J Appl Physiol 101: 771–777, 2006.
First published June 15, 2006; doi:10.1152/japplphysiol.00644.2005.
Ambulation in simulated fractional gravity using lower body positive
pressure: cardiovascular safety and gait analyses
Adnan Cutuk,1 Eli R. Groppo,1 Edward J. Quigley,2
Klane W. White,1 Robert A. Pedowitz,1 and Alan R. Hargens1
1
University of California, San Diego, Department of Orthopaedic Surgery; and 2Children’s
Hospital and Health Center, San Diego, Motion Analysis Laboratory, San Diego, California
Submitted 31 May 2005; accepted in final form 18 May 2006
Cutuk, Adnan, Eli R. Groppo, Edward J. Quigley, Klane W.
White, Robert A. Pedowitz, and Alan R. Hargens. Ambulation in
simulated fractional gravity using lower body positive pressure: cardiovascular safety and gait analyses. J Appl Physiol 101: 771–777, 2006.
First published June 15, 2006; doi:10.1152/japplphysiol.00644.2005.—
The purpose of this study is to assess cardiovascular responses to
lower body positive pressure (LBPP) and to examine the effects of
LBPP unloading on gait mechanics during treadmill ambulation. We
hypothesized that LBPP allows comfortable unloading of the body
with minimal impact on the cardiovascular system and gait parameters. Fifteen healthy male and female subjects (22–55 yr) volunteered
for the study. Nine underwent noninvasive cardiovascular studies
while standing and ambulating upright in LBPP, and six completed a
gait analysis protocol. During stance, heart rate decreased significantly from 83 ⫾ 3 beats/min in ambient pressure to 73 ⫾ 3 beats/min
at 50 mmHg LBPP (P ⬍ 0.05). During ambulation in LBPP at 3 mph
(1.34 m/s), heart rate decreased significantly from 99 ⫾ 4 beats/min in
ambient pressure to 84 ⫾ 2 beats/min at 50 mmHg LBPP (P ⬍ 0.009).
Blood pressure, brain oxygenation, blood flow velocity through the
middle cerebral artery, and head skin microvascular blood flow did
not change significantly with LBPP. As allowed by LBPP, ambulating
at 60 and 20% body weight decreased ground reaction force (P ⬍
0.05), whereas knee and ankle sagittal ranges of motion remained
unaffected. In conclusion, ambulating in LBPP has no adverse impact
on the systemic and head cardiovascular parameters while producing
significant unweighting and minimal alterations in gait kinematics.
Therefore, ambulating within LBPP is potentially a new and safe
rehabilitation tool for patients to reduce loads on lower body musculoskeletal structures while preserving gait mechanics.
exercise; cranial circulation; unloading; fractional body weight;
ground reaction force; motion analysis; joint range of motion; stride
length
AFTER INJURY OR SURGERY to the lower extremities, patients are
often unable to walk while supporting their entire body weight
(BW). This may be due to pain, weakness, instability, or a
requirement to protect surgically repaired tissues and implanted prostheses. However, immobilization causes muscle
and bone loss, particularly in the lower limb (4). Furthermore,
elderly patients are at heightened risk of injury or reinjury upon
reambulation (8, 22). Because early mobilization and rehabilitation are highly desirable during the immediate postoperative
period, elderly patients require the assistance of devices to
decrease forces acting on the joints, bones, muscles, and
ligaments of the lower extremities. Devices that are currently
used for unloading of the lower extremities during rehabilita-
Address for reprint requests and other correspondence: A. R. Hargens, Dept.
of Orthopaedic Surgery, Univ. of California, San Diego, 350 Dickinson St.
#121, MC 8894, San Diego, CA 92103-8894 (e-mail: ahargens@UCSD.edu).
http://www. jap.org
tion include walkers, parallel bars, therapist-assisted waist
belts, overhead suspension harnesses, and swimming pools (1,
26). All of these have disadvantages associated with their use
that interfere with patients’ rehabilitation and call for development
of the novel rehabilitation techniques. Optimally, rehabilitation
protocols should promote normal joint ranges of motion (ROMs)
(30). However, many of the gait deviations observed in neurologically impaired patients result from their inability to bear weight
adequately through their affected lower extremities during the
loading phase of the gait cycle (5, 11, 34).
A new device for loading and unloading lower extremities
while walking and running has been developed by our laboratory (19). This device consists of a treadmill enclosed within a
waist-high chamber. A seal is achieved between the patient and
the chamber through the use of a neoprene kayak-type skirt that
fastens over a lip in the aperture opening of the chamber. For
unweighting, the pressure inside the chamber is increased
above the external, ambient (atmospheric) pressure using an air
compressor. This pressure differential causes an axial buoyant
force that is proportional to the pressure differential multiplied
by the cross-sectional area of the patient’s body at the waist
seal (19), similar to a “piston effect.” The net ground reaction
force (GRF) then becomes the difference between the patient’s
weight and this axial force. In this way elderly and postsurgery
patients can ambulate on the treadmill under simulated fractional gravity.
Safety evaluation and studies of healthy individuals are
necessary before the use of the LBPP rehabilitation concept by
elderly patients with potential cardiovascular comorbidities.
The purpose of this study is to 1) assess the impact of lower
body positive pressure (LBPP) on the cardiovascular system,
specifically on head perfusion, heart rate (HR), and blood
pressure changes in healthy subjects standing and ambulating
on a treadmill; and 2) examine the effects of LBPP unloading
on gait mechanics during treadmill ambulation. We hypothesize that HR, blood pressure, brain blood flow velocity, head
perfusion, and brain oxygenation do not change significantly
during upright standing and treadmill ambulation in LBPP. We
further hypothesize that GRF decreases linearly with LBPP,
whereas gait mechanics remain unaffected.
MATERIALS AND METHODS
Our study was divided in two parts, first testing our hypothesis on
cardiovascular safety of LBPP and, second, focusing on LBPP unweighting effects and its influence on walking and running gait. The
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subjects were healthy nonsmokers and none took any medication at
the time of the study. All subjects came to the laboratory without any
restrictions to their diet and fluid intake. Before testing, subjects were
informed about all aspects of the experiment and familiarized with all
measurement devices. All subjects provided informed, written consent. The protocol was approved by the University of California, San
Diego Human Subjects Institutional Review Board.
Subjects wore neoprene shorts with a kayak-type skirt attached at
the level of the waist. Subjects then stepped into the LBPP chamber
and a comfortable seal was achieved by securing the neoprene skirt
over a lip in the top of the chamber. The compliance properties of the
skirt were such that subjects were able to walk and run comfortably on
an electronically controlled treadmill (model Q65, Quinten Instruments, Seattle, WA) without leaning on the seal for support. Seal
height was adjusted level with each subject’s iliac crest, so that the
seal itself exerted little or no vertical force in ambient pressure or
LBPP. In cardiovascular part of the study, pressure in the chamber
was applied in graded manner. In the gait analysis part of the study
pressure was adjusted in graded manner until the desired level of
unloading was achieved. The subject’s total net GRF was composed
of BW reduced by the axial upward force produced by the pressure
differential between inside and the outside of the chamber acting
across the body cross-sectional area at the waist level (Fig. 1).
In the cardiovascular safety component of our study, nine healthy
subjects (6 men, 3 women; age 22–55 yr) underwent testing protocol
on 2 separate days. The average weight of the subjects tested was
83.8 ⫾ 4.3 kg. On day 1, subjects stood on a spring weighing scale
(Sunbeam Products, Boca Raton, FL), and their BW at ambient
pressure was measured and recorded. LBPP was then applied in a
graded manner at 10, 20, 30, 40, and 50 mmHg with the subjects in
stationary standing position. On the basis of our pilot data, each level
of LBPP was maintained for 1 min before data collection to allow
subjects to acclimatize to the reduced gravity and achieve physiological steady state but with essentially no time interval between the
different levels. We found that all recorded parameters reached steady
state within this time frame. Data collected in stationary position
included GRF and forehead skin microvascular blood flow, measured
with laser (Doppler flow meter blood perfusion monitor 403A, Laserflo, Vasamedics, St. Paul, MN). Subjects then exited the chamber
and returned later for additional testing. This allowed their cardiovascular parameters to normalize in ambient pressure before additional
testing.
On day 2 of testing, subjects first stood in the chamber while the
LBPP was applied at graded levels of 10, 20, 30, 40, and 50 mmHg.
Identical to day 1, each LBPP level was maintained for 1 min before
data collection and with no delay between the different levels. Each
subject’s HR, systolic blood pressure (SBP), diastolic blood pressure
(DBP), brain blood flow velocity, and brain tissue oxygenation were
recorded. HR was measured by use of a four-lead ECG monitor
(model Q6500, Quinten Instruments), whereas SBP and DBP were
measured utilizing a manual cuff applied to the left arm. Near-infrared
spectroscopy continuously monitored brain tissue oxygenation under
temporal bone (Run-Man CWS-2000, NIM). Brain blood flow velocity through the middle cerebral artery was measured with transcranial
Doppler ultrasound (Transpect TCD, Medasonics, Mountain View,
CA). To minimize data collection variability, one experimenter obtained all the blood pressure measurements for all the subjects, and
another experimenter collected head perfusion, vascular flow velocity,
and oxygenation data.
After these measurements, the weighing scale was removed from
the chamber. The treadmill was started and subjects were instructed to
walk at constant speed of 3 mph (1.34 m/s). The LBPP was applied at
0, 25, and 50 mmHg (order randomized). One minute was allowed for
acclimatization to LBPP before measurements of HR and blood
pressure. Brain blood flow velocity, microvascular flow, and brain
tissue oxygenation were not measured during the dynamic part of the
test because of large motion artifacts created by an ambulating
subject.
Six healthy adult men participated in the unweighting and gait
analysis part of the study. The mean age of the subjects was 30 yr
(range ⫽ 22– 42), mean height was 1.86 m (range ⫽ 1.78 –1.98), and
mean weight was 91.2 kg (range ⫽ 76.2–108.4). All subjects were
relatively active and had no known gait abnormalities. A spring
weighing scale was placed under each subject, and the chamber
pressures needed to generate specific levels of unloading (100, 60, and
20% bodyweight) were determined. The scale was then removed and
subjects walked at 3 mph (1.34 m/s) and ran at 6 mph (2.68 m/s) while
the chamber pressure was adjusted to achieve each of the three BW
conditions. Conditions were performed in randomized order for all
subjects. It was previously demonstrated that walking kinematics
rapidly adapt to simulated reductions in gravity (12); consequently, all
data were acquired after 1 min of exercise to allow subjects time to
acclimatize to the reduced gravity.
Kinematic parameters included GRF; knee, and ankle sagittal
ROM; and stride length. Along with walking exercise, running was
explored from a comparison standpoint as well as to provide a
baseline for future rehabilitation of competitive athletes. The vertical
component of GRF was collected bilaterally at 2 kHz by using
force-sensitive insoles (Electronic Quantification, 600 Galahad Road,
Plymouth Meeting, PA) placed in each subject’s shoes. Force-sensitive insole systems have good correlation and small root mean square
error compared with force plate systems (6). Force pads were calibrated using a standard spring scale and a two-point calibration at 0
and 100% static BW. GRF signals were smoothed with a 50-ms
Fig. 1. Schematic diagram of the lower body positive
pressure (LBPP) treadmill chamber. A buoyant force
(F1) equal to the pressure differential (⌬P) between the
pressure inside the chamber (P1) and outside the chamber (P2) multiplied by the cross-sectional area of the
body and the flexible waist seal (Axy) counteracts the
downward gravitational force (Mg), thus reducing the
net vertical ground reaction force (GRF). During ambulation, an inertial force (Maz,CM) equal to the mass of
the individual multiplied by the body’s acceleration also
contributes to the net GRF.
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LBPP AMBULATION
simple moving average filter. Peak GRF was defined as the maximum
vertical force produced at the foot during the gait cycle. Heel strike
and toe off were defined as the time at which the GRF signal exceeded
three standard deviations above baseline noise, and temporal markers
were placed at each heel strike and toe off. Stride length was
calculated by multiplying the time between consecutive heel strikes
by the treadmill speed.
The positions of reflective markers attached to lateral thigh (midway between greater trochanter and the knee), lateral femoral condyle,
lateral malleolus, calcaneus, and forefoot were sampled at 60 Hz using
a video motion measurement system (Vicon Motion Systems, Spectrum Point, Lake Forrest, CA). Using these data, dynamic knee and
ankle joint angular ROM in the sagittal plane was calculated. Because
the pelvis and hip were obscured by the chamber frame, hip ROM data
could not be obtained. Dynamic knee ROM was calculated by subtracting the angle of maximum extension from the angle of maximum
flexion about the knee during the gait. Ankle ROM was calculated by
subtracting the angle of maximum plantar flexion from the angle of
maximum dorsiflexion about the ankle. All gait data were time
normalized to a percentage progression through a gait cycle using the
heel strike from the GRF signal as the beginning and end of each gait
cycle. Data were averaged across all subjects to form a composite
signal consisting of a mean and variance for each parameter collected.
Data are presented as means ⫾ SE for all parameters.
Statistical analyses were performed using one-way or two-way
ANOVA for initial effects. When significance was detected (P ⬍
0.05), post hoc individual comparisons were made using the Scheffé’s
correction.
RESULTS
While standing inside the LBPP chamber, subjects’ HR
decreased from 83 ⫾ 3 beats/min at ambient pressure to 73 ⫾
3 beats/min at 50 mmHg LBPP with statistical significance
(P ⬍ 0.05) between HR at ambient pressure and that measured
at LBPP levels greater than 20 mmHg (Fig. 2A). With initiation
of walking at 3.0 mph (1.34 m/s) in ambient pressure, HR
increased significantly (P ⫽ 0.009) from 83 ⫾ 3 beats/min at
rest to 99 ⫾ 4 beats/min while walking. With addition of the
LBPP during walking, HR decreased from 99 ⫾ 4 beats/min at
ambient pressure to 84 ⫾ 2 beats/min at 50 mmHg of LBPP
with statistically significant decreases (P ⬍ 0.009) observed at
both 25 and 50 mmHg of LBPP (Fig. 2B).
During standing, SBP trended higher from a resting value of
130 ⫾ 6 mmHg in ambient pressure to 135 ⫾ 6 mmHg at 50
mmHg of LBPP (Fig. 3A). This increase of SBP, however, was
not statistically different for the stationary upright subjects at
all levels of LBPP (P ⫽ 0.464). Similarly, standing in various
LBPP conditions tended to increase DBP from 75 ⫾ 6 mmHg
in ambient pressure to 81 ⫾ 5 mmHg at 50 mmHg of LBPP
(Fig. 3A), but again the increase was not statistically significant
(P ⫽ 0.36). During walking, DBP increased from 73 ⫾ 3
mmHg at ambient pressure to 83 ⫾ 4 mmHg at 50 mmHg of
LBPP (P ⫽ 0.053) (Fig. 3B).
Blood flow velocity through the left middle cerebral artery,
as measured with a transcranial Doppler, did not increase
significantly between levels of LBPP ranging from 74.9 ⫾ 8.5
cm/s in ambient pressure to 78.4 ⫾ 10.1 cm/s at 50 mmHg of
LBPP (Table 1). Forehead skin microvascular blood flow
remained unchanged from 6.4 ⫾ 0.4 to 5.8 ⫾ 0.4
ml䡠min⫺1 䡠100 g⫺1 of tissue at various levels of LBPP. Finally,
brain tissue oxygenation varied from 0.4 ⫾ 6.8 mV at ambient
pressure to 15.0 ⫾ 10.9 mV at 50 mmHg LBPP without
statistical significance associated with this trend (P ⫽ 0.875).
J Appl Physiol • VOL
Fig. 2. A: heart rate (HR) during standing in various levels of LBPP (mean ⫾
SE). *Statistically significant difference (P ⱕ 0.05) from HR during standing
in ambient (0 mmHg LBPP) pressure. B: HR during standing and walking in
various levels of LBPP (mean ⫾ SE). *Statistically significant difference (P ⬍
0.05) from HR during standing in ambient (0 mmHg LBPP) pressure. **Statistically significant difference (P ⬍ 0.05) from HR measured during walking
in ambient pressure.
Standing subjects’ weight decreased linearly from 84 ⫾ 5 kg
at ambient pressure to 13 ⫾ 3 kg at 50 mmHg LBPP. There
were significant differences (P ⫽ 0.001) between the subjects’
weight at 0 mmHg and that measured at 10, 20, 30, 40, and 50
mmHg, indicating an inverse relationship between increasing
LBPP and the net GRF (Fig. 4).
For gait analysis all subjects were able to walk and run
comfortably at each of the decreased BW conditions. Peak
GRF decreased significantly as LBPP was increased during
both walking and running (Fig. 5). Peak GRF during running
was significantly higher than peak GRF during walking. Normalized to the peak GRF during walking at 100% BW, the
peak GRF recorded during walking at 60% BW was 0.71
(⫾0.06) and walking at 20% BW was 0.30 (⫾0.09). Running
at 100% BW was 1.52 (⫾0.10), running at 60% BW was 1.21
(⫾0.08), and running at 20% was 0.58 (⫾0.15) times the peak
GRF during walking at 100% BW.
LBPP exercise did not significantly change ankle ROM (Fig.
6). Ankle ROM at 100, 60, and 20% BW during walking was
30.6 (⫾9.3), 31.4 (⫾5.2), and 33.9° (⫾6.4), respectively.
ROM at 20% BW during running was similar to the ROM at
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LBPP AMBULATION
Fig. 4. Weight reduction during stance with increasing LBPP (mean ⫾ SE,
n ⫽ 9). *Statistically significant difference (P ⬍ 0.05) compared with that for
subjects standing in ambient pressure.
Fig. 3. A: blood pressure (BP) in subjects standing in various LBPP levels
(mean ⫾ SE). B: BP in subjects standing and walking in various LBPP levels
(mean ⫾ SE).
significant changes were observed with LBPP during running
compared with the 100% BW condition (Fig. 7). Knee ROMs
at 100, 60, and 20% BW during running were 57.5 (⫾8.5), 63.2
(⫾12.4), and 55.8 (⫾10.9), respectively.
A significant increase in stride length was observed during
running compared with walking at all BW conditions, but
LBPP produced no significant changes compared with the
100% BW condition (Fig. 8). Stride lengths at 100, 60, and
20% BW during walking were 140.6 (⫾5.3), 148.8 (⫾8.5), and
159.4 cm (⫾9.5), respectively. Stride lengths at 100, 60, and
20% BW during running were 191.7 (⫾11.3), 211.9 (⫾10.8),
and 244.7 cm (⫾6.7), respectively. In addition, five of the six
subjects exhibited a float phase between steps while walking at
20% BW that is characteristic of a running gait. However,
patterns of knee and ankle joint motion for this condition were
more consistent with a walking than a running pattern.
DISCUSSION
100% BW during walking. There was an expected significant
increase in ankle ROM with running compared with walking.
Ankle ROM at 100, 60, and 20% BW during running was 44.5
(⫾5.8), 44.8 (⫾1.4), and 36.3 (⫾6.4), respectively.
There was a trend to lower dynamic knee ROM with
increasing LBPP during walking, although this change was not
significant (P ⫽ 0.09). Knee ROM at 100, 60, and 20% BW
during walking were 50.2 (⫾7.3), 45.2 (⫾5.1), and 36.7
(⫾4.6), respectively. Knee ROM increased significantly during
running compared with walking at 60 and 20% BW, but no
Prevalence of conditions such as transient ischemic attack,
stroke, cerebral vascular disease, hypertension, and coronary
artery disease is high in adult and elderly populations undergoing rehabilitation (24a). Sudden systemic and head blood
flow and pressure changes in patients with these medical
conditions can produce serious and even catastrophic outcomes. For the LBPP chamber to be effective as a rehabilitation tool, it must safely reduce load in lower extremities, while
not provoking brain hemorrhages or other potentially negative
cardiovascular effects. Our study confirms our hypothesis that
Table 1. Transcranial blood flow velocity, head microcirculatory blood flow, and brain tissue oxygenation
in subjects standing in various LBPP levels
Chamber Pressure, mmHg
Head microcirculatory blood flow,
ml䡠min⫺1䡠100 g of tissue⫺1
NIRS oxygenation, mV
TCD blood flow velocity, cm/s⫺1
0
10
20
30
40
50
6.4⫾1.2
0.4⫾6.8
74.9⫾8.5
6.1⫾1.0
2.9⫾7.9
77.8⫾8.8
5.9⫾1.1
8.6⫾8.3
80.7⫾8.0
5.8⫾0.9
7.5⫾8.4
80.7⫾8.9
6.1⫾1.0
12.1⫾11.8
77.9⫾8.0
5.8⫾1.0
15.0⫾10.9
78.4⫾10.1
Values are means ⫾ SE. LBPP, lower body positive pressure; NIRS, near infrared spectroscopy; TCD, transcranial Doppler.
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Fig. 5. Peak vertical GRF during walking and running within LBPP (mean ⫾
SE). Data presented as amplitude normalized to the peak GRF observed during
the 100% body weight (BW) walking condition. *Significant difference (P ⬍
0.05) from the 100% BW condition during walking. †Significant difference
(P ⬍ 0.05) from the 100% BW condition during running. ‡Significant
difference (P ⬍ 0.05) between a running condition and the corresponding
walking condition with the same level of LBPP.
Fig. 7. Dynamic knee ROM during walking and running within LBPP
(mean ⫾ SE). ‡Running ROM was significantly increased (P ⬍ 0.05) above
ROM for walking at the same level of LBPP.
mean arterial pressure (MAP), SBP, and DBP, although trending upward, do not significantly increase in standing and
treadmill ambulating subjects exposed to 0 –50 mmHg of
LBPP. To our knowledge, no one to date has determined
whether blood pressure rises during upright, submaximal
treadmill exercise in LBPP. Reneman and associates (27)
document that changes in pressure around a limb are fully
transmitted from the skin to the muscle. Elevation of tissue
pressures in lower extremities generated by LBPP slightly
increases central venous return and MAP in supine and seated
subjects (2, 14, 23). Nishiyasu and coworkers (23, 24) demonstrate that systemic blood pressure response of subjects
during seated and supine cycle ergometer exercise in LBPP is
dependent on the intravascular fluid shift from lower extremities to torso and the subject’s body position. Similarly, Shi et
al. (28) document that MAP increases 3– 6 mmHg at 20 –30
mmHg LBPP and by 4 –15 mmHg at 40 –50 mmHg LBPP in
supine subjects. However, Nishiyasu and associates utilize a
chamber that encloses subjects in seated or supine position,
whereas we study subjects standing and ambulating fully
upright on an exercise treadmill. This methodological difference could account for the slight difference in the LBPP effect
on cardiovascular parameters observed in our study. Furthermore, we propose that upright treadmill exercise has greater
application potential in patient rehabilitation than ergometric or
supine exercise.
To our knowledge, our study is also the first to assess
whether LBPP causes any sudden shifts in microvascular
perfusion, oxygenation, and blood flow velocity in the head.
Our results confirm our hypothesis that head capillary perfusion, macrovascular circulation, and oxygenation are unchanged by the application of the LBPP. Although changes in
these parameters are not detrimental per se, they may suggest
that cerebral blood pressure and flow are adversely affected. As
previously mentioned, this is especially important when this
exercise device is utilized by adult and elderly patients. There-
Fig. 6. Dynamic ankle range of motion (ROM) during walking and running
within LBPP (mean ⫾ SE). LBPP produced no significant changes in ROM.
‡Running ROM was significantly increased (P ⬍ 0.05) above ROM for
walking at the same level of LBPP.
Fig. 8. Stride length during walking and running within LBPP (mean ⫾ SE).
‡Running stride length was significantly increased (P ⬍ 0.05) above the stride
length for walking at the same level of LBPP.
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fore, this cerebrovascular safety assessment is a very important
first step before the studies on an elderly patient cohort are
carried out. Blood flow velocity through the middle cerebral
artery does not change with application of different levels of
LBPP. This is expected considering autoregulation of brain
blood flow. Furthermore, head skin microvascular flow and
frontal lobe oxygenation remain unaffected by various levels of
LBPP. Independent of intravascular fluid shifts from lower
extremities, the heart still must overcome the gravity gradient
between the chest and the head, which remains unaffected by
LBPP exposure. The small sample size limits us from extrapolating that the head blood pressure and flow are completely
unaffected by the LBPP but allows us to conclude that this
potential change is within clinically safe range.
For this device to be effective as a rehabilitation tool, it
should also provide adequate reduction of the forces acting on
lower extremities while maintaining gait parameters. The total
force imposed on the rehabilitating joint is equal to the sum of
the internal or muscular forces and the external forces due to
gravity or GRF. Our study shows that by altering the chamber
pressure, we can precisely modulate the external load on
subject’s lower extremities; hence, 50 mmHg of LBPP provides 85% reduction in static external lower extremity loading.
Furthermore, LBPP produces significant reductions of GRF
during both walking and running. The observed decrease in
peak GRF with increasing LBPP indicates that external forces
affecting the lower extremities are reduced.
Devices that are currently used for unloading rehabilitation
include walkers, parallel bars, therapist-assisted waist belts,
overhead suspension harnesses, and water immersion therapy.
However, each of these devices has practical limitations.
Canes, crutches, walkers, and parallel bars require significant
upper body strength to unload the lower extremities, limiting
their usefulness in an elderly patient population. Additionally,
the amount of weight relieved is neither constant nor quantifiable (17), and the energy expended by the patient using these
appliances is higher than that of healthy subjects (10). Therapist-assisted waist belts require a therapist to be present during
the patients’ entire rehabilitation. Currently, overhead suspension harness systems are widely used for rehabilitation (7,
15–17, 20, 25, 29, 33). A harness support system has been used
effectively in total hip arthroplasty patients to reduce hip
extension deficits, improve gait symmetry, and increase
strength compared with a standard physical therapy protocol
(20). However, there are some concerns about the ability of
such systems to support large percentages of BW (⬎75%)
comfortably (7, 17).
Finally, water immersion therapy may increase the risk of
infection and wound dehiscence during the immediate postoperative period, thus delaying the beginning of patient’s active
rehabilitation. The viscous drag associated with water immersion therapy probably causes abnormal muscle activation patterns and joint ROMs (3, 9, 18). Although patients treated with
water immersion therapy have less joint effusion, exercise in
water may not be as effective as traditional land-based therapy
in regaining maximum muscle performance (32). In contrast,
our results during walking within LBPP suggest that decreased
total force in the lower extremity protects healing tissues and
musculoskeletal implants in the immediate postoperative period while maintaining normal gait and possibly reducing
postoperative edema and joint effusion.
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Our results for running suggest that LBPP unloads lower
body tissues similar to the effect of LBPP during walking
exercise or standing. Thus running within LBPP may benefit
athletes in their high-demand postoperative rehabilitation, allowing a running workout without the negative effects of
increased lower extremity loading. Whether potential reduction
in total joint force during rehabilitation is beneficial to protect
healing tissues requires further investigation and is beyond the
scope of the present study.
Our gait analysis also demonstrates that LBPP does not
significantly affect ankle or knee ROM. There is a trend to
lower dynamic knee ROM with increasing LBPP during walking, but this trend is not statistically significant. Hip ROM and
lumbar positions cannot be monitored with our methodological
design, because the top of the LBPP chamber obscures the
view of hips and waist. Hence, it is conceivable that these gait
parameters could be affected by LBPP. Additionally, knee
ROM measured during running is lower than previously reported values of 90.5° at 3.4 m/s and 103.1° at 5.0 m/s (21).
Slow running speeds (2.68 m/s) in this study probably contribute to this difference. In addition, the waist seal used to
maintain positive pressure in the LBPP chamber may limit
upward displacement during ambulation, further decreasing
knee ROM. However, knee ROM during walking at 100% BW
is comparable to previously reported values (31).
All variables are measured in healthy subjects without impairments for three reasons. The degree of unloading (reduction of GRF) that can safely and comfortably be achieved with
LBPP must be determined before patients attempt to use this
modality for rehabilitation. This is critical, as many of the
prospective patients have weight-bearing restrictions. Moreover, data detailing the forces experienced by the lower extremities at various pressures are needed to prevent reinjury or
exacerbation of existing injuries. From a scientific perspective,
analysis of the effects of LBPP on gait is required on subjects
without impairments so that future studies may determine
which gait effects are due to patient’s injuries and which
effects are due to the LBPP apparatus itself. Finally, cardiovascular safety must be established before any patients with
preexisting medical conditions are subjected to LBPP exercise.
Exercise in LBPP may help prevent muscle atrophy associated with bed rest or disuse, while at the same time decreasing
joint forces, protecting healing tissues and prostheses, and
decreasing pain associated with ambulation. Some patient
groups who may benefit from this form of partial weight
bearing during their postoperative recovery include those with
ligament and meniscus repairs (13), joint replacements, and
complicated fractures. Placing these patients in an LBPP chamber early during their postoperative period may allow them to
ambulate earlier in their recovery. Future studies are planned to
investigate the efficacy of exercise within LBPP for patients
with hip and lower extremity injuries and disorders.
In conclusion, our study is the first to examine LBPP effects
on cephalic and systemic cardiovascular parameters and to
quantify the antigravity and gait effects of LBPP during upright
standing, walking, and running. This information provides
insight into cardiovascular and cerebrovascular responses to
LBPP in upright subjects and provides initial data about the
safety of the LBPP chamber as an exercise modality before its
use in rehabilitation by adult or elderly postoperative patients.
Application of the LBPP to healthy subjects in stationary
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LBPP AMBULATION
upright position and ambulating on a treadmill presents minimal risk in terms of head perfusion and vascular flow while
offloading lower extremities and maintaining normal gait.
ACKNOWLEDGMENTS
We thank Brandon R. Macias for technical support.
GRANTS
This research was supported by the Toby Freedman Endowment Fund,
University of California, San Diego Chancellor’s Associates, and by an
Orthopaedic Research and Education Foundation Center of Excellence Grant.
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