J Forensic Sci, January 2007, Vol. 52, No.1

doi: 10.llllIj.1556-4029.2006.00296.x
Available online at: www.blackwell-synergy.com

Betty A. Michalewicz, I M.S.; Theodore C. Chan, 2 M.D.; Gary M. Vilke, 2 M.D.; Susan S. Levy, I Ph.D.;
Tom S. Neuman, 2 M.D.; and Fred W Kolkhorst,I,3 Ph.D.

Ventilatory and Metabolic Demands During
Aggressive Physical Restraint in Healthy Adults

ABSTRACT: We investigated venti1story and metabolic demands in healthy adults when plsced in the prone maxima1 restraint position (PMRP),
i.e., hogtie restraint. Maximal voluntary ventilation (MVV) was measured in seated subjects (n = 30), in the PMRP, and when prone with up to
90.1 or 102.3 kg of weight on the back. MVV with the heaviest weight was 70% of the seatedMVV (122 ± 28 and 156 ± 38Umin, respectively;
p<O.OOI). Also, subjects (n~27) were placed in the PMRP and struggled vigorously for 60 sec. During the restrained struggle, ventilatory
function (VJ MVV) was 44% of MVV in the resting PMRP. While prone with up to 90.1 or 102.3 kg on the back, the decrease in MVV was of no
clinical importance in these subjects. Also, while maximally struggling in the PMRP, VB was still adequate to supply the ventilatory needs.

KEYWORDS: forensic sciences, maximal voluntary ventilation, minute ventilation, oxygen consumption, positional asphyxia

Law enforcement and prehospital care personnel often confront
violent, dangerous individuals who must he physically restrained
to ensure safety to themselves and to those around them. Author·
ities have developed a numher of physical restraint techniques to
control and subdue such individuals in the field (1,2). One of
these, the prone maximal restraint position (pMRP), also referred
to as the hogtie or hobble restraint, has heen used extensively by
field personnel (3,4). When in the PMRP, an individual is prone
with hislher wrists secured behind the back, ankles bound together, and wrists and ankles tied together using handcuffs, cords,
chains, or hobble devices (3,5).
Reports of sudden deaths in individuals in the PMRP have ap·
peared at least since the 1980s, which have created controversy
regarding the safety of these restraint positions (3,6-8). Some au·
thors have suggested that the PMRP may prevent adequate chest
and abdominal movement, which places the individual at risk of
asphyxiation (7-9). Asphyxiation that is caused by body position
has been referred to as 'positioua1 asphyxia" (10). However, a
recent study reported that, although PMRP by itself resulted in a

small, restrictive ventilatory pattern compared with seated measurements, there was no evidence of hypoventilation, hypercapnia,
or hypoxemia (11).
Additionally, police officers often apply force to the back to
better control an agitated person during the restraint process. This
additional force has been hypothesized to constrict the chest and
abdomen more than the PMRP alone (12-14) and lead to as·
phyxiation (IS). Chan et al. (3) examined the effects on po1mon·
ary function from 11.4 to 22.7 kg applied to the back of prone
subjects. Although the PMRP with or without force applied to the
back led to a restrictive pattern on pulmonary function testing,

there was no evidence of hypoxia or hypercapnia.
lDepartment of Exercise and Nutritional Sciences, San Diego State University. San Diego, CA 92182.
2Department of Emergency Medicine, Medical Center, University of Cal-

ifornia, San Diego, CA 92103.
3San Diego State Heart Institute, San Diego State University, San Diego,

CA 92182.
Received II Dec. 2005; and in reWed form 19 June 2006; accepted 1 July
2006; published 8 Dec. 2006.
Copyright © 2006 by American Academy of Forensic Sciences

Nevertheless, it would be safe to assume that law enforcement
officers are able, and do, apply more than 22.7 kg of force to the
back of suspects placed in the PMRP, particularly if the individ·
uals are violent. Cases of sudden death of restrained individuals
often involve those who continued to struggle after heing reo
strained (8). This study was undertaken to determine whether the
use of force greater than 22.7 kg might inhibit ventilatory function
such that it became a clinically important consideration in the
analyses of such deaths. Moreover, we were interested in whether
individuals stroggling during periods of physical restraint were
able to approach the limits of their ventilatory function. This
study, which utilized a randomized, cross-over, controlled design,
had two parts. For the first part, the effects on maximal voluntary
ventilation (MVV) in subjects were examined while in the PMRP
and while prone with up to 102.3 kg of weight positioned on their
back. The second part of the study was an investigation of max·
imally struggling subjects while in the PMRP on cardiopulmonary
measurements to determine the effect of PMRP on ventilatory

function.
Methods
Subjects

Thirty volunteer male and female subjects were recruited to
participate in the study. Exclusionary criteria included any history
of pulmonary or cardiac disease (as screened with the Physical
Activity Readiness-Questionnaire), current recreational drug use,
or other significant illness or disability that would limit the ability
to perform the exercise regimen required for the study. Each po.
tential participant was screened hefore testing by a physician in·
vestigator to ensure that helshe was free of acute illness or injury.
Also, urine specimens were collected and tested for the presence
of the major metabolites of common drugs of abuse (i.e., phen·
cyclidine, benzodiazepines, cocaine, amphetamines, THe, morphine, and barbiturates) and tricyclic antidepressants (Triage@
Dru~s of Abuse Panel plus Tricyclic Antidepressants Test, Bio·
site Inc., San Diego, CAl. Individuals were excluded if the im·
munoassay detected any of these substances in the urine. Informed
consent was obtained from each individual before participation,

171

172

JOIJfIIW. OF FORENSK: SCIENCES

.-I aIb;ecb: wbo ~ the Itudy Mft! I i - 6. .1 com~ 'Ibe ~ ...,. cIcaip &lid pcICDooI ""' cP~ by Ibc
Dkco SCIlc UrdYC:l:li.ty ad !be Urd'l'Cl'lily of
c.liC0!Diiw,
Dieso Imtirutioaa1 11m- Bo.oI (IIlB). mel
prot' , ... " " ' followed in accmdaDa: with the cdDcaJ.1taDdardr.
caNi"''''' br IlIe IllBI.

s.a
s....

PIUt J-Politicll tlIId WC!i8ht EJfteU 0It Ma:tiIIwl Vollmulry
VMrilarIcM (atVV)

The finI ~ oflhlll\udy evalllllld the effectJ of IUbjllCtl in the
PMRP .Ild .beD prone trirh v.ying trt:i&Ja I:. tile t.:k upoo the
MVV. A Ucea.od AmericaD 'JborIc:ie Soclcq (Ai'S) ~
..... haicD, ,.,."..... od Il11 MVV tcaIiDc (UodGnphice Cl'XJD 5yIU!IIl, MedkaI <lnFIDcI c..p.noaa, St. J>.1. MN). .-I eEb IriaI
_ repe&Imd III: leu!: hrEe.. ~ were obbiIB:I ill ac..
CCll'dEI::e wi1tI ATS ilbiDitlidi fm iCfC' dlriNlicy f;Dd ~
(16). bw MVV d.ca ""' zamatm:d h ... bc:ipt. padcl', mel
(l7).
MVV w.. meuured with rubjrdl Rart.d IIl4 .bile i1I. the
PMRP. Ia..sdi!iDD, MVV wulDlCllllled with Plbjoeu pMIC and
thme d.i.ft'.nm YmiJhts plIced OIl the back. The onim' of theIe
IIlfIUUI'lmlIalta WIll; nndomly choIeD to !IIIpte pouible ordm
effcett of rcpcdii'¥e tcltiJI&. For tile ICIlCd potItion, II!bjcetl !1It
OIl a chair wiIbout n::IInIird wilh !heir Ccct flat CD IDe azOIaId and
tbrtir bad: upript .,.mat Ibe cbir baclL DvriDr Ibe PMRP trW,
~ _
pUud prtlIIC I:. I D - * mat with the bead
rumcd II) !be Ddt. 'Ik ~' wriIU IUId IUIkb _
ti&bdJ
bowId IopdJcr beblDd !be bid: IIIiD& • maniaIa& faIIric cd
(H&. 1). n. __ -' .om _
~ IMJl.a !up!B. nil
poIIitioa. 1n.I Ii.-.r iii thI: PMRP t.m ill pre..... IIIIdB &lid
_
ftIP(IrtI (11). Far chc prtlIIC tria1a wi.ch wciahta. ali&bt (LW).
mocknlll (MW), mil bcayY wQg.ht (HW) in the Corm cC caD""
bq:l. eath &led wtlb. lI.4q aI ieMllbat, were plM:ed 0Il1vbjecu' bac:b. The C&IlTU bqs were eymIy poIitlMod ~ the
portarior IbfVIden and mid bIc:1I: lad !ICCarOd by a ~madc
'ICII _
by the lubja:! that had IlnIpI to Imld !ho CIlIa".. hap in
place. N we thIDized !hat law .mu.CUlI£Id: af'fIdab vroald .pply
leu wclJbt 10 the bIck when rertraiJliD& a llJbIet t.diridual, we
\IICd difforiq wei&btI depcading OIl tilt rubject'. -.eiPl SubjrI:b.tao weiJbed" tb. 68q __ 1MIId wilb 22.7:q (LW).
56.1q (MW), .-I !iO..5Iq (HW) wbile IIIlhjec:IIa weipin( IIXft
thao 68q 1i'I:n ~ IIriIII lot.lq (LW). 6&.2q (MW). aDd
101311 (HW). Ia &II fM:. ..
MVV . . . .
'ed
l1ICII

_.M.,

i.........iwJy . . ~ the ~ ~ the Iriah,
IUb;cctI aced ia a.-d p;IIitioD. fm c. tSmiD.

-,.

Ptur l--OvrIl~

111,_,-. DIulIfI JI4rtoroal

.Pot Iht: ~ put of the Itady, oxygca OOZI~ (t'Oil,

miIlule VlIDIi1aIiOl1 (li's; ~ rate x tidal vo1ume).1I!Id bHrt
rate (HR) 1nIn: IDIWIInd dllriD8 a 6O-111C mnjm.J ItrQQIe while

lubjCCU were.ill tho PMRP. Pulmonary Vo,; il a _
of I)'Jtcmie CII;y~ udllatiOD, .hicb:rcl1cctl the mctabalic ~ .., !he
muimel VO:t (m-J quaoli1illl the .subic capadIy. Subject.
were placed pmt'lII 011 a ~ dr;,Med c. 4Ocm:&om the 1Ioor.
A flcc DIiIIIt Wh a 1ow"-t:eU!~ two-WlY brcaIhlJI.a ftlve 1n.I
pI.ocd ova the IIIOIIII:h aad - . . aDd Ik..,ja:t _ pnriri ....
aacb Iba!: IIiII'I:I« bAd ...... O'IU the atp of!hc II)I!hA
1KJIIld _ arI.l:e . . IIoor Of ~ ~. the IlrUQle. AIr. . .
aft ad appliod 10 110 edp ~thI: mMt_ UICd. to preYCDt m
kIIb. The .ubjcx:u.' wriltl mil &DkIca .en: tiJhtly ""-' ~
behind the bad: t.hIilK to the PMRP described ahn\tr). Subjed:a
then ~(hc4 a mvimal IIIrIIgIc---in an Ittempt 10 eac:ape Cbe
IeIIraiJIt- far 60_ wbile cardiopalmnnaty ~U WCft
tabm (TrusMaJ. 2400. Parvo Modics IDe., Smdy. IJI). V.t.l
encoorapmmIt .... provided IIIrt!qhout eaci1ltrL FIIlIr:Iwioa: Cbe
6O-see ItnIftlc, lubjCCUlaid qgieIIy 011. Iht: !D8llI'e" for 5 miD 10
l'tIOOVC:t W"hiIe ~ IIIir
iii be analyzed. Br:foro t«:b
lei!, the p i analy.-..-I pncunoI...oo...-_c:alibnud, HR
_ moai!arrd
a ~ b'wmlitter: Mtvbecl iii !hc cIlIIIt
(PolAr 1Dcc!laaict. Eempdc, PiDIMd). All r'll' _ _ _
laxatbi .. zbD ~ uf I.s.- izdcznJa.
ftJr ~ !he _
cmtinp'lrnm-y .....-en _

.d

!XJQll_"""

.au,

~

.-ru... a

'Nn';.w ~ tal 011

a

tqW* .,..

I\iIjecU pcdarmed a tradmilI mat at 1.61 mit«. whilD~_
IIlDIW)' ~ wm: IIICUIIlaI. The initial. cIGn&ioa _
0'J(p
aDd .... iDcmued 29&1IYIIrY miaute IIIII.il the mbjlllCt readied YO~ fati&uc. V~ CDeOIID.JC!DCII- Uo provided dllJiaa
1cItiDJ· 'Ibe crlIcWi far reaeI!iDa Vo:-. wczc acIdoviD&: (1) a
p1atcmiD ~ (::I: 2m1JkWmiD); (2) II1HR::I: 15bpm oflht: l i p
pmdictM mesb".1 HR; mdIm: (3) a ~ em..... n.tIo
(RHR) fI 1.l0 or abo'¥e. All Albje<:1:l .meved at leut two fI
the a:iIa:i&.

""""'" .....

.

.

Toanalyome. dledl'«:tolthePMRP ad weight brleOD MVV a

1JDD..• •y I ,

"IJt

lklzImmni'II_~

a'

L .. ANOVA _

. . . . . Inl

I'wt ""

were 11-'. iii foUaw up Id",it\cont dl.fI'.erer........

CIt "'.. let
0.05 far .n 1er15 fI signifielnec. All IItItbtital
analylel ware pedormfld IIIiDa SPSS v. 11.0 (SPSS (aQ" Cdcqo, a). N'.-y,fi.. pIIlCIII1 confjdrwp.. inllnl1l
calculated to ~ MVV v.J.uea ia the vmoos pnrJtI_ ..,jib
the: predicted MVV and to MVV mcuurcd in Iht: ICatod politUJa.
To analyome. CIrdiopulmaaIry meuarcmtDta dllliq ml.·lm,!
ItragIe, we cnmpu.! ~ WriDa; PRMP rtrug\tI with
iii aMelI the impIct of rsIniDt I:. OlIJPIl nwmwpiOl'l. lA .dditioa, we CIOCq*od VII aDd Vr/MVV ratio to _
r:apKi1J I11III ~ dIDq: PRMP IInIaJc IIId die mui mel

caw .......

............

---

m.-'*!Cry

Thirty.ubjDda (15 mm mil 15 _ ) "II]( ':1 1 the fint part
of Iht: study " .... ini ... the dfectI of !be P'MRP and ~Jht
011. MVV. Db from tMl lubjeett 1i'I:n t:lduded from die..".

MICHALEWICZ ET AL. • PHYSIOLOGICAL DEMANDS DURING RESTRAINT

TABLE l-Subject characteristics (mean ± SD).
Total (n
Age (yea:r)
Weight (kg)
Height (m)
BM!

~

30)

24.5 ± 3.5
72.6 ± 14.2
1.72 ± 0.09
24.5 ± 3.5

Men (n

~

15)

25.0 ± 4.2
81.4 ± 13.9
1.76 ± 0.10
26.1 ± 3.5

Women (n ~ 15)
28.9 ± 6.2
63.9 ± 7.3
1.67 ± 0.06
22.9 ± 2.7

because they were psychologically unable to tolerate restraint
(neither subject left the trial for any complaint other than they
were frightened of being restrained. Both these subjects elected to
withdraw from the study before the exercise period commenced
and before any data were collected). Subject characteristics are
presented in Table 1. All subjects were healthy and at least moderately active [~ = 50.2 ± 7.8 mIJkglmin; body mass
index (BMI) = 24.5 ± 3.4]. Overall, 50% of the subjects weighed
less than 68 kg, which included 12 women and three men.

Part i-Position and Weight Effects on MW
The results of the MVV measurements under various conditions
are presented in Table 2. Because there was a significant departure
from sphericity (p = 0.001; E = 0.646), Greenhouse-(Jeisser adjusted values were used to evaluate the significance of the main
effects. Confidence intervals that included 100% were not considered different from the predicted or the seated MVV. MVV in the
seated position was 156 ± 38 Umin, which was 122% higher than
predicted (CIg, = 104-140). Conversely, MVV while prone with
the HW was lower (85%) than predicted MVV (Clgs = 72-98).
All measured MVVs differed from each other (p<0.001),
except the comparison of the PMRP and MW trials. MVVs of
the treatment trials were also compared with the seated MVV.
MVV of the PMRP trial and when prone with MW and HW were
significantly less than the seated MVV, although the C1gs from the
LW trial suggested no difference from the seated MVV
(CI.s = 77-100).

Part 2-Cardiopulmonary Measurements During Maximal

Struggle
Valid data were obtained from only 27 subjects for this part of
the study. The struggle was physically difficult for subjects. In
spite of continued verbal encouragement, the intensity of movement was visibly waning in all subjects by the end of the 60sec trial. This was supported by the RER---<:alculated as
2:
V02-that averaged 1.16 ± 0.14. Only one subject, a female,
failed to achieve an RER greater than 1.05 during the maximal

veo

173

i ':

;=_E

~

8070

•

•

b- -I!l.A, 6",

'A'A,
~

'6 'A,

~

'1 :
~

40

Z

30

l

20

'5

10

..J:I

~

6,~

'6'A'6'

Restraint

Slru

I

O~~W~.~

o

60

A

HR

' 6 '6--b--6'6'A'6'A

va,
Passive
recovery
____~____~
____
~~__~__--,
120

180

240

300

360

Time{s)

FIG. 2-Percent differences ofcardiopulmonary valuesfrom peak treadmiU
values during and after the restrained struggle trial.

restrained struggle. Values in excess of 1.0 indicated that an individual was hyperventilating, and the values observed at the end
of the maximal struggle were sintilar to those observed at the end
of their maxima1 treadmill test (1.21 ± 0.07). At the end of the
struggle, Vo 2 and VE were 40% and 42%, respectively, of the peak
values achieved during the maximal treadmill test (Fig. 2). Also,
HR at the end of the struggle was 84% of peak HR from the
treadmill test. The VEIMVV ratios (using MVV measured in
the seated position) of peak VE during the treadmill test and at
the end of the restrained struggle were 89% and 44%, respectively
(Table 3).

Discussion
Although sudden death has occurred in individuals placed in the

PMRP, the cause of death and whether body position was a factor
remain controversial. Some have suggested that PMRP prevents
adequate chest wall, abdominal, and diaphragmatic movement,
leading to hypoventilatory respiratory compromise and risk for
death from positional asphyxia (8,18). Prior studies in healthy
subjects have found no evidence of significant hypoventilation
when subjects were placed in the PMRP (2,8). Our results in this
study appear to support these findings. MVV in all of the treatment conditions remained above 80% of predicted, well within the
normal range (19,20).
While by itself the PMRP does not appear to compromise ventilatory capacity unduly, the restraining process also frequently
includes applying force to the back. Weight applied in the prone
position has been hypothesized to further compress the chest and

TABLE 2-Measured MVV values (n = 30).
MVV(Umin)

Position
Seated
PMRP
LW
MW
HW

Mean ±SD
156 ±
128 ±
137 ±
122 ±
109 ±

38
29
27
31
28

Percentage of Seated MVV

Range
75-243
65-193
85-189
61-197
57-167

%

82"
88
78"
70"

Percentage of Predicted MVV

a.,

%

C1"

68-96
77-100
64-93
54-116

122
100
107
95
85'

104-140
99-102
97-117
87-103
72-98

"'Below MVV measured in the seated position.
tBclow predicted MVV.
Note: All measured MVV values differed from each other except for the PMRP and MW comparison (p< 0.001). PMRP, prone maximal restraint position; LW,
prone position with low weight (22.7 or 34.1 kg) applied on subject's back; MW, prone position with moderate weight (56.8 or 68.2 kg); HW, prone position with
heavy weight (90.2 or 102.3 kg); MVV, maximal voluntary ventilation. For the three conditions in which weight was placed on the back, the lighter weight was used
for subjects who weighed less than 68kg (n = 15) and the heavier weight for those who weighed more than 68kg (n = 15).

174

JOURNAL OF FORENSIC SCIENCES

TABLE 3-Cardiopulmonary peak values during maximal treadmill test and
at the end of the restrained struggle (n = 27).

Restrained Struggle

Treadmill

Mean± SD

liO:z

(mI1kgImin)
VB (l./min)
VT (L)
RR (brea1llslmin)
IIR (beatslmin)
RER

50.2
140.1
2.8
56
190
1.21

±
±
±
±
±
±

7.8
36.7
0.7
8
12
0.07

Range

29.1-65.8
84.2-206.5
1.8-4.7
39-70
166-221
1.09-1.35

Mean± SD
19.8 ±
57.6 ±
1.0 ±
60±
160 ±
1.16 ±

5.4
23.3
0.4
14
19
0.14

Range

11.6-30.9
29.4-113.3
0.6-2.3
35-1!8
105-196
0.92-1.35

HR., heart rate; RER, respiratory exchange ratio.

abdomen, which might lead to hypoventilatory respiratory compromise (IS) and diminished ventilatory function to the point of
asphyxiation (20). One of the goals of this paper was to isolate the
effects of weight applied in the prone position. Our results indicated that with 90.2-102.3 kg of weight applied to the back of our
subjects, MVV was decreased to 8S% (CI.s = 72-98) and 70%
(CI.s = S4-4l6) of the predicted and previously measured MVVs,
respectively. Despite these decreases, these MVVs are still within
published Clos for men (12) and women (14).
Even though the decreases in pulmonary function as a result of
weight force applied to the back remain with the normal clinical
parameters for a healthy person at rest, the circumstances of
PMRP-related sudden death cases are very different. Sometimes,
the victim has been involved in high-intensity exercise (e.g., running, fighting) before being restrained and, afterwards, will continue to resist the restraint violently. It has been suggested that
under these circumstances, oxygen consumption may exceed ventilatory capacity in individuals placing them at risk for respiratory
compromise (21). All such, we also measured V0 2 and VB while in
the PMRP and compared them with similar measurements from
maximal treadmill tests. Our results indicated that with maximal
struggle while in the PMRP, V0 2 and VB were less than 42% of
peak values obtained from a maximal treadmill test. In general,
the most metabolically and ventilatory demanding type of exercise
occurs when large muscle groups
resulting in the highest
work in a rhythmic fashion (e.g., rwming, cycling). It is likely that
PRMP limits subjects from using these large muscle groups in
rhythmic movements, thus resulting in the low V0 2 and VB we
observed.
Ventilatory constraint is often determined by measuring how
close VB at maximal exercise intensity approaches MVV (18). At
V02max, individuals with normal lung function ventilate at 6070% of their MVV (22). Accordingly, in our study, the peak V";
MVV during the maxirual treadotill test was 72% of the measured
MVV. On the other hand, during PRMP struggle, VB IMVV was
ouly 36% of the measured MVV. Our findings of clinically normal
MVVs with PMRP and prone weight in phase I, as well as the
lower
and VpfMVV during PRMP struggle suggest that our
subjects appeared to have adequate ventilatory reserve when
struggling while restrained. Furthermore, the extremely low V";
MVV ratio at the end of maximal struggle, compared with the
actual MVV measured with weight on the back, suggests that
should weight be applied while individuals were in the PMRP it
would be well tolerated as well. Clearly, this remains to be proven
in future studies.
Based on these findings, as well as previously published studies,
we suggest that factors other than ventilatory failure associated
with the restraining process may be responsible for the sudden
unexpected deaths of restrained individuals. Although autopsy

va,

va,

evidence is often unrevealing as to the cause of the death, those
individuals who die at times seem to succumb suddeuly (23),
which is a pattern generally inconsistent with a respiratory death.
Some individuals have been repurted to die suddenly while restrained without force applied to the back (6,7,10,23), restrained
in a supine, sitting, or side position (9, 17), or even without being
restrained (24). Other factors, such as excited delirium, drug intoxication, stress, trauma, and catecholamine hyperstimulation.
are considered to be the most likely factors in these sudden deaths
(2,3,19). In addition, studies indicate that many of these individuals have an abnormally enlarged heart on autopsy, likely related
to chronic stimulant drug abuse (S). Not only is there a greater risk
for cardiac dysrhythmias and sudden death in those with cardiomyopathy, but recent investigations suggest that individuals with
this condition have decreased capillary density in their endocardium, placing them at risk for chronic and perhaps acute cardiac ischemia (2S). Our results, as well as those of others (2,3,IS),
suggest that in deaths associated with the PMRP, factors other
than ventilatory compromise may playa more important role.
Clearly, this study has a number of limitations. First, our subjects were young and generally healthy and may not reflect the
population of individuals who are restrained in the field sening. It
should be noted that the baseline-measured MVVs of subjects
were 122% of predicted, suggesting that our subjects were both
highly motivated, and had a high aerobic fitness level (26). In the
actual field setting, underlying medical conditions and other differences from our subject population (e.g., age, weight, etc.) might
theoretically influence the outcome.
Second, we could not reproduce all conditions during which
this type of restraint method is used in the field. In particular,
while we had subjects restrained and maximally exening themselves, we could not reproduce the psychological or other physiologic stresses associated with a field pursuit, struggle, or
trauma). During the trials with weight applied to the back, the
weight was disttibuted evenly over the back, unlike in a field
situation in which force is applied to the back frequently with a
knee that focuses the force over a smaller area. In addition, a small
number of our subjects did opt out of the study out of fear of the
restraint Clearly, in the field setting, individuals are unlikely to

have such a choice as to whether they are restrained or not; however, it is difficult to understand how such factors might affect
ventilation.

Therefore, how such factors may or may not contribute to these
deaths will require future study; however, an animal model suggests that restraint alone (without affecting an animal's ability to
breathe) increases the death rate in animals treated with cocaine
(27). This implies that the physiologic effects of restraint involve

more than ventilation alone, which is consistent with the results of
this and prior studies (3,11). Second, we placed weight on sub-

jects' back when MVV was measured in the prone position, but no
force was applied on subjects when positioned in the PMRP. Such
a model has its limitations and does not necessati1y duplicate the
sequence of events that may take place in any given field situation;
however, we do feel this represents a first step in investigating a
very complex arena. Third, the exertion and struggle of our subjects in PMRP was of short duration and also may not reflect a
field situation where prolonged struggles can occur. Furthermore,
our subjects exerted themselves on a voluntary basis, although
they were verbally encouraged by the investigators to struggle as
much as possible throughout the I-min periud. This voluntary nature may not exactly reflect field situations where individuals are
often under the influence of drugs or are mentally incapacitated.
On the other hand, our subjects did exercise to exhaustion or near

MICHALEWICZ ET AL. • PHYSIOLOGICAL DEMANDS DURING RESTRAINT

exhaustion and their significantly increased HR suggests high levels of exertion on their part.
Finally, of course, none of our subjects used illicit drugs. However, none of the illicit drugs frequently used in the setting we are
trying to simulate have an effect upnn ventilation and therefore it
appears to be more likely that the role such drugs play in these
deaths is through some other mechanism than their effect upon
ventilation. There are other factors in the field that that are also
dil'ferent from our serting that might theoretically affect our results
(e.g., a gymnastic mat rather than the actual outside surface, and
the even distribution of the weight force we used across the back
rather than it being localized to a smaller area); however, it will
await future studies to determine wbether these factors play any
role in such deaths.
In sururuary, this study attempted to investigate the impact of
varying weight force upon the back in healthy individuals in the
prone position. We recognize the dil'ferences between the laboratory setting and actual field conditions; nonetheless, we found no
clinically important restriction of ventilatory reserve when subjects were placed in the PMRP or when prone with up to 90.2 or
102.3 kg of weight on their back. Likewise, when subjects were
maximally struggling for 60 sec while in the PMRP, there were no
clinically important limitations of metabolic or ventilatory functions. Based on these observations in healthy subjects, we conclude that PMRP and prone pnsitioning with moderate weight
force on the back do not in and of themselves restrict metabolic or
ventilatory demands to any clinically important degree. As such,
factors other than isolated ventilatory failure should be considered
wben evaluating deaths occurring in the serting of restraint in
the field.

Acknowledgments
The authors wish to acknowledge the contribution of Mohammed Najeed, RT, UCSD Medical Center, for performing the MVV
measurements in this study.
This study was funded in part by an unrestricted grant from the
University of California, San Diego, Department of Emergency
Medicine Research Fund. Portions of this study were presented at
the Society for Academic Emergency Medicine, New York, NY,
May 2005.

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Additional information and reprint requests:
Kolkhors~ Ph.D.
Department of Exercise and Nutritional Sciences
San Diego State University
5500 Campanile Drive
San Diego
CA 92182-7251
E-mail: fred.kolkhorst@sdsu.edu

Fred W.