Clinical Science (2001) 100, 199–206 (Printed in Great Britain)
Cardiovascular regulation during head-up tilt in
healthy 20–30-year-old and 70–75-year-old men
Tim J. GABBETT, Shane B. WESTON, Rod S. BARRETT and Greg C. GASS
School of Physiotherapy and Exercise Science, Faculty of Health Sciences, PMB 50 Gold Coast Mail Centre,
Griffith University Gold Coast, Queensland 9726, Australia
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This study compared the heart rate, finger arterial pressure (AP) and electromyographic (EMG)
activity of selected anti-gravity muscles during the initial and prolonged phases of orthostatic
stress in healthy young and older men. Beat-by-beat recordings of heart rate, finger systolic
pressure, diastolic pressure and mean AP were made during supine rest and 5 min of 90 m headup tilt (HUT) in 18 young (23p1 years) and 15 older (73p1 years) men. The EMG activity of the
soleus, tibialis anterior and vastus medialis muscles was recorded. During the first 30 s following
90 m HUT (immediate response), the young men exhibited significant (P 0.05) decreases in
finger systolic pressure, diastolic pressure and mean AP, followed by a sustained increase in
finger AP during the 5 min following 90 m HUT (prolonged response). The immediate and
prolonged finger AP and diastolic pressure responses were not significantly different (P 0.05)
from the values at supine rest for the older men. The mean root mean square EMG activity of the
soleus, tibialis anterior and vastus medialis muscles during 90 m HUT was not significantly
different (P 0.05) from that at supine rest for either group. These results demonstrate that,
when compared with healthy older men, young men show larger reductions in finger AP during
the initial phase of orthostatic stress. However, during the prolonged phase of orthostatic stress,
older men maintain resting finger AP, whereas young men demonstrate a reflex overshoot in
finger AP. Finally, differences in lower-limb anti-gravity muscle activation do not account for the
contrasting finger AP responses of healthy young and older men.
INTRODUCTION
Orthostasis can be generally defined as any process that
produces physiological responses similar to those induced by the upright, stationary posture [1]. When
humans assume the upright posture, a significant volume
of blood is displaced from the thorax into the legs. Under
conditions where the muscle pump is inactive, blood
pools in the lower extremities, and venous return, enddiastolic volume and stroke volume are significantly
reduced [2]. Despite a rise in heart rate (HR), the
reduction in stroke volume may lead to a reduction in
cardiac output and arterial pressure (AP) [2]. Total
peripheral resistance is increased in an attempt to restore
AP towards a homoeostatic level [2,3].
There are conflicting reports regarding the relative
cardiovascular responses of young and older subjects to
an orthostatic challenge. It has been reported that young
and older subjects exhibit similar reductions in stroke
volume, cardiac output [4] and AP [5] during the initial
phase (first 30 s) of standing and head-up tilt (HUT). In
contrast, White et al. [6] and Taylor et al. [7] demonstrated that older subjects had superior control of AP
compared with younger subjects during prolonged
standing, HUT and lower-body negative pressure. In
addition, a recent investigation showed that over 50 % of
Key words: aging, anti-gravity, finger arterial pressure, orthostasis, sympathetic outflow.
Abbreviations : AP, arterial pressure ; DBP, diastolic blood pressure ; EMG, electromyographic ; HR, heart rate ; HUT, head-up tilt ;
MAP, mean arterial pressure ; NA, noradrenaline ; SBP, systolic blood pressure.
Correspondence : Mr Tim J. Gabbett, at present address School of Health Science, Griffith University Gold Coast, PMB 50 Gold
Coast Mail Centre, Queensland 9726, Australia (e-mail t.gabbett!mailbox.gu.edu.au).
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healthy older men aged 65–75 years exhibited immediate
increases (without subsequent decreases) in arterial pulse
pressure, systolic blood pressure (SBP), diastolic blood
pressure (DBP) and mean arterial pressure (MAP) during
acute 90 m HUT [8]. On the basis of these equivocal
findings [4–8], it is unclear whether young and older
subjects respond similarly to an orthostatic challenge.
Factors which may influence AP control during
orthostasis include sympathetic outflow [6,9,10] and
lower-limb anti-gravity muscle activation [3,9,11–13].
However, the interaction of sympathetic outflow and
lower-limb anti-gravity muscle activation on orthostatic
responses has not been addressed in young and older
men. The present study compared the cardiovascular
responses of healthy 20–30-year-old and 70–75-year-old
men during 90 m HUT. In addition, we investigated the
influence of sympathetic outflow and electromyographic
(EMG) activity of selected lower-limb anti-gravity
muscles on orthostatic responses in these individuals.
METHODS
Subjects
Groups of 18 healthy young men (20–30 years) and 15
healthy older men (70–75 years) participated in the study.
The mean (pS.E.M.) age, height and body mass of the
young subjects were 23p1 years, 178p2 cm and
83p2 kg respectively, and the corresponding values for
the older subjects were 73p1 years, 175p2 cm and
83p4 kg respectively. Subjects were non-smokers with a
normal resting twelve-lead ECG. On their first visit to
the laboratory, all young subjects had a supine resting
blood pressure of 140\90 mmHg, while older subjects
had a supine resting blood pressure of 160\90 mmHg.
Each subject completed a comprehensive medical examination and an incremental exercise test to volitional
exhaustion without any clinically significant findings.
Subjects were not prescribed or taking any medication
that was likely to interfere with cardiovascular or
sympatho-adrenal responses. To control for chronic
physical activity levels, only individuals who were
physically inactive to recreationally active were included
in the study. Young subjects were required to have
a peak oxygen consumption of between 40.0 and
50.0 ml:min−":kg−", while older subjects were required
to have a peak oxygen consumption of between 20.0 and
30.0 ml:min−":kg−". All subjects received a clear explanation of the study, including the risks and benefits of
participation, and written consent was obtained. All
experimental procedures were approved by the Griffith
University Ethics Committee for Human Experimentation, and subjects were familiarized with all experimental procedures.
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Peak oxygen consumption
Before being accepted into the study, subjects underwent
an incremental exercise test to volitional exhaustion on an
electronically braked cycle ergometer (Lode Excalibur,
Groningen, The Netherlands) to ensure that their aerobic
capacity was in the normal range for healthy untrained
young and older men. Exercise tests were increased using
5 W (15 W:min−") or 10 W (30 W:min−") increments
every 20 s until volitional exhaustion for the old and
young men respectively. Peak oxygen consumption was
determined for each subject using open circuit
spirometry. The pneumotach (Hans Rudolph 3830,
Kansas City, MO, U.S.A.) and the O and CO analysers
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(Exerstress OX21, CO21, Sydney, Australia) were calibrated before and after each test using a 3-litre syringe
and precision reference gases. Oxygen consumption,
CO output, pulmonary minute ventilation and the
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respiratory exchange ratio were recorded as 20 saveraged data throughout the incremental exercise test.
HR and rhythm were monitored continuously and
recorded throughout the exercise period using a modified
CM5 electrode configuration [14]. Peak exercise values
were determined as the highest values obtained during
the incremental exercise test.
90 m HUT protocol
A custom-made padded tilt-table was used to induce
orthostatic stress. The tilt-table rotated around a central
axis, and could be adjusted manually from the horizontal
to the vertical position. Each subject was familiarized
with the operation of the tilt-table and with the sensation
of moving rapidly from a supine to an upright position
within 2 s. Familiarization sessions were identical for all
subjects.
The 90 m HUT test was administered at least 3 days
after the subject was familiarized with the tilt-table. All
tests were conducted in a temperature-controlled environment (22–24 mC ; dry bulb). Subjects were
encouraged to void their bladder before beginning the
test. The subject lay supine and ECG electrodes were
applied in the CM5 position [14]. Three 10 cm-wide
adjustable straps were used to secure the subject to the
tilt-table. The straps were firmly, but comfortably,
applied around the chest, thigh and lower leg of the
subject. Beat-by-beat measurements of HR were
obtained from an ECG, while continuous finger SBP,
DBP and MAP values were obtained non-invasively
using a Finapres device (Ohmeda 2300, Louisville, CO,
U.S.A.), which was applied to the subject’s left hand. The
subject’s left arm was placed across his chest and the
palmar aspect of the left hand was positioned in the right
anterior axillary region at the level of the fifth intercostal
space. The fingers of the left hand were positioned in the
mid-axillary region. A Velcro hand strap connected the
Finapres to the chest strap, thereby maintaining the hand
Orthostasis in young and older men
(and Finapres) at heart level throughout the entire
experimental procedure.
Before testing, a 21-gauge venous catheter was inserted
into a right antecubital vein under sterile conditions.
Subjects were instructed to remain relaxed, maintain
normal respiration and avoid all muscle contraction.
Following cannulation, all subjects rested in the supine
position for 20 min. To minimize any anticipatory
changes in HR or finger AP, each subject was randomly
assigned an additional rest period ranging from 2 to
10 min before 90 m HUT. On completion of the
randomized 2–10 min rest period, subjects were tilted
rapidly to the upright, vertical position within 2 s.
Subjects remained in the upright position for a period of
5 min. Beat-by-beat HR and finger AP values were
recorded continuously throughout 90 m HUT. A
cushioned footplate supported the subject during 90 m
HUT. No conversation was made between the subject
and investigators throughout the 90 m HUT experiment.
While the Finapres device has been shown to accurately
track relative beat-by-beat changes in direct intra-arterial
pressure [15], the absolute AP value from the Finapres
device may be inaccurate in some individuals [15,16].
Furthermore, finger AP may not always be representative
of central AP [17]. With this in mind, before each 90 m
HUT test, Finapres AP measurements were compared
with auscultatory AP measurements to ensure a valid
measurement of AP. The auscultatory and Finapres
SBP\DBP measurements for the young subjects before
the 90 m HUT test were 123p2 mmHg\73p2 mmHg
and 123p2 mmHg\69p2 mmHg respectively, and those
for the older subjects were 141p4 mmHg\78p3 mmHg
and 142p4 mmHg\74p2 mmHg respectively. Furthermore, in a previous investigation (T. J. Gabbett, unpublished work) we detected no significant differences
between auscultatory and Finapres AP measurements,
recorded at supine rest, after 30 s of 90 m HUT and
throughout 15 min of 90 m HUT (n l 8).
EMG recording
The EMG activity of the right vastus medialis, soleus and
tibialis anterior muscles was recorded throughout supine
rest and 90 m HUT. Before applying the electrodes, the
skin was shaved, cleansed and lightly abraded. Sentry
bipolar Ag\AgCl 10 mm disc, pre-gelled disposable
surface electrodes were placed on the skin superficial to
each of the muscle bellies, in parallel with the muscle fibre
orientation of the underlying muscle. The EMG signal
was collected at 500 Hz using a Biopac EMG 100
differential amplifier, interfaced with a Biopac system
(MP 100 ; SDR Clinical Technology, Middle Cove, NSW,
Australia) and Acqknowledge 2.0 Software. Amplifier
gains were set to maximize resolution, and were typically
set to 5000. The samples were digitized by a 12-bit
analogue-to-digital converter and were stored on a PC
for subsequent analysis. Following de-meaning of the
raw EMG and band-pass filtering (20–250 Hz), the root
mean square of the raw EMG signal for each muscle was
calculated [18] using a window of 200 ms. A 2 s period
immediately before 90 m HUT and 10 s after 90 m HUT
was used to calculate the mean pre- and post-90 m
HUT root mean square signal. Before and after the
90 m HUT test, we instructed subjects to contract the
vastus medialis, soleus and tibialis anterior muscles in
order to ensure correct placement and attachment of
electrodes. The contraction of these muscles resulted in a
significant increase in EMG activity. Additionally, in a
separate investigation (T. J. Gabbett, R. S. Barrett and
S. B. Weston, unpublished work) we asked subjects (n l
22) to stand as still as possible for a 30 s period, while the
EMG activity of the vastus medialis, soleus and tibialis
anterior muscles was recorded. During the 30 s standing
test, there was a sustained increase in the EMG activity of
all three muscle groups. These results confirm the validity
of surface EMG for detecting changes in tibialis anterior,
vastus medialis and soleus muscle activity during acute
orthostatic stress.
Venous blood sampling
A 10 ml blood sample was obtained on completion of the
20 min rest period. An additional 10 ml sample was
obtained after 5 min of 90 m HUT. Plasma samples were
analysed for noradrenaline (NA) and adrenaline concentrations using HPLC with electrochemical detection.
The remaining 5 ml sample was analysed for haematocrit
and haemoglobin concentration using a Coulter Counter
(Coulter Electronics ; Model T660). Changes in plasma
volume were calculated using the procedure described by
Greenleaf et al. [19], with NA and adrenaline concentrations adjusted for changes in plasma volume.
Statistical analysis
Differences in physical characteristics between the young
and older men were compared using unpaired t-tests. The
HR, finger MAP, SBP, DBP and EMG responses
throughout each 1 min of 90 m HUT were compared
using ANOVA with repeated measures, to determine
differences between groups and any group-by-time
interactions. When required, comparisons of group
means were performed using Scheffe’s multiple comparison procedure. Changes in NA concentration,
adrenaline concentration, haematocrit, haemoglobin
concentration and plasma volume between supine rest
and 90 m HUT were evaluated using paired t-tests. The
level of significance was set at P 0.05, and all data are
reported as meanspS.E.M.
RESULTS
Physical characteristics
The physical characteristics and peak exercise responses
of the subjects are presented in Tables 1 and 2. Young
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T. J. Gabbett and others
Table 1
men
Peak exercise responses of healthy young and older
Values are meanspS.E.M. Significant difference compared with young subjects :
*P 0.05.
Parameter
Young
Peak O2 consumption (litres:min−1)
Peak O2 consumption
(ml:min−1:kg−1)
Peak HR (beats:min−1)
Peak ventilation (litres:min−1)
Peak power (W)
Old
3.83p0.13
46.5p1.1
2.14p0.12*
25.9p0.9*
188p2
161.2p6.7
321p7
148p4*
99.4p6.2*
164p9*
Table 2 Supine resting HR, and finger SBP, DBP and MAP
before 90 m HUT for healthy young and older men
Values are meanspS.E.M. Significant difference compared with young subjects :
*P 0.05.
Parameter
−1
HR (beats:min )
Finger MAP (mmHg)
Finger SBP (mmHg)
Finger DBP (mmHg)
Young
Old
61p2
84p2
129p3
66p2
60p2
95p3*
145p7*
72p3*
subjects had significantly higher (P 0.001) peak oxygen
consumption, peak ventilation, HR and power than the
older subjects (Table 1). No significant differences (P 0.05) were found between groups for height or body
mass. When compared with young subjects, older subjects had significantly higher (P 0.05) resting finger
SBP, DBP and MAP (Table 2).
Immediate (first 30 s) and prolonged
(1–5 min) steady-state cardiovascular
responses
The mean (pS.E.M.) beat-by-beat HR and finger MAP,
SBP and DBP responses during the initial (first 30 s) and
prolonged (1–5 min) phases of 90 m HUT are shown in
Figure 1. During the initial 30 s of 90 m HUT, a significant
increase (P 0.05) in HR was observed for both the
young and older men ; however, the rate of change and
the peak change in HR were lower (P 0.001) in the
older men. 90 m HUT resulted in significant decreases
(P 0.05) in finger SBP, DBP and MAP in the young
men. Finger MAP and DBP were not significantly
different (P 0.05) from the values at supine rest for the
older men. The peak changes in finger SBP (young men,
k20p4 %\k26p5 mmHg ; older men, k10p3 %\
k14p4 mmHg), DBP (young men, k17p3 %\
k11p2 mmHg ; older men, k4p4 %\k3p3 mmHg)
and MAP (young men, k16p3 %\k14p3 mmHg ;
older men, k6p4 %\k6p4 mmHg) during 90 m HUT
were significantly greater (P 0.05) for the young
subjects.
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2001 The Biochemical Society and the Medical Research Society
Figure 1 Immediate and prolonged HR, and finger SBP, DBP
and MAP responses during 90 m HUT in healthy 20–30-yearold (solid line) and 70–75-year-old (dotted line) men
Compared with the older subjects, young subjects had
a significantly greater (P 0.01) increase in HR during
the 5 min of 90 m HUT. In addition, the changes in finger
MAP and DBP were significantly greater (P 0.01) in
the young subjects during the 5 min of 90 m HUT.
Anti-gravity muscle EMG
Typical examples of raw EMG activity in one young and
one older subject during supine rest and 90 m HUT are
shown in Figure 2. The soleus, tibialis anterior and vastus
medialis muscles were relatively quiescent during the
supine rest period. Immediately upon the onset of 90 m
HUT, a transient rise in EMG activity was observed in all
subjects for all three muscle groups measured. However,
no significant differences (P 0.05) in the mean root
mean square activity of the soleus, tibialis anterior and
gastrocnemius muscles before and after 90 m HUT were
detected for either group.
Changes in plasma volume and in NA and
adrenaline concentrations
Compared with values at supine rest, we observed
significant increases (P 0.001) in the haematocrit and
the haemoglobin concentration, and significant decreases
in plasma volume, in both groups following 5 min of 90 m
HUT. While young subjects had a significantly higher
Orthostasis in young and older men
Figure 2 Original ECG, finger AP and EMG tracings for one young and one older subject during supine rest and the initial phase
of 90 m HUT
Traces from top to bottom : ECG ; finger AP ; tibialis anterior EMG ; soleus EMG ; vastus medialis EMG.
(P 0.05) resting haematocrit than older subjects
(43.8p0.6 % and 41.1p1.1 % respectively), no significant differences (P 0.05) were observed between young
and older subjects for changes in haematocrit (young,
j1.8p0.2 % ; old, j2.1p0.1 %) or haemoglobin concentration (young, j1.0p0.1 g:dl−" ; old, j1.0p0.1
g:dl−") during 90 m HUT. Accordingly, the decreases in
plasma volume following 5 min of 90 m HUT were similar
(P 0.05) in the young (k9.2p0.8 %) and older
(k10.6p0.4 %) subjects. No significant differences (P 0.05) existed between groups for supine resting concentrations of NA (young, 361p21 pg:ml−" ; old,
376p58 pg:ml−")
and
adrenaline
(young,
27p4 pg:ml−" ; old, 31p6 pg:ml−"). Whereas 90 m HUT
evoked a significant (P 0.01) increase in the NA
concentration in young subjects (j54p14 pg:ml−"), the
NA concentration during 90 m HUT was not significantly
different from that at supine rest for the older subjects
(j4p32 pg:ml−" ; P 0.05). The plasma adrenaline
concentration during 90 m HUT was not significantly
different (P 0.05) from that at supine rest in both the
young (29p3 pg:ml−") and the older (30p4 pg:ml−")
subjects.
DISCUSSION
The results of the present study demonstrate that, when
compared with healthy older men, young men have
larger reductions in finger AP during the initial phase of
orthostatic stress. However, during the prolonged phase
of orthostatic stress, older men maintain resting finger
AP, whereas young men demonstrate a reflex overshoot
of finger AP, which corresponds with an increase in
sympathetic outflow. Finally, differences in activation of
lower-limb anti-gravity muscles do not account for the
contrasting finger AP responses of healthy young and
older men.
During the first 30 s of 90 m HUT, the young subjects
experienced a significant reduction in finger MAP of
16p3 % (k14p3 mmHg), while the change in finger
MAP during 90 m HUT in the older individuals
(k6p4 %, k6p4 mmHg) was not significant. These
findings suggest that healthy older men have an enhanced
ability to minimize reductions in finger AP during the
initial stages of orthostatic stress. The present finding of
maintained finger AP during the initial phase of orthostatic stress in older individuals is in disagreement with
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other investigators [5], who reported that young and
older individuals had similar reductions in finger AP
during the first 30 s of active standing. It is possible that
the differences between the present and previous [5]
findings result from differences in the choice of orthostatic challenge [20–23]. Alternatively, the rapid (" 2 s)
HUT used in the present study, as opposed to a much
slower (" 6 s) transition from supine to vertical in
previous investigations [5], may offer some explanation
for the novel finding of maintained finger MAP during
orthostatic stress in older men. Conceivably, the reduction in finger AP during orthostatic stress in the older
subjects of Wieling et al. [5] may have resulted from
metabolic vasodilation associated with the prolonged
(active) transition from supine to standing [20].
While the young subjects in the present study had
significantly greater reductions in finger SBP, DBP and
MAP during the first 30 s of 90 m HUT, the subsequent
increases in finger DBP and MAP during the 5 min of
90 m HUT were significantly greater in young than in
older subjects. Indeed, young subjects demonstrated a
sustained ( 6p2 mmHg) overshoot in finger MAP
during the 5 min of 90 m HUT, while finger MAP in the
older men did not differ significantly from that at supine
rest. Tanaka et al. [24] demonstrated that young subjects
with orthostatic intolerance had larger reductions in
finger AP and slower AP recovery during the first 30 s of
standing than healthy age-matched controls. In addition,
abnormal cardiovascular responses during the initial
phase of orthostatic stress have been shown to be
predictive of orthostatic intolerance and other orthostatic
disorders [24,25]. The present finding of increased finger
MAP during prolonged HUT, despite significant immediate reductions in finger MAP, may indicate an
uncoupling of the initial and prolonged cardiovascular
responses during 90 m HUT in healthy young men.
Furthermore, in contrast with the findings of Tanaka et
al. [24] and Wieling et al. [25], the present results
demonstrate that, in healthy young men, acute orthostatic
hypotension may not predispose to clinically abnormal
regulation of orthostatic AP.
It has been suggested that the AP response during
orthostatic stress is influenced by the resting AP [5,26].
Additionally, it has been reported that baroreflex sensitivity is attenuated with advancing age [27], and that the
greatest reductions in AP during orthostatic stress occur
in individuals with the highest resting AP [26]. On the
basis of this evidence, the present finding of maintained
finger MAP during orthostatic stress in healthy older
men is unexpected, given that these individuals were "
50 years older and had a higher resting finger MAP
(95p3 mmHg compared with 84p2 mmHg) than the
younger men. While the maintenance of finger AP during
the initial phase of 90 m HUT differs from the results of
others [5], the finding of smaller increments in HR
during orthostatic stress in older individuals is consistent
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2001 The Biochemical Society and the Medical Research Society
with other studies [4–7,28], and may reflect the reduced
β-receptor sensitivity of older individuals [29] or the
attenuated increase in sympathetic outflow. Despite the
smaller increase in HR, finger MAP was maintained well
during orthostatic stress in the older men. Given that the
reduction in stroke volume during the initial phase of
HUT is reported to be similar in young and older men
[4], these findings suggest that the older men in the
present study had a larger increase in total peripheral
resistance during acute orthostatic stress than the
younger men [5].
The rate and magnitude of central blood volume
reduction determines the cardiovascular response to
orthostatic stress [3]. In addition, the extent of the central
blood volume reduction during orthostatic stress is
dependent on vascular compliance [2]. Elderly individuals demonstrate lower vascular compliance than
young individuals [30], and it has been suggested that the
higher resting AP of elderly individuals is due, at least in
part, to this decrease in vascular compliance [31]. Indeed,
the present finding of higher finger SBP and pulse
pressure in older subjects (Table 2), coupled with their
smaller reflex increases in HR and AP (Figure 1), may
suggest that the young subjects had a higher arterial
vascular compliance than the older subjects. It is possible
that reduced vascular compliance in the older subjects
would limit the reduction in blood volume during
orthostasis, thereby minimizing the reduction in AP, and
hence allaying the need for a reflex increase in HR and an
overshoot of AP [30].
We found that, during 90 m HUT, plasma NA concentrations were significantly increased compared with those
at supine rest in young men, but that there was no
significant change in the older men. These results suggest
that, in healthy young men, the regulation of HR and
finger AP during orthostatic stress is dependent on an
increase in sympathetic outflow. In addition, these results
confirm our previous findings that, in healthy older men,
regulation of finger AP during HUT is mediated through
factors in addition to, or other than, sympathetic outflow
[8]. It is well documented that the arterial baroreceptors
provide the critical regulation of AP during orthostatic
stress [32], and that significant reductions in AP result in
reflex increases in sympathetic outflow [33]. The finding
of unchanged finger AP in response to 90 m HUT in the
older subjects may be the cause or the result of the
unchanged sympathetic outflow (as estimated from
plasma NA concentration) in these individuals. However,
a possible influence of an age-induced reduction in
elastin, coupled with increases in collagen [29], on the
sensitivity of the baroreceptor response must also be
considered. It is possible that the older men experience a
diminished deformation of the carotid and aortic baroreceptors, resulting in limited transmission and processing of afferent information within the central nervous
system. The insignificant changes in NA concentrations
Orthostasis in young and older men
in the older subjects may therefore reflect the absence of
a typical sympathetic efferent response to the upright
posture.
Previous investigators have suggested that the antigravity musculature may play a role in the maintenance
of AP during orthostatic stress [2,3,13]. In the present
study, EMG recordings of the vastus medialis, soleus and
tibialis anterior muscles during 90 m HUT did not differ
significantly from those at supine rest for both the young
and older subjects. These results differ from other recent
investigations that have shown sustained increases in the
EMG activity of the soleus [12], gastrocnemius [11] and
tibialis anterior [12] muscles during 60–75 m HUT. It has
been demonstrated recently that 60 m HUT induces
sustained increases in EMG activity, while 60 m head-up
suspension does not [12]. In the present study, subjects
were secured to the tilt-table with straps applied to the
chest, thigh and lower leg. Therefore it is possible that the
protocol used in the present study closely resembled
head-up suspension, and this may provide some explanation for the insignificant changes in EMG activity
during the orthostatic challenge. The present results do
not discount an influence of intentional muscle contractions and muscle tension on HR and AP responses
during orthostasis [13]. However, these findings demonstrate that, during 90 m HUT, healthy young and older
men have distinct HR and finger AP responses in the
absence of significant differences in lower-limb antigravity muscle activity.
In summary, the results of the present study demonstrate that, when compared with healthy older men,
young men have larger reductions in finger AP during the
initial phase of orthostatic stress. However, during the
prolonged phase of orthostatic stress, older men maintain
resting finger AP, whereas young men demonstrate a
reflex overshoot in finger AP, which corresponds with an
increase in sympathetic outflow. Finally, differences in
lower-limb anti-gravity muscle activation do not account
for the contrasting finger AP responses of healthy young
and older men.
ACKNOWLEDGMENTS
We thank the volunteer subjects for their support of this
project. The technical assistance provided by the Princess
Alexandra Hospital (Brisbane, Australia) in the plasma
catecholamine assay is also appreciated.
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Received 9 March 2000/12 September 2000; accepted 2 November 2000
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2001 The Biochemical Society and the Medical Research Society