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doi:10.1093/gerona/glq001
1
IN the elderly, loss of muscle strength and power in the
lower extremities is associated with functional limita-
tions in the activities of daily life, such as walking, stair
climbing, and chair standing. Muscle atrophy with aging
(sarcopenia), the progressive loss of muscle mass, has
been implicated as a primary factor in the loss of muscle
strength in older adults (1). The maintenance of muscle
volume (MV), therefore, seems to be critical in maintain-
ing the activities of daily life in the elderly. Several previ-
ous studies indicated that MV is a strong independent
predictor of physical disability or mortality (2–4). Other
studies, however, showed that MV measured by imaging
methods, such as magnetic resonance imaging (MRI) and
computed tomography (CT), or muscle mass measured by
dual x-ray absorptiometry (DXA) had poor associations
with physical function and mortality (5–7).
Skeletal muscle tissue holds a large amount of water,
which is partitioned into intracellular water (ICW) and
extracellular water (ECW; the sum of interstitial fluid and
blood plasma) fractions. Therefore, skeletal muscle con-
tains not only muscle cell mass but also ECW, which may
not be related to muscle strength (8). Therefore, we hypoth-
esized that MV is correlated more strongly with muscle
strength if the ECW volume is excluded from the MV. To
our knowledge, however, due to the difficulty assessing
regional ECW and ICW, previous studies have not examined
ECW and muscle strength.
Recently, estimating the regional muscle mass, ICW,
and ECW became possible using segmental multifrequency
bioelectrical impedance spectroscopy (S-BIS) (9–11) (see
detail comments for S-BIS on Supplementary Material
[S1]). The aims of this study were to examine the changes
in ICW and ECW in the lower leg with age using S-BIS
and to determine whether MV correlates more strongly
with muscle strength when the ECW volume is excluded
from the MV.
Extracellular Water May Mask Actual Muscle Atrophy
During Aging
Yosuke Yamada,1,2,3 Dale A. Schoeller,2 Eitaro Nakamura,1,4 Taketoshi Morimoto,5 Misaka Kimura,5 and
Shingo Oda1
1Graduate School of Human and Environmental Studies, Kyoto University, Japan.
2Department of Nutritional Sciences, University of Wisconsin-Madison.
3Current address: The Fukuoka University Institute for Physical Activity, Japan.
4Department of Sport Science, Kyoto Iken College of Medicine and Health, Japan.
5Kyoto Prefectural University of Medicine, Japan.
Address correspondence to Yosuke Yamada, PhD, The Fukuoka University Institute for Physical Activity, 8-19-1 Nanakuma, Jonan-ku,
Fukuoka 814-0180, Japan. Email: yyamada@fukuoka-u.ac.jp
Background. Skeletal muscle tissue holds a large volume of water partitioned into extracellular water (ECW) and
intracellular water (ICW) fractions. As the ECW may not be related to muscle strength directly, we hypothesized that
excluding ECW from muscle volume would strengthen the correlation with muscle strength.
Methods. A total of 119 healthy men aged 20–88 years old participated in this study. Knee isometric extension
strength, vertical jump, and standing from a chair were measured as indices of muscle strength and power in the
lower extremities. The regional lean volume (LV), total water (TW), ICW, and ECW in the lower leg were estimated
by anthropometry (skinfold and circumference measurements) and segmental multifrequency bioelectrical impedance
spectroscopy (S-BIS). Then, we calculated the ECW/TW and ICW/TW ratios.
Results. Although ICW and the LV index decreased significantly with age (p < .001), no significant changes in ECW
were observed (p = .134). Consequently, the ECW/TW ratio increased significantly (p < .001) with age (young adult, 27.0 ±
2.9%; elderly, 34.3 ± 4.9%; advanced elderly, 37.2 ± 7.0%). Adjusting for this by including the ICW/TW ratio in our
models significantly improved the correlation between the LV index and strength-related measurements and correlated
with strength-related measurements independently of the LV index (p < .001).
Conclusions. The ECW/TW ratio increases in the lower leg with age. The results suggest that the expansion of ECW
relative to ICW and the LV masked actual muscle cell atrophy with aging.
Key Words: Intracellular water—Extracellular water—Muscle strength—Muscle power—Muscle volume.
Received September 30, 2008; Accepted December 21, 2009
Decision Editor: Luigi Ferrucci, MD, PhD
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YAMADA ET AL.
2
Methods
Participants
A total of 2,844 Japanese adults aged 18–97 years were
recruited through the announcements in neighborhood as-
sociations, local community centers, and health promotion
centers; local newspapers in Kyoto and neighboring cities;
and to those who received a free routine physical fitness test
from 2002 through 2008 at Kyoto Prefecture University of
Medicine. From those participants, a total of 119 healthy
male volunteers consisting of 50 young (20–31 years) and
69 independently living community-dwelling elderly (60–
88 years) adults were randomly selected as participants for
this study after providing written informed consent. The
elderly participants were divided into two categories, el-
derly (60–74 years) and advanced elderly (≥75 years), based
on the Japanese medical insurance system. The eligibility
criteria were as follows: the ability to walk more than 400
m, climb stairs, and take a bath without assistance; the
absence of dementia and the capability to understand the
informed consent procedure; and no replacement arthro-
plasty or artificial pacemaker, pathological edema or lym-
phedema, definite kidney or digestive disease, hormone
replacement therapy, any symptoms of dehydration or over
hydration, or any acute or chronic diseases that influenced
body composition or hydration status. The study protocol
was approved by the ethics committee of Kyoto Prefectural
University of Medicine. Body mass of each participant was
measured to the nearest 0.1 kg, with the participants dressed
in light clothing. Barefoot standing height was measured to
the nearest 0.1 cm using a wall-mounted stadiometer.
Segmental Multifrequency Bioelectrical
Impedance Spectroscopy
Bioelectrical impedance was measured using a logarith-
mic distribution of 140 frequencies ranging from 2.5 to 350
kHz (MLT-30; Sekisui Medical, Tokyo, Japan) using dispos-
able electrodes. MLT-30 is the successor to the MLT-100,
which was validated against DXA to measure fat-free mass
in previous studies that included elderly participants (12,13).
The impedance of the right lower leg was measured between
5 and 10 minutes after the participant had laid down to avoid
the immediate (1–2 minutes) shifts in body fluids from the
extremities to the thorax when changing to a supine position
but not the slow phase of the shifts that continues for over 3
hours. Two injecting electrodes were placed on the dorsal
surfaces of the hand and foot on the right side of the body
(Figure 1). Sensing electrodes were placed on the right ar-
ticular cleft between the femoral and tibial condyles and the
anterior surface of the ankle between the protruding portions
of the tibial and fibular bones for lower leg measurements
(14). Participants were asked to abstain from strenuous exer-
cise and alcohol intake for 24 hours and from eating a meal
or drinking more than 0.5 L of water for 4 hours preceding
Figure 1. The setup for the measurement of segmental multifrequency
bioelectrical impedance spectroscopy in the lower leg.
the experiments. The room temperature was kept at about
22°C. Acquisition, storage, and analysis of data were per-
formed with the software supplied with the bioimpedance
analyzer. The instrument was calibrated against a reference
resistance and checked before measurements. The resistance
of the extracellular water compartment (RECW) and that of
the total water compartment (RTW) for the lower leg was
determined by extrapolation after fitting the spectrum of bio-
impedance data to the Cole–Cole model using the supplied
software. The resistance of the intracellular water compart-
ment (RICW) was calculated as 1/[(1/RTW) − (1/RECW)]. The
segmental ECW and ICW in the lower leg were calculated
using the following equations (10,11): ECW = rECW ×
L/RECW and ICW = rICW × L/RICW, where r represents fac-
tors for extracellular (rECW = 47 Wcm) and intracellular
(rICW = 273.9 Wcm) resistivity, respectively, L is segmental
length, RECW is segmental extracellular resistance, and RICW
is segmental intracellular resistance. The volume of TW was
calculated as the sum of ECW and ICW. The between-day
coefficients of variation (CV) for repeated ECW, ICW, and
TW measures in our laboratory were 2.0%, 3.4%, and 2.4%,
respectively, and the day-to-day intra-class correlation
coefficients (ICC[3,1]) were 0.969, 0.896, and 0.944.
Anthropometric Measurements
Right calf skinfold thickness was measured in the stand-
ing position using a calibrated Eiken skinfold caliper (TK-
11258; National Institute of Health and Nutrition, Meikosha,
Japan) with a constant pressure of 10 g/mm2 (15). Right calf
maximum circumference was measured by an investigator
with appropriate training in the procedure using a standard
measuring tape. Measurements were repeated twice, and the
means were used in the data analysis. If the values disagreed
by more than 5%, a new series of measurements was ob-
tained. The fat and lean (muscle + bone) cross-sectional area
(CSA) was estimated from calf skinfold (S) and circumfer-
ence (C) using previously developed equations: fat = [S·C/2]
− [p·S2/4] and lean = (C − p·S)2/4p (16). The physiological
CSA of muscle is linearly correlated with its volume in vivo
(17), and maximal muscle torque is more closely related to
MV than anatomical CSA (18). Therefore, we also calculated
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MUSCLE STRENGTH AND WATER DISTRIBUTION 3
the lean volume (LV) index as the lean CSA multiplied by
segmental length to match the volume dimension.
Muscle Strength and Power in the Lower Extremities
The maximal muscle isometric strength (MS) was mea-
sured in the sitting position on a custom-made dynamom-
eter chair at a knee angle of 90°, as described elsewhere
(19). The ankle was attached to a strain-gauge system via
belts around the pelvis and shoulders. After familiariza-
tion with the test, participants were encouraged to produce
maximal knee extension force. Three maximal efforts,
separated by a 1-minute rest period, were conducted, and
the highest recorded value was accepted as the result.
Maximal leg power was assessed by a vertical jump test,
as described elsewhere (16,20), performed on a specially
designed measuring scale (Jump Meter MD, TKK5106;
Takei Scientific Instruments, Niigata, Japan) (20). Each par-
ticipant performed two maximal squat jumps. A trained in-
vestigator supported the participant after landing if they
were likely to lose balance, although most did not lose bal-
ance on landing. The participants were given several famil-
iarization trials before the test. The vertical jump index
(VJI) was calculated using the following equation: VJI (m/
kg) = jump height × body weight (16).
Chair stand (sit-to-stand) frequency was measured as
participants stood up and sat down as quickly as possible
on a firm, padded armless chair with a seat that was 0.43 m
from the ground. The back of the chair was supported
against a wall, and participants were instructed to fold their
arms across their chest during the test. The number of rep-
etitions over a period of 30 seconds was recorded as previ-
ously described (21). The between-day CVs for repeated
MS, VJI, and chair stand frequency measures were 11.8%,
11.5%, and 8.9%, respectively, and the intra-class correla-
tion coefficients (ICC[3,1]) between two tests separated by
approximately a month were 0.839, 0.856, and 0.838.
Data Analysis
Results are presented as the means ± SD. Group differ-
ences were analyzed using one-way analysis of variance
(ANOVA), followed by Tukey’s post hoc test. Pearson’s
correlation coefficients were calculated between regional
body composition and strength-related variables (VJI, MS,
and chair stand). Partial correlation coefficients were also
calculated with fat CSA as a covariate. The ICW/TW ratio
was calculated and multiplied by lean CSA and the LV
index to eliminate the relative expansion of ECW. Pearson’s
correlation coefficients were compared statistically using
the methods described by Meng and colleagues (22). Mul-
tiple linear regression analyses were applied to examine
whether the ratio of ICW/TW significantly improved the
relationships between the LV index and strength-related
variables. All analyses were performed using SPSS 12.0 for
Windows. For all analyses, an alpha of .05 was used to
denote statistical significance. The data were judged to be
distributed normally according to the Kolmogorov–Smirnov
test (p > .05).
We compared our results with those of an epidemiological
study (InCHIANTI) (6) that used quantitative CT to mea-
sure the calf muscle CSA of the participants aged 20–102
years. We obtained data for young men aged 20–29 years,
elderly men aged 65–74 years, and advanced elderly men
aged 75–85 years from Table 2 in the literature (6) and cal-
culated the percent decrease from young adults to elderly
and to advanced elderly.
Results
Table 1 shows the physical characteristics, strength-
related characteristics, and regional body composition in
the lower leg of the participants. Six young (12.0%), nine
elderly (20.5%), and three advanced elderly adults (12.0%)
had body mass indices (BMIs) >25 kg/m2, and one young
Table 1. Physical Characteristics, Strength-Related Characteristics, and Regional Body Composition in Lower Leg of the Participants (n = 119)
Young (n = 50), M ± SD
(minimum, maximum)
Elderly (n = 44),M ± SD
(minimum, maximum)
Advanced Elderly (n = 25), M ± SD
(minimum, maximum)
Physical characteristics
Age (y) 22 ± 2 (20, 31) 69 ± 3 (62, 73)*** 79 ± 4 (75, 88)***,†††
Height (cm) 171.7 ± 5.3 (158.0, 183.0) 165.6 ± 6.3 (150.3, 182.6)*** 159.8 ± 6.4 (146.7, 175.2)***,†††
Weight (kg) 65.7 ± 8.8 (50.0, 99.5) 64.1 ± 9.0 (42.9, 84.1) 56.3 ± 11.0 (38.1, 80.5)***,†
BMI (kg/m2)22.3 ± 3.4 (17.3, 39.8) 23.3 ± 2.8 (17.4, 29.7) 22.0 ± 3.8 (16.1, 33.1)
Strength-related characteristics
Vertical jump index (m·kg) 32.5 ± 5.4 (17.1, 44.2) 18.8 ± 4.9 (6.9, 28.2)*** 14.5 ± 4.2 (6.5, 23.3)***,†
Muscle isometric strength (kg) 59.2 ± 8.4 (40.0, 79.0) 32.6 ± 8.6 (15.8, 48.8)*** 27.5 ± 7.4 (9.2, 41.3)***,†
Chair stand (frequency/30 s) 38.6 ± 4.0 (30.0, 48.0) 23.7 ± 4.9 (16.0, 33.0)*** 21.2 ± 6.5 (12.0, 39.0)***
Regional body composition in lower leg
Fat CSA (cm2)13.1 ± 9.1 (1.7, 44.2) 18.1 ± 8.9 (2.9, 41.3)* 17.1 ± 9.3 (3.4, 43.1)
Lean CSA (cm2)98.4 ± 19.5 (63.7, 162.2) 86.3 ± 13.5 (62.6, 120.3)** 74.7 ± 14.1 (54.5, 118.2)***,†
LV index (m3)3.78 ± 0.76 (2.33, 6.29) 3.17 ± 0.58 (2.22, 4.67)*** 2.65 ± 0.52 (1.86, 4.08)***,††
Notes: Elderly: 60–74 years old; advanced elderly: 75–88 years old. Fat and Lean CSA was estimated by skinfold and circumference measurements. LV index
was calculated as Lean CSA and segment length. BMI = body mass index; CSA = cross-sectional area; LV = lean volume.
Significantly lower than young adults (*p < .5, **p < .01, *** p < .001).
Significantly lower than younger elderly adults (†p < .05,††p < .01,†††p < 0.001).
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YAMADA ET AL.
4
health status for each group were very similar to those of
healthy participants in previous studies (23,24)
Using one-way ANOVA, significant decreases in height
and weight, but not in BMI, were observed with age. With
the exception of fat CSA, all of the other strength-related
variables and regional body composition variables de-
creased significantly as the age of group advances.
The water distribution in the lower leg in each age group
is shown in Figure 2. Estimated ICW decreased significantly
(p < .001) with aging (young 1,126 ± 222 mL; elderly,
784 ± 190 mL; and advanced elderly, 653 ± 233 mL), but no
significant (p = .134) changes in estimated ECW were ob-
served with aging (young 414 ± 72 mL; elderly, 402 ± 74
mL; and advanced elderly, 374 ± 101 mL; Figure 2A).
Therefore, the ICW/TW ratio decreased significantly (p <
.001) with aging (young 73.0 ± 2.9%; elderly, 65.7 ± 4.9%;
and advanced elderly, 62.8 ± 7.0%), and the ECW/TW ratio
increased significantly with age (young 27.0 ± 2.9%;
elderly, 34.3 ± 4.9%; and advanced elderly, 37.2 ± 7.0%;
Figure 2B).
The ICW/TW ratio was significantly correlated with VJI
(r = .640, p < .001), MS (r = .600, p < .001), and chair stand
(r = .583, p < .001) in the 119 men enrolled in this study.
Furthermore, partial correlations were calculated after ad-
justing for fat CSA. The ICW/TW ratio was still signifi-
cantly correlated with VJI (r = .628, p < .001), MS (r = .582,
p < .001), and chair stand (r = .567, p < .001).
Table 2 shows the correlation coefficients between
strength-related variables and regional body composition
for the entire study population. TW, lean CSA, and the LV
index were significantly correlated with VJI, MS, and chair
stand. All of these correlations improved significantly when
the ICW/TW ratio was multiplied by lean CSA and the LV
index (p < .01). In multiple regression analysis (Table 3),
the ratio of ICW/TW was a significant predictive variable
independently of the LV index for estimating strength-
related variables (p < .001). The correlation coefficient be-
tween LV and ICW/TW was .484, and the variance inflation
factor was 1.306 and did not reach the level for significant
(>5.0) multicollinearity.
Table 4 shows the percent decreases in calf muscle area,
lean CSA, and ECW-eliminated lean CSA (lean CSA ×
ICW/TW) from young adults to the elderly. The percent de-
crease of the lean CSA in our study was almost the same as
and one advanced elderly had BMIs of >30 kg/m2. The
height, weight, and BMI in the present study were not
different from those described by the National Health
and Nutrition Survey of Japan 2007. Further description of
demographic status is shown in the Supplementary Material
(S2) and Table S1. The physical function, demographic, and
0
500
1000
1500
2000
Elderly Advanced elderly
Hydrated volume in lower leg (ml)
ICW (ml)
ECW (ml)
***
***, †
n.s. n.s.
A)
0
10
20
30
40
50
Young Elderly Advanced elderly
The ratio of ECW/TW (%)
***
***, †
B)
Young adult
Figure 2. Water distribution in the lower leg estimated by S-BIS (mean ±
SD). (A) ***significantly lower intracellular water (ICW) than young adult
(p < .001); †significantly lower ICW than elderly adults (p < .05). No signifi-
cant main effect was observed in extracellular water (ECW; p = .134). The total
bar shows the sum of ICW and ECW (total water [TW]). (B) The ECW/TW
ratio increased significantly (p < .001) with aging.
Table 2. Correlation Coefficients Between Strength-Related Variables and Regional Body Composition in the Lower Leg
Water Measurements Lean CSA LV Index
TW ICW Lean CSA Lean CSA × I/T LV Index LV Index × I/T
Vertical jump index 0.723 0.766*** 0.661 0.741*** 0.744 0.803***
Muscle isometric strength 0.663 0.703** 0.560 0.648*** 0.633 0.701***
Chair stand 0.516 0.583*** 0.414 0.528*** 0.457 0.555***
Notes: ICW = intracellular water; I/T = ratio of intracellular water to total water; Lean CSA = lean cross-sectional area; LV index = lean volume index;
TW = total water.
All correlation coefficients were significant (p < .001).
Significantly higher than the correlation coefficient in its left column (**p < .01, ***p < .001).
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MUSCLE STRENGTH AND WATER DISTRIBUTION 5
ECW directly using only the resistance and segment length
(9–11,30). Moreover, previous studies have indicated that
S-BIS succeeds in monitoring the fluid changes in the
segmental limbs during position changes, hemodialysis,
exercise, and 72-hour head-down bed rest (9,11,30,31). For
further discussion, regarding the scientific consensus related
to bioelectrical impedance and the differences between
whole-body methods and S-BIS, see the Supplementary
Materials (S1).
We observed a relative increase in ECW in the lower leg in
successively older cohorts when estimated using S-BIS
(Figure 2). No previous reports have described age-related
changes in segmental water distribution in vivo. Tanaka and
colleagues (32) reported that people with cervical spinal cord
injury (quadriplegia) had a higher ECW/ICW ratio in the
limbs than noninjured individuals and that the relative expan-
sion of ECW was related to the decrease in muscle mass. The
retention of water in the extracellular space may occur
concurrently with the loss of muscle contractile tissue.
Although we did not measure the MV anatomically, if this
MV of young adults is 1,300 cm3 (14) and the ECW is 400
cm3, the ECW-eliminated MV is 900 cm3. If this MV of the
elderly adults were 1,100 cm3 and the ECW is still 400 cm3,
the ECW-eliminated MV were 700 cm3 and age-related mus-
cle atrophy were expressed using the percent decrease from
the value of young adults, then the percent decrease in MV is
(200/1,300) = ~15%, whereas the percent decrease in ECW-
eliminated MV is (200/900) = ~22%. Therefore, if we did not
eliminate ECW from the total MV, the rate of actual MV de-
cline would be underestimated. As shown in Table 4, the per-
cent decrease in the lean CSA in our study was similar to the
percent decrease in calf muscle area in the study by Lauretani
and colleagues (6). However, the percent decrease in the
ECW-eliminated lean CSA in our study was higher than that
in the lean CSA or calf muscle area. These results support the
idea that relative expansion of the ECW would mask the ac-
tual muscle cell atrophy in the previous studies.
the percent decrease in calf muscle area in the study by Lau-
retani and colleagues (6). The percent decrease in the ECW-
eliminated lean CSA in our study was higher than that in the
lean CSA or calf muscle area.
Discussion
At the whole-body level, although some studies have
reported no age-related differences (25), many others have
indicated age-related increases in the ECW/ICW ratio (26–
28). The human body consists of various types of tissue, and
we therefore hypothesized that the measurement of seg-
mental water distribution would improve the relationship
between muscle size and strength. Recent studies have indi-
cated that S-BIS can be used to estimate the segmental ECW
and ICW (9–11). The traditional whole-body impedance
method is based on the assumption that the hydrated portion
of the body is cylindrical in shape. Although this assump-
tion breaks down at the whole-body level (29), individual
segments more closely resemble cylinders. Therefore,
S-BIS can be used to estimate the segmental ICW and
Table 3. Multiple Linear Regression Analysis for Predicting Muscle Function in the Lower Extremities
Dependent Variables Independent Variables
Coefficients
Standardized Unstandardized
pbBSEE
Vertical jump index (m/kg) LV index (m3) .567 6.688 0.735 <.001
ICW/TW (%) .365 0.528 0.090 <.001
Constant −34.471 5.417 <.001
R2 = .655
Muscle isometric strength (kg) LV index (m3) .447 9.419 1.560 <.001
ICW/TW (%) .383 0.990 0.191 <.001
Constant −55.976 11.506 <.001
R2 = .513
Chair stand (frequency/30 s) LV index (m3) .229 2.729 0.996 <.01
ICW/TW (%) .472 0.689 0.122 <.001
Constant −26.571 7.344 <.001
R2 = .380
Note: ICW/TW = ratio of intracellular water to total water in the lower leg; LV index = lean volume index; SEE = standard error of estimate.
Table 4. Change in the Segmental Composition of the Lower Leg
During Adulthood
Calf Muscle
Area* (cm2)
Lean CSA
(cm2)
Lean CSA × I/T
(cm2)
Young adults 83.3 98.4 71.8
Elderly adults 72.2 86.3 56.7
Advanced elderly adults 64.8 74.7 46.9
Present decrease from young
to elderly adults
13.3 12.3 21.1
Present decrease from young
to advanced elderly adults
22.2 24.1 34.7
Notes: I/T = ratio of intracellular water to total water; Lean CSA = lean
cross-sectional area measured at the largest outer calf diameter.
* The values from the literature of Lauretani colleagues (6) InCHIANTI
study, derived from quantitative computerized tomography scans obtained at
66% of the tibia length, proximal to the anatomic marker that is the region with
the largest outer calf diameter.
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YAMADA ET AL.
6
Several recent studies have indicated changes in muscle
composition (ie, intramuscular adipose tissue and muscle
density) with aging in vivo. Kent-Braun and colleagues (33)
reported that the intramuscular fat in the tibialis anterior
muscle as estimated by MRI increased with aging, and Gal-
ban and colleagues (34) reported that the diffusion tensor–
magnetic resonance imaging (DT-MRI) signal intensity
decreased significantly in the muscles of the lower leg with
aging. Other studies have found significant decreases in the
muscle density estimated by CT with aging related to lower
extremity function (35,36). Although the relationships
among ECW, intramuscular fat, muscle density of CT, and
DT-MRI remain unknown, the previous and present results
suggest that quantification of the intramuscular noncontrac-
tile tissue is critical for accurate assessment of muscle qual-
ity. Furthermore, S-BIS is the least costly of these methods.
The present results suggest that the relative expansion of
ECW with aging may be one reason for the age-related
decrease in specific force. Metter and colleagues (37) noted
that the relationship between specific force and age is de-
pendent on how muscle mass is estimated. Even the most
reliable methods, such as CT or MRI, have several assump-
tions and limitations. Bartok and Schoeller (9) reported that
the decreasing MV estimated by MRI in the lower leg after
short-term head-down bed rest mainly reflected the decreas-
ing ECW. Berg and colleagues (38) also demonstrated that
calf muscle area estimated by CT significantly decreased
by 5.5% and the radiological density of muscle showed a
simultaneous increase of 4.8% during the initial 120 minutes
of bed rest. MV estimated by CT decreased significantly
after changing position from standing to lying down. There-
fore, the MV estimated using these imaging methods in-
cludes not only the muscle cell volume but also ECW, which
is not related to muscle strength (8). Simultaneous measure-
ment using S-BIS and imaging methods should be effective
for assessing actual muscle tissue changes.
Although the proportion is small, fat tissue also holds
water. We examined partial correlations with the fat CSA as
a covariate. The partial correlation coefficients were not dif-
ferent from the single correlation coefficients. Therefore,
the differences in subcutaneous fat CSA did not affect the
results obtained in the present study. We did not measure
muscle mass CSA but instead measured lean (muscle + bone)
CSA; thus, quantifying the effects of bone mass was not
possible in this study. However, the correlation coefficients
between strength-related variables and the LV index were
almost the same as those between strength-related variables
and TW, which is mainly associated with the MV. Therefore,
although further investigations are required, this suggests that
the bone tissue mass did not influence our results.
A limitation of our study is that the vertical jump test and
chair stand test rely not only on the muscles of the lower
legs but also on the muscles in the thighs and hips, which
were not measured using bioelectrical impedance spectros-
copy. If we could measure the thigh segments as well as the
lower legs by S-BIS or measure the muscle power of each
muscle groups separately, our results might be reinforced.
We also measured S-BIS for the thigh segment, but the
reactance showed poor reproducibility at both low and very
high frequencies, especially in the advanced elderly (data
not shown). This poor reproducibility may be attributable to
the difficulty in distinguishing between trunk and thigh
segments due to the noncylindrical shape of the pelvis or
instrumental limitations for these values of reactance. This
poor reproducibility markedly affected Cole–Cole model-
ing in the thigh segment because this segment presented a
smaller resistance than the lower leg due to its large CSA.
Improvements in the instrument are needed. The measure-
ment of the size and strength of specific muscle groups is
important for discussing the effect of the relative expansion
of ECW on the age-related change in a specific force. Other
potential limitations of this study were that the recruitment
method used was not entirely random and that the study
sample was small. We enrolled only generally healthy indi-
viduals. Further studies are required to provide standardized
values for each generation.
In conclusion, our study found that the ICW/TW ratio
decreased and the ECW/TW ratio increased significantly in
the lower leg with aging. The ICW/TW ratio was signifi-
cantly correlated with muscle strength and power in the
lower extremities. This indicated that the expansion of ECW
relative to ICW and the LV masks actual muscle cell atrophy
during aging. S-BIS is an affordable, noninvasive, easy-to-
operate, and fast method and would complement anthropo-
metric or other measurements when assessing sarcopenia in
the limbs. The findings reported here suggest that measure-
ment of ECW and ICW using S-BIS may fill a gap in the
measurement of muscle atrophy with aging.
Funding
Supported by a research grant to Y.Y. from Research Fellowships from
the Japan Society for the Promotion of Science for Young Scientists
(19-1440).
Supplementary Material
Supplementary material can be found at: http://biomed.gerontology
journals.org/
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