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ORIGINAL ARTICLE
Effect of carrying a weighted backpack on lung mechanics
during treadmill walking in healthy men
Paolo B. Dominelli •A. William Sheel •
Glen E. Foster
Received: 4 April 2011 / Accepted: 9 September 2011 / Published online: 23 September 2011
ÓSpringer-Verlag 2011
Abstract Weighted backpacks are used extensively in
recreational and occupational settings, yet their effects on
lung mechanics during acute exercise is poorly understood.
The purpose of this study was to determine the effects of
different backpack weights on lung mechanics and
breathing patterns during treadmill walking. Subjects
(n=7, age =28 ±6 years), completed two 2.5-min
exercise stages for each backpack condition [no backpack
(NP), an un-weighted backpack (NW) or a backpack
weighing 15, 25 or 35 kg]. A maximal expiratory flow
volume curve was generated for each backpack condition
and an oesophageal balloon catheter was used to estimate
pleural pressure. The 15, 25 and 35 kg backpacks caused a
3, 5 and 8% (P\0.05) reduction in forced vital capacity
compared with the NP condition, respectively. For the
same exercise stage, the power of breathing (POB)
requirement was higher in the 35 kg backpack compared to
NP (32 ±4.3 vs. 88 ±9.0 J min
-1
,P\0.05; respec-
tively). Independent of changes in minute ventilation, end-
expiratory lung volume decreased as backpack weight
increased. As backpack weight increased, there was a
concomitant decline in calculated maximal ventilation, a
rise in minute ventilation, and a resultant greater utilization
of maximal available ventilation. In conclusion, wearing a
weighted backpack during an acute bout of exercise altered
operational lung volumes; however, adaptive changes in
breathing mechanics may have minimized changes in the
required POB such that at an iso-ventilation, wearing a
backpack weighing up to 35 kg does not increase the POB
requirement.
Keywords Pulmonary mechanics Backpack Lung
volumes Work of breathing Ventilatory capacity
Introduction
Carrying a weighted backpack is a common practise in
various occupational and recreational settings; with weights
ranging from under 5 kg to the extreme of 68 kg in some
military and expedition settings (McCaig and Gooderson
1986). Commonly, recreational outdoor backpackers, carry a
fourth to a third of their body weight. A fully equipped
firefighter wears fire protective clothing, a self-contained
breathing apparatus with their respective air cylinders and
often carries other equipment in a backpack; all of which can
restrict chest-wall movement and alter breathing mechanics
(Butcher et al. 2007). Despite their widespread use, weighted
backpacks have been shown to have a negative effect on
resting pulmonary function. Specifically, backpack usage
has been shown to reduce forced vital capacity (FVC) and
forced expired volume in 1 s (FEV
1.0
) approximately 12%
with a 25 kg backpack; without a coincidental decrease in
the FVC/FEV
1.0
ratio (Bygrave et al. 2004; Legg and Cruz
2004; Muza et al. 1989; Legg and Mahanty 1985). Backpack-
induced reductions in vital capacity (VC) probably do not
hinder ventilation during low intensity exercise due to the
reserve in ventilatory capacity. However, during intense
exercise, ventilatory demand increases 10–15 times and the
backpack-associated restrictions in lung capacity may have
detrimental effects by reducing the ventilatory reserve.
Communicated by Susan A. Ward.
P. B. Dominelli A. W. Sheel G. E. Foster
School of Kinesiology, The University of British Columbia,
Vancouver, BC, Canada
P. B. Dominelli (&)
6108 Thunderbird Blvd., Vancouver, BC V6T 1Z3, Canada
e-mail: paolo321@interchange.ubc.ca
123
Eur J Appl Physiol (2012) 112:2001–2012
DOI 10.1007/s00421-011-2177-8
During dynamic exercise in healthy individuals, tidal
volume is increased mainly by decreasing end-expiratory
lung volume (EELV) (Henke et al. 1988; Sharratt et al.
1987; Younes and Kivinen 1984; Johnson et al. 1992). The
decrease in EELV serves to optimize diaphragm length and
allows tidal volume to increase without excessive increases
in end-inspiratory lung volume. When the chest-wall is
loaded or restricted, EELV decreases below levels that
would be expected in an unaltered state (Wang and Cerny
2004; Babb et al. 2002; Tomczak et al. 2010). Loading of
the chest-wall occurs in pathological states, such as obesity
(Salome et al. 2010), or can be externally imposed, such as
applying weight directly to the chest (Sharp et al. 1964)or
wearing a weighted garment on the torso (Wang and Cerny
2004). Similarly, chest-wall restriction can occur in disease
states, such as interstitial lung disease (Marciniuk et al.
1994), or by applying inelastic straps to the thorax
(Tomczak et al. 2010; O’Donnell et al. 2000; Miller et al.
2002). The lower operational lung volume hinders expi-
ratory flow and is related to the development of expiratory
flow limitation (EFL) (Tomczak et al. 2010). Operational
lung volumes tend to increase when EFL is present, caus-
ing breathing to become more tachypnoeic and increasing
the power of breathing (POB) requirement (McClaran et al.
1999; Johnson et al. 1992). However, when the chest-wall
is restricted, this compensatory increase in operational lung
volumes may not be possible.
Wearing a weighted backpack could have the dual
negative effect of mass loading and restricting the chest-
wall. Both loading and restricting of the chest-wall could
cause an increase in inspiratory muscle work and lead to
respiratory muscular fatigue (Tomczak et al. 2010). While
the mass loading effect of the backpack could abnormally
decrease EELV, possibly leading to EFL (Ofir et al.
2007), the restriction from backpack straps could hinder
any increase in operational lung volumes. The purpose of
this study was to determine the effect of light, moderate,
and heavy backpack weights on resting pulmonary func-
tion and respiratory mechanics during treadmill walking.
We hypothesize that increasing backpack weight is asso-
ciated with decreasing FVC and FEV
1.0
. More impor-
tantly, we hypothesized that a heavily weighted backpack
results in the greatest degree of mechanical constraint to
ventilation during treadmill walking. Specifically, we
anticipated a decrease in EELV, an increase in breathing
frequency and an increased POB requirement while
treadmill walking with a heavily weighted backpack
compared with an un-weighted backpack. To our knowl-
edge, this is the first study that has instrumented subjects
with oesophageal balloons to estimate pleural pressure and
assess changes in respiratory mechanics during treadmill
walking while wearing a light, moderate, and heavy
weighted backpack.
Materials and methods
Subject characteristics
Seven healthy young (\40 years) men participated in this
study. Subjects provided informed consent to participate
and all procedures were approved by the Clinical Research
Ethics Board at the University of British Columbia. All
subjects were non-smokers, had no history of cardiopul-
monary disease, had normal pulmonary function, regularly
engaged in physical activity at least 3 days a week and had
previous experience carrying a weighted backpack for
extended periods of time ([6 h per day for several days).
Subjects were recruited from the university student
population.
Experimental protocol
Subjects underwent basic anthropometric measurements
(height, weight) and were instrumented with an oesopha-
geal balloon catheter (see detailed Methods below). After
10 min of resting metabolic and ventilatory measurements,
the subjects completed five randomized exercise bouts on a
treadmill: no backpack (NP), an un-weighted backpack
(NW) or a backpack weighing 15, 25 or 35 kg. In between
exercise trials, the subjects removed the weighted backpack
and rested for at least 15 min or until their heart rate nor-
malized. Inspiratory capacity (IC) maneuvers were per-
formed during the last 30 s of each stage to determine
operational lung volumes. Maximum expiratory flow-vol-
ume (MEFV) curves were generated immediately before
and after the exercise bout, while the subjects were
standing on the treadmill wearing the appropriate backpack
(except for the NP condition).
Oesophageal pressure and POB
The method for determining oesophageal pressure has been
detailed previously (Milic-Emili et al. 1964;Guenetteetal.
2007). Briefly, a balloon tipped latex catheter (no. 47-9005;
Ackrad Laboratory, Cranford, NJ) was advanced approxi-
mately 45 cm past the nostril, after the application of a local
anaesthetic (lidocaine hydrochloride 2%, Xylocaine, Astra-
Zeneca Canada Inc. Mississauga, ON, Canada). Air was
removed from the balloon by having the subjects perform a
valsalva maneuver and then 1 ml of air was injected, as per
manufacturer guidelines. Mouth pressure was monitored via a
port in the mouthpiece. Both oesophageal and mouth pressure
were measured using a piezoelectric pressure transducer
(±100 cmH
2
O; Raytech Instruments, Vancouver, BC, Can-
ada), which was calibrated against a digital manometer
(2021P, Digitron, Torquay UK). The POB was calculated by
averaging and integrating several trans-pulmonary pressure-
2002 Eur J Appl Physiol (2012) 112:2001–2012
123
tidal volume loops (Otis 1964) using customizable software
(Bibo, LabVIEW software V6.1; National Instruments, Aus-
tin, TX) to estimate the work. Subsequently, the work of
breathing was multiplied by breathing frequency to indicate
the power output requirement of the respiratory system and
expressed in Joules min
-1
(J min
-1
).
Treadmill exercise trials
Subjects completed five randomized exercise trials on a
treadmill (model TMX425C, Full Vision Inc, Newton, KA).
The NW, 15, 25 and 35 kg backpack stages consisted of
two 2.5-minute stages; 4.0 km h
-1
at a 10% grade followed
by 4.0 km h
-1
at a 15% grade. The NP trial consisted of
five 2.5-min stages; 4.0 km h
-1
at a 10% grade, 4.8 km h
-1
at a 10% grade, 4.0 km h
-1
at a 15% grade, 4.8 km h
-1
at
a 15% grade and 5.6 km h
-1
at a 15% grade. Additional
stages were conducted in the NP condition to ensure suffi-
cient overlap in ventilation with weighted backpack
trials. The MEFV curve was obtained immediately before
and after each stage, with the associated backpack still
being worn.
Backpack packing and fitting
A standard size and commercially available backpack was
used (Bora 80, Arcteryx, Vancouver, BC), which was an
appropriate size for all of the individuals tested. The
backpack itself weighed 3.1 kg; therefore, the NW back-
pack condition had a slight external load. The backpack
was packed according to manufacturer recommendations
and common practise. The least dense item was placed in
the very bottom of the pack with the heaviest positioned in
the middle of the backpack closest to the wearer’s back.
Other low density items were inserted until the pack was
near capacity to ensure there was no shifting or movement
of items within the pack. The backpack was then weighed
and fitted to the subject as per manufacturer specifications.
Subjects were informed to securely fasten the backpack’s
stabilizing straps and hip-belt to mimic how they would
wear a backpack for an extended trip. The hip-belt and
sternum straps were fastened during all data collection to
simulate the real-life situation.
Flow, volume, and metabolic measurements
Inspiratory and expiratory flow was measured using two
independent pneumotachographs (model 3813, Hans
Rudolph, Kansas City, MO), one attached to the inspiratory
side and one attached to the expiratory side of a two-way
non-rebreathing valve (model 2700B, Hans Rudolph),
respectively. The pneumotachographs were calibrated at
various flow rates using a 3 l syringe with the volume
being obtained through numerical integration of the flow
signal. Ventilatory and mixed expired metabolic parame-
ters were gathered using a customized metabolic cart
consisting of calibrated expired and inspired pneumota-
chographs (model 3813, Hans Rudolph) and calibrated O
2
and CO
2
analyzers (Model S-3-A/I and Model CD-3A,
respectively, Applied Electrochemistry, Pittsburgh, PA).
MEFV curves
All data used in MEFV curve analysis were collected while
the subject was standing on the treadmill with, when
applicable, the backpack still being worn. The method used
to collect the MEFV curve has been previously described
(Guenette et al. 2010). Briefly, to account for any post-
exercise bronchodilation, FVC maneuvers were performed
immediately before and after each exercise trial. Moreover,
to minimize the effect of thoracic gas compression, several
expiratory efforts were superimposed on each other to
create a composite MEFV curve. The final MEFV curve
was determined by aligning all volumes to the greatest
FVC and using the greatest expiratory flow at any given
lung volume from FVC to the assumed residual volume
using the various efforts before and after exercise.
EFL and operational lung volumes
Expiratory flow limitation was determined by superimposing
an average composite of tidal breaths within the MEFV
curve. Approximately 10 tidal breaths immediately prior to
an IC maneuver were averaged using a customizable soft-
ware program (Bibo, LabVIEW software V6.1; National
Instruments). The position of the tidal breath within the
MEFV curve was determined by the IC maneuver. Prior to
the experimental protocol, all subjects had practice per-
forming the IC maneuver with and without visual feedback.
To perform the maneuvers, the subjects were instructed ‘‘at
the end of a normal breath out, take a quick and forceful
breath all the way in.’’ To ensure the subjects reached total
lung capacity, peak oesophageal pressure during the exercise
IC maneuvers was compared to the peak pressure during
resting IC measures. The resting maneuvers were performed
with the appropriate backpack being worn. During analysis,
the IC maneuvers were corrected for pneumotachograph
drift by selecting six breaths prior to the maneuver and
‘‘zeroing’’ end expiratory volume. The position of the EELV
of the tidal breath on the volume axis of the MEFV curve was
determined by subtracting the IC volume from the FVC
volume. The magnitude of EFL was calculated by dividing
the volume of the tidal breath that contacted the MEFV curve
by the tidal volume and expressed the value as a percent of
tidal volume. End inspiratory lung volume (EILV) was cal-
culated by adding tidal volume and EELV.
Eur J Appl Physiol (2012) 112:2001–2012 2003
123
Ventilatory capacity
The method used to estimate ventilatory capacity ( _
VECAP)
has been described in detail previously (Jensen et al. 1980;
Johnson et al. 1995). This method determines the theoret-
ical ventilation if the subject breathed exclusively along
their MEFV curve for a given tidal volume and EELV. All
calculations utilized the variables from each of the asso-
ciated backpack conditions. Briefly, approximately ten
exercise tidal breaths were averaged and placed within the
MEFV curves according to the calculated EELV. The
volume of the tidal breath was divided into volume equal
segments; ranging from 40 to 60 ml. Each volume segment
was divided by the corresponding maximal expiratory flow
rate to give an estimated minimal expiratory time. All
equal volume segments were summed to give a minimal
expiratory time for the tidal breath. Inspiratory time was
determined by using the inspiratory-to-total breathing cycle
time ratio. Maximal breathing frequency was determined
after the minimal tidal breath time was calculated. Calcu-
lated tidal volume and maximal breathing frequency were
multiplied to give the _
VECAP for each workload with each
backpack condition. Ventilatory reserve ( _
VERES) is the
difference between _
VECAP and measured _
VE.
Data processing
Raw data (flow, volume, ventilatory and mixed expired
metabolic parameters) was recorded continuously at 200 Hz
using a 16-channel analog-to-digital data acquisition system
(PowerLab/16SP model ML 795, ADI, Colorado Springs,
CO) and stored on a computer for subsequent analysis
(LabChart v6.1.3, ADInstrument, Colorado Springs, CO).
Statistical analysis
Repeated measures ANOVA was used to compare the
different backpack conditions. When significant Fratios
were detected, Tukey’s post hoc analysis was applied to
determine where the differences resided (Statistica v6.1,
Statsoft Inc., Tulsa, OK, USA). The level of significance
was set at P\0.05 for all statistical comparisons. Values
are presented as mean ±SE.
Results
Subject characteristics
Anthropometric values for the subjects are presented in
Table 1. None of the subjects were classified as obese
according to the World Health Organizations cut-off point
for obesity (BMI [30 kg m
-2
). All subjects fell within
the manufacturers’ suggested height range for proper fit of
the backpack. Every subject engaged in at least one activity
within the previous year that required carrying a backpack
of at least 15 kg for several hours.
Pulmonary function
Table 2summarizes the pulmonary function gathered
immediately before and after each backpack exercise trial.
There was no significant difference between the NP and
NW condition for any measured variables. Wearing any
weighted backpack (i.e. all weighted backpack conditions)
resulted in a significant reduction in FVC values, when
compared to the NP condition (3, 5 and 8% reduction for
15, 25 and 35 kg backpack, respectively). There was no
difference between backpack conditions in any of the
expiratory flow parameters measured (peak expiratory flow
and forced expiratory flow at 75, 50 and 25% of FVC).
Exercise data
Descriptive data obtained during all exercise bouts is pre-
sented in Table 3. Two additional stages during the NP trial
were used for ventilatory analysis and is presented as fol-
lows: _
VE (49 ±2.4 and 62 ±2.7 l min
-1
for the fourth
and fifth stage; respectively), _
VECAP (201 ±19 and
210 ±20 l min
-1
for the fourth and fifth stage; respec-
tively) and for _
VE=_
VECAP (25 ±2.0 and 31 ±3.3% for
the fourth and fifth stage; respectively). As predicted
a priori, there was a main effect for stage in all of the
parameters (P\0.01) except for _
VECAP (P=0.53). Fig-
ure 1depicts the effect of increasing backpack weight on
both _
VO2and _
VCO2. The increasing metabolic demand as
backpack weight increased was also predicted a priori.
There was no effect of backpack weight on V
T
(P=0.73).
Breathing frequency approached significance between the
NP and NW condition (P=0.08), but was significantly
increased at 15, 25 and 35 kg backpack weights compared
with NP (P\0.01). The POB requirement was also similar
between the NP and NW condition (P=0.17), but with a
significant increase between the NP and 15, 25, 35 kg
Table 1 Anthropometric measurements (n=7)
Mean ±SE Range
Age (years) 27.6 ±2.1 22–38
Height (cm) 179.6 ±2.1 173–188
Mass (kg) 80.2 ±3.3 67.8–90.8
BMI (kg m
-2
) 24.8 ±0.7 22.4–27.7
BSA (m
2
) 2.00 ±0.05 1.81–2.16
BMI body mass index, BSA body surface area
2004 Eur J Appl Physiol (2012) 112:2001–2012
123
backpacks (P\0.001). The 35 kg backpack condition had
a higher required POB compared with all other backpack
conditions (P\0.01). The 25 kg backpack POB require-
ment was similar to the 15 kg condition (P=0.20) and
higher than the NP and NW (P\0.001). However, as seen
in Fig. 2when adjusted for minute ventilation, all of the
backpack conditions appear to fall within the same con-
tinuum for POB requirement. The percent of time spent on
inspiration (T
i
) and expiration (T
e
) did not show any effect
of stage (45.2 vs. 45.4%, P=0.94; 54.7 vs. 54.6%,
P=0.95, respectively). None of the subjects had a respi-
ratory exchange ratio of 1.0 or above during any of the
exercise stages.
Operational lung volumes and ventilatory constraints
Figure 3illustrates the changes in operational lung vol-
umes for the five backpack conditions. No effect of back-
pack weight was seen in EILV (P=0.50), however, there
was a stage effect (62.8 vs. 67.9% FVC, P=0.01; stage 1
and 2, respectively). Conversely, EELV demonstrated a
strong effect of decreasing EELV as backpack weight
increased (Fig. 3), while not showing any effect of stage
(P=0.23).
Two out of the seven subjects developed EFL. For both
individuals, this occurred in the second stage (4 km h
-1
at
15% grade) of the 35 kg backpack condition and was rel-
atively minor (7 and 12% of tidal breath). The MEFV
curve, along with tidal breaths, for the subject who showed
12% EFL is seen in Fig. 4. The subject had a similar _
VE
for the two exercise stages depicted, but is 12% EFL in the
35 kg backpack trial. Notable differences include an 8%
smaller FVC and a lower EELV (26 vs. 29% FVC; for
35 kg and NP condition) in the 35 kg backpack condition.
Consequently, the subject had a lower _
VECAP and venti-
lated at a higher percent of their _
VERES.
Figure 5shows the changes in _
VE, _
VERES and _
VECAP
for the exercise stages. As the backpack weight increased,
_
VECAP increases while the _
VECAP declines, resulting in
decreasing _
VERES along with a greater utilization of _
VECAP
(Table 3). Indeed, while the NP and NW condition show no
difference in percent utilization of _
VECAP (19 vs. 26%,
P=0.06; respectively), the 35 and 25 kg condition both
use a greater percent than the NP, NW and 15 kg backpack
(P\0.01). Figure 6shows two matched iso-ventilation
exercise stages for both the NP and 35 kg backpack con-
dition. While the _
VE are not different, the 35 kg condition
utilizes a significantly greater proportion of the _
VECAP.
Discussion
Major findings
The purpose of this study was to determine the effects of
three different weights of backpacks on resting pulmonary
function and respiratory mechanics during treadmill walk-
ing. The major findings of our study are fourfold. First, the
heaviest backpack was associated with the greatest reduction
in resting FVC and FEV
1.0
, with minimal changes in the FVC
to FEV
1.0
ratio and no change in expiratory flows. Second,
the POB requirement did not differ between backpack con-
ditions when comparing similar ventilation. Third, EELV
during the exercise bout declined as the backpack weight
increased. Finally, increasing the backpack weight caused a
Table 2 Pulmonary function during different backpack trials
NP NW 15 kg 25 kg 35 kg
FVC (l) 6.0 ±0.2
c,d,e
5.9 ±0.2
d,e
5.8 ±0.2
a,d,e
5.7 ±0.2
a,b,c,e
5.5 ±0.2
a,b,c,d
FEV
1.0
(l) 4.7 ±0.2
d,e
4.6 ±0.2 4.6 ±0.2 4.5 ±0.2
a
4.4 ±0.2
a
FEV
1.0
/FVC (%) 77 ±1.2
e
77 ±1.2
d,e
79 ±1.0
e
79 ±0.7
b
81 ±1.0
a,b,c
PEF (l s
-1
) 11.6 ±0.6 11.8 ±0.7 11.6 ±0.6 11.6 ±0.7 11.4 ±0.7
FEF
75
(l s
-1
) 9.2 ±0.7 9.0 ±0.6 9.0 ±0.5 8.9 ±0.4 8.6 ±0.5
FEF
50
(l s
-1
) 5.7 ±0.4 5.7 ±0.3 5.6 ±0.4 5.6 ±0.4 5.5 ±0.5
FEF
25
(l s
-1
) 2.3 ±0.2 2.3 ±0.2 2.3 ±0.2 2.3 ±0.3 2.4 ±0.3
Values are mean ±SE
NP no backpack, NW non-weighted backpack, FVC forced vital capacity, FEV
1.0
forced expired volume in 1 s, PEF peak expiratory flow, FEF
75
forced expiratory flow at 75% FVC, FEF
50
forced expiratory flow at 50% FVC, FEF
25
forced expiratory flow at 25% FVC
a
P\0.05 compared to NP
b
P\0.05 compared with NW
c
P\0.05 compared to 15 kg
d
P\0.05 compared to 25 kg
e
P\0.05 compared with 35 kg
Eur J Appl Physiol (2012) 112:2001–2012 2005
123
decrease in _
VERES, through changes in both _
VECAP and _
VE.
Our findings indicate that exercising while wearing a heavy
backpack changes respiratory mechanics and places addi-
tional metabolic stress on an individual.
Pulmonary function
We found no difference in FVC between the NP and NW
condition, indicating that the restrictive nature of the back-
pack straps or the minimal weight of the backpack alone
(3.1 kg) was insufficient to change FVC. When the back-
pack weight was increased to 15 kg, the FVC was reduced
relative to the NP condition (Table 2). Increasing the
backpack weight to 25 and 35 kg caused a further decline in
FVC (Table 2). Additionally, FEV
1.0
was only different
between the two heaviest backpacks (25 and 35 kg) and the
NP condition. None of the backpack conditions significantly
changed any expiratory flow values (Table 2). Our findings
are consistent with others who have looked at resting lung
function while wearing a weighted backpack (Legg and
Mahanty 1985; Bygrave et al. 2004; Legg and Cruz 2004;
Muza et al. 1989). The heaviest backpack in the present
study (35 kg) caused an 8% decrease in FVC; similarly, the
heaviest backpack utilized by Muza et al. (1989) (30 kg)
caused a 6% reduction in FVC. The reduced FVC could
arise from either an increase in residual volume or a decrease
in total lung capacity, or both. A decreased total lung
capacity is primarily due to restriction of the chest-wall from
changes in chest-wall compliance or inspiratory muscle
fatigue (West 2008). Previous studies that have loaded the
chest-wall, using a weighted vest, attributed the decrease in
FVC to decreases in total lung capacity (Wang and Cerny
2004). Increased residual volume occurs during loading of
the chest or abdomen; whereby the closing volume is
increased due to increased transpulmonary pressure, but its
occurrence during chest-wall loaded exercise is controver-
sial (Sharp et al. 1980; Jenkins and Moxham 1991). Wearing
a weighted backpack both restricts and loads the chest-wall;
therefore, without the use of a full body plethysmograph to
determine residual volume while wearing the backpacks, we
are unable to determine the precise mechanism(s) for the
reduction in FVC.
The FEV
1.0
did not decrease to the same degree as the
FVC (6 vs. 8%; respectively, for the 35 kg backpack
Table 3 Metabolic and ventilatory response to each exercise stage for NP, NW, 15, 25, and 35 kg backpack conditions
Con Var Stage Var Stage
12 1 2
NP _
VO2(l min
-1
)1.9 ±0.1 2.3 ±0.1 _
VE(l min
-1
)29 ±2.1 42 ±2.6
NW 2.0 ±0.1 2.6 ±0.1 36 ±2.1 47 ±2.7
15 2.2 ±0.1 2.7 ±0.1 38 ±2.1 53 ±3.4
25 2.5 ±0.1 3.0 ±0.1 44 ±2.2 61 ±3.6
35 2.6 ±0.1 3.2 ±0.1 47 ±2.2 69 ±4.0
NP _
VCO2(l min
-1
)1.3 ±0.1 2.0 ±0.1 _
VECAP (l min
-1
)185 ±17 190 ±15
NW 1.6 ±0.1 2.2 ±0.1 169 ±15 172 ±19
15 1.6 ±0.1 2.4 ±0.1 155 ±5 151 ±11
25 1.9 ±0.1 2.7 ±0.2 131 ±9 139 ±9
35 2.0 ±0.1 3.0 ±0.1 130 ±8 137 ±1
NP Fb (bpm) 17 ±2.0 21 ±1.7 _
VE=_
VECAP (%) 16 ±1.9 22 ±1.7
NW 22 ±2.3 25 ±2.2 22 ±1.7 29 ±2.6
15 24 ±1.1 27 ±1.5 24 ±1.2 36 ±1.8
25 24 ±1.5 30 ±2.5 34 ±2.4 45 ±3.0
35 27 ±1.7 34 ±3.5 38 ±2.7 52 ±3.2
NP V
T
(l) 2.1 ±0.2 2.4 ±0.2 POB (J min
-1
)17±2.7 32 ±4.3
NW 1.9 ±0.2 2.2 ±0.1 25 ±3.2 41 ±4.0
15 1.8 ±0.1 2.3 ±0.1 31 ±6.1 55 ±8.4
25 2.0 ±0.1 2.3 ±0.2 35 ±5.9 66 ±6.4
35 2.0 ±0.1 2.4 ±0.2 40 ±4.3 88 ±9.0
Values are mean ±SE
Con condition, Var variable, NP no backpack, NW non-weighted backpack, 15 kg 15 kg backpack, 25 25 kg backpack, 35 kg 35 kg backpack,
Stage 1 4.0 km h
-1
at 10% grade, Stage 2 4.0 km h
-1
at 15% grade, V VO2oxygen consumption, VCO2carbon dioxide production, _
VE minute
ventilation, F
b
breathing frequency, bpm breaths per minute, V
T
tidal volume, _
VECAP ventilatory capacity, POB power of breathing
2006 Eur J Appl Physiol (2012) 112:2001–2012
123
compared to NP), resulting in a slightly increased FEV
1.0
/
FVC ratio. An increased FEV
1.0
/FVC is indicative of
restrictive conditions, as there is nothing obstructing
expiratory flow at high lung volumes. The similarity in
peak expiratory flow values for all backpack conditions
indicates that while FVC decreased, the largest volume
change (-8%) was not enough to reduce the FVC to the
effort independent segment of the flow-volume curve. At
high lung volumes, peak expiratory flow is highly depen-
dent on the subject’s motivation to expire forcefully.
However, once a minimal amount of volume is expired, the
maximal expiratory flow achievable at a given volume is
effort independent (Hyatt and Flath 1966). Conversely,
studies that used several inelastic straps tightly wrapped
around the thorax decreased FVC significantly more than
the present study (-40%) and report a decrease in peak
expiratory flow (Tomczak et al. 2010). A similar effect can
be seen when the chest-wall or abdomen are loaded using
heavy weights directly placed on the subject (Sharp et al.
1964) or by using weighted vests (Wang and Cerny 2004).
The above methods of loading cause the mass to be con-
fined to a relatively small area of the individual, and can
change individual’s flow-volumes loops. To eliminate
loading in a confined area, we used a well-padded
Fig. 1 Oxygen consumption ( _
VO2,filled circle) and carbon dioxide
production ( _
VCO2open circle) during each backpack conditions.
Values are mean ±SE of the two stages combined. NP no backpack,
NW unweighted backpack; (a) significantly different from NP;
(b) significantly different from NW; (c) significantly different from
15 kg; (d) significantly different from 25 kg; (e) significantly
different from 35 kg. P\0.05
Fig. 2 The power of breathing at different minute ventilations for the
five backpack conditions and each stage ±SE. Filled circles
represent no backpack condition. Open circles represent the
unweighted backpack condition. Filled triangles represent the 15 kg
backpack condition. Open triangles represent 25 kg backpack con-
dition. Filled squares represent 35 kg backpack condition
Fig. 3 Operational lung volumes for each backpack condition during
the exercise trials. Values are relative means of the two stages
combined ±SE. There was no effect between stages for EELV, while
EILV showed a stage effect. Open circles represent EILV, filled
circles represent EELV. FVC forced vital capacity, EILV end-
inspiratory lung volume, EELV end-expiratory lung volume, NP no
backpack, NW unweighted backpack; (a) significantly different from
NP; (b) significantly different from NW; (c) significantly different
from 15 kg; (d) significantly different from 25 kg; (e) significantly
different from 35 kg. P\0.05
Eur J Appl Physiol (2012) 112:2001–2012 2007
123
backpack that spreads the added mass over a larger area
and the use of the hip belt to transfer some weight to the
hip.
Power of breathing
Our subjects showed the expected rise in the POB
requirement as _
VE increased (Guenette et al. 2007; John-
son et al. 1992). As seen in Fig. 2, there does not appear to
be any effect of backpack weight, as all the points fall in
the usual curvilinear continuum. Rather, the heaviest
backpacks were associated with the highest required POB
because they also had the largest _
VE. The higher _
VE seen
in the heaviest backpack condition is due to the greater
metabolic demand of carrying an increased weight. This
finding differs from others who have shown that loading or
restricting the chest-wall does increase the required POB
(Tomczak et al. 2010; Miller et al. 2002; O’Donnell et al.
2000). Chest-wall restriction causes a decrease in the static
and dynamic compliance of the chest-wall; which in turn
increases the amount of pleural pressure needed to move a
given volume. Furthermore, significant restrictions of the
chest-wall decrease the amount of available expiratory flow
for tidal breathing, leading to the development of EFL
(Tomczak et al. 2010). When a person becomes flow lim-
ited, increasing positive pleural pressure will not result in
increased expiratory flows (Hyatt 1983). The excessive
positive pleural pressure causes the POB output to increase.
Loading of the chest-wall will also decrease the compli-
ance of the lung and chest-wall (Sharp et al. 1964). The
heaviest backpack (35 kg) in the current study was a suf-
ficient stressor to elicit mechanical changes in ventilation,
but not enough to alter the required POB. However, it is
possible that there was an adaptive response to the loading
created by the backpack, like changing operational lung
volumes, and this is what minimized the POB requirement
for the heavier backpacks.
The weighted backpacks caused a decrease in EELV
(see discussion below); causing the subjects to breathe on a
segment of the pressure–volume curve that has high com-
pliance. Further decreasing EELV (by wearing a heavier
backpack) or increasing both EELV and tidal volume
(increased ventilation or hyperinflation from EFL) would
result in breathing on parts of the pressure–volume curve
that are much less compliant. Breathing at either extreme
Fig. 4 MEFV curves and exercise tidal breath loops for a single
subject during the no backpack (NP) and 35 kg backpack trial. Solid
lines represent the NP trial, dashed lines represent the 35 kg backpack
trial. The exercise breath loops are from the 5.6 km h
-1
at 15% grade
and 4.0 km h
-1
and 15% grade for the NP and 35 kg backpack trial;
respectively. The NP and 35 kg backpack exercise trials had similar
minute ventilations (68 and 66 l min
-1
, respectively) but different
ventilatory capacities (166 and 111 l min
-1
, respectively) resulting in
a different percent usage of ventilatory capacity (43 and 59%,
respectively). During the 35 kg backpack trial, the subject is 12%
expiratory flow limited and ventilated at a 3% lower end expiratory
lung volume. The anchored zero volume represents a relative residual
volume for each trial. EFL expiratory flow limitation, V
T
tidal
volume, V
FL
volume flow limited
Fig. 5 Changes in ventilation, ventilatory capacity and ventilatory
reserves during the five backpack conditions. Values are means of the
two stages combined ±SE. Open squares represent ventilatory
capacity. Shaded area is the ventilatory reserve. Open triangles
represent ventilatory reserve. Filled circles represent measured
ventilation. NP no backpack, NW unweighted backpack, _
VE minute
ventilation, _
VECAP ventilatory capacity, _
VERES ventilatory reserve;
(a) significantly different from NP; (b) significantly different from
NW; (c) significantly different from 15 kg; (d) significantly different
from 25 kg; (e) significantly different from 35 kg. P\0.05
2008 Eur J Appl Physiol (2012) 112:2001–2012
123
of the pressure–volume curve, due to the additional stress
of a heavy backpack, could lead to a greater required POB.
Yet, when subjects are free to adopt their own breathing
strategy, the intra-subject variability in POB is probably
larger than the compliance or EFL-related changes from
our heaviest backpack.
Operational lung volumes
As the weight of the backpack increased, there was a
concomitant decrease in EELV (Fig. 3). During a bout of
progressive exercise without any restriction or loading of
the chest-wall, EELV decreases as _
VE increase (Henke
et al. 1988). While each backpack condition increased _
VE
(Fig. 5), the changes in EELV appeared independent of
_
VE. For all backpack conditions, there was an increase in
_
VE between stages. Yet, for the weighted backpack con-
ditions, the EELV did not change between stages, despite
the increase in _
VE. This finding would suggest that the
weighted backpack was more responsible for the changes
in EELV rather the changing metabolic and ventilation
demands. This is illustrated, in Fig. 4, where the subject
has a lower EELV in the 35 kg condition relative to the NP
condition, despite a similar _
VE. Two of our subjects
showed some degree of EFL. However, the EFL was quite
minimal in both cases (7 and 12% of tidal volume) and
only occurred during the last stage of the 35 kg backpack
condition. When an individual becomes EFL, especially
more severely, a rise in EELV is normally seen (Johnson
et al. 1999), yet, this is not a universal finding (Mota et al.
1999; Dominelli et al. 2011). Increasing EELV allows for
higher available expiratory flow and minimizes the energy
wasted on generating pleural pressure beyond what is
needed for maximum flow. Yet, if EELV continues to rise
and approaches or exceeds functional residual volume, the
diaphragm becomes forced to contract in a mechanically
compromised length and the POB required can increase.
Since EFL was only detected during the last stage with the
heaviest backpack, we were not able to determine if our
subjects would have increased EELV to avoid or minimize
EFL. We speculate that the degree of EFL would increase
if our minimally affected subjects had increased their _
VE
further, because EELV would be constrained to a low level
from the heavy backpack.
The EILV did not appear to change between backpack
conditions; however, it did show an increase between
stages. The increase in EILV between stages can be
attributed to the increased tidal volume with similar EELV.
However, we must caution that our measurement of oper-
ational lung volumes relied upon the IC maneuver, which
forces the use of relative values. The absolute value for
EILV could have changed between conditions, but because
we expressed the values relative to FVC (which also
decreased between conditions) any absolute change may
not be seen.
Ventilatory demand and capacity during exercise
When exercising with the added load of a weighted back-
pack, _
VE increases due to the greater metabolic load
(Fig. 5; Table 3). In healthy subjects not carrying a
weighted backpack, the increasing _
VE is coupled with a
relatively stable _
VECAP, causing the _
VERES to slowly
decline as exercise intensity increases. An individual’s
‘‘true’’ maximal _
VECAP is an inherent attribute that is not
trainable. Rather, _
VECAP is related to one’s intrinsic
respiratory anatomy such as lung and airway size. To asses
one’s _
VECAP, there are two commonly used techniques: the
maximum voluntary ventilation maneuver and a measure-
ment which is derived from tidal flow-volume loops placed
within the MEFV according to EELV. We chose not to use
the maximum voluntary ventilation measure as it repre-
sents an unrealistic breathing pattern that would not show
Fig. 6 Percent of ventilatory capacity utilization at two iso-ventila-
tion exercise stages. The two groups of points (A) and (B) have minute
ventilations that are not statistically different (48 ±2.0 vs.
49 ±2.4 l min
-1
and 69 ±3.9 vs. 62 ±2.7 l min
-1
,P[0.05; for
Aand B, 35 kg and NP, respectively), while the 35 kg backpack
condition uses significantly more of their ventilatory capacity
(38 ±2.7 vs. 25 ±2.0% and 52 ±2.2 vs. 31 ±3.3%, P\0.05;
for A and B, 35 kg and NP, respectively). Values are mean ±SE.
The values for the NP condition are from stages 4 and 5, while the
35 kg condition is using stage 1 and 2. Filled circles represent no
backpack condition. Filled squares represent 35 kg backpack condi-
tion. _
VE minute ventilation, _
VECAP ventilatory capacity, NP no
backpack
Eur J Appl Physiol (2012) 112:2001–2012 2009
123
the weighted backpack effects (Johnson et al. 1999). By
accounting for exercise-related changes in EELV and tidal
volume, the latter form of measurement gives a more
accurate assessment of _
VECAP at a specific workload. As
the weight of our backpacks increased, we found that both
_
VECAP and _
VERES declined. The decrease in _
VERES indi-
cates that the ventilatory demand is approaching ventila-
tory capacity. Although, demand never approached 100%
of capacity, approaching such a value could lead to relative
alveolar hypoventilation which can be associated with
various systemic consequences (exercise-induced arterial
hypoxemia) (McClaran et al. 1998). Figure 6shows the
effects of the 35 kg backpack on _
VE=_
VECAP. The points
labelled ‘‘A’’ and ‘‘B’’ have similar _
VE, yet the _
VE=_
VECAP
percentage is significantly higher in the 35 kg backpack
condition, indicating that the same absolute _
VE demand
represents greater proportion of available capacity. The
‘‘Demand vs. Capacity’’ model has been used to describe
why some population groups are more susceptible to a
variety of pulmonary system limitations (Dempsey 1986).
Methodological considerations
Several of our outcome variables (EFL, operational lung
volumes, _
VECAP) are all dependent on the correct perfor-
mance and analysis of the IC maneuver. To ensure accu-
racy we took several precautions. First, prior to any data
collection, each subject had extensive practice performing
the maneuver. Second, using an oesophageal balloon
catheter allowed us to determine if peak negative oesoph-
ageal pressure was consistent within conditions. Third,
during analysis, volume was corrected for pneumotacho-
graph drift. With appropriate precautions, the IC maneuver
has been used extensively and previously shown to be
accurate and reproducible in both healthy and clinical
populations (Henke et al. 1988; Mota et al. 1999; Johnson
et al. 1992).
Operational lung volumes were determined using the IC
maneuver, not a fully body plethysmograph, or comparable
instrument. Therefore, operational lung volumes are all
expressed in relative terms, rather than absolute values. As
mentioned above, the IC maneuver is accurate and appro-
priate when measuring operational lung volumes during
exercise because TLC is not thought to change. Thus,
expressing operational lung volumes in relative terms
reflects actual changes in lung volumes. However, in the
present study, the added weight of backpack probably
caused FVC to reduce because TLC occurred at lower
absolute lung volumes. Therefore, while the relative
operational lung volumes could be unchanged (as was
EILV) the absolute lung volume could be different.
To ensure the magnitude of EFL was not overestimated,
we utilized two techniques to ensure a correct MEFV curve
generation. First, to minimize the effect of thoracic gas
compression, the subjects performed graded FVC maneu-
vers. Second, to account for exercise-induced bronchodi-
lation, the subjects performed graded and non-graded FVC
maneuvers before and after each bout. Both techniques
have been shown to minimize overestimation of EFL
(Guenette et al. 2010).
All of our subjects utilized an identical backpack and
packing was done in a similar manner by the same
experimenter. However, while the selected backpack was
an appropriate size for all individuals, a ‘‘perfect’’ fit could
not be guaranteed. Slight variations in subjects’ anatomical
feature (back-length, chest and shoulder girth) could result
in a less than ideal backpack fit, resulting in discrepancies
in our results (Bygrave et al. 2004). Additionally, the
sternum and waist straps’ tension were not standardized
between subjects. Subjects were instructed to fasten both
straps in a similar manner as if they were undertaking a
backpacking trip, therefore, due to their experience, all
subject securely fastened the straps. However, there still
could be slight variations in the tension which would result
in differential loading of the chest-wall. All the subjects
were satisfied with the overall fit of the backpack.
Weighted backpacks and occupational relevance
Some occupations, such as firefighting, must exercise at
high intensities for a prolonged period while carrying a
weighted backpack. Previous studies (Butcher et al. 2007)
have shown that a fully equipped firefighter must cope with
ventilatory constraints elicited by their equipment. In the
previous study (Butcher et al. 2007), the effects of only one
backpack weight were studied. However, as we have
shown, different weights of backpacks can have signifi-
cantly different effects on the respiratory system. It is
reasonable to assume that the constraints shown by Butcher
et al. (2007) would only be amplified by a heavier back-
pack. Severely compromising the respiratory system can
lead to the systemic problem of compromised blood flow
(Harms et al. 1997).
High altitude porters could be especially vulnerable to
backpack-related changes in the respiratory system. These
porters are required to carry a heavy backpack for pro-
longed periods, in environmental hypoxia (Minetti et al.
2006). Environmental hypoxia forces an individual to have
a significantly higher _
VE for any given workload (com-
pared to a normoxic environment) in order to maximize
blood oxygenation. Furthermore, cardiac function at alti-
tude could be significantly decreased from the combined
effect of pulmonary hypertension (Eldridge et al. 2006) and
2010 Eur J Appl Physiol (2012) 112:2001–2012
123
the excessive pleural pressure due to EFL (Stark-Leyva
et al. 2004). The combined effects of hypoxia and a heavy
backpack would undoubtedly lead to impaired ventilatory
mechanics. The added ventilatory constraints are perhaps
why some high altitude porter carry their load suspended
from their forehead rather than the typical backpack style
(Bastien et al. 2005).
Conclusion
Wearing a weighted backpack appears to cause real phys-
iological changes in lung function and breathing mechan-
ics. Compared to not wearing a backpack, a 35 kg
backpack reduced FVC and EELV, both of which resulted
in a decreased _
VERES. Yet, when comparing similar ven-
tilations, the required POB was not different. Aside from
the increased metabolic demand from carrying a heavy
load, the ventilatory changes associated with a heavy
backpack does not alter the energetic cost of breathing.
However, more extreme stressors like environmental
hypoxia or carrying an excessively heavy load may further
compromise the respiratory system.
Acknowledgments We thank our subjects for their enthusiastic
participation. This study was supported by the Natural Science and
Engineering Research Council of Canada (NSERC). G.E. Foster was
supported by a post doctoral fellowship from NSERC.
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