phosphomonoester (PME) concentrations
(means ± SE) from 6 subjects during 80-Hz stimulation protocols
(A) and 20-Hz stimulation protocols (B). Before
onset of stimulation protocols, points for PME represent fluctuations
of random noise about zero, as no consistent peak was observable above
noise in spectra. Shaded areas, duration of stimulation
protocol.
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Fig. 7.
pH values (means ± SE) from 6 subjects during 80-Hz
stimulation protocols (A) and 20-Hz stimulation protocols
(B). Shaded area, duration of stimulation protocol. Note:
increasing variance during recovery of pH is due to loss of area under
Pi peak in some subjects, a phenomenon that has been noted
previously (6).
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Fig. 8.
A: ATP cost per contraction associated with the
stimulation protocols. Bars, means ± SE. P < 0.05:
*Significantly different from 20-Hz protocol with corresponding train
duration; ¶significantly different from protocol with next highest
duration train at 20 Hz; significantly different from protocol with
next highest duration train at 80 Hz. B: ATP cost associated
with 1st 600 ms (force generation) and final 1,200 ms (force
maintenance). *P < 0.05, significantly different from all
other costs of contraction. C: ATP cost associated with
600-ms blocks corresponding to beginning, middle, and end of an
1,800-ms tetanus. Bars represent means ± SE. *Significantly
different from 80-Hz train for the same time frame (P < 0.05); significantly different from cost of 0- to 600-ms block for
the same frequency (P < 0.01); ¶trend toward a
difference in cost of 0- to 600-ms block for the same frequency
(P < 0.10).
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Although the costs per contraction in response to the 20- and 80-Hz
protocols were significantly different, there were no significant
differences between the 20- and 80-Hz protocols for the cost of force
maintenance during the 600- to 1,200-ms and 1,200- to 1,800-ms time
periods (Fig. 8C). Thus differences in the ATP cost per
contraction between the two protocols appeared to be a function of the
initial force-generating portion of the train.
For each protocol, the total ATP consumption was the sum of the PCr
breakdown by creatine kinase (CK) and the glycolysis that occurred,
because there was no net decline in ATP. Interestingly, the percentage
of ATP consumption due to PCr breakdown varied across the 20- and 80-Hz
protocols (Fig. 9). For the 20-Hz
protocols, there were no significant differences in this percentage for
any of the different durations, although it had decreased by 1,800 ms.
For the 80-Hz protocols, the percentage of ATP cost accounted for by CK
was greatest for the protocol using the shortest trains, and it
declined as the duration of the trains increased (Fig. 9). In addition,
for the 1,200- and 1,800-ms protocols, the percent ATP cost associated
with PCr breakdown was greater for the 20-Hz than for the 80-Hz
protocols.

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Fig. 9.
Percentage of ATP consumption associated with breakdown
of PCr for each protocol. Bars represent means ± SE. Because no
net change in [ATP] was observed during the protocols, the remainder
of ATP consumption in each protocol was due to glycolysis. P
< 0.05: *significantly different from all other 80-Hz protocols;
significantly different from 80-Hz, 600-ms protocol; ¶significantly
different from 20-Hz protocol with the corresponding train duration.
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DISCUSSION |
This study used in vivo 31P-NMR to analyze the
metabolic cost associated with stimulation trains of different
frequencies and durations in the human MG. As we hypothesized,
attaining force appeared to be metabolically more costly than
maintaining it, resulting in higher ATPase rates during short
contractions than during long contractions. These results are
consistent with earlier work demonstrating that shorter contractions
were more metabolically costly than longer, sustained contractions in
both human (4, 8, 36) and animal (21, 35)
muscle. As a consequence of the lower metabolic cost of force
maintenance, the net ATPase rate declined as contraction duration increased.
These findings are consistent with work in single muscle fibers that
demonstrated a markedly greater rate of ATP hydrolysis at the onset of
a twitch (9) and during initiation of a contraction than
during the steady-force phase (20). They also may explain the observation that there is a force-dependent component of the labile
heat associated with the onset of stimulation in single muscle fibers
(7). The greater ATP turnover associated with attaining a
high force would account for the differences in total labile heat
observed with an increase in force (7). Moreover, although
our results were obtained from electrically stimulated muscle, evidence
suggests that the anaerobic ATP cost of such contractions is comparable
to brief high-intensity voluntary contractions (30).
This study examined the ATP turnover in human muscle associated with a
wide range of stimulation trains. Responses to the specific trains
tested here can be compared with those in a number of separate,
previous experiments. Meyer and Foley (27), in comparing
human and animal ATPase rates during brief tetani (~1 s), suggested
that the maximum ATPase rate for human muscle was ~2.0
µmol · g
1 · s
1. The value
of 7.00 mM/s found for the 300-ms, 80-Hz train in the present study can
be converted to similar units (3), resulting in a value of
4.69 µmol · g wet
wt
1 · s
1, a much higher value than
that predicted. However, the rate for the 1,800-ms, 80-Hz train, when
similarly converted, becomes 2.03 µmol · g wet
wt
1 · s
1, nearly the expected
value. Newham et al. (28) reported an ATP turnover of 9.24 mM/s for 50-Hz, 100-ms contractions in the human adductor pollicis
muscle. This is greater than the highest ATPase rate found in the
present experiment, but the 100-ms contraction time is also shorter
than any used here. Because we observed that mean ATPase rate declined
as contraction duration increased, it seems reasonable to assume that
the rate would increase if contraction duration were smaller than
durations used here. Walter et al. (38) found peak ATP
turnover rates of 4.78 mM/s during voluntary maximal-effort
plantarflexion exercise performed at a rate of 2-3 Hz. This is
similar to the ATPase rate seen for the 80-Hz, 600-ms trains (5.35 mM/s) in the present study. Our results are also comparable to those
obtained from biopsy studies of the human quadriceps femoris muscle.
Chasiotis and colleagues (8) and Bergström and
Hultman (4) reported ATPase rates of 6.9 and 6.6 mmol · kg dry wt
1 · s
1 in
response to 20-Hz, 1,600-ms and 20-Hz, 800-ms trains, respectively. In
the present experiments, values of 6.1 and 7.85 mmol · kg
1 · s
1 (after
conversion from mM/s) were obtained for the 20-Hz, 1,200 ms and 20-Hz,
600-ms trains, respectively. Thus our results appear to be compatible
with previous work that used both NMR and biopsy methods.
In addition to confirming findings of earlier energy cost studies, the
present work also allowed us to separate the costs of force generation
and force maintenance. Surprisingly, the cost of maintaining force
during the final 1,200 ms of the 1,800-ms trains was comparable for 20- and 80-Hz stimulation, despite the greater force produced at 80 Hz. The
difference in metabolic cost at longer-contraction durations (1,200 and
1,800 ms) appeared to be a function of the greater cost of generating
force at the onset of contraction. Because AM-ATPase activity has been
shown to be proportional to force (3, 6, 29, 37), the lack of a difference in the cost of force maintenance at the two frequencies was unexpected.
The overall metabolic costs observed in this study are the result of
the AM-ATPase and the noncontractile ATPases
(Na+-K+ pump and SR-Ca2+ pump) that
are thought to account for 20-40% of the total ATP consumption
(2, 21, 22). The Na+-K+-ATPase is
believed to contribute to the total ATP demand to a small degree
(22, 27). Thus the bulk of the noncontractile cost is
thought to result from the activity of the SR-Ca2+-ATPase.
It is possible that differences in SR-Ca2+-ATPase activity
could account for some of the differences in ATPase rate that we
observed for the different protocols. However, there is evidence from
single-fiber experiments (37) to suggest that the
SR-Ca2+-ATPase was running at its maximum rate during all
of the protocols used in the present experiments. Based on the
force-pCa and ATPase-pCa curves presented in the work of Stienen et al.
(37), the SR-ATPase rate plateaued at a pCa that
corresponded to a force of ~30% maximum. Even the 300-ms, 20-Hz
train in the present study, which produced the lowest forces, surpassed
this level. Thus if the SR-Ca2+-ATPase was running at its
maximum rate for all of the protocols, the differences we observed in
total ATPase rate were probably primarily the result of differences in
the AM-ATPase activity. We did not, however, make any attempt to
separate the metabolic costs of the different ATPases in our
experiments. Because of this, and because of differences in species,
preparations (single fiber vs. in vivo), and activation methods (direct
Ca2+ activation vs. nerve stimulation) between the work by
Steinen et al. and our own, we cannot rule out the possibility that
differences in the cost of Ca2+ handling may have
contributed to our results. We believe that any such potential effect
is likely to be small. In fact, differences in SR-ATPase activity would
bias against our finding that ATPase rates were greater for
short-duration trains than for long-duration trains. Thus, we think
that differences in AM-ATPase rate account for the bulk of our findings.
The mechanisms behind the greater rate of ATP turnover at the onset of
contraction vs. during the steady-force state remain unclear, even if
we attribute them to differences in AM-ATPase rate. It may be due to
sarcomere shortening that is occurring at the onset of the contraction
(19, 29), cross-bridge cooperativity decreasing the rate
of detachment as the contraction progresses (15), or
transient buildup of Pi due to the rapid PCr breakdown at
the initiation of contraction (9). Recent studies of the cross-bridge cycle, however, have suggested that the rate-limiting step
in the cross-bridge cycle (and thus ATP turnover) is a strain-dependent isomerization that occurs after the power stroke and Pi
release (for recent reviews see Refs. 15 and 16). When
force on the cross bridge is high, as it is during the force plateau of
isometric contractions, the forward rate of this reaction step slows,
and the rate constant of cross-bridge detachment
(gapp) also slows. This is in agreement with
recent studies by Sieck and colleagues (33, 34), who
employ a model suggesting that isometric ATPase rate is a function of
the isometric force (F) and gapp, as presented in Eq. 7, where k is a constant representing the
number of half sarcomeres per fiber divided by the mean force per cross
bridge
|
(7)
|
If, at the onset of contraction, gapp is
very high, a high ATPase rate will result. As force reaches a plateau,
gapp decreases and, although the force remains
high, the total ATPase rate will decrease. Such a decline in
gapp with force development has been observed in
single muscle fibers (G. C. Sieck, personal communication). If the
relative decline in gapp during the plateau of
the 80- vs. the 20-Hz stimulation trains was proportional to the
differences in forces produced at the two frequencies, it could explain
the comparable costs of force maintenance observed during the 20- and
80-Hz trains used in the present study. Of course, some direct measure
of cross-bridge cycling is needed to confirm this hypothesis.
We also found that there was a progressive decline in the percentage of
the ATP turnover associated with PCr breakdown, because contraction
duration increased during the 80-Hz stimulation protocols (Fig. 9).
This decline was not apparent during the 20-Hz protocols, although
there was a trend toward a decrease when the 600-ms protocol was
compared with the 1,800-ms protocol (P < 0.07). No net
change in [ATP] was observed during any of the protocols, and so the remainder of the ATP turnover in each case was the result of
glycolysis. Together, these findings suggest that a greater proportion
of the ATP synthesis during the stimulation protocols was taken on by
glycolysis as the duration of the contraction increased. Conley et al.
(10) demonstrated that activation of glycolysis was a function of the number of stimulation pulses during twitch stimulation and that the glycolytic rate was dependent on muscle activation frequency, suggesting that glycolysis was regulated in a feed-forward manner by Ca2+ and not by a feedback mechanism related to
metabolic by-products of stimulation. These observations may explain
the present observation that the decline in the percentage of ATP
resynthesis due to PCr breakdown with increasing contraction duration
was more rapid at 80 Hz. At 20 Hz, the total number of stimulation
pulses would accumulate more slowly, delaying the onset of glycolysis,
and the lower frequency would produce a lower glycolytic rate.
Conclusions
Our results confirm the assertion that attaining force is more
costly, in metabolic terms, than maintaining that force
(14). This finding helps to explain previous studies that
showed greater ATP turnover during brief intermittent contractions vs.
longer sustained contractions when either the total contraction time (4, 8, 21) or total number of contractions
(32) was kept constant. It may also help to explain why
the FTI (32) does not always predict fatigue during
isometric contractions. From the difference in the cost between force
generation and force maintenance, it appears that the mean ATPase rate
declines during contraction, consistent with single-fiber experiments
that examine both twitch (9) and tetanic (10)
contractions. Finally, the observation that maintaining force with
80-Hz stimulation was no more metabolically costly than maintaining a
lower force with 20-Hz stimulation suggests an increased economy of
contraction at higher frequencies. This finding may be related to
current models of the cross-bridge cycle and ATP consumption.
 |
ACKNOWLEDGEMENTS |
We thank Dr. Samuel C. K. Lee and Michael Vardaro for their
assistance in data collection.
 |
FOOTNOTES |
Partial funding for this study was provided by the University of
Delaware Office of Graduate Studies and the Foundation for Physical
Therapy (D. W. Russ), and National Institutes of Health Grants
HD-33738 (K. Vandenborne), HD-42164 (S. A. Binder-Macleod), and
RR-2305.
Portions of these data were previously presented at the American
Physiological Society Meeting on the Integrative Biology of Exercise,
held in Portland, ME, in September, 2000.
Address for reprint requests and other correspondence: S. A. Binder-Macleod, 323 McKinly Laboratory, Dept. of Physical Therapy, Univ. of Delaware, Newark, DE 19716 (E-mail:
sbinder{at}udel.edu).
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.
10.1152/ajpendo.00285.2001
Received 2 October 2001; accepted in final form 18 October 2001.
 |
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