Unchanged Muscle Deoxygenation Heterogeneity
During Bicycle Exercise After 6 Weeksof Endurance Training
Ryotaro Kime, Masatsugu Niwayama, Masako Fujioka, Kiyoshi Shiroishi,Takuya Osawa, Kousuke Shimomura, Takuya Osada, Norio Murase,
and Toshihito Katsumura

Abstract The purpose of this study was to examine the changes in muscle oxygen
saturation (SmO2) level and its heterogeneity after 6 weeks of endurance training
using multi-channel near infrared spatially resolved spectroscopy (NIRSRS). Nine
healthy subjects participated in this study (Male ¼ 6, Female ¼ 3, age: 27 5 years, height: 168.7  7.4 cm, weight: 62.4  12.4 kg). The subjects performed a30 W ramp incremental bicycle exercise test until exhaustion before and afterendurance training. The NIRSRS probe was attached to the left vastus lateralis
muscle along the direction of the long axis. The subjects performed bicycle exercise
for 30 min/day, 3 days/week for 6 weeks. The work rate during training was set at_ 2peak and increased every 5%VO
_ 2peak when the subjects could maintain the60%VO.
work rate three times consecutively. After training, VO2peak was significantlyincreased (Pre: 42.7  9.9 ml/kg/min, Post: 52.3  7.2 ml/kg/min, p < 0.001)_ 2peak was significantly
and the mean SmO2 within measurement sites at VO
decreased (Pre: 56.1  1.1 %, Post: 53.3  2.2 %, p < 0.05). Conversely, theheterogeneity of the SmO2 during exercise was not changed by training. Theseresults suggest that the functional heterogeneity of O2 balance did not change due
to endurance training, and the O2 balance heterogeneity may not interfere with O2exchange in the activating muscle in healthy individuals.

1 Introduction
We have previously reported that muscle deoxygenation level at exhaustion wasnegatively correlated with peak oxygen uptake (peak VO2), and the results
suggest that O2 availability may be enhanced in higher oxidative capacity
muscles. Basically, blood flow increases in relation to local metabolic rate [1],and the O2 balance is distributed heterogeneously in a single muscle [2, 3, 4]. The
R. Kime (*)
Department of Sports Medicine for Health Promotion, Faculty of Medicine, Tokyo
Medical University, Tokyo, 160-8402, Japane-mail: kime@tokyo-med.ac.jp

E. Takahashi, D.F. Bruley (eds.), Oxygen Transport to Tissue XXXI,Advances in Experimental Medicine and Biology 662,
DOI 10.1007/978-1-4419-1241-1_51, Ó Springer ScienceþBusiness Media, LLC 2010

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nonuniform distribution of O2 balance may interfere with O2 exchange in theactivating muscle [5]. We hypothesized that muscle oxygen extraction may beenhanced by endurance training, and the adaptation will lead to muscle oxygensaturation (SmO2) distribution in the activating muscle being more homogeneous at higher work rates as nonuniform O2 balance distribution may interferewith O2 exchange in the activating muscle. There have been no studies reportingthe changes in muscle deoxygenation heterogeneity by endurance training in an
exercising muscle. Therefore, we examined the changes in muscle oxygen
saturation (SmO2) level and its heterogeneity after 6 weeks of endurance training using multi-channel near infrared spatial resolved spectroscopy (NIRSRS).

2 Methods
2.1 Subjects
Nine healthy untrained volunteers (six males and three females, age: 27  5 year
height: 168.7  7.4 cm,weight: 62.4  12.4 kg) participated in this study. Allsubjects were briefed about the experimental protocol, and written informed
consent was obtained before the experiment. The institutional review board of
the Tokyo Medical University approved the research protocol.

2.2 Experimental Design.
The peak oxygen uptake ( VO2peak) and SmO2 distribution in the vastus lateralis(VL) muscle were determined before and after 6 weeks of endurance training.
As for the endurance training, the subjects performed bicycle exercise for
30 min/day,rate during training was set. 3 days/week for 6 weeks. The work.
at 60 % VO2peak and increased every 5 % VO2peak when the subjects could
maintain the work rate three times consecutively.
The subjects performed
an incremental bicycle exercise until exhaustion.
to determine the VO2peak and muscle deoxygenation
heterogeneity in the VL.
muscle. During the test, pulmonary VO2 and carbon dioxide production(VCO2) were assessed breath-by-breath with an online metabolic system(AE-300 Minato, Japan). Pedal frequency of 60 rpm was maintained by keeping
time with a metronome.

2.3 Multichannel NIRSRS System
We used two wavelength light-emitting diode NIRSRS (ASTEM Co, Japan).
The probe of the present system consisted of two light sources and one photodiodedetector, and the optode distance was 20 and 30 mm, respectively. The measurement probes were attached to the left VL muscle along the direction of the long

Unchanged Muscle Deoxygenation Heterogeneity During Bicycle Exercise

355axis. The eight measurement probes were arranged vertically and the most distal
site was channel 1 and the most proximal site was channel 8. As leg length is shorterin females than in males, the seven measurement probes were used to cover the left
VL muscle in the female subjects (Fig. 1). The SmO2 was derived from the relative
absorption coefficients obtained from the slope of light attenuation over a distancemeasured at two focal points from the light emission. The relative absorption
coefficients are converted to relative concentrations of oxygenated and deoxygenated-Hb. Therefore, this device can essentially calculate tissue oxygen saturation.A previous study has reported that fat layer thickness affects SmO2 [6]. Incontrast, Niwayama et al. [7] recently reported that the SmO2 can be quantified by
the correction of fat layer thickness effects and the specifications of the NIRSRS
were fully described. In this study, we measured fat layer thickness on eachmeasurement site in VL muscle to correct these effects using an ultrasound device(LogiQ3, GE-Yokokawa Medical Systems, Japan) by placing an ultrasound probe
on the same sites as the NIRSRS probes had been placed. The SmO2 at rest and atVO2peak were defined as the SmO2 averaged over the last 5 s of each period.

Fig. 1 NIRSRS probes (left) and the probe position (right)

Relative Dispersion (RD) of StO2 was calculated as RD ¼ (SD/Mean) * 100( % ) as an index of heterogeneity.

2.4 Statistics
All data are expressed as means  SD. All parameters during exercise werecompared among pre- and post-endurance training by using a paired t-test.

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Also, the RD at rest and VO2peak were compared using a paired t-test. The
level of significance was set at p  0.05.

3 Results
The VO2peak was significantly enhanced by 6 weeks of endurance training (Pre: 42.7
 9.9 ml/kg/min, Post: 52.3  7.2 ml/kg/min, 22.5 % up-regulation, p < 0.001).
At rest, the SmO2 of each measurement site in the VL was not significantly different
before or after training (Pre-T: 70.0  8.2 %, Post-T: 71.5  4.6 %, mean  SD).
In contrast, the SmO2 at VO2peak was significantly lower after training at increasingly proximal sites, but not at the distal sites (Fig. 2). Also, the mean SmO2 in theVL muscle was significantly lower after training (Pre-T: 56.1  9.9 %, Post-T: 52.7
 11.9 %, p < 0.05). The RD (an index of heterogeneity) was significantly lower at
rest than VO2peak at both pre- and post-training (p < 0.05). The RD was not
significantly different between pre- and post-training at each rest period or VO2peak,respectively (Fig. 3). Fat layer thickness was not significantly different after endurance training (Pre-T: 4.70  1.78 cm, Post-T: 4.56  1.65 cm, mean  SD).

Fig. 2 SmO2 in VL muscle atVO2peak before and afterendurance training. *: p <0.05, **:p < 0.01

Fig. 3 Relative dispersion(RD) of SmO2 in VL muscle
before and after endurance
training

Unchanged Muscle Deoxygenation Heterogeneity During Bicycle Exercise

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4 Discussion
We investigated changes in SmO2 and its distribution in VL muscle at rest andduring bicycle exercise at VO2peak by 6 weeks of endurance training using themulti-channel NIRSRS system. This study reveals that muscle deoxygenation inVL was more enhanced at VO2peak, but the muscle deoxygenation heterogeneity
in VL was unchanged by endurance training. Recently, some PET studies havedemonstrated that muscle O2 extraction was higher and muscle blood flow
heterogeneity in the exercising muscle was lower in endurance-trained men[2, 3], but not during dynamic exercise because of movement artifacts. In
addition, there has been no intervention study to determine whether muscle
deoxygenation heterogeneity is reduced by endurance training. This is the firststudy reporting unchanged muscle deoxygenation heterogeneity in a single
muscle during bicycle exercise by endurance training.
Some studies have reported that muscle O2 extraction was enhanced afterendurance training as demonstrated by a lower venous femoral PO2 during
exercise [8]. Therefore, it is suggested that O2 extraction may be improved in theactivating muscle due to endurance training.
In this study, the SmO2 in the VL muscle was significantly enhanced after 6
weeks of endurance training, especially at proximal sites, but not at distal sites.
One of the reasons may be due to the small sample size. In addition, as the SmO2
was already significantly lower at distal sites than proximal sites during exercise at
pre-training, the difference in muscle deoxygenation change between distal andproximal sites post-training might be due to the inability to further deoxygenate.
The RD was significantly lower at rest than VO2peak at both pre- and posttraining. This implies that the dynamic O2 balance between muscle VO2 and O2
supply is more uniform at rest than during bicycle exercise.
We hypothesized that SmO2 distribution in the activating muscle would be
more homogeneous at higher work rates after endurance training [9]. However,SmO2 distribution in the VL muscle during exercise was not significantly
different between pre- and post-training, even though a previous study demonstrated that the muscle blood flow heterogeneity in the exercising muscle waslower in endurance-trained men [2]. The different results in these studies may beexplained by the differences of muscle group measurement, measurement depthand measurement devices. As the muscle deoxygenation heterogeneity in theactivating muscle was not significantly different after endurance training, O2balance distribution in the activating muscle may not be related to whole bodyoxidative capacity in healthy individuals.
In conclusion, after 6 weeks of endurance training, the SmO2 in VL muscle
was significantly enhanced, especially at increasingly proximal sites. In contrast, the SmO2 distribution in VL muscle during exercise was not significantly
different between pre- and post-training.
Acknowledgments The authors are grateful for revision of this manuscript by Christa
Colletti. We also thank Mikiko Yonemitsu and Ayaka Sato for their helpful technical

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assistance. This study was supported in part by Grant-in-Aid for Young Scientists from theJapan Society for Promotion of Science 18700536 to R. K.

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