Physiologic responses during rest on a sleep system at varied

Transcription

ERGONOMICS, 2002,
VOL.
45,
NO.
11, 798 ± 815
Physiologic responses during rest on a sleep system at varied
degrees of ®rmness in a normal population
RYAN LAHM{ and PAUL A. IAIZZO{*
{Department of Biomedical Engineering, University of Minnesota, MN 55455,
USA
{ Departments of Anesthesiology, Physiology and Surgery, University of
Minnesota, Mayo Mail Code 107, 420 Delaware Street SE, Minneapolis, MN
55455, USA
Keywords: Spinal alignment; Mattress interface pressures; EMG.
This study explores the hypothesis that a high degree of sustained muscle activity
associated with a sub-optimal spinal orientation may compromise an individual’s
ability to relax or initiate sleep. Data from 22 participants who were considered to
be part of a normal, back-pain-free population were used in these studies.
Participants laid down on a mattress in a foetal position (i.e. on their sides) at
three varying bed pressures while EMG activities, heart rates, blood pressures,
subjective comfort levels and spinal alignment data were recorded. Minor eVects
of mattress in¯ation pressures were associated with changes in EMG activity,
heart rate, blood pressure and/or subjective comfort. In contrast, spinal alignment
assessment revealed signi®cant diVerences between the three diVerent in¯ation
pressures studied (827.4, 2413.2 and 3999.0 Pa). It was concluded that in a
population of normal participants, although mattress in¯ation pressure induced
signi®cant changes in spinal alignment, these changes were of little physiological
consequence. Nevertheless, this data provides baseline information needed to
assess similar correlations in a symptomatic population (e.g. those with either
acute or chronic neck or back pain).
1. Introduction
It is widely accepted that the amount and quality of sleep a person gets has direct
eVects on their mood, behaviour, motor skills and overall performance in their work
and leisure life (Brendel et al. 1990, Smith and Maben 1993, Dotto 1996, Meney et al.
1998, Schlesinger et al. 1998). Many people have signi®cant problems with sleep
deprivation and may go to great lengths to ®nd a solution to the problem, since sleep
directly in¯uences quality of life. Often, these problems are due to sleep disorders
such as insomnia or sleep apnoea (Bonnet and Arand 1998). Sleep deprivation can
also be caused by chronic back pain. Since the mattress on which a person lays has a
direct eVect on the person’s comfort level, it follows that the mattress may also
in¯uence someone’s ability to initiate and maintain sleep. Mattresses with diVerent
®rmness characteristics may elicit distinct responses in certain people.
*Author for correspondence. e-mail: iaizz001@tc.umn.edu
Ergonomics ISSN 0014-0139 print/ISSN 1366-5847 online # 2002 Taylor & Francis Ltd
http://www.tandf.co.uk/journals
DOI: 10.1080/00140130210159968
Supine comfort and mattress firmers
799
The hypothesis of this study was that a high degree of sustained muscle activity
associated with a sub-optimal spinal orientation may compromise an individual’s
ability to relax or initiate sleep. The speci®c aims of this study were three-fold: (1) to
study spinal alignment at diVerent mattress in¯ation pressures using video imaging
analysis; (2) to study physiologic responses such as heart rate, blood pressure, and
electrical activity of the back musculature while laying on an air mattress in¯ated to
diVerent pressures; and (3) to identify how interface pressure and area changes as
mattress in¯ation pressure is altered.
This study was performed to gain new insights as to how these responses,
speci®cally spinal alignment, activity of the back musculature, mattress interface
pressure, heart rate and blood pressure, are aVected by the support a given mattress
provides. To obtain varied support pressure under standardized conditions, a sleep
system with an adjustable air bladder (Select Comfort, Plymouth, MN) was used.
2.
Methods
2.1. Participants
The Human Subjects Committee at the University of Minnesota approved this study
protocol. These investigations involved 22 participants (15 males, 7 females) ranging
in age from 20 to 51 years. These participants were selected because they could be
considered to be part of a normal population. Speci®cally, this meant that each
participant had not undergone back surgery within 6 months of the study, had no
sleep disorders, and could be considered to be healthy and without back pain.
2.2. Procedure
Participants took part in only one study session, which is described below. After
signing a consent form, each participant was asked to stand in a relaxed position
with their feet shoulder-width apart and their hands at their sides. While standing,
the participants had small, round stickers placed on their backs to depict the location
of their spinous processes. The stickers ranged from the C7 spinous process down to
T12. Participants then had seven sets of electromyogram (EMG) electrodes placed at
central locations of seven of their major back muscles. The seven major muscles
targeted in this study were: the left and right cervical paraspinals, left and right
trapezius, left and right thoracic paraspinals, and left or right lumbar paraspinals
depending on the side on which the participant was laying (i.e. right lumbar
paraspinals were studied for participants laying on their left side and vice versa; see
®gure 1(a)). Acquisition of EMG signals for later analysis was done using Nicolet
disposable disk surface electrodes (Nicolet Biomedical Inc., Madison, WI). The
signals were ampli®ed and ®ltered with a Grass DC ampli®er (Grass Medical
Instruments, Quincy, MA), and stored on a VCR tape. Once the electrodes were
a xed to the skin and secured with tape, a picture was taken of each participant’s
back as they stood with hands at their sides and feet shoulder-width apart; this
position was taken to represent the participant’s back in a neutral position (®gure
1(b)). These pictures were also used to help to diagnose the presence of scoliosis.
Initial heart rate and blood pressure readings were obtained when the participants
were standing. The participants then were asked to lay in a foetal position on a Select
Comfort TM (Plymouth, MN) air-in¯atable mattress using a pillow (®gure 2). They
were told to lay on the side that was most comfortable for them or the side on which
they typically slept. Each participant’s position on the mattress was such that their
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R. Lahm and P. A. Iaizzo
a
b
Figure 1. (a) Seven back muscles were targeted in this study: left and right cervical
paraspinals, left and right trapezius, left and right thoracic paraspinals, and left or right
lumbar paraspinal. (b) Participant standing with hands at sides and feet shoulder-width
apart.
Figure 2.
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Participants laid in the foetal position on a Select ComfortTM air-in¯atable
mattress.
body was typically centred on the bed in both axes and their head and neck were
oriented parallel with the sides of the bed. Their legs were positioned such that the
angle between the femur and thorax was approximately 1208 and the angle between
the femur and tibia was about 908. The sleep system with an adjustable air pressure
bladder unit (Ultra Series, Select Comfort, Plymouth, MN) was then set to one of the
mattress in¯ation pressure settings (827.4, 2413.2 or 3999.0 Pa). The three pressures
were studied in a random order from participant to participant. These settings were
chosen to represent a low, mid-range, and high mattress ®rmness level. Participants
were asked to minimize movement and relax for 30 min while continuing to lay on
their sides. After 30 min, participants then stood up and walked for approximately
5 min in order to normalize back muscle activity. The same procedure was repeated
for the remaining two in¯ation pressures. Heart rate and blood pressure readings
were taken at the beginning and end of each 30-min trial. EMG recordings were
obtained throughout the duration of each 30-min trial at each of the three bed
pressures. Digital images of the participants’ backs were taken approximately 20 ±
25 min into each of the three trials. Additionally, mattress interface pressure pro®les
were obtained at each of three bed pressures. At the end of the entire experimental
session, participants were asked to give relative subjective comfort information
pertaining to each bed pressure.
2.3. Spinal alignment analysis
The analysis of spinal alignment was done using video image analysis. Although xray images provide more details of the spine, they are associated with known risks
(Lorimore et al. 1978, Archer 1995, Doll 1995, Giver et al. 1995, McGinley and
Miner 1995, Miller 1995, Berlin 1996, Soleo et al. 1996, Nakagawa et al. 1997,
Wingren et al. 1997, Steenvoorde et al. 1998). Since the objective of the study was to
quantify the alignment of the spinous processes and not speci®c anatomical
variations in the detailed structure of the bones that comprise the spine itself, video
image analysis was chosen instead of x-ray analysis. The collection of video images
was also easier and therefore less disturbing to the resting participants.
802
The video images taken during each experiment were acquired with a JVC Digital
Video Camera Model GR-DV1 (JVC, Wayne, NJ); the images were stored on a JVC
Mini Digital Video Cassette. For analysis, the images were downloaded onto a
Macintosh Quadra 840AV (Apple Computer Inc., Cupertino, CA) computer via
Photoshop software (Adobe Systems Inc., San Jose, CA).
To analyse spinal alignment via video images, a region shading process (®gures
3(a) and (b) were chosen. Two diVerent regions were chosen for shading. Region 1
on each image represented the ®rst spinal alignment quanti®cation region. This area
is that which one would obtain by shading the region encompassed by a straight
line drawn between the endpoints of the spinal region of interest (®rst and last
stickers) and the curved line represented by the spine itself. This area represented
the amount the spine was de¯ected in relation to being perfectly straight. It does not
take into account the relationship of the spine to the horizontal axis, which is
parallel to the ¯oor. Region 2 is the second spinal alignment quanti®cation region.
It does take into account the relationship of the spine to the horizontal axis. This
area was obtained by shading the region between the curved line represented by the
spine and the horizontal axis as it passes through the most distal spinous process
(T12).
Once these regions had been shaded using Photoshop, a LabView (National
Instruments, Austin, TX) pixel-counting program was used to count the number of
pixels each area represented. Since the video camera was not always situated the same
distance from each participant, and therefore the pixel resolution would be diVerent
from session to session, a method to normalize area measurements was needed. To
do this, the pixel count for each picture was converted to square millimetres by using
a ruler scale that was captured along with each image of the back.
2.4. Mattress interface pressure pro®le analysis
Mattress interface pressures and areas were analysed using the Ergocheck system
(Version 2.62, ABW GmbH, Hillerse, Germany). This system provided an interface
pressure pro®le using a two-dimensional grid equipped with 5.163.2 cm load cells.
It could also be used to determine maximum pressure, mean pressure and total
interface area measurements. These three parameters were analysed for each of the
three mattress in¯ation pressures for each individual (827.4, 2413.2 and 3999.0 Pa).
For an example of a typical pressure distribution display, see ®gure 4.
2.5. EMG analysis
The stored EMG signals were transferred to a computer and analysed by means of a
data acquisition program written in LabView. This program provided a printout of
each signal on all seven channels (gain = 8) for the duration of each 30-min session,
as well as the associated RMS (root mean square) values. All DC and environmental
60 Hz components of the signal were subtracted. The printouts allowed for visual
assessment of electrical activity that occurred on all channels.
For the majority of each 30-min session, only an electrocardiogram (ECG) trace
could be observed on each channel for all participants and there was an absence of
any muscular activity. However, at intermittent times throughout each session, there
were some easily discernible EMG bursts which signalled the presence of muscular
activity. Visual inspection allowed the isolation of those subjects who exhibited
signal bursts above and beyond the baseline ECG trace. A subset of 16 participants
exhibited this muscular activity. Each channel in the subset was initially analysed
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a
b
Figure 3. (a) Illustration of region 1 spinal alignment quanti®cation measurement acquired
using digital video images. (b) Illustration of region 2 spinal alignment quanti®cation
measurement acquired using digital video images.
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Figure 4. Representation of mattress interface pressure distributions measured using the
Ergocheck system (participant laying on right side).
visually to understand which of the channels showed detectable changes in activity as
each session proceeded. Total RMS values over time were obtained as well; a typical
plot obtained from LabView of EMG signals and associated RMS values for each
channel is provided in ®gure 5. These RMS values were then used to calculate
percentage changes in muscle activity over time. In addition, these values were
compared between in¯ation pressures in order to detect any diVerences.
2.6. Cardiovascular response
In the course of each session, heart rate and blood pressure measurements were also
obtained using an automated blood pressure and heart rate cuV (Accutorr NonInvasive Blood Pressure Monitor, Datascope Corp., Paramus, NJ). In a subset of
eight participants, measurements were only taken while laying down (once at the
beginning and once at the end); for another subset of 10 participants, initial standing
heart rates and blood pressures were taken as well. This allowed the observation of
how the two parameters changed compared to a normalized value.
2.7. Subjective parameters
Subjective comfort was examined using two types of qualitative scales. For a subset
of eight participants, individuals used a scale to rank each of the three bed pressures
according to which was the most comfortable. A second subset of 11 participants
was asked to rate on a scale of 1 to 10 (1 being the least comfortable and 10 being the
most comfortable) their relative comfort for each of the three in¯ation pressures of
the sleep system.
Figure 5.
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Typical EMG readout for participant laying in a foetal position and associated
RMS values. Spikes indicate presence of ECG signal.
2.8. Statistical analysis
One-way analysis of variance (ANOVA) was used through the statistics package
InStat 1.14 (Graphpad Software, Inc., San Diego, CA). Bonferroni correction was
used since multiple sample comparisons were employed. All values are presented as
the mean value+standard deviation.
3. Results
3.1. Participant characteristics
The participant population had a mean age of 25.2+8.7 years of age, mean stature
of 175.5+10.0 cm, and mean body weight of 73.1+16.7 kg. The males (n = 15) had
a mean age of 26.9+10.1 years of age, mean stature of 179.2+9.1 cm and mean
weight of 78.8+16.2 kg. Females (n = 7) had a mean age of 21.6+1.4 years of age,
mean stature of 167.6+7.0 cm, and mean weight of 60.8+10.4 kg.
3.2. Spinal alignment
Statistical analysis was performed on region 1 and region 2 data. Shown in table 1
are the results of these analyses for each participant at each sleep system in¯ation
pressure. The t-test performed on the region 1 data comparing means at each of the
three in¯ation pressures showed that its area decreased with increasing sleep system
in¯ation pressure, but the trend was not statistically signi®cant (p = 0.1125). The
region 1 means for the overall participant population (n = 22) at the sleep system
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Table 1. Spinal alignment for regions 1 and 2.
Region 1 (mm2)
In¯ation pressure (Pa)
Region 2 (mm2)
In¯ation pressure (Pa)
Subject
827.4
2413.2
3999.0
827.4
2413.2
3999.0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
2987.1
4251.6
961.3
3193.5
490.3
2129.0
5122.6
5483.9
316.1
2722.6
3890.3
4825.8
3741.9
1651.6
2206.5
1838.7
1877.4
3303.2
2116.1
1703.2
4509.7
4612.9
851.6
3832.3
361.3
2335.5
2690.3
2038.7
3064.5
3406.5
625.8
1761.3
2819.4
3954.8
3612.9
922.6
1858.1
1219.4
1025.8
3006.5
1580.6
1503.2
3600.0
4374.2
561.3
3658.1
729.0
1980.6
445.2
2219.4
3471.0
3316.1
1638.7
2200.0
1980.6
3980.6
3264.5
877.4
2283.9
1322.6
993.6
3071.0
961.3
1225.8
2735.5
3632.3
9341.9
16580.6
6909.7
2400.0
7425.8
6129.0
7148.4
7451.6
6412.9
5019.4
15058.0
10993.5
5329.0
7735.5
13554.8
14761.3
9858.1
7825.8
8774.2
7748.4
6251.6
12419.3
9967.7
10219.3
5309.7
1709.7
3619.4
4329.0
5438.7
3748.4
4025.8
3122.6
13309.7
9277.4
4045.2
6245.2
10793.5
11677.4
7974.2
7109.7
6206.4
5483.9
4606.4
7619.3
5729.0
9806.4
2038.7
1787.1
3483.9
4271.0
3980.6
2735.5
1935.5
3580.6
13677.4
7703.2
3522.6
6180.6
10683.9
10438.7
7232.2
6354.8
4883.9
5032.3
3522.6
6832.2
in¯ation pressures of 827.4, 2413.2 and 3999.0 Pa were 2903.2+1509.7,
2290.3+1206. 5 and 2116.1+1135.5 mm 2 , respectively. The region 2 data also
showed a similar trend; namely, area decreased with increasing bed pressures. Unlike
the region 1 data, this data set did show statistical signi®cance (p50.01). The region
2 means at the same bladder in¯ation pressures were 8871.0+3632.3, 6632.3+3109.7
and 5703.2+3180.6 mm2 , respectively.
3.3. Mattress interface pressures
Average and maximum mattress interface pressure means were calculated for each
in¯ation pressure. The analysis of maximum pressure data showed no signi®cant
trends. At the lowest in¯ation pressure, the maximum mean was 39.7+7.8 mmHg
(n = 20). At the middle in¯ation pressure, the maximum mean was 38.5+6.8 mmHg,
and at the highest in¯ation pressure, it was 40.7+7.2 mmHg. The same was true of
the average pressure data, which showed a mean of 13.0+0.9 mmHg at the lowest
in¯ation pressure, 13.0+1.1 mmHg at the middle in¯ation pressure, and
13.0+1.1 mmHg at the highest in¯ation pressure. Mattress interface area revealed
no trends. The mean was 0.4+0.1 m2 at each in¯ation pressure.
3.4. EMG
Initial visual analysis was completed and the percentage of each 30-min session that
contained signi®cant muscular activity (activity greater than baseline) for each muscle
at each bed pressure was assessed. To illustrate this method, refer to ®gure 5, which
shows all seven channels of EMG traces for 1 min of an experimental session. One
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can use channels 1 and 2 as examples. A close look at channel 1 will show an EMG
burst from the beginning to the end. Therefore, the percentage of signi®cant muscular
activity for this time span is 100%. In contrast, channel 2 contains only a short burst
of EMG activity (above the baseline ECG trace which appears as periodic spikes on
the output) approximately halfway through this time period. This burst lasts about
5 s. This channel shows approximately 8% signi®cant muscular activity over this time
period since the overall time span is 60 s. In general, for these normal participants,
visual assessment provided little or no correlative changes in EMG activity in any of
the muscles studied. Although there were changes in activity throughout each session,
no trends were found when comparing the three in¯ation pressures.
In order to conduct a more quantitative analysis of EMG activity over time at
each mattress in¯ation pressure, average RMS values were calculated at 5-min
intervals for each 30-min session at each in¯ation pressure. Table 2 gives a
representation of these RMS values at the mattress in¯ation pressure of 827.4 Pa.
The values are given for the ®ve participants showing the greatest changes in
muscular activity as determined by the previously described visual analysis.
Once these values were calculated for each in¯ation pressure, a total RMS was
also calculated for the entire 30-min session. Tables 3(a), (b) and (c) show the
percentage change of each 5-min session in relation to the total RMS value at each
in¯ation pressure.
For many of the sessions, the ®rst and last 5-min intervals typically showed
greater activity than the intermediate intervals. This is true because of the nature of
the protocol. The participants, after initially laying down on the mattress, usually
took a few moments to get comfortable. This would include a period of time where
they would `toss and turn’ to readjust their position on the mattress. As a result,
large bursts of activity would be seen during this time. However, these bursts were
not directly due to the overall comfort of the mattress itself. The greater activity
shown in the last 5 min can be explained by the fact that toward the end of each
session, participants were alerted to the fact that the session was coming to an end,
sometimes causing them to stir. This activity is also clearly not linked to any
physiological response to the mattress.
The primary objective of this particular EMG analysis was to understand how
muscular activity changes over time and if any diVerences in activity exist between
the three mattress in¯ation pressures. In order to quantify the changes in muscle
activity over time, two similar analyses were employed.
In the ®rst analysis strategy, all the RMS values for each 5-min interval were
averaged for the ®ve most active participants and were then compared to one
another. This analysis explored the possibility of signi®cant changes in muscle
activity over time at a given mattress in¯ation pressure. When the interval RMS
values were compared, it was found that signi®cant diVerences were present.
However, when the ®rst and last intervals were omitted from calculations, due to the
reasons explained above, these diVerences lost signi®cance. The results of the initial
comparisons showed p-values of 50.0001 at 827.4 Pa, 50.0009 at 2413.2 Pa and
50.0008 at 3999.0 Pa. The results of the second comparisons after deletion of the
®rst and last interval values showed p-values of 0.3261 at 827.4 Pa, 0.7991 at 2413.2
Pa and 0.9931 at 3999.0 Pa.
The second analysis strategy used an aggregate percentage change for each
participant in order to evaluate muscle activity change over time. This was done by
averaging each individual percentage change value for a participant in order to get
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Table 2.
RMS values at mattress in¯ation pressure of 827.4 Pa.
Muscle
Minutes
0±5
Minutes
6 ± 10
Minutes
11 ± 15
Minutes
16 ± 20
Minutes
21 ± 25
Minutes
26 ± 30
1
LCP
RCP
LT
RT
LTP
RTP
RLP
20.9
37.4
15.7
28.8
22.8
37.2
29.6
14.4
6.7
14.9
15.2
19.2
28.7
21.0
32.2
9.8
15.5
13.5
19.7
35.3
16.0
33.5
6.6
18.5
10.4
19.0
27.8
14.5
43.0
7.1
12.7
7.6
19.0
25.1
14.1
58.4
8.1
9.9
9.7
19.3
27.2
14.2
2
LCP
RCP
LT
RT
LTP
RTP
RLP
28.2
101.0
23.8
44.5
45.2
45.1
40.3
11.0
15.8
16.9
58.7
30.8
41.9
122.2
11.5
13.4
40.1
80.9
26.5
29.8
16.2
9.8
11.1
13.0
36.4
25.5
20.6
15.8
8.2
8.6
2.5
3.3
25.3
20.4
15.7
45.5
52.0
69.1
132.7
58.6
64.5
74.8
3
LCP
RCP
LT
RT
LTP
RTP
LLP
33.5
23.6
13.7
24.9
28.7
68.7
58.7
25.0
17.4
4.6
8.4
21.5
77.2
63.8
31.2
32.8
13.2
8.3
21.3
67.2
56.6
25.2
15.9
4.1
1.7
20.4
73.3
60.5
24.4
15.0
3.7
1.5
19.9
71.8
61.0
80.3
65.4
66.6
63.5
76.7
76.0
62.8
4
LCP
RCP
LT
RT
LTP
RTP
LLP
37.3
57.7
20.1
28.3
39.6
26.9
34.3
35.9
36.2
2.9
8.0
14.2
8.4
19.0
37.9
100.6
6.1
34.3
14.4
8.8
18.7
36.2
50.5
3.3
14.8
14.3
8.9
18.8
38.6
68.8
5.0
27.5
14.4
8.8
18.6
33.8
39.8
3.0
7.5
14.3
8.6
18.6
5
LCP
RCP
LT
RT
LTP
RTP
RLP
25.2
32.2
11.9
24.7
55.5
14.5
22.7
27.5
30.2
10.1
26.7
55.0
12.3
25.9
28.9
29.8
8.3
25.4
54.1
11.3
25.9
29.2
28.6
6.4
24.4
53.3
10.1
27.1
30.0
26.6
5.2
24.1
52.5
9.8
27.5
54.1
65.5
34.6
37.2
58.3
29.5
41.0
Subject
RMS = root mean square; LCP = left cervical paraspinal; RCP = right cervical paraspinal; LT = left trapezius; RT = right trapezius; LTP = left thoracic paraspinal; RTP = right
thoracic paraspinal; LLP = left lumbar paraspinal; RLP = right lumbar paraspinal.
one value that represented the percentage change over time for each in¯ation
pressure. This single value was then compared between each in¯ation pressure for
the same ®ve participants used in the previous analysis. The comparison results
showed that no signi®cant changes were apparent between in¯ation pressures. The
total mean percentage change value at low in¯ation pressure was 28.7+9.9%, at
medium in¯ation pressure it was 28.4+9.8%, and at high in¯ation pressure it was
25.5+8.2%.
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Table 3(a). Percentage change (%) of each 5-min session in relation to total RMS value at
827.4 Pa in¯ation pressure.
Muscle
Session
RMS
mean
1
LCP
RCP
LT
RT
LTP
RTP
RLP
33.8
12.6
14.5
14.2
19.8
30.2
18.2
738.1
66.3
7.5
50.7
13.2
18.7
38.5
757.2
747.0
2.2
6.7
3.3
75.0
13.2
74.7
722.3
6.0
74.9
70.8
14.5
712.3
70.6
747.9
21.5
727.0
74.2
78.1
720.7
21.6
743.8
712.5
746.6
74.2
716.8
722.6
42.2
735.8
732.2
731.5
72.8
710.0
722.0
2
LCP
RCP
LT
RT
LTP
RTP
RLP
19.0
33.6
27.6
59.4
35.3
37.1
47.5
32.5
66.7
713.8
725.0
21.9
17.9
715.2
742.2
753.1
738.5
71.3
712.8
11.6
61.1
739.7
760.2
31.3
26.6
724.8
719.5
765.9
748.7
767.1
752.9
738.7
727.7
744.5
766.7
756.9
774.4
791.1
794.5
728.4
744.9
767.1
58.2
35.3
760.2
55.2
39.7
42.6
36.5
3
LCP
RCP
LT
RT
LTP
RTP
LLP
36.6
28.4
17.7
18.1
31.4
36.2
60.6
78.4
716.9
722.3
27.5
78.7
75.1
73.0
731.7
738.5
773.8
753.2
731.6
6.3
5.1
714.8
13.5
725.0
754.0
732.0
77.1
76.6
731.2
743.9
776.9
790.6
735.1
1.3
70.2
758.9
747.0
779.3
791.5
736.6
70.9
0.7
54.4
56.6
73.5
71.6
59.0
4.8
3.6
4
LCP
RCP
LT
RT
LTP
RTP
LLP
36.6
59.0
6.7
20.1
18.5
11.7
21.3
1.8
72.1
66.6
29.0
53.1
56.4
37.8
72.0
738.5
756.3
760.0
723.2
728.5
710.9
3.4
41.4
710.0
41.5
722.3
725.2
712.1
71.2
714.3
751.6
726.1
723.0
724.0
711.9
5.2
14.3
725.3
27.1
722.3
725.2
713.0
77.6
732.4
755.7
762.4
722.8
726.8
712.9
5
LCP
RCP
LT
RT
LTP
RTP
RLP
32.5
35.5
12.8
27.1
54.8
14.6
28.4
722.5
79.2
77.0
78.7
1.3
70.4
720.0
715.4
715.0
720.6
71.3
0.5
715.4
78.5
711.1
715.9
735.0
76.2
71.2
722.6
78.7
710.1
719.5
749.7
710.0
72.8
730.6
74.3
77.6
725.0
758.9
710.9
74.2
733.1
73.1
40.0
45.8
63.1
27.1
6.0
50.5
30.8
Subject
Minutes Minutes Minutes Minutes Minutes Minutes
0±5
6 ± 10
11 ± 15
16 ± 20
21 ± 25
26 ± 30
3.5. Cardiovascular responses
Heart rate and blood pressure mean values were compared and showed no
signi®cant trends. Heart rate means for mattress in¯ation pressures of 827.4, 2413.2
and 3999.0 Pa were 68.2+9.5, 67.8+9.6 and 69.0+10.1 beats/min, respectively.
Blood pressure data showed the following: in order of increasing mattress in¯ation
pressure, the mean values were 96.4+10.6, 94.6+10.3 and 94.4+7.9 mmHg.
810
Table 3(b). Percentage change (%) of each 5-min session in relation to total RMS value at
Muscle
Session
RMS
mean
1
LCP
RCP
LT
RT
LTP
RTP
RLP
12.0
5.0
19.6
20.3
24.9
37.6
68.2
46.9
70.8
28.2
35.4
4.8
41.2
56.0
727.7
774.6
741.8
768.2
728.6
753.9
736.4
718.0
776.0
752.6
734.5
727.4
742.2
736.6
730.3
711.0
762.0
737.6
749.0
739.4
735.9
734.3
785.0
768.2
783.4
733.8
757.8
735.5
18.0
3.3
65.0
62.0
53.5
55.2
14.4
2
LCP
RCP
LT
RT
LTP
RTP
RLP
14.7
11.5
40.1
29.8
16.0
35.2
9.0
37.1
41.6
21.9
714.1
37.0
19.3
55.5
72.9
78.9
17.9
21.5
75.5
16.1
715.4
711.6
726.8
1.5
33.9
79.5
74.0
726.3
75.7
741.3
35.2
23.2
714.0
721.6
727.1
726.3
739.1
741.3
738.9
711.8
721.3
729.4
718.5
31.1
764.3
755.7
718.0
3.6
726.9
3
LCP
RCP
LT
RT
LTP
RTP
LLP
34.6
11.0
8.0
5.3
26.2
26.3
9.3
19.9
41.6
53.6
63.9
17.1
1.8
17.3
710.2
715.0
725.4
740.3
714.6
0.3
3.2
2.7
715.8
735.0
729.7
78.5
70.7
72.7
74.3
712.7
714.7
737.1
74.7
5.5
70.1
7.2
72.3
4.5
715.1
15.2
72.2
77.5
720.9
725.3
745.4
754.8
710.8
75.1
713.2
4
LCP
RCP
LT
RT
LTP
RTP
LLP
44.4
50.7
6.8
6.1
18.9
20.0
82.3
10.1
33.6
71.8
75.2
44.0
49.8
70.0
1.6
70.6
751.0
759.7
717.4
715.9
70.3
1.2
77.3
751.4
761.5
716.6
721.5
0.3
72.0
710.6
750.8
761.4
714.8
720.7
0.0
75.1
714.8
749.9
758.1
714.5
719.2
70.1
77.1
717.1
751.1
762.8
715.4
721.8
0.1
5
LCP
RCP
LT
RT
LTP
RTP
RLP
20.4
33.7
13.1
28.1
18.1
22.3
45.3
37.6
41.5
33.7
24.0
20.0
33.3
22.8
79.3
26.7
763.4
724.0
7.6
725.8
71.5
711.4
8.8
769.6
730.9
73.5
747.8
712.8
712.0
716.9
771.9
734.6
711.4
753.2
723.4
71.7
753.7
770.8
733.3
714.6
740.9
731.5
18.8
746.3
69.2
47.7
73.8
54.1
28.4
Subject
0±5
6 ± 10
11 ± 15
16 ± 20
21 ± 25
26 ± 30
Statistical analysis was also done on the percentage decrease of heart rate and the
percentage decrease of blood pressure relative to values determined during standing;
these data were also not signi®cantly altered with diVerent mattress in¯ation
pressures. The following is data for the percentage decrease of heart rate: at 827.4 Pa,
the mean value was 11.7+18.1%, at 2413.2 Pa the mean value was 10.5+21.6% and
at 3999.0 Pa it was 9.0+21.3%. Concerning the percentage decrease in blood
811
Table 3(c). Percentage change (%) of each 5-min session in relation to total RMS value at
Muscle
Session
RMS
mean
1
LCP
RCP
LT
RT
LTP
RTP
RLP
36.8
28.4
12.2
23.9
21.5
48.1
61.6
48.8
35.2
13.3
41.9
15.6
716.0
0.6
760.8
754.9
752.0
769.2
722.1
711.4
722.2
745.8
741.9
765.1
728.1
1.4
715.3
712.4
712.7
738.0
765.9
738.3
724.5
753.4
76.8
737.0
738.4
767.2
761.3
2.7
39.5
73.5
37.8
54.3
70.1
55.5
19.2
23.6
30.6
2
LCP
RCP
LT
RT
LTP
RTP
RLP
14.8
13.9
37.0
41.4
27.2
28.3
13.2
44.8
11.0
11.4
8.5
10.3
26.8
732.8
79.3
74.4
720.8
729.2
3.9
14.5
0.6
729.4
765.0
78.1
757.0
77.6
753.7
726.0
715.7
721.9
730.9
21.7
70.8
41.3
76.2
717.4
723.8
20.5
17.4
74.4
733.2
717.3
79.3
50.7
17.5
22.0
72.9
71.6
70.2
3
LCP
RCP
LT
RT
LTP
RTP
LLP
37.9
21.6
11.9
9.5
65.0
31.4
11.1
710.5
72.1
74.6
1.0
9.6
0.8
16.2
715.5
726.8
758.1
778.3
4.4
2.5
71.5
717.9
723.9
753.7
771.7
76.2
10.8
79.3
5.6
6.7
25.1
731.0
77.5
0.4
77.4
713.8
726.3
762.5
780.5
712.9
711.7
76.8
34.1
41.9
59.2
66.3
10.2
74.1
5.6
4
LCP
RCP
LT
RT
LTP
RTP
LLP
16.7
89.2
14.7
9.7
18.7
4.5
65.0
741.7
8.0
739.5
717.2
711.5
59.5
75.6
7.9
5.3
775.4
778.4
730.3
760.4
75.2
751.5
72.3
711.5
4.7
79.4
730.2
3.6
759.2
74.3
764.1
778.8
735.8
751.2
4.0
758.4
75.3
764.0
780.0
736.4
757.5
3.1
18.6
72.4
73.6
71.4
55.3
34.6
70.2
5
LCP
RCP
LT
RT
LTP
RTP
RLP
36.5
24.3
56.3
82.5
13.9
21.0
61.3
64.7
741.5
12.0
7.3
1.4
9.9
18.2
762.4
722.2
13.2
4.9
0.8
8.4
13.5
767.0
79.5
71.0
3.7
0.4
6.4
4.6
764.5
77.8
1.5
71.2
70.2
71.4
77.2
765.1
72.4
76.4
75.7
70.9
76.7
714.2
43.0
45.5
713.7
710.1
71.7
76.8
715.8
Subject
0±5
6 ± 10
11 ± 15
16 ± 20
21 ± 25
26 ± 30
pressure, the means with increasing mattress in¯ation pressure were 22.7+4.9,
24.4+6.0 and 25.1+6.3%.
3.6. Subjective comfort parameters
With regard to the comfort ranking procedure done on the subset of eight
participants, no statistical trends were determined as to any preference toward
812
speci®c mattress ®rmnesses; however, participants did tend to prefer the mid-range
bed pressures to the two outer extremes. The second analysis using the 0 ± 10 comfort
scale showed that participants preferred the middle mattress in¯ation pressure to the
extremes, but these ®ndings were not statistically signi®cant. The means (0 ± 10 scale)
in order of increasing in¯ation pressure were 6.8+1.8, 7.1+1.3 and 6.4+1.6.
4. Discussion
The ®rst aim de®ned for this study was to investigate spinal alignment at diVerent
mattress in¯ation pressures using video image analysis. Such analysis provided a
very quick, e cient, and safe alternative to x-rays for studying spinal alignment. Xrays, although more detailed, take quite a signi®cant amount of time to acquire and,
as was mentioned earlier, can pose a danger to participants.
The data acquired from the video images were shown to be quite signi®cant.
They showed that the spine was in much better alignment at mid-range and higher
mattress in¯ation pressures. These results suggest that people in the normal
population tend to be more comfortable at higher mattress in¯ation pressures.
However, this may only apply to the normal population. It is possible that if one
were to look at spinal alignment responses in a symptomatic population, completely
diVerent reactions may be observed.
It should also be noted that participants were laying with their heads on a pillow,
and this most likely has a signi®cant eVect on spinal alignment. Most people use
pillows when they sleep at night, and therefore pillows were used as a standard. It is
true that some people may be more comfortable without a pillow, and this may be a
cause for poorer or better spinal alignment than with a pillow.
The second aim of this study was to study physiological responses to laying on an
air mattress at diVerent in¯ation pressures. It was found that for the parameters of
heart rate, blood pressure, and EMG activity of the back musculature few, if any,
trends were present. In particular, the results obtained from the EMG analysis were
a bit unexpected. It was originally hypothesized that when the spine is in an
abnormal alignment while laying on a mattress, the activity of the back musculature
will signi®cantly increase due to discomfort. This was clearly not what was observed
in this study. While spinal alignment did ¯uctuate quite a bit between mattress
in¯ation pressures, EMG activity did not follow suit.
The third and ®nal aim was to identify how interface pressures and areas change
as mattress in¯ation pressure is altered. It was clearly seen that redistribution of
interface pressures occurred for all participants when mattress in¯ation pressures
were changed. Although these redistribution trends were observed, no signi®cant
diVerences in interface pressures relative to each mattress in¯ation pressure setting
were found. Thus, it was concluded that the normal participant population does not
have characteristic biases with regard to mattress interface pressures and areas
relative to mattress in¯ation pressures.
The authors’ colleagues at Select Comfort have observed that the Ergocheck
system is quite sensitive to body positioning. Future studies must take into account
how diVerent body positioning may aVect interface pressure distributions. Normalization of body positioning on the mattress is important for decreasing variability of
measurements.
It is clear from the calculations that the measurements studied in this population
are highly variable, especially when analysing EMG and spinal alignment data. This
would suggest that in order to more con®dently compare measurement diVerences
813
between interface pressures, one should recruit a larger population. As further work
is done in this area, it will be critical for researchers to be aware of this requirement.
The issues addressed in this study are largely ignored in the current literature.
This fact is surprising considering how much the comfort of a person’s mattress can
aVect how well one sleeps and, consequently, the quality of daily life.
One study was done with a software package that does assessments of spinal
deformities based on three-dimensional images (Vandegriend et al. 1995). This
system is typically used by clinicians to assess symptomatic conditions.
Although there are many studies that deal with EMG measurements of back
muscles, there were none found that had similar intentions of this study, namely,
using EMG measurements to assess diVerences in comfort between diVerent
mattresses and mattress in¯ation pressures.
Another report was aimed at ®nding diVerences in contact pressures and
subjective comfort ratings between diVerent types of mattresses (Buckle and
Fernandes 1998). It found no signi®cant diVerences between contact pressures
analysed on diVerent mattresses, or subjective comfort ratings versus these contact
pressure measurements. The report concluded that subjective comfort factors are
probably dependent on variables other than contact pressures. It should be noted
that the Buckle and Fernandes study (1998) was not only done with an all-female
participant population, but the participants were also tested while laying supine as
opposed to in the foetal position. Both this and the previously discussed studies seem
to support the idea that contact pressures are not eVective indicators of certain
diVerences between mattress characteristics, including comfort.
There are limitations of the current study that should be accounted for in future
work. For example, it might be bene®cial to recruit a participant population that has
a more equally distributed age pro®le, as it may be possible that age distribution
could have an eVect on certain characteristic outcomes. Also, instead of having three
signi®cant bed pressures tested, the protocol could include ®ve or more diVerent
mattress pressures, thus allowing for a more detailed analysis of the mattress
in¯ation pressure distribution.
Another possible enhancement would be to study additional muscles in the back
and/or utilize additional methodologies for assessing changes in EMG activities (e.g.
integrated activity or power spectral analysis) (Roy et al. 1995, Ng and Richardson
1996, Mannion et al. 1997, Greenough et al. 1998, Kankaanpaa et al. 1998, Peach
and McGill 1998, Roy and Oddsson 1998). The muscles chosen for this study were
selected because it was thought that they would be the ones most likely to elicit
responses associated with unfavourable spinal alignments. The fact that little
activation was seen from changing spinal alignments does not exclude the possibility
that there are other muscles aVected by compromised spinal alignment. Nevertheless,
such investigation may be more insightful when direct comparisons are made
between normal participants and those suVering from back pain.
It has been suggested that to conduct this as a sleep study would give a better
insight into the comfort of the mattress. This may involve measuring `time to initiate
sleep’ as well as `time to sleep termination’. Finally, this study assumed that most
people sleep on their sides; this may not always be the case. It could be that more
people sleep on their backs than anticipated. In these cases, it would be bene®cial to
design the study such that participants would lay supine rather than on their sides.
A bene®t of this work is the development of a novel approach to quantify the
curvature of the spine. Past studies have used the Cobb method to accomplish this
814
type of measurement (Ylikoski and Tallroth 1990, Janke et al. 1997). Although the
Cobb method for spinal curvature measurement is accepted as accurate, investigations have found this method to exhibit fairly signi®cant intra-observer variability
(Morrissy et al. 1990, Diab et al. 1995). Although intra-observer variability was not
measured in the current study, it is possible that the `area under the curve’ method
may give better results with regard to intra-observer variability. Even if there is no
signi®cant decrease in intra-observer variability, this new method also has the other
bene®ts mentioned earlier, including: less patient risk due to x-rays, less time to
acquire images, easier implementation, and greater cost eVectiveness.
5. Conclusions
It was shown that a normal participant population elicited a wide variety of
responses regarding a number of diVerent physiological parameters. Since the
participant population was normal, it is somewhat intuitive that such results were
obtained. This quantitative approach to analysis in which responses of participants
are assessed relative to diVerent mattress in¯ation pressures, has strong potential to
gain new insights into supine comfort on a mattress and its in¯uence on an
individual’s ability to relax or initiate sleep. It is suggested that the next step to
continue with this research approach is to recruit a symptomatic patient population
of people with chronic back pain and/or scoliosis. The present study serves as an
observation of baseline data that will be important when studies are done involving a
symptomatic population.
Acknowledgements
The authors wish to thank Select Comfort for their support as well as the use of their
equipment and also Gary Williams for his help with computer issues.
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Physiologic responses during rest on a sleep system at varied

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