Oxygen uptake kinetics following 20 days of unilateral lower limb suspension.

Abstract

The purpose of the present study was to examine the effect of unilateral lower limb suspension (ULLS) deconditioning on oxygen uptake kinetics. Eight healthy males underwent ULLS for 20 days and performed a series of 6-min square-wave transitions from rest to 60-W single-leg cycling exercises just before and after ULLS. To characterize the kinetics of the oxygen uptake response, a single exponential model was applied to the data until the end of the fast component omitted the first 15 s of the on-transit using a nonlinear least-squares fitting procedure. The following results were found: (i) the time constant of oxygen uptake was unchanged before and after ULLS; (ii) although there was no significant difference in the baseline and the asymptotic amplitude of the fast component, the asymptote, i.e., the absolute asymptotic amplitude of the fast component (the sum of the baseline and the asymptotic amplitude), and the end exercise oxygen uptake were decreased after ULLS; (iii) the contribution of the slow component to the total response of oxygen uptake was unchanged at pre- and post-ULLS. In conclusion, the asymptote in the fast component and the end exercise oxygen uptake were decreased after 20-d ULLS, though the response speed and the amplitude of the slow component of oxygen uptake were not changed. It is suggested that deconditioning as a result of limb disuse affects oxygen uptake response.

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347
The Journal of Physiological Sciences Vol. 56, No. 5, 2006
J. Physiol. Sci. Vol. 56, No. 5; Oct. 2006; pp. 347–353
Online Sep. 28, 2006; doi:10.2170/physiolsci.RP005606
Oxygen Uptake Kinetics Following 20 Days of Unilateral Lower Limb
Suspension
Norio HOTTA1, Kohei SATO2, Keisho KATAYAMA3, Shunsaku KOGA4, Kazumi
MASUDA5, Motohiko MIYACHI6, Hiroshi AKIMA3, and Koji ISHIDA3
1Graduate School of Medicine, Nagoya University, Nagoya, Japan; 2Research Institute of Physical Fitness, Japan Women’s College of
Physical Education, Tokyo, Japan; 3Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya, Japan;
4Applied Physiology Laboratory, Kobe Design University, Kobe, Japan; 5Faculty of Education, Kanazawa University, Kanazawa,
Japan; and 6Laboratory of Physical Activity and Health Evaluation, National Institute of Health and Nutrition, Tokyo, Japan
Abstract: The purpose of the present study was to examine the
effect of unilateral lower limb suspension (ULLS) deconditioning
on oxygen uptake kinetics. Eight healthy males underwent ULLS
for 20 days and performed a series of 6-min square-wave transi-
tions from rest to 60-W single-leg cycling exercises just before
and after ULLS. To characterize the kinetics of the oxygen up-
take response, a single exponential model was applied to the
data until the end of the fast component omitted the first 15 s of
the on-transit using a nonlinear least-squares fitting procedure.
The following results were found: (i) the time constant of oxygen
uptake was unchanged before and after ULLS; (ii) although
there was no significant difference in the baseline and the as-
ymptotic amplitude of the fast component, the asymptote, i.e.,
the absolute asymptotic amplitude of the fast component (the
sum of the baseline and the asymptotic amplitude), and the end
exercise oxygen uptake were decreased after ULLS; (iii) the
contribution of the slow component to the total response of oxy-
gen uptake was unchanged at pre- and post-ULLS. In conclu-
sion, the asymptote in the fast component and the end exercise
oxygen uptake were decreased after 20-d ULLS, though the re-
sponse speed and the amplitude of the slow component of oxy-
gen uptake were not changed. It is suggested that decondition-
ing as a result of limb disuse affects oxygen uptake response.
Key words: oxygen uptake fast component, oxygen uptake slow component, limb disuse.
In general, to analyze oxygen uptake ( O2) kinetics in
exercising humans, O2 is usually measured at the mouth
by means of the breath-by-breath method. The response
speed of O2 to exercise, i.e., the time constant of O2
kinetics of the fast component in which O2 increases ex-
ponentially and reaches a steady state within 2–3 min after
the onset of exercise, is affected by O2 delivery systems
consisted of central and peripheral circulation, and the
function of O2 utilization, such as the activity of oxidative
enzymes in the muscle [1, 2]. In cases where the intensity
of exercise is heavy, O2 increases moderately after the
end of the fast component, and this is known as the slow
component. Although the mechanisms remain specula-
tive, it is thought that the recruitment of fast twitch fibers
or the added recruitment of muscle fibers, which have not
yet participated in muscle contraction, is the primary
cause [3].
It is known that the fast component of O2 kinetics is
faster in trained than in untrained individuals [4]. This
would indicate that because the time to start producing
lactic acid obstructing muscular contraction is shortened,
burdens of exercise decrease. Further, Carter et al. [5] re-
vealed that the O2 slow component was attenuated by 6
weeks of endurance training. This would imply that the
subjects were able to accomplish the same level of exer-
cise without the mobilization of added muscular fibers or
fast twitch fibers. The endurance tolerance of each muscle
fiber would be increased. Therefore it is conceivable that
endurance training would speed up
would lead to the elimination of the O2 slow component
so that humans could continue exercising without as much
effort in comparison with before endurance training.
On the contrary, little is known about the effect of inac-
tivity on O2 kinetics. An investigation performed by
Convertino et al. [6] revealed that O2 kinetics in an up-
right position became slowed after bed rest decondition-
ing. To our knowledge, their research is the only one of its
kind that has compared O2 kinetics before and after in-
activity, so their research is thought to be very beneficial.
However, there are times in which humans are forced into
not only whole body inactivity, such as bed rest, but also
into limb disuse, such as when an arm or leg is set in a cast
or in limb suspension. Until now there have been no stud-
ies that have compared O2 kinetics before and after limb
O2 kinetics and
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Received on May 27, 2006; accepted on Sep 25, 2006; released online on Sep 28, 2006; doi:10.2170/physiolsci.RP005606
Correspondence should be addressed to: Norio Hotta, Research Center of Health, Physical Fitness and Sports, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya, 464-8601 Japan. Tel: +81-52-789-5927, Fax: +81-52-789-3957, E-mail: hotta-n@med.nagoya-u.ac.jp
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disuse. Previous studies have reported that even limb dis-
use attenuates muscle oxidative capacity [7], function of
peripheral circulation [8, 9], and muscle strength [10].
Thus even limb disuse might have an influence on
kinetics.
We have therefore decided to investigate the effects of
limb disuse on O2 kinetics. Slightly more than a decade
ago, unilateral lower limb suspension (ULLS) was devel-
oped by Berg et al. [10] to expose the lower limb to un-
loading, and in their research they clearly showed that
muscle atrophy and a decrease in muscle strength occur in
the suspended leg after ULLS, similar to the results of bed
rest. We recognized that ULLS could be the equivalent of
limb disuse and measured
using only the ULLS leg before and after 20 d of ULLS.
O2
O2 during cycling exercise,
METHODS
Subjects. Eight healthy men, whose average age,
height, and body mass were 20.0 ± 0.2 (mean ± SD) years
old, 169.6 ± 6.6 cm, and 62.8 ± 5.4 kg, respectively, vol-
unteered to participate in this study. The subjects were in-
formed of the experimental protocol and the possible risks
involved in this study before writing their consent. Ap-
proval of this study was given by the ethics committee of
the Research Center of Health, Physical Fitness and
Sports at Nagoya University.
Experimental procedure. Initially, preliminary testing
was carried out to familiarize all subjects with the experi-
mental procedures and apparatuses. The actual experi-
ments were performed, on a day different from the prelim-
inary testing, just before (Pre) and after 20 days of ULLS
(Post).
ULLS. Each subject’s left lower limb was suspended us-
ing a harness in the same way as in a previous study [10].
All ambulatory activities were conducted for 20 days by
means of crutches. The harness was made of tubular nylon
strips and allowed for the suspension of the left leg by
means of a strap secured around the waist and attached to
the foot. The harness was fastened so that the toe of the
subject was lifted approximately 15 cm above ground lev-
el, and the knee was fixed at an angle of 90–130° (180° =
full knee extension).
To prevent deep venous thrombosis, we made sure that
each subject wore a knee-length compression stocking,
drank water frequently, and performed light exercise of
the ankle joint, which has been recommended by some
airlines to prevent economy-class syndrome, at least 3
times a day during ULLS. The subjects were allowed to
move the hip, knee, and ankle joints. To reduce joint stiff-
ness, they were required to move their leg musculature in
the morning and evening. To monitor compliance, the
subjects kept diaries and were interviewed about their ac-
tivities 2–3 days a week during ULLS. We then measured
the skin temperatures of the leg calves by using a data log-
ger (HR3087, YOKOGAWA, Japan). The sensors were
put on the calves of both legs for approximately 10 min.
We also measured the circumferences of the thighs and
calves at the same spots at Pre and Post.
Exercise test. The subjects performed a series of 6-min
square-wave transitions from rest to 60-W single-leg cy-
cling exercise using an electromechanically braked bicy-
cle ergometer (Aerobike 800, COMBI, Japan) three times.
Resting intervals were taken until HR returned to within 5
beats/min of the pre-exercise value according to the meth-
ods of a previous study [6]. The pedaling rate was kept
constant at 60 rpm with the aid of a metronome. The posi-
tion of the saddle for each subject before and after ULLS
was kept the same.
Measurements. Respiratory parameters were deter-
mined by means of the breath-by-breath method. The gas
fractions were analyzed by a mass spectrometer (ARCO-
1000, Arco System, Japan) every 10 ms. Inspired and ex-
pired gas volumes were measured by a Fleisch pneumot-
achometer (WLCU-5201, Westron, Japan). Breath-by-
breath data were analyzed continuously using customized
software on a computer to calculate O2, carbon dioxide
output ( CO2), expiratory minute ventilation (
respiratory exchange rate (RER). Heart rate (HR) was
measured beat-to-beat from the R spike using an electro-
cardiogram through a bioamplifier (AB 621G, Nihon Ko-
hden, Japan). Beat-to-beat arterial blood pressure (BP)
waveforms were measured by means of a Portapres Model
2 noninvasive BP monitor (TNO-TPD BMI, The Nether-
lands). Systolic BP (SBP) and diastolic BP (DBP) were
obtained at the instantaneous pressure outputs, and mean
BP (MBP) was calculated from the formula: MBP = 1/3
(SBP-DBP) + DBP. They were converted from analog to
digital data using an A/D converter (CBI-3133B, Inter-
face, Japan) at a sampling frequency of 100 Hz and stored
on a computer (PC-LM700, NEC, Japan).
Kinetic analysis. To remove nonphysiological datum
points resulting from coughing and sneezing, any breaths
more than four standard deviations away from the mean of
the surrounding six breaths (3 before and 3 after) were de-
leted according to the methods presented in a previous
study [11]. The breath-by-breath data were aligned with
the onset of exercise and then linearly interpolated be-
tween each breath to yield a datum point at 1-s intervals.
Ensemble averaging was carried out across three repeti-
tions. Beat-to-beat HR and BP data were analyzed in the
same way as the breath-by-breath data. Also,
were averaged every 15 s.
Many investigators observe
moderate (< anaerobic threshold, AT) exercise without the
slow component and heavy (>AT) exercise with the slow
component, and the analyzing procedures are different.
However, we measured O2 response to exercise only at
60 W. Therefore to confirm whether the slow component
existed, the on-transit of O2 response to the exercise at
E), and
O2 data
O2 response to both
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Pre was modeled as mono-exponential, beginning after
the first 15 s and continuing to the 3rd min, and beginning
after the first 15 s and continuing to the end (6th min) for
each subject. As a result, since time constants of the latter
in all subjects were larger when compared with those of
the former (Fig. 1), we determined that the intensity was
>AT and that the slow component was contained in the
O2 response.
The resulting O2 response was modeled with nonlin-
ear least-squares fitting procedures to an exponential re-
sponse by computer software (Origin 6.1). The on-transit
of O2 response to 60-W intensity exercise was modeled
as a mono-exponential, beginning after the first 15 s (the
end of phase I, the start of phase II) and continuing to a
new steady state of the fast component (the end of phase
II) [12].
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O2 (t) = O2,BL + ∆ O2,fast (1 – e–(t–TD)/τ)
where O2,BL is the baseline, ∆ O2,fast is the asymp-
totic amplitude to which O2 projects (Fig. 2), τ is the
time constant of the response, and TD is the delay time.
To find the end of the fast component, i.e., the start of the
slow component (the end of phase II and the start of phase
III), two alternative indexes were used following the
methods used in a previous study [13]: (i) the mainte-
nance of the flat profile of the residual plot, as judged by
visual inspection; (ii) the demonstration of a local thresh-
old for increase in the chi-squared value (Fig. 2). The
magnitude of the O2 slow component ( O2,sc) (Fig. 2)
was taken as the difference between the amplitude of the
final measured value averaged for the last 30 s ( O2,end)
and the asymptote, i.e., absolute amplitude, in the fast
component, demonstrated as the sum of
∆ O2,fast ( O2,fast) (Fig. 2). The percentage of contri-
bution of the slow component to the total response of
O2 (% O2,sc) thus equals: (( O2,end –
∆ O2,fast) × 100.
As for
1 min before the onset of exercise and for 15 s before the
3rd min and 6th min and defined them as BL, 3rd and 6th.
Statistical analysis. All values were expressed as means
± SD. We compared the variables using the Wilcoxon
signed-ranks test. Statistical analyses were computed by
software (StatView 5.0), and the level of significance was
set at 5%.
O2,BL and
O2,fast)/
E, RER, HR, and MBP, we averaged them for
RESULTS
Effect of ULLS on the muscle
As for the circumference of the thigh, the difference in
the ULLS leg between Pre and Post (–1.49 ± 1.2 cm) was
significantly (P < 0.05) larger than that of the control leg
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200
400
600
800
1000
1200
1400
0
0
100
100
200
200
300
300
00 100 100200 200 300300
Subject 1Subject 1
VO2(ml? min-1)
.
Time (s)Time (s)
A
B
Time (s)
0 100200300
0
200
400
600
800
1000
1200
1400
00 100 100200 200 300300
?VO2,fast
VO2,fast
.
Time (s)
VO2(ml? min-1)
.
Subject 1Subject 1
.
VO2,sc
.
A
B
fast component slow component
Fig. 1.
tive subject with model fits. The case
with the mono-exponential fit limited to
until 3 min after the start of exercise (A),
and with the entire response modeled
with a mono-exponential (B). Note that
response speed is different between A
and B (τ = 47.8 and 96.1 s).
O2 response for a representa-
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Fig. 2.
resentative subject with model
fits and residuals. The case
with the entire response mod-
eled with a mono-exponential
omitted for the first 15 s (A) and
with the mono-exponential fit
limited to the fast component
omitted for the first 15 s (B).
Note that the residuals of B,
shown below, are smaller than
those of A. ∆
O2,fast, asymp-
totic amplitude of the fast com-
ponent (the increase in
from the baseline to the asymp-
tote of the fast component);
the baseline and the asymptotic amplitude);
and O2 at the end of exercise).
O2 response for a rep-
O2
O2,fast, the asymptote, i.e., the absolute asymptotic amplitude of the fast component (the sum of
O2,sc, the amplitude of the slow component (the difference between O2,fast
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(–0.28 ± 0.6 cm). Similarly, the amount of change in the
circumference of the calf was significantly (P < 0.05) dif-
ferent between the ULLS leg (–1.33 ± 0.9 cm) and the
control leg (0.16 ± 0.6 cm). The skin temperature of the
ULLS calf was found to be lower than that of the control
leg, and the mean difference was 1.3 ± 0.9°C throughout
the ULLS term.
O2 kinetics
Figure 3 shows the ensemble-averaged 15-s interval
values of O2 before and after ULLS in the 8 subjects.
The changes in parameters of O2 kinetics are presented
in Table 1. Although O2,BL and ∆ O2,fast were un-
changed, O2,fast and O2,end were decreased signifi-
cantly (P < 0.05) after ULLS. τ did not change between
before and after ULLS, and no significant difference be-
tween Pre and Post was detected in O2,sc and % O2,sc.
Respiratory and circulatory parameters other than
O2
We display respiratory and circulatory parameters oth-
er than O2 in Table 2. Significant differences between
Pre and Post were not detected in HR and MBP with re-
gard to BL, 3rd and 6th. BL, 3rd and 6th of RER was also
unchanged before and after ULLS. As for
BL at Post was significantly (P < 0.05) lower than that of
Pre, 3rd and 6th were unchanged.
E, although
DISCUSSION
The major results were as follows: (i)
O2,end were declined after ULLS, though τ and
% O2,sc were not changed; (ii) HR and MBP responses
were not altered before and after ULLS.
O2,fast and
Limb disuse
In this study, the circumferences of the thigh and calf
were decreased. In fact, an accompanying study by Akima
et al. (unpublished observation) revealed by means of
magnetic resonance imaging (MRI) that muscle atrophy
in the thigh occurred after ULLS in the same way as in
previous studies [10, 14, 15] and that the degree of muscle
atrophy as a result of ULLS was unchanged as compared
to that of bed rest. In addition, calf skin temperature of the
ULLS leg was lower than that of the control leg similar to
a previous study [16]. Thus it is evident that the ULLS leg
was exposed to disuse for 20 days.
Response speed of the fast component
It is known that the response speed of the fast compo-
nent is controlled by O2 delivery to and utilization by the
exercising muscles [1, 2]. Previous studies have demon-
strated that limb disuse by casting attenuated the muscle
oxidative capacity in the immobilized limb [7], that ULLS
reduced the femoral artery diameter in the side of the sus-
pended leg [9], and that the number and luminal diameter
of capillaries in the muscles of rats decreased after hind
limb suspension [8]. Accordingly, there was a possibility
that ULLS would slow response speed of the fast compo-
nent. Contrary to our expectations, however, the response
speed of O2, i.e., τ, was unchanged after ULLS (Table
1), and this result does not agree with a previous study,
which revealed that O2 kinetics were slowed after bed
rest [6]. These conflicting results could have been caused
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200
400
600
800
1000
1200
1400
-600 60120 180240300360
Pre
Post
Time (s)
VO2(ml?min-1)
.
Fig. 3. Ensemble-averaged 15-s interval values of
fore and after ULLS. The filled circles indicate the averaged
values of O2 before ULLS, and the open squares show
them after ULLS (n = 8 subjects). Values are means ± SD.
O2 be-V·
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Table 1. O2 response parameters for 60-W exercise before and after ULLS.
Pre Post
O2,BL(ml·min–1)
(s)
(ml·min–1)
(ml·min–1)
(ml·min–1)
(ml·min–1)
(%)
O2,BL, baseline; τ, time constant; ∆
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365 ± 65
33.9 ± 11.3
813 ± 65
1,178 ± 106
1,272 ± 107
93.2 ± 46.1
11.6 ± 5.7
O2,fast, asymptotic amplitude of the fast component;
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35.3 ± 11.9
772 ± 67
1,110 ± 96*
1,156 ± 122*
46.1 ± 61.4
5.9 ± 7.8
τ
∆
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O2,fast
O2,fast
O2,end
O2,sc
O2,sc%
Values are means ± SD; n = 8 subjects.
O2,fast, the asymptote, i.e., the absolute asymptotic amplitude of the fast component (the sum of the baseline and the asymptotic
amplitude); O2,end, exercise end value; O2,sc, the amplitude of the slow component; %
the slow component to the total response of O2. * Significant difference between Pre and Post (P < 0.05).
O2,sc, the percentage of contribution of
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by the following. First, bed rest induces cardiac muscle at-
rophy and attenuates central circulation so that HR is up-
regulated at rest and during exercise [6, 17, 18]. In fact,
from the results that O2 kinetics in a supine position
were not altered after bed rest, Convertino et al. [6] con-
cluded that the attenuation of the capability of central cir-
culation could slow O2 kinetics. On the contrary, it
should be hard for limb disuse to reduce the capability of
central circulation because the subjects went about their
daily routines as usual, except for the fact that the ULLS
leg was suspended in a harness. Actually, in this study HR
and MBP at rest and their responses to exercise were un-
changed after ULLS (Table 2). Therefore the cause of un-
changed τ of O2 in this study was possibly due to unal-
tered central circulation. Second, because a previous
study [6] did not do an analysis using exponential fitting,
whereas we did, the difference in how the data were ana-
lyzed might have produced this discrepancy.
The amplitude of the fast component
The subjects performed the same work (60 W and 60
rpm) at Pre and Post, and
changed before and after ULLS in the same way as in a
previous study [6] (Table 2). Moreover, the energy sub-
strate that was utilized during exercise should be the same
before and after ULLS because RER during exercise was
not changed after ULLS (Table 2). Nevertheless, the as-
ymptote in the fast component (the absolute amplitude of
the fast component), i.e., O2,fast, fell after ULLS (Table
1). Although a significant difference was not detected in
the asymptotic amplitude of the fast component, i.e.,
∆ O2,fast (Table 1), the resting value of O2 before the
start of exercise, i.e., O2,BL, was found to be unaltered
at Pre and Post (Table 1). Therefore it is assumed that the
E during exercise was un-
amplitude of the fast component tended to be down-regu-
lated after ULLS.
With regard to the reason for the declined asymptote of
the fast component, the following possibilities might ac-
count for it. Many investigators have reported that in-
creasing the percentage of the recruitment of the fast
twitch fibers, i.e., type II fibers, decreases the amplitude
of the fast component [3, 19, 20]. In this study, after
ULLS, muscle strength was weakened (Dr. Akima’s un-
published observation). Thus relative work intensity at
Post should be increased as compared with that of Pre, and
for that reason the recruitment of larger high-threshold fi-
bers, which are mostly type II fibers, could be facilitated
more after ULLS, taking into consideration the size prin-
ciple [21]. Therefore the reason why the asymptote of the
fast component was down-regulated after ULLS could be
possibly due to the alteration of the recruitment of muscle
fibers. Another possibility proposed by Koga et al. [22]
was that under the circumstances in which O2 delivery to
and utilization by the working muscles were compro-
mised, the amplitude of the fast component was more sen-
sitive to a limitation in O2 delivery than was the associated
τ . Therefore it is possible that ULLS weakened the capa-
bility of O2 delivery in peripheral circulation and utiliza-
tion in the muscles and that the limited O2 delivery might
have down-regulated the asymptote of the fast compo-
nent. In any case, further research is required because the
recruitment of muscle fibers, the function of O2 delivery
in the peripheral circulation, and the O2 utilization of the
muscles were not evaluated.
The slow component
The hypothesis that the recruitment of type II fibers or
the added recruitment of muscle fibers, which have not yet
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Table 2. Respiratory and circulatory parameters for 60-W exercise before and after ULLS.
Pre Post
E
(l·min–1) BL13.7± 2.312.8± 2.2*
3rd41.9± 7.441.8± 8.2
6th45.1± 9.743.9± 6.3
RERBL0.83±0.050.79± 0.05
3rd1.02± 0.04 1.01±0.05
6th0.99± 0.030.98±0.04
HR (bpm)BL85± 11 82± 10
3rd 127± 13125± 13
6th135±15 131± 15
MBP (mmHg)BL 79±8 87± 10
3rd 104±8 100± 10
6th103±999± 13
Values are means ± SD; n = 8 subjects.
blood pressure; BL, baseline; 3rd, third minute value; 6th, end exercise value. * Significant difference between Pre and Post (P <
0.05).
E, expiratory minute ventilation; RER, respiratory exchange rate; HR, heart rate; MBP, mean
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participated in muscle contraction, as the mechanism of
the slow component is convincing [3]. Previous studies
have demonstrated that even when the amplitude of the
fast component is declined because of a facilitated recruit-
ment of type II fibers, the value of O2 at the end of exer-
cise is unchanged in >AT exercise [3, 20]. Koga et al. [22]
also revealed that even when the amplitude of the fast
component is down-regulated as a result of the restriction
of O2 delivery in circumstances in which O2 extraction
and delivery were compromised, the end exercise O2 is
the same as that of the control condition because of the
compensation of the elevated amplitude of the slow com-
ponent in >AT exercise. However, in this study in which
exercise intensity was determined >AT, in spite of the fact
that the asymptote of the fast component fell, the O2,end
after ULLS was lower than before ULLS, and % O2,sc
was unchanged after ULLS. The reason for this would
possibly be that the exercise intensity (60 W), which was
utilized in this study, was just above AT, where the slow
component was less likely to be observed. Many investi-
gators employ 50% of the difference between AT and peak
oxygen uptake ( O2 peak) as the exercise intensity to ob-
serve the slow component. Accordingly, if we had also
utilized the exercise intensity that many investigators em-
ploy, the amplitude of the slow component should have in-
creased after ULLS. Alternatively, some human studies
show the magnitude of atrophy of type II fibers to be larg-
er than that of slow twitch fibers (type I fibers) in the vas-
tus lateralis muscle after bed rest or unloading [14, 23,
24]. Additionally, in this study it was speculated that type
II fibers were more actively recruited during muscle con-
tractions in the fast component after ULLS in comparison
with before ULLS. So after the fast component, it might
be possible that type II fibers, which have a higher O2 cost
per tension development compared with type I fibers,
might be more difficult to recruit in muscle contractions
after ULLS as the additional recruitment of muscle fibers
to replace the fatigued fibers. As a result, the amplitude of
the slow component did not increase much, leaving
O2,end after ULLS lower than before ULLS.
What the declined asymptote in the fast
component of O2 implies
What does the declined asymptote in the fast compo-
nent, i.e., O2,fast, imply? If we simply take it to mean
the subjects were performing the same exercise while uti-
lizing less O2, the exercise efficiency would be higher.
However, if we take the decreased
more facilitated recruitment of type II fibers or a delayed
response in O2, this would indicate that the energy-sup-
plying system becomes dependent on glycolysis more
than oxidation and that more lactate is produced. This pos-
sibility is supported by reports demonstrating that muscle
oxidative capacity decreases after inactivity [7, 24].
Therefore after ULLS, the subjects might be harder
O2,fast to mean a
pressed to carry out the exercise tasks. Further research
will be needed, however, because no lactate data were tak-
en.
In conclusion, the asymptote in the fast component and
the end exercise O2 were decreased after 20-d ULLS,
though the response speed and the amplitude of the slow
component of O2 were not changed. It is suggested that
muscle deconditioning as a result of limb disuse affects
O2 response.
We appreciate the cooperation of the subjects in the present study. We
would also like to thank Drs. B.J. Whipp, Y. Ikegami and N. Kasuga for ad-
vising us, Dr. N. Kondo for lending us his apparatus, and Mr. J. Myerson for
reviewing the English in this manuscript. This study was supported by a
Grant-in-Aid for Scientific Research from the Japanese Ministry of Educa-
tion, Culture, Sports, Science and Technology (grant no. 17650193).
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