REPORT DOCUMENTATION PAGEForm Approved

OMB No.0704-0188

valid OMB @ntrol number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADoRESS.' f . KEl ,uRf uAtE (uu-MM-YYYY)

Dec 2010Z , T {EPUI { I IYPE

Joumal Article3. DATES COVERED (From - To)Jan 2009 - Dec 2009

4 . i l t L E A N U l ' U t t i l t L E

Acceleration Tolerance After Ingestion of a Commercial Energy Drink5A , U (JN IKAUI NUMtsEKF48650-04-D-6472

5b, LiKAN I NUMIJE]{

N/ACC. I- 'K9('T{AM ELEMEN I NUMI'EK

62202F6. AUTHOR(S)

Walker TB1. Balldin U2. Fischer J2. Storm W2. Warren GLa

5d. PROJECT NUMBER7757CE. IAl iK NUMtsEK

P8CI . WUKA UNI I NUMT'EK

097. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

'Air Force Research Laboratory

B iosciences and Performance Division

Brooks City-Base, TX782352wyle, Inc

Life Sciences Group1290 Hercules Dr.H^rref^n TY 77n{R

'Division of Physical TherapyGeorgia State University, GA

8. PERFORMING ORGANIZATION REPORTNUMBER

9. SPONSORTNG / MONTTORTNG AGENCY NAME(S) AND ADDRESS(ES)Air Force Materiel Command

Air Force Research Laboratory

7l I Human Performance WingHuman Effectiveness Directorate

Biosciences and Performance Division

Biobehavior, Bioassessment & Biosurveillance Branch

Brooks Citv-Base. TX 78235

1 0. sPoNsoR/MoNtToR's AcRoNYM(S)7l I HPWRHP;711 HPWRHPF

1 1. SPONSOR/MONITOR'S REPORTNUMBER(S)

12. DISTRIBUTION / AVAILABILITY STATEMENTApproved for public release; distribution unlimited

,I 3. SUPPLEMENTARY NOTESHuman Use Protocol # F-WR-2009-01l5-H

14. ABSTRACTThis study examined the ability ofa commercial energy drink to enhance acceleration tolerance, strength under G-load, and cognitive performanceimmediately prior to and following acceleration exposure. Eight experienced centrifuge subjects completed three separate experimental accelerationexposures following ingestion of I 1.5 ml/kg body weight of a) a commercial energy drink, providing 5.0 mg caffeine/kg body weight, b) a commercialenergy drink without caffeine or c) placebo. The acceleration exposures consisted ofa relaxed gradual onset run to peripheral light loss, a rapid onset runto 6.0 G for l5 s, and a simulated air combat maneuver (SACM) run of repeated alternations between 4.5 G for l5 seconds and 7G for l5 seconds untilvolitional exhaustion. Cognitive tests were performed prior to and after the acceleration profiles. Relaxed G-tolerance was significantly higher under thecaffeine session, whereas SACM duration did not differ among the drink conditions. Hip adductor muscle strength was lower during the placebo sessionthan during the other two sessions. Cognitive reaction time was faster post-acceleration than pre-acceleration, and faster under the caffeine condition thanthe placebo condition. We conclude that consumption ofa caffeine-based energy drink enhances relaxed G-tolerance and may increase strength, but doesnot impact acceleration duration. We further conclude that cognitive reaction time is improved by the caffeinated drink, as well as by the physical exertionduring the acceleration exposure.

15. SUBJECT TERMSCaffeine, G-tolerance, Cenhifu ge

16. SECURIry CLASSIFICATION OF:U

17.LIMITATIONABSTRACTOF

U

18. NUMBEROF PAGES

7

19a. NAME OF RESPONSIBLE PERSONThomas B. Walker

a. REPORTU

b. ABSTRACTU

c. THIS PAGEU

19b. TELEPHONE NUMBER finclude areacode)

. 8-98)Prescribed by ANSI Std. 239.18

Acceleration Tolerance After I ngestionof a Commercial Energy Drink

RESEARCH ARTICLE

Wllrrn TB, BerloIN U, FtscurR J, SToRM W WAnnEN GL. Accelera-tion tolerance after ingestiott of a commercial energy drink. AviatSpace Environ Med 2010;81:1-7.

Background: Caffeine ingestion has been demonstrated to increasephysical performance in some si tuat ions. This study examined the abi l i tyof a commercia l energy dr ink contain ing caf fe ine to enhance accelera-tion tolerance and strength under C load. Methods: Eight experiencedcentr i fuge subjects completed three separate exper imental accelerat ionexposures fo l lowing ingest ion of

' l 1 .5 ml 'kg-r bodyweight of 1) a com-

mercia l energy dr ink, provid ing 5,0 mg caf fe ine/kg bodyweight ; 2) acommercia l energy dr ink wi thout caf fe ine; or 3) a p lacebo. The accel-erat ion exposures consisted of a re laxed gradual onset run to per ipherall ight loss, a rapid onset run to 6 C for 15 s, and a s imulated ai r combatmaneuver (SACM) run of repeated al ternat ions between 4.5 C for 15 sand 7 C for 15 s unt i l vol i t ional exhaust ion. Resu/ts; Relaxed C to ler-ance was 13% higher under the caf fe inated energy dr ink session,whereas SACM durat ion did not d i f fer amonq the dr ink condi t ions. Hipadductor muscle strength was 37o/o lower dluring the placebo sessionthan dur ing the other two sessions. Conclusion: Consumot ion of acaf fe ine-baied energy dr ink may enhance re laxed C to lerance and mayincrease strength, but does not impact accelerat ion to lerance durat ion.Keywords: caffeine, C tolerance, centrifuge.

A DVANCED FIGHTER aircraft are capable of oper--f1.ating in high-G environments and are often limitedby the physiological capabilities of the aircrew. Aircrewmembers must perform an anti-G straining maneuver(AGSM) just prior to and during a high-G aircraft ma-neuver to prevent G-induced loss of consciousness(GLOC). The inability to maintain and repeatedly per-form an AGSM can result in the loss of life and aircraft.Millions of dollars have been spent on the developmentof life support equipment to help prevent GLOC, yetperformance of a proper AGSM remains the most effec-tive protection against GLOC. Ergogenic aids contain-ing caffeine, such as energy drinks, are readily availableand may prove to enhance performance of the AGSMduring high G via reduction of muscle fatigue associ-ated with repeated isometric contractions. Recent stud-ies have demonstrated that relatively low doses ofcaffeine are effective in improving exercise performance(6,22) and may specifically enhance muscular strength(13,25) and. muscular endurance (74,78,25). Particular tothe type of contraction used in the AGSM, Plaskett andCafarelli (23) and Meyers and Cafarelli (20) have dem-onstrated that caffeine significantly increases time tofatigue during isometric contractions, making it a po-tentially valuable aid in the high-G combat envirorunent.V/hile one study (10) of rhesus monkeys failed to show

THonaas B. Wl.rxrn, Url BanorN, ]osElrr Frscrrrn,Wrnrarr Sronnr, arto GonooN L. WannrN

any effect of caffeine on cardiovascular function or re-laxed G tolerance (without performing AGSM), the effi-cacy of caffeine in humans to aid an active AGSM hasnot been investigated. This study was conducted toevaluate the ability of a caffeine-based energy drink toserve as an inexpensive yet effective aid by enhancingthe performance of AGSM in a high-G environment.

METHODS

Subjects

There were 10 volunteer subjects, including 2 women,mean age 30.2 1- 7.4yr, who were recruited from theBrooks City-Base human centrifuge subject panel. Cen-trifuge panel members are prescreened for appropriatehealth and fitress and demonstrate the capabilitv toconsistently tolerate exposures up to +9 G, #he.r #ear-ing standard G-protection ensembles. The research pro-tocol for this study was reviewed and approved by theAir Force Research Laboratory Institutional ReviewBoard prior to subject recruitment and the subjects gavewritten informed consent before participating. Femalesubjects provided a negative pregnancy test within 72 hprior to each of their centrifuge exposures.

Experimental Design

This study employed a repeated measures design anddouble blind procedures. Each subject participated inthree dosing conditions. The design goal was to balancethe order of the dosing conditions to avoid confoundingdue to any potential carry-over or learning effects. Whileperfect balance is not possible with 10 subjects and 3 treat-ments, we created a partially balanced model in whicheach of the 6 permutations of the 3 conditions was usedonce and 4 were repeated. These 10 combinations wererandomly entered into a list and as a subject entered thestudy they were assigned to the next combination in the

From the 7|1th Human Performance Wing, Brooks City-Base, TX.This manuscript was received for review in May 2010. It was

accepted for publication in September 2010.Address correspondence and reprint requests to: Maj. Thomas

B. Walker, USAF, BSC,711 HPW/RHPFA,2485 Giilingham Dr., BrooksCity-Base, TX 78235; thomas.walker@brooks.af .mil.

Reprint & Copyright O by the Aerospace Medical Association,Alexandria, VA.

DOL 1,0.3357 / ASEM.2833.2010

Aaiation, Space, and Enaironmental Medicine. Vol. 87, No. 12. December 2010

ENERGY DRINK & ACCELERATION-WALKER ET

list. The conditions consisted of ingesting 1) a commer-cially available caffeinated energy drink (Full Throttle@,produced by the Coca-Cola Company, containing 9.0 mgcaffeine per 1. oz fluid); 2) a modified version of theenergy drink comprised of the same ingredients but forthe removal of caffeine and guarana; or 3) a placebo ver-sion of the drink with all of the 'energy'ingredients re-moved (i.e., no high-fructose corn syrup, B-vitamins,ginseng, guarana, L-carnitine, or taurine). In this paperthe three dose conditions will be respectively referredto as caffeinated energy drink, non-c-affeinated energydrink, and placebo. All drinks were prepared by theCoca-Cola Company and administered to the subjects insealed bottles which had coded labels so as to obscureeach drink's composition.

Procedure

Dosing and testing schedules: The three experimentalsessions were conducted at approximately the sametime of day for each subject, with approximately 72 hlapsing between sessions to allow for recovery. Subjectswere asked to avoid strenuous physical exercise on theday before and the same day as the trials. Immediatelyprior to each trial the subjects received a brief medicalexam and were asked about their general condition andhealth, including potential caffeine intake. Subjects ab-stained from caffeine consumption for 1.4 h, and fromfood and tobacco consumption for 6 h, before each ses-sion. Dosing was based on each subject's weight and, foreach experimental session, administered in two drink-ing portions. In each experimental condition, the firstdrink consisted of 10.95 ml of drink per kg bodyweight;the second 5.48 ml of drink per kg bodyweight. For thecaffeine trial this volume resulted in a caffeine dose of5.0 mg caffeine per kg of bodyweight. This dosage wasbased on previous work by one of the co-investigators.In a study investigating the effects of a caffeinated sportsdrink (6), it was found that a nearly identical dosage(5.3 mg'kg-t) improved both endurance exercise perfor-mance and knee extensor strength. Furthermore, thebeneficial effect of caffeine on muscle strength has beenfound to be independent of the dosage used, at leastover the range of 1 to 9 mg'kg-l (25). The first drinkwas issued to the subject 1-3 d prior to the day of eachexperimental session for self-administration by the sub-ject prior to reporting to the centrifuge facility. Subjectswere instructed to keep the drink refrigerated until in-gesting it 2.5 h prior to the scheduled centrifuge run.Subjects ingested the second drink immediately on ar-riving at the centrifuge facility 30-45 min prior to thecentrifuge n:n. Each drink was consumed within 2 min.After ingesting the second drink, the subject was givena brief medical examination and instrumented withelectrocardiograph (ECG) monitoring leads as requiredfor centrifuge exposure. The subject then completedbrief subjective mood and alertness surveys. On com-pletion of the suweys, he/she was emplaced in the cen-trifuge gondola for acceleration testing, which required15-30 min depending on the subject's G-tolerance. Oncompletion of acceleration testing an 8-ml sample of

AL.

venous blood was drawn from the subject for subsequentassay of serum caffeine. Serum caffeine concentrationwas determined using a homogeneous enzyme immu-noassay technique using commercially available reagents(Emit Caffeine Assay; Dade-Befuing Syva, Cupertino,CA). Subjects then completed the mood and alertnesssurveys for a second time, were debriefed, and departedthe facility.

Acceleration profiles and G tolerance measures: All centri-fuge exposures were conducted in the 711th HumanPerformance Wing's_human centrifuge capable of G on-set rates of 6 Gr ' s-1. The 5.1-m rotating arm producescentrifugal force and the free swinging action of thegondola orients the human subject such that the resul-tant G vector is aligned with the subject's z-axis, produc-ing +G, (so that blood is forced from head to feet) as ismost common in tactical aviation. For this study an F-16seat (30'back tilt) was installed in the centrifuge gon-dola and subjects did not wear an anti-G suit. Subjectswere sequentially exposed to the same three accelera-tion profiles during each experimental drink session.Baseline (resting) heart rate (from the ECG recordingsusing the Gould Bioelectric amplifier 13-5615-58, ValleyView, OH, and a Astro-Med MT9Sk2 recorder, WestWarwich, RI) and blood pressure data were collectedat each session prior to conducting the accelerationprofiles.

Gradual onset profile: This profile measured what iscommonly known as relaxed G-tolerance. Each subjectwas exposed to this profile once for each of the threeconditions. Subjects were exposed to a profile consistingof 0.1 G . s-r onset rate to a maximum of +9 G, whilemaintaining a relaxed state (i.e., no AGSM) to the pointof 100% loss of peripheral vision or 50% loss of centralvision. Subjects were highly trained in the centrifuge be-fore participating in the protocol and were well prac-ticed at remaining as relaxed as possible during theseexposures. The outcome measures were maximum GIevel attained, systolic blood pressure (SBP), diastolicblood pressure (DBP), and heart rate (HR) measured ata cofiunon G level during the centrifuge run. A "com-mon G level" is defined as the lowest maximum G levelattained by a subject across the three experimental ses-sions and was determined for each subject individually.It was necessary to compare blood pressures and heartrate at a common G level to avoid bias when comparingthe three drink conditions. For example, if a subject wentto *5 G, under one drink and +9 G. under anotherdrink, his/her blood pressures and/or heart rates mightdiffer simply due to the additional stress of the higher G,not because of a difference caused by the drinks.

Rapid onset profile: This profile employed an onset rateof 6 G .s-1 to +6 G, where the subject remained for 15 s,during which the subject did perform AGSM. Each sub-ject was exposed to this profile once for each of the threeconditions. Outcome measures were the duration oftime at 6 G, HR, SBP, and DBP (measured at a commonpoint in time, as per the argument presented in the aboveparagraph), and estimated subjective maximum effortrequired to perform AGSM (where maximum effort is

Aoiation, Space, and Enaironmental Medicine. Vol. 81., No. 12 . December 2010

ENERGY DRINK & ACCELERATION-WALKER ET

the effort level required to perform an adequate strain-ing maneuver during high-G, measured on a modi-fiedBorg scale of 0-11, where 0 : no effort, and 1l- : extremematmal physical effort).

Simulated aerinl combat maneuaer: Each subject was ex-posed to a simulated aerial combat maneuver (SACM)profile once for each of the three conditions. This profileconsisted. of up to 15 repeated alternations between+4.5 Gzfor 15 s ar:d- +7 G, for 15 s, during which the sub-ject performed AGSM as needed until the subject self-terminated due to fatigue, light loss, or completion of 15alternations. Immediately prior to the start of the profilethe subjects performed a maximum voluntary isometriccontraction (MVIC) of their hip adductor muscles, dur-ing which strength was measured. During the first 5 s ofeach +7 G, exposure the subjects repeated the MVIC aspart of their AGSM. Outcome measures were duration(i.e., time at G), HR, SBR DBP, MVIC strength (all mea-sured at a corrunon point in time, i.e., at the lowest dura-tion observed for all three conditions in each subject),and subjective effort. MVIC strength was measured bya padded force transducer [Load Cell MLP-150 (lb)Tiansducer Techniques, Temecula, CA] located betweenthe subject's knees. Arterial blood pressure was recordedduring all the G exposures by a noninvasive photo-plethysmographic technique (Portapres@, TNO, Delft,The Netherlands) with a pressure cuff around the mid-phalanx of the third finger on the left hand. The forearmand hand were supported by a sling and the hand posi-tioned at heart level and enclosed in a preheated gloveto avoid vasoconstriction of the finger blood vessels bya cool environment.

Subjectiae measures: One instrument from the Auto-mated Neuropsychological Assessment Metrics (ANAIW24)battery and one paper-and-pencil survey, the Profileof Mood States (POMS; 19) were selected to assess alert-ness and affective state immediately before and aftereach subject's centrifuge exposure. Each survey sessionrequired about 3 min for the subjects to complete, al-ways in the following sequence.

Alertness: The ANAM battery offers an automatedversion of the Stanford Sleepiness Scale that maintainsthe original seven-point scale, rating subjective sleepi-ness from lt1-vety alert, wide awake, and energetic" to'//-ysyy sleepy and cannot stay awake much longer."

Affect: Subjective evaluations of mood were acquiredusing the POMS. This survey consists of 55 adjectivesdescribing feeling and mood to which the subject re-sponds according to a five-point scale ranging from"Not at all" to "Extremely." Subjects were instructed toindicate mood status for "how you feel right now" withregards to each item. A standardized "state" measure isgenerated for each of six mood categories: anger, confu-sion, depression, fatigue, tension, and vigor.

Statistical Analyses

For each of the centrifuge acceleration variables, a re-peated measures analysis of variance (ANOVA) wasperformed to test for differences among the three drink

AL.

conditions. When a significant drink effect was found,Fisher's LSD procedure was used to identify specific dif-ferences among the three drinks. For each subjectivemood variable, a repeated measures ANOVA with twofactors was performed to test for drink main effects, pre-versus post-acceleration main effects (hereafter referredto as'acceleration' main effect), and drink-by-accelerationinteraction. For the StanJord Sleepiness Scale (alerbress),nonparametric procedures (Friedman's and Wilcoxonsigned rank tests) were used to test for acceleration anddrink effects. The statistical package SPSS 11.5 was usedfor all statistical analyses. Alpha : 0.05 was used as thelevel for statistical significance for all tests.

RESULTS

Of the 10 subjects completing this study, only 8 wereincluded in the analyses. One subject experiencedG-induced loss of consciousness on the majority of therapid onset and SACM rides. We concluded that, whilethis subject was an experienced rider who usually per-formed satisfactorily when wearing a G suit, the G lev-els used in this study were slightly too high for thisindividual's unprotected innate G tolerance. Anothersubject, upon post-study analysis of the serum data, wasfound to have significant serum caffeine levels (> 5 pM)during all three of the experimental conditions, suggest-ing that the subject did not abstain from caffeine as di-rected by the protocol. Unfortunately, the loss of thesetwo subjects resulted in our experimental design noIonger being balanced. In fact, of the eight subjects ana-lyzed, six of them experienced the placebo run beforethey experienced the caffeine run. The potential impactof this imbalance is addressed in the Discussion sectionbelow. Finally, due to technical problems, blood pres-sure data was not always available for all centrifugeruns of each subject. Thus, for each blood pressure out-come measure, only subjects with data for all three con-ditions were included in the analysis.

Serum Caffeine Leoels

For the eight subjects used in this study, serumcaffeine levels averaged33.37 -F 8.45 pM under the caf-feinated energy drink, 2.49 -+ 1.09 pM under the non-caffeinated energy drink, and 2.70 '+ 1.38 pM under theplacebo drink, thus con-firming adherence to the proto-col with respect to the use of c#feinated products andstudy drink intake.

Ac celer ation Toler ance

The centrifuge acceleration measures are summarizedin Table I. There were four outcomes of primary interestin this study: relaxed gradual onset G tolerance; rapidonset G exposure duration (with AGSM); SACMG-exposure duration; and MVIC strength (measured ata corrunon point in time during the SACM). For two ofthese (rapid onset G-exposure duration and SACM du-ration), no significant differences were found amongdrink conditions. For relaxed gradual onset G tolerance,there was a significant drink main effect [MSE : 0.183,

Aaiation, Space, and Enuironmental Medicine. Vol. 81, No. 12. December 2010

ENERGY DRINK & ACCELERATION-WALKER ET AL.

F(2,74) : 9.69, P : 0.0021. Follow-up f-tests revealedthat G tolerance was significantly higher under the caf-feinated energy drink condition than under the non-caffeinated energy drink and placebo conditions by0.9 +G,(P : 0.0O1) and 0.7 +G"(P : 0.077), respectively.

There was no statistical difference among the threedrink conditions with respect to the MVIC control datameasured before the centrifuge mns. MVIC strengthmeasured at a cornmon time point during the SACMexposures was lower than MVIC control for all threeconditions, with the largest drop occurring during theplacebo condition. The drink main effect was significantfor MVIC strength measured at a common time pointduring the SACM IMSE : 225.2, F(2,1.4) : 3.96, P :0.043] and post hoc tests revealed that the means weresignificantly higher under the two energy drink condi-tions than under the placebo condition by 29% and32o/o,respectively (P : 0.043 in each case).

No significant drink effects were found for any ofthe physiologic measures (SBP, DBR HR). Subjects'per-ceived effort required to complete the various profilesalso did not differ among drink conditions. Finally, be-cause of the nature of the cardiac arrhythmia data andthe small sample size, no statistical tests were performedon these measures. However, the distribution of the data(see Table I) suggests a trend toward more subjects be-ing affected and more overall occurrences of arrhyth-mias under the caffeinated energy drink condition thanunder the other two conditions.

Mood States and Alertness

No significant drink by acceleration interactions ordrink main effects were detected for any of the six POMSmood states. Acceleration main effects were found forconfusion, fatigue, and vigor. The average standardizedscore for confusion significantly increased from 34.5 be-fore acceleration to 37 .8 after acceleration [MSE : 16.21.F(I,7) : 8.43, P : 0.0231, fatigue significantly increasedfrom36.7 to 43.3 [MSE : 68.667 ,F(1.,7) : 7 .77 , P : 0.027),and vigor significantly decreased from 49.4 to M.6 [MSE :25.85, F(1,,7) : 70.48, P : 0.0141. The scale range forconfusion is 30-80, fatigue 34-77, andvigor 30-76. Thus,the pre- to post-acceleration changes in score for thesethree mood factors, while statistically significant, werevery small and, in absolute terms, indicative of the sub-jects' being in a positive and alert affective state bothbefore and after exposure to acceleration. For theStanford Sleep Scale subjective alertness scores, nosignificant drink, acceleration, or interaction resultswere found. The overall average score was 1.8 beforeacceleration exposure and 2.4 following exposure,both scores indicating a high level of alertness (scalerange : 7 to7).

DISCUSSION

The primary finding of this investigation was that in-gestion of a caffeinated energy drink, delivering 5.0 mgof caffeine per kg bodyweight, did not significantly in-fluence acceleration endurance for subjects while per-

forming the AGSM during rapid onset or SACMexposures. However, ingestion of a caffeinated energydrink did result in a significant improvement in relaxedgradual onset G-tolerance, and appeared to increase hipadductor strength levels measured during the perfor-mance of the AGSM. -

An effective AGSM can improve G tolerance by over3 +Gz(11), but can be very fatiguing. Acceleration toler-ance is usually related to the ability to maintain a suffi-cient heart level and cerebral arterial blood pressure.Before a gray-out, blackout, and/or G-LOC, retinal andcerebral arterial blood pressure usually fall drastically.During relaxed (no AGSM) G-exposure the cardiovascu-lar response, through arterial and cardiac baroreceptors,increases heart rate and blood pressure within 6-9 s inan attempt to counteract the C-induced decrease inblood pressure.. Heart level blood pressure is partiallyrestored in 10-15 s through this baroreceptor effect (3).The endurance to withstand repeated G loads is alsorelated to the ability to maintain an effective respira-tory and muscular AGSM. During G exposures withan AGSM, blood pressure is immediately elevated1) through muscle contraction of the legs and abdomen,causing peripheral vasoconstriction; and 2) tfuough therespiratory straining maneuvet which increases intra-thoracic pressure and heart contractility.

Caffeine is known to stimulate the cardiovascular andcentral nervous systems through the activation of thesympathetic neryous system (7),but also to cause relax-ation of smooth muscles. It influences cardiovascularstress reactivity and potentiates the body's stress re-sponse (15). In response to caffeine ingestion, bloodpressure rate of increase appears to be highest duringthe first 30 min, with a smaller rise during the next30 min, followed by a weak response after an hour (21).

Caffeine has also been well demonstrated to enhanceexercise performance, although its ergogenic benefitsseem highly dependent upon the duration, intensity,and mode of exercise. In the athletic arenas most closelyresembling performance of the AGSM under G expo-sure, the literature is incomplete, but suggests caffeineingestion may be beneficial. For example, Beck et al. (2)observed that ingestion of a caffeinated energy drink en-hanced performance of a one-repetition maximum (RM)bench press, but did not improve performance of a10-RM bench press or a 1.0-RM leg extension exercise, nordid it improve performance of repeated 30-s Wingatetests. However, Andersen et al. (1) observed that shortdistance (2000-m) rowing_performance was improvedafter ingesting 6 mg 'kg-'bodyweight of caffeine, theimprovement being most apparent within the first 500 m.Meyers and Caferelli (20) determined that ingesting6 mg'kg-'bodyweight of caffeine increased time to ex-haustion during submaximal knee extension. In a recentmeta-analysis, Warren et al. (25) concluded that caffeineis ergogenic for both strength and high-intensity muscu-lar endurance exercise. However, they suggested thatthe strength benefits of caffeine ingestion may be great-est in the knee extensors, which are not primary actorsin the AGSM.

Aoiation, Space, and Enoironmental Medicine. Vol. 81, No. 1,2. December 2010

ENERGY DRINK & ACCELERATION-WALKER ET AL.

TABLE I. ACCELERATION DATA MEANS (SD).

G-Profile

Drink Condition

Variable Caffeine Non-Caff. Placebo

Resting

Relaxed Cradual Onset Run

Rapid Onset Run (wi th ACSM)

Simulated Air Combat Maneuver(wi th ACSM)

Arrhythmias (PVCs & bigeminis)

SBP (mmHg)DBP (mmHg)HR (bpm)C-level AttainedSBP (mmHg)DBP (mmHg)HR (bpm)Durat ion (s at 6 C)SBP (mmHg)DBP (mmHg)HR (bpm)Perceived EffortDurat ion (s)SBP (mmHg)DBP (mmHg)HR (bpm)Max MVIC contro lMVIC at Common CPerceived Effort# of subjects wi th arrhythmia# of occurrences of arrhythmia.

o

o

8B558844B785

B8B888

146 (8)_74 (11 \63 (12)6 .9 (1 .6 ) *214 (36)124 (17)9 6 ( r 1 )1s (o)

237 (23)138 (20)1 3 9 ( 1 8 )6.4 (1 .4)206 (1 31 )2s2 (ss)1 s 3 ( 2 1 )1 51 (20)88 (23)76 (23\

e . 0 ( 1 . 7 )6

49+

129 (23)63 (1 s)67 (21)6 .0 (1 .2 )2O9 (4O)1 1 4 ( 1 3 )99 (14)14 (2 \

2O3 (43)124 (36\134 (21)6 . 4 ( 2 . 1 )173 (92)264 (31] '132 (211148 (23)87 (17)78 (28)

e . 1 ( 1 . 6 )35

. r38 (1 7 )72 (13)64 (1 s)6 .2 (1 .5 )177 (s9)1 1 0 ( 3 0 )91 ( i 9 )1s (o)

2s6 (1 8 )152 (25\132 (2O\6 .3 (2 .4 )2O4 (102)243 (671137 (28t146 (23194 (24)59 (26)**7 . 9 ( 2 . 1 )

41 7

t I nc l udes l2b igem in i s , a l l f r om l sub jec t .Nob igem in i swe reseen fo r t heno -ca f f e i neandp lacebocond i t i ons .i Signi f icant ly h igher than the no caf fe ine and placebo means (P < 0.05, Fisher 's LSD post hoc tests) .** Significantly lower than the caffeine and no caffeine means (P < 0.05, Fisher's LSD post hoc tests).

Caffeine-induced increases in blood pressure andheart rate, along with potential increases in muscularstrength and endurance, would theoretically be benefi-cial for increasing G tolerance. Maximal MVIC control(pre-SACM) means did not differ among the drink con-ditions. MVIC during the SACM run dropped (com-pared to the MVIC control) by 10 to 14% under the twoenergy drink conditions and by 37% under the placebocondition. In general, the MVIC strength reduction dur-ing the SACM likely reflects the inability of the subjectsto devote complete attention to performing MVIC dueto the need to concentrate on achieving a good AGSM.The fact that the drop was less under the two energydrink conditions than under placebo suggests that caf-feine (and/or the other "energy" ingredients in the no-caffeine energy drink) did have a positive effect onmuscular endurance. In fact, our analysis showed thatMVIC strength during the SACM was significantlyhigher in both of those conditions compared to placebo.Despite this finding, we did not see any statistical differ-ences or even a trend toward differences in SACM endur-ance among the drink conditions. SACM endurance wasthe variable of primary interest for this study as that mea-sure is strongly applicable to the air combat environment.Apparently the impact of caffeine on muscular endur-ance was insu-fficient to positively affect SACM endur-ance in our scenario of high-intensity muscular work.

Notably, G tolerance was significantly higher by +0.9and *0.7 G, during the gradual onset mns under thecaffeinated energy drink condition compared to thenon-caffeinated energy drink and placebo, respectively.Since the lower body muscles are relaxed during thiscondition and the breathing muscles are engaged for no

more thim normal breathing (no AGSM), the improvedG tolerance is not explained by an improved stimulatingeffect on the voluntary muscles. However, the stimulat-ing effect of caffeine on the cardiovascular system dis-cussed above (7) could have contributed to the improvedG tolerance during the relaxed gradual onset run. Thisenhanced cardiovascular response while riding relaxedwas apparently mild and was not revealed by an in-crease in the registered blood pressure. During SACMthe cardiovascular response was likely overwhelmed/masked by the voluntary increase in muscle tension andintrathoracic pressure exerted while performing theAGSM. For probably the same reason, this effect was notseen during the rapid onset runs to * 5 G, with AGSM.However, this apparent lack of effect during the * 6 G,rapid onset run could also be due to the fact that all butone subject reached the maximum run time of 15 s dur-ing all three conditions. In other words, the G-level mayhave been too low and the duration too short during therapid onset runs to reveal potential benefits fromcaffeine.

Our results did not show any statistically significantdifferences in systolic or diastolic blood pressure amongthe drink conditions during either the resting or G-exposlrre conditions. There was a slight hend to increasedsystolic blood presswe during the caffeinated energydrink condition while resting before the centrifuge runs,but it was not statistically significant. Possibly, the in-crease in blood pressure usually associated with caffeinewas not su-fficient to have any further effect over the al-ready strong baroreceptor response during increased Gor when straining maneuvers were used to increaseblood pressure for improved G tolerance. It is also possible

Aaiation, Space, and Enaironmental Medicine. Vol. 81, No.72. December 201.0

I

ENERGY DRINK & ACCELERATION-WALKER ET AL.

that the lack of significant results was partially a prod-uct of the reduced sample size. (Due to technical prob-lems, we were able to measure blood pressure in onlysix subjects during the resting condition and in four sub-jects during the increased G exposures).

Surprisingly, heart rate was not significantly higherunder the caffeinated energy drink condition than un-der the other drinks either at rest or at the various Gexposures. An increased heart rate under caffeine is ex-p"ct"d through its activation of the sympathetic nervoussystem both during rest and during physical activity,but this effect was perhaps overwhelmed by increasedsympathetic activity due to a G-anticipatory effect be-fore the centrifuge runs or the even stronger sympatheticactivity during the hard physical work when strainingmaneuvers were used at increased G.

Caffeine is known to induce and increase the fre-quency of cardiac arrhythmias (4,9). Since arrhythmiasare often seen during exposures to high G loads in thecentrifuge (25), one could expect that caffeine should in-crease these arrhythmias at high G loads. However, weobserved relatively few arrhythmias in this study (onlya total of 71 occurrences during the entire 72 G expo-sures). The number of subjects experiencing arrhyth-mias and the total occurrences of arrhythmias tended tobe higher under the caffeinated energy drink conditionthan under the non-caffeinated energy drink and pla-cebo conditions, suggesting that caffeine may have someimpact on the number of arrhythmias normally experi-enced during high G acceleration. Importantly, however,none of the 72 centrifuge exposures had to be stoppeddue to serious arrhythmias.

In addition to its physiological effects, caffeine in-gestion has been observed to influence mood (16,17).In studies evaluating the administration of caffeine,especially in sleep-deprived subjects, enhanced or re-covered performance is typically accompanied by adecrease in fatigue scores and an increase in vigorscores (8,1,6,77) compared to the effects of a placebo.Because variations in mood have been demonstrated toaffect both physiological response to exercise (5) andperceived exertion during exercise (12), we suspectedthe caffeinated energy drink SACM trials may havebeen perceived as Iess taxing. However, we found nodifferences in any of the POMS factors among the caf-feinated energy drink, non-caffeinated energy drink,and placebo conditions in our well-rested subjects priorto or after acceleration exposure. Nor did we observesignificant differences in SACM RPE between drinkconditions. The lack of significant differences in thecurrent study between conditions for POMS fatigueand vigor may be due, at least in part, to the subjectshaving been well rested and in a positive, can-do stateof mind for all conditions.

There are several limitations to this study, as follows.1) A limitation conunon to most human centrifuge stud-ies is the inability, due to safety concerns, to expose sub-jects to G levels greater then 9.0 *G,. That limitation iswhy many protocols, including the current study, useSACM duration, rather than maximal G tolerance, as a

prime parameter. Fortunately, although maximum Gtolerance would be very decisive from a pure physiolog-ical perspective, SACM duration is very relevant to ac-tual operations of high performance aircraft. 2) Anotherlimitation is our inability to accurately measure bloodpressure at high C levels. Current equipment appearsunreliable above approximately 6 +G".3) We used arandomized semibalanced design to administer thedrink conditions. Unfortunately, given our relativelysmall starting sample, the elimination of two subjectscaused the design to become trnbalanced. As indicatedearlier, of the eight subjects, six of them received the pla-cebo drink in an earlier session than they received thecaffeinated drink. That is, the drink condition and orderof presentation were confounded. If a learning trendwere present, this would influence the test for drink dif-ferences. Given the experience level of our centrifugepanelists, we are com-fortable that this was not a prob-Iem for the majority of the centrifuge variables studied.However, since performing a good MVIC while simulta-neously performing an AGSM may have improved withrepeated trials, we cannot say with complete confidencethat the increased strength seen in the caffeinated drinkcondition is entirely due to the drink.4) Finally, we wereIimited by our inability to insure that our subjects'moti-vation levels were identical between trials. The SACMprofile is very taxing and, although we provided thesame level of encouragement for each run, internal mo-tivation levels may have fluctuated.

Based on the results of this investigation, we concludethat ingestion of a caffeinated energy drink mildly in-creases relaxed G tolerance, but does not increase the ef-fectiveness of one's AGSM under G. Aircrew should notexpect to experience enhanced AGSM performance fol-lowing caffeine consumption. Therefore, given the con-siderable strength and muscular endurance necessary toeffectively perform an AGSM, fighter and trainer air-crew remain best served by maintaining a rigorousphysical training program and by refining their AGSMtechnique.

ACKNOWLEDGMENTSThis research was conducted under the auspices of Wvle, Inc. The

research was funded by a grant from the Coca-eola Compiny, Atlanta,GA.

The authors wish to thank Dr. Maxime Buyckx of Coca-Cola forhis assistance in designing the experimental protocol, Mr. Mac Baker,Capt. julia McGregor, Mr. Tom Beltran, and Mr. Wayne Isdahl fortheir support with data collection throughout this protocol, and Maj.Michael Rutter and Capt. Chris Vojta for their support as medicalmonitors during this protocol.

Authors and ffiliations: Thomas Walker, M.S., Ph.D., HumanPerformance Wing, and Ulf Balldin, M.D., Ph.D., Joseph Fischer, B.A.,M.S., and William Storm, M.S., Ph.D., Wyle, lnc., Brooks City-Base, TX;and Gordon L. Warren, M.S., Ph.D., Georgia State University, Aitanta,

REFERENCES1. Anderson ME, Bruce CR" Fraser SF, Stepto NK Klein & et al.

Improved 2000-meter rowing performance in competitiveoarswomen after caffeine ingestion. Lrt J Sport Nutr ExercMetab 2000; 10:464-75.

2. Beck TW, Housh Tj, Schmidt RJ, Johnson GQ, Housh DJ, et al. Theacute effects of a caffeine-containing supplement on skength,

Aaiation, Space, and Enaironmental Medicine. Vol. 81, No. 12. December 2010

ENERGY DRINK & ACCELERATION-WALKER ET

muscular endurance, and anaerobic capabilities. J StrengthCond Res 2006 ; 20 :506-1,0.

3. Banks RD, Brinkley JW, Allnutt R, Harding RM. Human responseto acceleration. In: Davis IR, ed. Fundamentals in aerospacemedicine, fourth ed. Philadelphia: Lippincott, Wil-liams &Wilkins; 2008:8F109.

4. Cannon ME, Cooke Cl McCarthy JS. Caffeine induced cardiacarrhythmia: an unrecognized danger of health food products.Med J Aust 2001,;17452W1.

5. Crews DJ. Psychological state and running economy. Med SciSports Exerc f992; 24:475-82.

6. Cureton KJ, Waren GL, Millard-Stafford ML, Wingo JE, TrilkJ, Buyckx M. Caffeinated sports drink: ergogenic effects andpossible mechanisms. Int J Sport Nutr Exerc Metab 2007;17:35-55.

7. Denaro CR Jacob III BP, Benowithz NL. Effects of caffeine withrepeated dosing. Eur J Clin Pharmacol L991,;40:273-8.

8. DoanBli HickeyPA, LiebermanH& FischerJR.Caffeinated tube foodeffect on pilot performance during a 9-hour, sirnulated nighttimeU-2 mission. Aviat Soace Environ Med2N6:77:L034-4O.

9. Dobmeyer DJ, Stine ItA, L"iur CV, Greenberg R, Schaal SF. Thearrhythrnogenic effects of caffeine in human beings. N Engl JMed 1983;308:814-24.

10. Florence G, Riondet L, Serra A, Etienne X, Huart B, et al.Psychostimulants and C tolerance in rhesus monkeys: effectsof oral modafinil and injected caffeine. Aviat Space EnvironMed 2005; 76:1.21.-4.

L1. Giliingham KK, Fosdick JP. High-G training for aircrew. AviatSpace Environ Med 1988; 59:12-9.

12. Harily CJ, Rejeski WJ. Not what, but how one feels: themeasurement of affect during exercise. J Sport Exerc Psychol7989;7L:304-77.

1.3. James RS, Kohlsdorf T, Cox VM, Navas CA. 70 pM caffeinetreatment enhances in vitro force and power output duringcyclic activities in mouse extensor digitorum longus muscle.Eur J Appl Physiol 2005; 95:74-82.

14. Kalmar JM. The influence of caffeine on voluntary muscleactiva tion. Med Sci Sports Exer c 2005 ; 37 :2173-9.

15. Lane JD, Adcock A, Wlliams RB, Kuhn CM. Caffeine effectson cardiovascular and neuroendocdne resoonses to acute

AL,

osvchosocial stress and their relationshio to level of habitualiuif"itr" consumption. Psychosom Me d igsl; sz:lzvze .

15. Lieberman HR, Tharion WJ, Shukitt-Hale B, Speckman KL,Tulley R. Effects of caffeine, sleep loss, and stress on cognitiveperformance and mood during U.S. Navy SEAL training.Psychopharmacology (Berl) 2002; 1,64:250-61,.

17. Lieberman HR, Wurhnan RJ, Emde GG, Roberts C, Coviella ILG.The effects of low doses of caffeine on human performance andmood. Psychopharmacology (Berl) 1 987; 92:308-12.

18. Mclellan TM, Bell DG, Lieberman HR, Kamimori GH. Theimpact of caffeine on cognitive and physical performanceand marksmanship during sustained operations. CanadianMilitary Joumal 2003-2004; 4:47 -54.

19. McNair DM, Lorr M, Droppleman LF. Profile of mood statesmanual. San Diego, CA: Educational and Industrial TestingServrce: 1yl1.

20. Meyers Bil, Cafarelli E. Caffeine increases time to fatigue bymaintaining force and not by altering firing rates duringsubmaximal isomehic conhactions. J Appl Physiol 2005;99:1,056-63.

21. Onrot J, Goldberg MR, Biaggiono I, Hollister AS, Kincaid D,Robertson D. Hemodynamic and humoral effects of caffeine inautonomic faiiure. N Engl J Med 1985;313:549-54.

22. Pasman WJ, Van Baak MA, Jeukendrup AE, de Haan A. The effectof different dosages of caffeine and endurance performancetime. Int J Sports Med 1,995;76:225-30.

23. Plaskett CJ, Cafarelli E. Caffeine increases endurance andattenuates force sensation during submaximal isometriccontractions. J Appl Physiol 2001 ; 91-:753544.

24. Reeves D, Winter K, Kane R, Elsmore I Bleiberg l. ANAM 2001user's manual: clinical and research modules. Special reportNCRF-SR-2001-1. San Diego, CA: National Cognitive RecoveryFoundation; 2001.

25. Warren GL, Park ND, Maresca RD, McKibans KI, Millard-Stafford ML. Effect of caffeine ingestion on muscular strengthand endurance: a meta-analysis. Med Sci Sports Exerc 2010;42:7375-87.

26. Whinnery JE. The electrocardiographic response to high +Gzcentrifuge haining. Aviat Space Environ Med 1990;61:71G21,.

Aaiation, Space, and Enaironmental Medicine . Vol. 81, No. 12. December 2070