DTIC ADA524537: Hypohydration Reduces Vertical Ground Reaction Impulse But Not Jump Height

0.36 MB·English

by  Defense Technical Information Center

Form Approved
REPORT DOCUMENTATION PAGE
OMB No. 0704-0188
The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,
gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of
information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188),
1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any
penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.
1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From - To)
2010 Journal Article-Eur. Journal of Applied Physiology
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Hypohydration Reduces Vertical Ground Reaction Impulse But Not Jump
Height
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
S.N. Cheuvront, R.W. Kenefick, B.R. Ely, E.A. Harman, J.W. Castellani, P.N.
Frykman, B.C. Nindl, M.N. Sawka
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
REPORT NUMBER
Thermal and Mountain Medicine Division
M09-59
U.S. Army Research Institute of Environmental Medicine
Natick, MA 01760-5007
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)
Same as #7 above.
11. SPONSOR/MONITOR'S REPORT
NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release; distribution unlimited
13. SUPPLEMENTARY NOTES
14. ABSTRACT
This study examined vertical jump performance using a force platform and weighted vest to determine why hypohydration (~4%
body mass) does not improve jump height. Methods: Jump height and other measures of functional performance from a force
platform were determined for 15 healthy and active males when euhydrated (EUH), hypohydrated (HYP) and hypohydrated while
wearing a weighted vest (HYPv) adjusted to precisely match water mass losses. Results: HYP produced a significant loss of body
mass [– 3.2 ± 0.5 kg (– 3.8 ± 0.6%); P<0.05], but body mass in HYPv was not different from EUH. There were no differences in
absolute or relative peak force or power among trials. Jump height was not different between EUH (0.380 ± 0.048 m) and HYP
(0.384 ± 0.050 m), but was 4% lower (P<0.05) in HYPv (0.365 ± 0.52 m) than EUH due to a lower jump velocity between HYPv
and EUH only (P<0.05). However, vertical ground reaction impulse (VGRI) was reduced in both HYP and HYPv (2-3%) compared
with EUH (P<0.05). Conclusions: This study demonstrates that the failure to improve jump height when hypohydrated can be
explained by offsetting reductions in both VGRI and body mass.
15. SUBJECT TERMS
dehydration, fluid balance, strength-to-mass ratio, jump performance
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF Samuel N. Cheuvront, Ph.D.
PAGES
Unclassified Unclassified Unclassified Unclassified 8 19b. TELEPHONE NUMBER (Include area code)
508-233-5607
Standard Form 298 (Rev. 8/98)
Reset
Prescribed by ANSI Std. Z39.18
EurJApplPhysiol(2010)109:1163–1170 Author's personal copy
DOI10.1007/s00421-010-1458-y
ORIGINAL ARTICLE
Hypohydration reduces vertical ground reaction impulse
but not jump height
Samuel N. Cheuvront • Robert W. Kenefick • Brett R. Ely • Everett A. Harman •
John W. Castellani • Peter N. Frykman • Bradley C. Nindl • Michael N. Sawka
Accepted:23March2010/Publishedonline:9April2010
(cid:2)USGovernment2010
Abstract This study examined vertical jump perfor- Keywords Dehydration (cid:2) Fluid balance (cid:2)
mance using a force platform and weighted vest to deter- Strength-to-mass ratio (cid:2) Jump performance
mine why hypohydration (*4% body mass) does not
improve jump height. Measures of functional performance
from a force platform were determined for 15 healthy and Introduction
active males when euhydrated (EUH), hypohydrated
(HYP) and hypohydrated while wearing a weighted vest A reduction in body mass, without equivalent losses of
(HYP ) adjusted to precisely match water mass losses. strength or power, increases the strength-to-mass ratio
v
HYP produced a significant loss of body mass [-3.2 ± (S/M) and should improve performance in sporting events
0.5 kg(-3.8 ± 0.6%);P\0.05], butbodymassinHYP where body mass is the principle form of gravitational
v
was not different from EUH. There were no differences in resistance (Harman 1994). The S/M is, therefore, critical
absoluteorrelativepeakforceorpoweramongtrials.Jump in explosive, body mass-dependent track and field events
height was not different between EUH (0.380 ± 0.048 m) such as high jump, long jump, triple jump, and pole vault
andHYP(0.384 ± 0.050 m),butwas4%lower(P\0.05) (Viitasalo et al. 1987; Williams 1998). But it is also of
inHYP (0.365 ± 0.52 m)than EUH due toa lower jump importance for weight class sports where greater strength
v
velocity between HYP and EUH only (P\0.05). How- or power confers an advantage against a competitor of
v
ever,verticalgroundreactionimpulse(VGRI)wasreduced the same body mass, such as wrestling (Williams 1998;
in both HYP and HYP (2–3%) compared with EUH Kraemeretal.2001).Itisadditionallyrelevanttoanysport
v
(P\0.05). In conclusion, this study demonstrates the where jumping plays a major role, such as basketball and
failuretoimprovejumpheightwhenHYPcanbeexplained volleyball (Hoffman et al. 1995). The precise balance
by offsetting reductions in both VGRI and body mass. between losses in body mass and losses in strength or
powerwilldecidewhetherornotaperformanceadvantage
is realized. However, it is important to recognize that rel-
ative performance may be improved by losses of both so
long as there is a net increase in the S/M.
CommunicatedbyJean-Rene´ Lacour.
Gradual losses of body mass (over days or longer)
S.N.Cheuvront(&)(cid:2)R.W.Kenefick(cid:2) achievedwithenergyandfluidrestrictionsusuallyresultin
B.R.Ely(cid:2)J.W.Castellani(cid:2)M.N.Sawka lossesofleantissuemass,fatmass,andwatermass(Nindl
ThermalandMountainMedicineDivision,
etal.2002;Welshetal.2008).Gradualmasslossesappear
USArmyResearchInstituteofEnvironmentalMedicine,
to reduce strength and power of the leg extensor muscles
KansasStreet,Natick,MA01760-5007,USA
e-mail:[email protected] (Nindletal.2002;Welshetal.2008)butproducenoeffect
(Fogelholmetal.1993),orsubstantiallyimpair(Chicharro
E.A.Harman(cid:2)P.N.Frykman(cid:2)B.C.Nindl
et al. 1998; Welsh et al. 2008) vertical jump performance.
MilitaryPerformanceDivision,
This reflects that the trade-off between lost strength and
USArmyResearchInstituteofEnvironmentalMedicine,
KansasStreet,Natick,MA01760-5007,USA lost mass yields no gain in S/M and provides no jumping
123
1164 Author's personal copy EurJApplPhysiol(2010)109:1163–1170
advantage;itmayevenhaveadetrimentaleffect.However, link between osmotic stress, neural motor unit activation,
acute (hours) losses in body water mass (hypohydration), andjumpheightmightthereforebeinferredbyquantifying
which are not confounded by energy deficits or losses of VGRIduringaverticaljumptestwithandwithoutchanges
contractileprotein,maypossiblypreserveorevenimprove to hydration state (body mass, plasma osmolality).
theS/M. Criticalreviews(Fogelholm1994;Judelsonet al. Although practical tests of jump performance (e.g.,
2007a) of hypohydration’s effect on strength and power, measured distance, flight time) have been the standard for
independentofbodymass,indicateasmallnegativeimpact hypohydration studies, the confounding effects of mass on
(1–3%) which might not appreciably reduce the S/M. acceleration (acceleration = force/mass) require separate
Acute hypohydration clearly does not impair vertical performance tests of the leg extensor muscles that are
jumpperformance(Gutierrezetal.2003;HayesandMorse independent of body mass (Gutierrez et al. 2003; Hayes
2009;Hoffmanetal.1995;Judelsonetal.2007b;Viitasalo and Morse 2009; Hoffman et al. 1995; Judelson et al.
et al. 1987; Watson et al. 2005), but it remains uncertain 2007b; Viitasalo et al. 1987; Watson et al. 2005). A more
why only one study (Viitasalo et al. 1987) has ever con- sophisticatedmeansofmeasuringverticaljumpheightisto
firmed the expected improvement in jump height that use a force platform (Hayes and Morse 2009). By analyz-
should accompany a reduction in body mass. Acute hyp- ing ground reaction forces, kinetic and temporal variables
ohydration should increase jump height, provided that produced by force–time curves can be used to determine
muscle contractile function remains normal, because the vertical position and velocity of the TBCM (Bosco
gravitational and inertial resistance to jumping are pro- et al. 1983; Cordova and Armstrong 1996; Harman et al.
portional to body mass (Harman 1994; Appendix). Since 1990). Force platforms also provide the advantage of
the net balance between losses in mass and strength or linking functional performance (jump height) to instanta-
power will determine whether or not jump height is neous measurements of force made during natural motion
increased, decreased, or unchanged by hypohydration, (functionalstrengthtesting)(Aagaardetal.2002;Cordova
seemingly small reductions in strength or power (Judelson and Armstrong 1996) and afford important calculations
etal.2007a)arepresumablyenoughtomaskthebenefitsof (power, velocity, VGRI) necessary for determining pre-
a lighter body mass when jumping. cisely how hypohydration might affect vertical jump per-
Although the effects of hypohydration on motor unit formance. Importantly, if mass could be held constant
activationhavereceivedsomeattention(Bigardetal.2001; while remaining hypohydrated (HYP), such as with a
Evetovich et al. 2002; Hayes and Morse 2009; Judelson weighted vest, insight might be gained into the potential
et al. 2007b), none of the methodologies thus far studied independent effects of mass changes on the S/M during a
offers insight toward understanding the reasons for vertical jump.
impaired vertical jumping. The large osmotic stress This studyexamined vertical jump performance using a
(*300 mmol/kg) that commonly accompanies hypohy- forceplatforminanefforttodeterminewhyhypohydration
drationfromsweatloss(FeigandMcCurdy1977;Kraemer (*4% body mass) does not improve jump height (Gut-
et al. 2001) can modulate opening of the blood–brain ierrez et al. 2003; Hayes and Morse 2009; Hoffman et al.
barrier (Rapoport 2000), alter neuronal firing of hypotha- 1995; Judelson et al. 2007b; Viitasalo et al. 1987; Watson
lamic osmoreceptor cells (Boulant and Silva 1988), and etal.2005).Inonetrial,aweightedvestwasusedtooffset
plausibly affect excitability of motor output pathways bodymasslossesfromthewaterdeficit,thusholdingmass
(Enoka and Stuart 1992). The kinetic impulse (time inte- constant in the S/M. Our hypothesis was that hypohydra-
grated moment of force or momentum) in the initial phase tion would impair vertical jump height when mass was
of muscle contraction (*30 ms) may be affected by replaced using a weighted vest. Conversely, without the
motoneuron recruitment and firing frequency (Aagaard vest, the reduction in mass would likely mask any
et al. 2002). Because it takes the knee extensor muscles impairment in jump height due to some combination of
comparativelylonger(C300 ms)(Thorstenssonetal.1976) offsetting strength and mass changes.
to reach peak force than the muscle contraction times
associatedwithrapidjumping(50–200 ms)(Aagaardetal.
2002),faster forcedevelopment inthe propulsionphase of Methods
jumpingisimportantformaximizingjumpheight(Harman
et al. 1990). The product of applied force and time during Subjects
the propulsion phase of jumping, or the vertical ground
reactionimpulse(VGRI),directlyimpactsjumpheightasit Fifteen male subjects volunteered to participate in this
isequivalenttothechangeinmomentum(productofmass study. All volunteers passed the Army Physical Fitness
and velocity) of the total body center of mass (TBCM) Test within the previous 6 months and received a general
(Cordova and Armstrong 1996; Harman et al. 1990). Any medical clearance prior to participation, thus all were
123
EurJApplPhysiol(2010)109:1163–1170 Author's personal copy 1165
considered physically fit and healthy. Use of alcohol, die- approximately shoulder width and the depth of the coun-
tary supplements, and any medication were prohibited. termovement (degree of knee bend) was self-selected
Subject characteristics taken on the first day of the study naturally between approximately 45(cid:3) and 90(cid:3) with the
were [mean ± SD (range)]: age 24 ± 5 (18–37) years, simple goal of jumping as high as possible (Harman et al.
body mass 84.5 ± 11.9 (66.0–107.5) kg, height 1990). Volunteers next performed a countermovement
1.78 ± 0.05 (1.69–1.86) m, body fat 16.5 ± 4.0 (10.7– jump from a 660 9 660 9 60 mm dual force plate plat-
25.9)%, and VO peak 47.0 ± 8.7 (37.4–52.4) ml/kg min. form (Leonardo v3.07, Orthometrix, Inc.) connected to a
2
Volunteers were blinded to the study hypothesis. The US PCforthe purposeofcollecting force dataandcalculating
Army Research Institute of Environmental Medicine jump height (described below). Subjects stood still on the
Human Use Review Committee approved this study. Sub- platform with one foot on each force plate for approxi-
jects were provided informational briefings and gave mately 10 s while a stable body mass measurement was
voluntary, informed written consent to participate. made. A 3-s countdown followed after which a maximal
Investigators adhered to AR 70-25 and US Army Medical countermovement jump was performed. Volunteers then
Research and Materiel Command Regulation 70-25 on the remained standing on the platform briefly before stepping
use of volunteers in research. off for a 1-min recovery. This procedure was repeated
three times and the highest single jump height of the day
Preliminary testing wasrecorded.Aperformancecoefficientofvariation (CV)
was calculated [(SD/mean) 9 100] using the highest jump
Two weeks before experimental testing, anthropometric height from the force platform determined on each of the
and fitness measurements were made to characterize the 3–5 practice days.
study population. Peak aerobic power (VO peak) was Vertical jump height and all other jump performance
2
determined using an incremental cycle ergometer protocol parameters were determined from force plate data using
with continuous gas exchange and body fat was estimated proprietary analytical software (Leonardo v3.07, Ort-
from three-site skinfold measurements (Jackson and hometrix, Inc.). Body weight, measured while the subjects
Pollock 1978). stood still, allowed acceleration of the TBCM to be cal-
All volunteers completed preliminary testing over a culated at each movement sample point (800 Hz). The
2-weekperiodinatemperateenvironment(20–22(cid:3)C)prior forceusedtocalculatethebody’sverticalaccelerationwas
toexperimentaltesting.Toachieveeuhydration,volunteers the vertical force reading from the force platform minus
were given *30 ml/kg of fluid-electrolyte beverage to body weight. The integral of acceleration over time is
consume after 1800 hours each evening supplemental to equivalenttothevelocityoftheTBCM,andtheproductof
daily ad libitum fluid intakes (Armstrong et al. 1998). Ten velocityandforcegivespower.Theintegralofthevelocity
volunteers reported to the laboratory for nude body mass overtimeisequivalenttothechangeinverticalpositionof
measurementseachoftenmorningsafteranovernightfast the TBCM (i.e., jump height). Because impulse is equiv-
(*8 h). The remaining five volunteers reported between alenttochangeinmomentum,VGRI(integralofforceover
three and five times due to schedule conflicts. An average time) was calculated as the product of body mass and the
of the first morning nude body mass measurements was change in jump velocity (Cordova and Armstrong 1996;
calculated for each volunteer and used as a baseline ref- Harmanetal.1990).Jumpheightwascalculatedusingthe
erence (Cheuvront et al. 2004) for later euhydrated (EUH) principle that potential energy (body weight 9 jump
body mass determinations. height)equalsthechangeinkineticenergy(0.5mV2,where
Allvolunteersperformedbetween3and5practicedays m is the body mass in kg and V is the vertical takeoff
of vertical jump testing to reduce training and learning velocity in m/s determined by the integration of vertical
effects. Practice trials were designed to mimic experi- accelerationovertimewhilethefeetareincontactwiththe
mental testing in every way except for heat exposure. force platform). Jump height using the Leonardo (v3.07,
Each day began with a 5-min warm-up on a cycle Orthometrix, Inc.) was recently (Frykman et al. 2009)
ergometerwherepedalcadencewasself-selected againsta validated using precise motion analysis software and was
constant 50 W workload(Lode Corival, TheNetherlands). significantly more accurate than typical jump and reach
Volunteers then performed three practice countermove- methods for assessing vertical jump height. Both relative
ment jumps from the floor. Briefly, the starting position measuresofforce(N/kg)andpower(W/kg)wereexamined
for all jumps was an upright posture with the hands tocharacterizeandcontrolforanyinherentbiasinabsolute
positioned to remain on the hips for the duration of the measures (e.g., N, W) owing to the purposeful manipula-
jump to allow assessment of lower extremity functional tion of body mass and the relationship: accelera-
strength only (Aagaard et al. 2002; Cordova and Arm- tion = force/mass. All jump measures are instantaneous
strong 1996; Harman et al. 1990). Feet were positioned at maximums calculated during dynamic motion.
123
1166 Author's personal copy EurJApplPhysiol(2010)109:1163–1170
Experimental trials mass lost from sweating. The vest itself weighed 0.60 kg
and was designed to hold as many as 20 flexible weights
Two experimental trials (EUH and HYP) were completed which ranged from 219 to 226 g each. This allowed
in counterbalanced, crossover fashion separated by 3–11 matching the mass of the weighted vest to the dehydration
days. Volunteers were always tested at the same time of mass losses with ±100 g precision. Weight placement in
day,instructedtolimitphysicalactivityfor24 h,anddrink thevestwasdividedequallybetweenthedorsalandcaudal
*30 ml/kg of fluid-electrolyte beverage supplemental to torso and ergonomically aligned between the xiphoid pro-
ad libitum fluid intakes the evening prior to testing (Arm- cess and navel to approximate the body center of mass.
strong et al. 1998). Volunteers were overnight fasted Like trials EUH and HYP, the order of jumps performed
(*8 h)uponarrivaltothelaboratory.Nudebodymasswas within HYP and HYP was counterbalanced.
v
measuredforallvolunteers(±50 g;MettlerToledo,Model
WSI-600, Toledo, OH). Both before and after heat expo- Statistical analysis
sure,asubsetof10volunteerssatfor20 mintoallowfluid
compartmentstabilizationpriortocollectinga7-mlvenous The effect of treatment (EUH, HYP, HYP ) on jump per-
v
blood sample for determination of plasma osmolality by formance parameters was determined using a one-way
freezing point depression (Fiske Micro-osmometer, Model repeated measures ANOVA. After a significant F test,
210, Norwood, MA). Dunnett’s post hoc test was used to compare HYP and
Heat exposures occurred in an environmental chamber HYP against EUH (control). The practical importance of
v
setto50(cid:3)C,20%rhwherethreecycles of30-mintreadmill differences in jump height between EUH and other trials
walking(3.5 mph,3.5%grade)and30-minofrest(sitting) wasexaminedbycomparingthemeanand95%confidence
were repeated (total 3-h exposure). The purpose of the limits of the percent differences against a pre-specified
walking exercise was to increase body heat storage and zone of indifference (Cheuvront et al. 2005). This proce-
induce sweating. The method of dehydration selected was dure is a corollary for significance testing which provides
considered the most appropriate as it is a legal means of insight into the magnitude and uncertainty of the true
losing mass in sport (Williams 1998), it is practical (i.e., (population) effect (Hopkins 2004; Nakagawa and Cuthill
sauna-like) and it produces a predictable hypertonic state 2007; Reichardt and Gollob 1997). It is also similar to
within the intravascular fluid space (Feig and McCurdy equivalence testing as it affords an evaluation of perfor-
1977). Water loss was determined from changes in body mance against an evidentiary standard other than zero
mass measured every 30-min. Under the conditions tested, (Batterham and Hopkins 2005; Ebbutt and Frith 1998;
water (sweat, urine) volume and body mass losses were Hopkins 2004). The pre-specified zone of indifference, or
considered equivalent. The objective was to lose 4% of trivial effect, in this study was the typical within-subjects
bodymass,whichwouldtheoreticallyimprovejumpheight jump height variability of 3.5%. This value is nearly
also by 4% if work done by the leg muscles remained identical to the vertical jump height CV of 3.6% reported
constant (Appendix). In EUH, volunteers drank 1 ml of by Viitasalo et al. (1987) for athletes accustomed to par-
0.05% NaCl–water solution to replace every 1 g of lost ticipatinginsportsthatfocusorrelyheavilyuponjumping.
mass. In HYP, no fluid was provided. A 45–90-min break Conventional 95% confidence limits of a mean difference
followed heat exposure where volunteers showered and arestatisticallysignificantwhentheyexcludethenullvalue
relaxed in a temperate environment (20–22(cid:3)C). The pur- (zero). However, the relevance of either bound of the
pose of this break was to allow core temperature to return confidence interval must also be considered (Curran-
torestingpre-heatexposurelevels(Cheuvrontetal.2006). Everett and Benos 2004). In this study, the practical
Importantly,therecoveryperiodremainedconstantwithin- importance of any significant difference was considered
subjects. Following the break, nude body mass was again unequivocalonlywhenthemajorityofthe95%confidence
measured and this value was compared to the pre-heat interval was outside the zone of indifference.
exposure value. If body mass was less than the value Sample size was calculated for comparisons of jump
measured prior to heat exposure, additional fluid was height, which was the central performance parameter of
provided. interest in this study. Applying conventional a = 0.05 and
Following the 45–90-min recovery, warm-up and jump b = 0.20 values, eight subjects were estimated to provide
testing commenced as described in ‘‘Preliminary testing’’. sufficient power to detect a 4% jump height difference
Thebestofthreeverticaljumpheightsdeterminedfromthe from EUH (Appendix) using the mean jump height
force platform was recorded in the same manner as pre- (0.370 ± 0.050 m) achieved during the initial 2 weeks of
liminary testing. Within the HYP trial, volunteers per- familiarization training coupled to the within-subjects CV
formed three jumps with (HYP ) and without (HYP) a (i.e., 3.5%) for an estimated effect size [1.0. However,
v
weighted vest (Uni-Vest, Pittsburgh, PA) that replaced the because experimental perturbations produce unique
123
EurJApplPhysiol(2010)109:1163–1170 Author's personal copy 1167
performance infidelity (Hopkins et al. 1999), increase the consistent with the effect of body mass manipulation on
observed CV, and decrease the anticipated signal-to-noise force production. Max power (W, W/kg) was lowest in
ratio, a sample size of 15 volunteers was tested to allow HYP . Jump height (Table 2) was not different between
v
detectionofdesireddifferenceswithaneffectsizeassmall EUHandHYP,butwas4%lower(P\0.05)inHYP than
v
as 0.50 after adjusting for repeated measures (Lipsey EUH. Jump height was significantly reduced in HYP as a
v
1990). All data are presented as mean ± SD except where direct result of a reduced jump takeoff velocity (Table 2).
indicated. A smaller (P\0.05) VGRI was calculated for both HYP
v
and HYP, which were 2–3% less than EUH (Table 2).
Figure 1 presents the percent change in jump perfor-
Results mance from EUH. The percent change was significantly
different for HYP but not for HYP. The means and 95%
v
Hydration confidence intervals [HYP : -4% (-1.8 to -6.2%) and
v
HYP: 1% (-1.6 to 3.4%)] provide the likely range of the
Inallvolunteers,thedailybodymassvariation(CV)afteran true change effects and illustrate why there is a difference
overnightfluidbolus(30 ml/kg)was0.6 ± 0.2%,similarto between EUH versus HYP but not between EUH versus
v
what has been reported previously when fluid intake was HYP (i.e., when confidence interval crosses zero,
controlled(Cheuvrontetal.2004).Bodymassuponarrival P[0.05). In addition, almost the entire confidence inter-
to the laboratory for testing (83.8 ± 10.9 kg) was within valinEUHversusHYPfallswithinthetrainingCV,while
±1% of the multi-morning average in all volunteers. The the majority of the confidence interval in EUH versus
corresponding plasma osmolality was \290 mmol/kg HYP is outside the same zone. This indicates that the
v
(289 ± 4 mmol/kg)forthesubsetof10volunteersprovid- significant reduction in jump performance in HYP is of
v
ingbloodsamples.Thus,allsubjectswereconsideredEUH sufficient magnitude to be considered important. Consid-
at the start of each trial (Sawka et al. 2007). In trial EUH, eration was also given to a trial order effect, but no effect
fluidintakewassufficienttorestorewaterlossesduringheat of trial order was found. In absolute terms, 12 of the 15
exposure, thus body mass was unchanged and plasma subjects performed worse in HYP while only 6 of 15
v
osmolality remained \290 mmol/kg (286 ± 4 mmol/kg). performed worse in HYP only (Fig. 2), the latter of which
In trial HYP, body mass was reduced by 3.2 ± 0.5 kg is consistent with chance. When comparing the theoretical
(-3.8 ± 0.6%; P\0.05) to 80.6 ± 10.3 kg and plasma improvementinjumpheight(Appendix)totheactualjump
osmolalityincreasedto300 ± 5 mmol/kg(P\0.05).Body heightinHYP(Fig. 2),11of15subjectsfailedtojumpas
mass was restored to EUH values of 84.0 ± 10.6 kg high as predicted (Appendix).
(0.3 ± 0.5%) in HYP by adding back the mass lost
v
(3.4 ± 0.5 kg)usingtheweightedvest.Therecoveryperiod
(45–90 min) from heat stress was considered sufficient to
return core temperatures to near resting levels (Cheuvront
et al. 2006), though they were not measured. Importantly, Table1 Absolute and relative measures of force and power during
thesmallobligatoryelevationincoretemperaturetypically verticaljumptesting
produced by hypohydration (B0.50(cid:3)C) does not impact
Trial Maxforce Maxpower
power performance outcomes (Cheuvront et al. 2006) and
N N/kg W W/kg
should not affect anaerobic chemo-mechanical energy
conversions(Bennett1984;Boscoetal.1983).Anymodest EUH 1766±281 21.04±1.75 3972±558 47.48±4.70
level of hyperthermia would also have been identical for HYP 1761±283 20.92±1.74 3902±590 46.54±5.29
v
comparisonsbetweentrialsHYPandHYPv. HYP 1721±267 21.35±2.13 3930±564 48.85±5.12
All data represent instantaneous maximums calculated during
Jump height and biomechanical performance
dynamicmotion
parameters
Table 1 provides absolute (N) and relative (N/kg) force as Table2 Performanceparametersduringverticaljumptesting
well as absolute (W) and relative (W/kg) power measure- Trial Jumpheight(m) Jumpvelocity(m/s) VGRI(Ns)
ments for the three experimental trials. Although no sig-
EUH 0.380±0.048 2.73±0.17 228.3±30.3
nificant differences were observed for peak values during
HYP 0.365±0.052* 2.67±0.19* 224.3±31.8*
the dynamic motion phase of the measurement, qualitative v
HYP 0.384±0.050 2.74±0.19 220.6±29.7*
trends indicate that the lowest absolute (N) and highest
relative (N/kg) force measurements occurred in trial HYP, *P\0.05fromEUH
123
1168 Author's personal copy EurJApplPhysiol(2010)109:1163–1170
%CV Training hypohydration; (3) a lower body mass in HYP did not
significantlyimprovejumpheightaspredictedbystandard
physicallaws(Appendix),apparentlyduetooffsettingS/M
HYPv vs. EUH changes.
The finding that EUH and HYP jump heights were not
different is consistent with other studies evaluating the
HYP vs. EUH impact of hypohydration on jump performance (Gutierrez
et al. 2003; Hayes and Morse 2009; Hoffman et al. 1995;
Judelson et al. 2007b; Viitasalo et al. 1987; Watson et al.
2005). The finding that HYP jump performance was less
v
-10-8 -6 -4 -2 0 2 4 6 8 10 than EUH seems to contradict Viitasalo et al. (1987) who
Change in Jump Height (%) observednoeffectsofhypohydrationonjumpperformance
with barbell loads of 20–80 kg. Though the subject popu-
Fig.1 PercentchangeinperformancefromEUHforbothHYPand
lation (athletes accustomed to jumping) might explain
HYP trials. Data are means; bars are 95% confidence intervals.
v
Shaded area represents zone of indifference (±3.5%) based on the differences in performance susceptibility to hypohydration
typicalperformancevariabilitymeasuredduringpracticesessionsand (Hakkinen et al. 1984), substantial barbell loads would
atheoreticalimprovementinperformance(*4%)inexcessoftheCV
alterstoredelasticenergy(Hakkinenetal.1984)andjump
(seetextandAppendixfordetails)
biomechanics far more and presumably renders obsolete
direct comparisons to an ergonomically weighted 3.4 kg
vest. The importance of the HYP performance effect
0.50 v
HYP magnitudeisevidencedbythefact thatthemajorityofthe
0.46 HYPv confidence interval for the percent change in jump height
m) Theory lies outside the typical noise of the measurement, while
( 0.42
th performance in HYP lies almost entirelywithin it (Fig. 1).
g Thetraditionaluseof95%confidenceintervalsappliedtoa
ie 0.38
H 3.5% CV evidentiary standard can even be viewed as
p
m 0.34 conservative (Hopkins 2004; Hopkins et al. 1999), thus
u
J strengtheningourinterpretationofimportance.Byvirtueof
0.30
comparison, it requires 8 days of chronic activity, sleep
deprivation, and underfeeding to achieve the same *4%
0.26
0.26 0.30 0.34 0.38 0.42 0.46 0.50 decrement in jump performance (Welsh et al. 2008)
EUH Jump Height (m) achievedhereinwitha-3.2 ± 0.5 kg(-3.8 ± 0.6%)total
body water deficit.
Fig.2 Hypohydrated jump height (y-axis) plotted as a function of
Measures of force and mass can explain both the pres-
euhydration jump height (x-axis). Dotted line represents the line of
ervation of jump height in HYP and the decrement
identity, where values falling above or below the line are higher or
lowerjumpheightsthaneuhydration,respectively.Valuesdenotedby observed inHYP . Peak force measures were not different
v
‘X’representthetheoreticalimprovementinjumpheightattributable amongthethreetrials(Table 1),butverticaljumpheightis
solelytoalighterbodymass(seeAppendixfordetails)
maximized by a delicate balance between peak force and
kinetic impulse (force 9 time) (Cordova and Armstrong
1996; Harman et al. 1990). The product of applied force
Discussion and time during the propulsion phase of jumping, or the
VGRI, is equivalent to the change in momentum
This was the first study to examine the impact of hypo- (mass 9 velocity) of the TBCM (Cordova and Armstrong
hydration on jump performance using sophisticated bio- 1996; Harman et al. 1990). Despite a lower VGRI, HYP
mechanical measures from a force plate, while holding jump velocity and jump height were preserved (Table 2).
massconstantintheS/M,thusenablingexaminationofthe This appears to be explained by a proportional decrease in
biomechanical trade-offs between mass loss and the body mass in HYP, since the similar reduction in kinetic
potential consequences of hypohydration on functional leg impulse in HYP impaired jump velocity and jump height
v
strength (Aagaard et al. 2002; Cordova and Armstrong when body mass was the same as EUH. Furthermore, the
1996)duringaverticaljump.Theprinciplefindingsofthis lower body mass in HYP did not confer the expected
study were: (1) hypohydration impaired VGRI; (2) when vertical jump advantage predicted by a *4% lower body
body mass was equated with trial EUH using a weighted mass (Appendix). Thus, the lower jump height in HYP ,
v
vest (HYP ), jump height was significantly reduced by coupled with the reduced VGRI in both HYP and HYP ,
v v
123
EurJApplPhysiol(2010)109:1163–1170 Author's personal copy 1169
reveals that the pernicious consequences of hypohydration Appendix
on jump height are observable only when mass is held
constant. Summary of calculation for theoretical jump height1
The physiological mechanism(s) by which hypohydra- improvement with hypohydration:
tionmightimpairthepotentialforimprovingverticaljump
force (cid:3)distance ¼force (cid:3)distance
1 1 2 2
atareducedbodymassarenotclear.Thereislittlesupport
bodyweight (cid:3)jumpheight ¼bodyweight (cid:3)jumpheight
1 1 2 2
for postulating that hypohydration changes metabolic jumpheight =jumpheight ¼bodyweight =bodyweight
2 1 1 2
energy stores (ATP/PC) (Montain et al. 1998), physiolog- jumpheight ¼jumpheight (cid:3)ðbodyweight =bodyweight Þ
2 1 1 2
ical buffering capacity (Bigard et al. 2001), alterations in
musclemembraneexcitability(Costilletal.1976),thermal where
influences on muscle function/elastic energy (Bennett
bodyweight ¼bodyweightbeforehypohydration
1984), or any other aspect of intra–extracellular muscle 1
bodyweight ¼bodyweightafterhypohydration
compartmentchanges(Evetovichetal.2002).Theinability 2
jumpheight ¼jumpheightbeforehypohydration
1
to improve vertical jump height in trial HYP and the dec-
jumpheight ¼jumpheightafterhypohydration
2
rement in jump height observed in trial HYP might have
v
motoneuralunderpinnings(Aagaardetal.2002;Enokaand
Stuart 1992) possibly related to the high osmotic stress
References
created by hypohydration (300 mmol/kg) (Boulant and
Silva 1988; Kraemer et al. 2001; Rapoport 2000). Alter-
AagaardP,SimonsenEB,AndersenJL,MagnussonP,Dyhre-Poulsen
natively, unpleasant sensations related to hypohydration,
P(2002)Increasedrateofforcedevelopmentandneuraldriveof
such as headache, lightheadedness or malaise, could also human skeletal muscle following resistance training. J Appl
reduce motivation mediated motoneural firing (Enoka and Physiol93:1318–1326
Stuart 1992). Though the precise mechanism(s) or sensory ArmstrongLE,HerreraSotoJA,HackerFT,CasaDJ,KavourasSA,
MareshCM(1998)Urinaryindicesduringdehydration,exercise,
pathwaysthatwouldimplicate‘centralfatigue’(Enokaand
andrehydration.IntJSportsNutr8:345–355
Stuart 1992; Gandevia 2001) in reducing VGRI in both Batterham AM, Hopkins WG (2005) Making meaningful inferences
HYP and HYP cannot be gleaned from the data at hand, aboutmagnitudes.Sportscience9:6–13
v
thefactthatVGRIwasreducedsimilarlyinbothHYPand Bennett AF (1984) Thermal dependence on muscle function. Am J
PhysiolRegulIntegrCompPhysiol247(16):R217–R229
HYP (Table 2) suggests a physiological, rather than
v Bigard AX, Sanchez H, Claveyrolas G, Martin S, Thimonier B,
mechanical (weight bearing), explanation for the lower Arnaud MJ (2001) Effects of dehydration and rehydration on
jump height in HYP . EMG changes during fatiguing contractions. Med Sci Sports
v
Exerc33(10):1694–1700
Bosco C, Luhtanen P, Komi PV (1983) A simple method for
measurement of mechanical power in jumping. Eur J Appl
Physiol50:273–282
Conclusions
BoulantJA,SilvaNL(1988)Neuronalsensitivitiesinpreoptictissue
slices:Interactionsamonghomeostaticsystems.BrainResBull
Hypohydration has no apparent net effect on jump height 20(6):871–878
due to offsetting reductions in VGRI and body mass. CheuvrontSN,CarterR,MontainSJ,SawkaMN(2004)Dailybody
massvariabilityandstabilityinactivemenundergoingexercise-
However, VGRI, jump velocity, and vertical jump height
heatstress.IntJSportNutrExercMetab14:532–540
wereallreducedwhenlostwatermasswasreplacedwitha
Cheuvront SN, Carter R, Castellani JW, Sawka MN (2005) Hypo-
weightedvest,indicatinggenuineperformanceimpairment hydrationimpairsenduranceexerciseperformanceintemperate
with hypohydrationotherwisemasked byalterations inthe butnotcoldair.JApplPhysiol99:1972–1976
CheuvrontSN,CarterR,HaymesEM,SawkaMN(2006)Noeffectof
S/M.Itis,therefore,plausiblethathypohydrationwillhave
moderate hypohydration or hyperthermia onanaerobic exercise
negative,butdifficulttoobserve,effectsonperformancein
performance.MedSciSportsExerc38(6):1093–1097
sports or occupations where the S/M is important (e.g.,
Judelson et al. 2007a; Kraemer et al. 2001; Welsh et al.
2008).
1 Assumeswork(force9distance)isunchangedbyhypohydration,
Acknowledgments The authors would like to thank all the volun- rather than impulse (force9time) (Viitasalo et al. 1987), since a
teerswhoparticipated inthisstudy.Theauthorsaregratefulforthe lighter body mass would be accelerated faster if force remained
assistance provided by Laura J. Palombo and SGT Daniel E. constant, thereby reducing impulse by shortening the time of force
Catrambone. We would also like to thank Dr. Scott J. Montain for application. As a result, the level of hypohydration (e.g., 4%) is
readingandremarkingonthismanuscript.Theopinionsorassertions equivalent to the theoretical percent improvement in jump height.
containedhereinaretheprivateviewsoftheauthor(s)andarenotto [Note:Ifimpulseremainedconstant,thedenominatoroftheequation
beconstruedasofficialorasreflectingtheviewsoftheArmyorthe abovewouldbecomesquaredandthetheoreticalimprovementwould
DepartmentofDefense. bedoublethelevelofhypohydration(e.g.,8%).]
123
1170 Author's personal copy EurJApplPhysiol(2010)109:1163–1170
Chicharro JL, Lopez-Mojares LM, Lucia A, Alvarez J, Calvo F, JacksonAS,PollockML(1978)Generalizedequationsforpredicting
Vaquero AF (1998) Overtraining parameter in special military bodydensityofmen.BrJNutr40:497–504
units.AviatSpaceEnvironMed69:562–568 Judelson DA, Maresh CM, Anderson JM, Armstong LE, Casa DJ,
Cordova ML, Armstrong CW (1996) Reliability of ground reaction Kraemer WJ, Volek JS (2007a) Hydration and muscular
forces during a vertical jump: implications for functional performance: does fluid balance affect strength, power, and
strengthassessment.JAthlTrain31(4):342–345 high-intensityendurance?SportsMed37(10):907–921
Costill DL, Cote R, Fink W (1976) Muscle water and electrolytes Judelson DA, Maresh CM, Farrell MJ, Yamamoto LM, Armstrong
following varied levels of dehydration in man. J Appl Physiol LE,KraemerWJ,VolekJS,SpieringBA,CasaDJ,AndersonJM
40(1):6–11 (2007b) Effect of hydration state on strength, power, and
Curran-EverettD,BenosDJ(2004)Guidelinesforreportingstatistics resistance exercise performance. Med Sci Sports Exerc
in journals published by the American Physiological Society. 39(10):1817–1824
AmJPhysiolRegulIntegrCompPhysiol287:R247–R249 Kraemer WJ, Fry AC, Rubin MR, Triplett-McBride T, Gordon SE,
EbbuttAF,FrithL(1998)Practicalissuesinequivalencetrials.Statist Koziris LP, Lynch JM, Volek JS, Meuffels DE, Newton RU,
Med17:1691–1701 Fleck SJ (2001) Physiological and performance responses to
EnokaRM,StuartDG(1992)Neurobiologyofmusclefatigue.JAppl tournamentwrestling.MedSciSportsExerc33(8):1367–1378
Physiol72(5):1631–1648 Lipsey MW (1990) Design sensitivity: statistical power for experi-
EvetovichTK,BoydJC,DrakeSM,EschbachLC,MagalM,Soukup mentalresearch.SagePublications,NewburyPark
JT, Webster MJ, Whitehead MT, Weir JP (2002) Effect of Montain SJ, Smith SA, Mattot RP, Zientara GP, Jolesz FA, Sawka
moderate dehydration on torque, electromyography, and mech- MN (1998) Hypohydration effects on skeletal muscle perfor-
anomyography.MuscleNerve26:225–231 mance and metabolism: a 31P-MRS study. J Appl Physiol
Feig PU, McCurdy DK (1977) The hypertonic state. N Eng J Med 84(6):1889–1894
297(26):1444–1454 NakagawaS,CuthillIC(2007)Effectsize,confidenceintervalsand
Fogelholm M (1994) Effects of bodyweight reduction on sports statisticalsignificance:apracticalguideforbiologists.BiolRev
performance.SportsMed18(4):249–267 82:591–605
Fogelholm GM, Koskinen R, Laakso J, Rankinen T, Ruokonen I NindlBC,LeoneCD,TharionWJ,JohnsonRF,CastellaniJW,Patton
(1993) Gradual and rapid weight loss: effects on nutrition and JF, Montain SJ (2002) Physical performance responses during
performanceinmaleathletes.MedSciSportsExerc25(3):371– 72h of military operational stress. Med Sci Sports Exerc
377 34(11):1814–1822
Frykman PN, Seay JF, Catrambone DE, Sharp MA (2009) Simulta- Rapoport SL (2000) Osmotic opening of the blood–brain barrier:
neous comparison of power output from three methods of principles, mechanism, and therapeutic applications. Cell Mol
vertical jump tests. Med Sci Sports Exerc 41(5 Suppl):S191– Neurobiol20(2):217–230
S192 ReichardtCS,GollobHF(1997)Whenconfidenceintervalsshouldbe
GandeviaSC(2001)Spinalandsupraspinalfactorsinhumanmuscle used instead of statistical tests, and vice versa. In: Harlow LL,
fatigue.PhysiolRev81(4):1725–1789 MulaikSA,SteigerJH(eds)Whatiftherewerenosignificance
Gutierrez A, Mesa JLM, Ruiz JR, Chirosa LJ, Castillo MJ (2003) tests?.Erlbaum,Mahwah,NJ
Sauna-induced rapid weight loss decreases explosive power in Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ,
womenbutnotinmen.IntJSportsMed24:518–522 Stachenfeld NS (2007) American College of Sports Medicine
Hakkinen K, Alen M, Komi PV (1984) Neuromuscular, anaerobic, position stand. Exercise and fluid replacement. Med Sci Sports
andaerobic performance characteristics ofelite power athletes. Exerc39(2):377–390
EurJApplPhysiol53:97–105 Thorstensson A, Karlsson J, Viitasalo HT, Luhtanen P, Komi PV
Harman EA (1994) The biomechanics of resistance exercise. In: (1976) Effect of strength training on EMG of human skeletal
BaechleTR(ed)Essentialsofstrengthtrainingandconditioning. muscle.ActaPhysiolScand98:232–236
HumanKinetics,Champaign ViitasaloJT,KyrolainenH,BoscoC,AlenM(1987)Effectsofrapid
Harman EA, Rosenstein MT, Frykman PN, Rosenstein RM (1990) weight reduction on force production and vertical jumping
Theeffectsofarmsandcountermovementonverticaljump.Med height.IntJSportsMed8(4):281–285
SciSportsExerc22(6):825–833 Watson G, Judelson DA, Armstrong LE, Yeargin SW, Casa DJ,
Hayes LD and Morse CI (2009) The effects of progressive MareshCM(2005)Influenceofdiuretic-induceddehydrationon
dehydration on strength and power: is there a dose response? competitive sprint and power performance. Med Sci Sports
EurJApplPhysiol(Epubaheadofprint) Exerc37(7):1168–1174
HoffmanJR,StavskyH,FalkB(1995)Theeffectofwaterrestriction Welsh TT, Alemany JA, Montain SJ, Frykman PN, Tuckow AP,
on anaerobic power and vertical jumping height in basketball YoungAJ,NindlBC(2008)Effectsofintensifiedmilitaryfield
players.IntJSportsMed16:214–218 trainingonjumpingperformance.IntJSportsMed29:45–52
Hopkins WG (2004) How to interpret changes in an athletic Williams MH (1998) Rating the sports ergogenics. The ergogenics
performancetest.Sportscience8:1–7 edge:pushingthelimitsofsportsperformance.HumanKinetics,
HopkinsWG,HawleyJA,BurkeLM(1999)Designandanalysisof Champaign,IL
research on sport performance enhancement. Med Sci Sports
Exerc31(3):472–485
123