Musculoskeletal adaptations to training with the advanced resistive exercise device

by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.Copyright @ 2010

Musculoskeletal Adaptations to Training withthe Advanced Resistive Exercise Device

JAMES A. LOEHR1, STUART M. C. LEE1, KIRK L. ENGLISH2, JEAN SIBONGA3, SCOTT M. SMITH4,BARRY A. SPIERING1, and R. DONALD HAGAN41

1Wyle Integrated Science and Engineering Group, Houston, TX; 2JES Tech, Houston, TX; 3Universities SpaceResearch Association, Houston, TX; and 4NASA Johnson Space Center, Houston, TX

ABSTRACT

LOEHR, J. A., S. M. C. LEE, K. L. ENGLISH, J. SIBONGA, S. M. SMITH, B. A. SPIERING, and R. D. HAGAN. Musculoskeletal

Adaptations to Training with the Advanced Resistive Exercise Device. Med. Sci. Sports Exerc., Vol. 43, No. 1, pp. 146–156, 2011.

Resistance exercise has been used as a means to prevent the musculoskeletal losses associated with spaceflight. Therefore, the National

Aeronautics and Space Administration designed the Advanced Resistive Exercise Device (ARED) to replace the initial device flown on

the International Space Station. The ARED uses vacuum cylinders and inertial flywheels to simulate, in the absence of gravity, the

constant mass and inertia, respectively, of free weight (FW) exercise. Purpose: To compare the musculoskeletal effects of resistance

exercise training using the ARED with the effects of training with FW. Methods: Previously untrained, ambulatory subjects exercised

using one of two modalities: FW (6 men and 3 women) or ARED (8 men and 3 women). Subjects performed squat, heel raise, and dead

lift exercises 3 dIwkj1 for 16 wk. Squat, heel raise, and dead lift strength (one-repetition maximum; using FW and ARED), bone mineral

density (via dual-energy x-ray absorptiometry), and vertical jump were assessed before, during, and after training. Muscle mass (via

magnetic resonance imaging) and bone morphology (via quantitative computed tomography) were measured before and after training.

Bone biomarkers and circulating hormones were measured before training and after 4, 8, and 16 wk. Results: Muscle strength, muscle

volume, vertical jump height, and lumbar spine bone mineral density (via dual-energy x-ray absorptiometry and quantitative computed

tomography) significantly increased (P e 0.05) in both groups. There were no significant differences between groups in any of the

dependent variables at any time. Conclusions: After 16 wk of training, ARED exercise resulted in musculoskeletal effects that were

not significantly different from the effects of training with FW. Because FW training mitigates bed rest–induced deconditioning, the

ARED may be an effective countermeasure for spaceflight-induced deconditioning and should be validated during spaceflight.

Key Words: RESISTANCE EXERCISE, SPACEFLIGHT, MUSCLE STRENGTH, BONE MINERAL DENSITY, iRED

Decreased muscle function and loss of bone strengthduring long-duration spaceflight may jeopardizecrew health and mission success by impairing or

limiting work performance or by increasing the risk ofmuscle injury or bone fracture. During bed rest, a spaceflightanalog, high-intensity resistance exercise protects againstmusculoskeletal deconditioning (6,5,12,27). Therefore, theNational Aeronautics and Space Administration (NASA)deployed the interim Resistive Exercise Device (iRED) in2001 as an exercise countermeasure during long-durationstays aboard the International Space Station (ISS). Despitethe availability of the iRED, results from long-duration ISSmissions (Q4 months) indicate that astronauts continue to

lose muscle mass, muscle strength, and bone mineral density(BMD) (16,31).

Several factors limit the effectiveness of the iRED asa countermeasure to spaceflight-induced musculoskeletal de-conditioning. First, the peak resistance is limited to 136 kg(300 lb) (26); this is less than the resistance shown to beeffective in previous bed rest studies (5,6,27). Further, con-sidering that body mass will not contribute to overall resis-tance in spaceflight, a 75-kg astronaut who performed asquat (SQ) with the iRED’s peak resistance on the ISSwould experience a load roughly equivalent to the resistancefrom performing a SQ with only 60 kg of load in normalgravity (20). Second, during the eccentric portion of themovement, the force is only È70% of the correspondingconcentric force (26), and a lack of eccentric resistance maycompromise strength gains because of a suboptimal intensity(9). Finally, the iRED provides a variable resistance inwhich force increases as the cable is extended farther fromthe iRED. During closed-chain exercises (i.e., SQ, heel raise(HR), and dead lift (DL)), the iRED may be providing in-sufficient resistance at the bottom of the movement, wherethe muscles are most active (11), and greater resistance at thetop, where the muscles are working less owing to an in-creased mechanical advantage (4).

Address for correspondence: James A. Loehr, M.S., Space Physiology andCountermeasures, NASA Johnson Space Center, 2101 NASA Parkway,Mail code: SK3, Houston, TX 77058; E-mail: [email protected] for publication October 2009.Accepted for publication April 2010.

0195-9131/11/4301-0146/0MEDICINE & SCIENCE IN SPORTS & EXERCISE�Copyright � 2010 by the American College of Sports Medicine

DOI: 10.1249/MSS.0b013e3181e4f161

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To address the limitations of the iRED, NASA developedthe Advanced Resistive Exercise Device (ARED), in thehope that its improved functionality would more completelyprotect the musculoskeletal system during long-durationspaceflight. Specifically, the ARED uses vacuum cylindersto provide a concentric resistance up to 272 kg (600 lb), aneccentric–concentric ratio of È90%, and a constant forcethroughout the range of motion. In addition, inertial fly-wheels were integrated into the resistance path of each vac-uum cylinder to simulate the inertial characteristics of freeweight (FW) exercise, an effective method to protect againstmuscle atrophy and bone loss during bed rest (27).

The purpose of this study was to compare the musculo-skeletal adaptations to 16 wk of resistance exercise trainingwith the ARED to training with FW in healthy, untrained,ambulatory men and women. First, we hypothesized that16 wk of training with the ARED or with FW would resultin significant increases in muscle strength, muscle volume,lean tissue mass, vertical jump (VJ) height, and BMD. Sec-ond, we hypothesized FW would increase muscle strength,muscle volume, lean tissue mass, and BMD to a greater extentthan ARED because of the differences in inertial character-istics of the ARED flywheels and FW. We further hypothe-sized that there would be a difference between the groups formarkers of bone formation and resorption and for blood con-centrations of anabolic and catabolic hormones.

METHODS

Subjects. Twenty-two volunteers (mean T SD; age =34 T 6 yr, body mass = 77.1 T 12.1 kg, height = 171 T 9 cm),16 men and 6 women, were recruited by the Human TestSubject Facility at NASA Johnson Space Center to partici-pate in this study. The ratio of men to women was chosenspecifically to mimic the distribution of men and womenin the astronaut corps (15). Before they were enrolled inthe study, all subjects passed a modified Air Force ClassIII physical examination, which included a comprehensiveevaluation to exclude subjects with metabolic, musculo-skeletal, or cardiovascular disease, soft tissue or joint inju-ries, and lumbar spine and hip BMD less than 2 SD belowthat of the healthy population mean measured by a dual-energy x-ray absorptiometry (DXA). Testing protocols werereviewed and approved by the NASA Johnson Space Center

Committee for the Protection of Human Subjects. Subjectsreceived written and verbal explanations of the study pro-cedures and provided written informed consent before par-ticipating. Two subjects withdrew from the study beforecompleting all the training and were not replaced. Onesubject had scheduling conflicts that did not allow adherenceto the prescribed training schedule, and the other withdrewbecause of a musculoskeletal injury unrelated to the study.

The subject screening, selection criteria, and dietary con-trols were similar to those used in a previous study from ourlaboratory (26). Subjects had not participated in a resistancetraining program for at least 6 months before entering thestudy and performed only the prescribed resistance exerciseprotocol during the study. They were instructed to maintaintheir prestudy aerobic exercise habits (intensity, frequency,and duration) and not to initiate any new exercise programsduring the study. A similar number of subjects (FW n = 6and ARED n = 8) in both groups participated in varioustypes of light to moderate aerobic exercise (i.e., jogging,walking, swimming) approximately 1–2 dIwkj1 for the du-ration of the study. Subjects were instructed to maintain theirdietary habits for the duration of the study and to not takeany nutritional supplements that might affect muscle per-formance, lean tissue mass, or bone metabolism.

Experimental design and procedures. The totallength of the study was 24 wk. It was composed of 6 wk ofpretraining tests, 16 wk of training, and 2 wk of posttrainingtests (Fig. 1). Subjects were not assigned randomly becausea planned NASA bed rest study required the only availableARED unit. The first subjects to be selected were assignedto the ARED training group to ensure that all testing andtraining were completed before the ARED was needed forthe planned bed rest study. The rest of the subjects wereassigned to the FW training group after the ARED trainingsubjects had completed the study.

Resistance exercise training protocol. Each groupperformed the parallel SQ, HR, and DL exercises (the pri-mary lower body exercises prescribed during spaceflight)3 dIwkj1 for 16 wk. The level of resistance used duringtraining for each group was based on a percentage of thepretraining and midtraining one-repetition maximum (1-RM)using the exercise hardware (FW or ARED) corresponding totheir group assignment. A periodized training protocol was

FIGURE 1—Timeline of the study.

MUSCULOSKELETAL EFFECTS OF ARED TRAINING Medicine & Science in Sports & Exercised 147

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used, in which the resistance varied in a predetermined fash-ion within each week and every subsequent week (2) (Fig. 2).The protocol was derived from the astronaut in-flight resistiveexercise prescription, which was developed by the AstronautStrength Conditioning and Rehabilitation specialists respon-sible for designing astronaut exercise protocols. The first ex-ercise day (day 1) of each week was considered the heavyresistance day, day 2 was the light resistance day (10% lessresistance than day 1), and day 3 was a moderate resistanceday (5% less resistance than day 1). For example, a heavyresistance day in which training was performed with loadsequivalent to 80% 1-RM would be followed by training with70% 1-RM for the light day and training with 75% 1-RMfor the moderate day. Subjects performed the same numberof sets and repetitions within each week, while the numberof sets and repetitions changed each week to correspond tothe given intensity. The exercise prescription included sev-eral warm-up sets performed before the training sets (highestresistance) during each session. Each subject was required tocomplete at least 90% of the total number of training sessions.

Exercise hardware. ARED training and testing wereperformed using a version of the NASA-designed device(NASA Johnson SpaceCenter, Houston, TX; Fig. 3A) identical

to the one constructed for use on ISS. The ARED wasdesigned to mimic two components of FW exercise: 1) con-stant resistance, provided by two vacuum cylinders; and 2) theinertial component of moving a constant mass, simulated bytwo flywheels within each cylinder’s resistance path (Fig. 3B).ARED bar exercises are ‘‘semi–free form’’ exercises; lateralmotion follows a fixed path with the ARED, similar to asmith machine, but unlike a smith machine, the ARED allowsfore-aft motion (Fig. 4).

Air is evacuated from inside the cylinders to create avacuum, and resistance is created when the pistons inside thecylinders are pulled away from the top of the vacuum-sealedcylinders. As the bar is lifted away from the platform, thevacuum inside the cylinders resists against the motion of thepiston, trying to pull the piston back to the starting position.Therefore, concentric resistance is created as the subjectpulls against the vacuum, and eccentric resistance is createdas the subject resists against the vacuum pulling the pistonback to the starting position. Because the vacuum is con-stant, the amount of resistance is adjusted by changing thelength of the lever arm through which the piston is pulled,thereby increasing or decreasing mechanical advantage. Toensure accurate resistance, a weekly four-point static cali-bration between 0 and 272 kg (600 lb) was performed usinga calibrated measurement tool and procedure designed byNASA for in-flight ARED calibration. In addition, a monthlyseven-point static calibration using a load cell was performedbetween 0 and 114 kg (250 lb) to validate a greater number ofresistances within the potential range of prescribed values.Resistance never varied more than T1.7 kg at any calibrationpoint throughout the study.

Testing and training for the FW SQ, HR, and DL wereperformed using a standard Olympic bar and weights. Alighter Olympic training bar (6.8 kg, 15 lb) was used forsmaller individuals whose warm-up resistance was less than20.5 kg (45 lb). The SQ exercise was performed using a halfrack (York Barbell Company, York, PA), and the HR wasperformed using a standard Smith machine (Bigger, Faster,Stronger 30052; Salt Lake City, UT). Because the lowest

FIGURE 2—Exercise intensity and volume across the training study byweek. Within a week, training intensity is represented by percentage 1-RM for the heavy day of training. Training resistance in the first 8 wkwas determined based on pretraining 1-RM measurements. Midtrain-ing 1-RM strength was measured during week 9 and was used to pre-scribe the training intensity for the remainder of the study.

FIGURE 3—A, The ARED as it was used in this study. B, The resistance was primarily provided by two vacuum cylinders, and forces due to inertiathat are experienced during FW exercises were simulated using a flywheel assembly.

http://www.acsm-msse.org148 Official Journal of the American College of Sports Medicine

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setting for the ARED bar height for the DL was 2 cm higherthan the height of the Olympic bar with standard Olympicplates, two dense foam pads were placed beneath the platesduring FW DL exercise to ensure identical starting positionson both devices.

1-RM testing. Each subject completed six 1-RM testingsessions during the pretraining period, two during themidtraining period, and two during the posttraining period(Fig. 1). During the pretraining period, three 1-RM tests wereconducted using the ARED and three using FW; all three 1-RM tests on a given device were performed consecutivelybefore testing with the other device. The first 1-RM sessionfor each modality was used to familiarize each subject withthe hardware and testing procedures as well as to establish apreliminary 1-RM. The pretraining 1-RM scores were definedas the highest resistance lifted during the subsequent two 1-RM test sessions. All six sessions were separated by at least5–7 d to avoid a training effect, fatigue, and muscle soreness.The FW and ARED midtraining and posttraining 1-RMsessions were conducted during 1-wk periods and were sep-arated by a minimum of 3 d. The order of devices used for the1-RM testing was randomized for each subject at each timepoint (pretraining, midtraining, and posttraining).

Tests to obtain 1-RM were conducted using a progressionof resistance from low warm-up sets to maximal efforts. Thefirst two sets consisted of eight repetitions at 50% and 60% of1-RM, respectively, followed by a set of five repetitions at70% 1-RM and a set of three repetitions at 80% 1-RM. Sub-sequent sets of one repetition were performed at 90% and100%, and until the subject could not lift the resistancethrough the desired range of motion using proper technique.Approximately 2–3 min of rest was given between each set.A 1-RM session was terminated if the subject failed ontwo consecutive attempts to lift a given resistance or if theirtechnique was poor. Certified Strength and ConditioningSpecialists evaluated the technique and form of each subjectaccording to National Strength and Conditioning Association

guidelines and oversaw all testing and training sessions. Val-ues for the initial 1-RM session for each device were ap-proximated on the basis of subject feedback and performance.The values for the remaining 1-RM test sessions were basedon the 1-RM achieved during the previous test session.

During the familiarization session, the depth and height ofthe SQ and HR, respectively, were established and recordedusing a linear encoder (Ergotest Technology, Langesund,Norway) and customized software program (LabVIEW 7.2;National Instruments, Austin, TX). The appropriate SQ depthwas achieved when the midline of the thigh was parallel to thefloor, and HR height was determined as the maximum heightachieved during the performance of the 80% 1-RM value.During all subsequent 1-RM tests, the linear encoder andsoftware were used to verify that the subject achieved theappropriate range of motion by generating an audible cue toindicate that the appropriate depth or height had been reached.The DL was considered successful if the subject was able tolift the bar from the floor to the upright position using propertechnique. A pilot study conducted in our laboratory revealedthree FW 1-RM sessions were needed to obtain an intraclasscorrelation coefficient (ICC) Q 0.90. In the current study, thereliability of FW and ARED 1-RM testing for the SQ, HR, andDL was between 4% and 6%.

VJ. Three countermovement VJ with a minimum of 30 s ofrest between jumps were performed in the pretraining,midtraining, and posttraining periods on the same day as, butbefore, the first FW 1-RM testing session. Subjects wereinstructed on proper jumping technique before each sessionand performed three or four practice jumps before performingthe test. VJ height, determined using a Vertec (Perform Better,Cranston, RI), was defined as the maximum height achievedduring the three repetitions. Using similar methods, Moir et al.(22) reported that these tests can be performed with a preci-sion of 2.4%.

Magnetic resonance imaging. Muscle volumes ofthe calf and thigh were determined before and after training

FIGURE 4—Demonstration of SQ performed using the ARED. Lines have been drawn to demarcate the yoke and upright support of the ARED, andthe angle between them throughout the range of motion, illustrating the fore-aft movement and the semi–free form motion of ARED exercise.


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using a Signa Horizon LX 1.5T MRI System (GeneralElectric, Piscataway, NJ) at a local hospital (Clear LakeRegional Medical Center, Webster, TX). The posttrainingmagnetic resonance imaging (MRI) was performed in theweek after 1-RM measurement to avoid potential fluid shiftsresulting from performing two 1-RM measures in the sameweek. A hospital MRI technician acquired the desired imagesunder the supervision of a member of the research staff.Subjects wore standard medical scrubs and removed all metal(such as jewelry) before the technician positioned the subjecton the MRI table for scanning. To control for the effects offluid shifts on these data, subjects were recumbent for aminimum of 15 min before data acquisition.

The subject’s feet were positioned in a holder to minimizemovement during image acquisition and to ensure repeatablepositioning. The imaged portion of the limb was suspended,whereas the nonimaged portion was supported with foam.Each scan was ‘‘landmarked’’ at the base of the patella, andthirty-two 1.0-cm slices were obtained for both the thigh andcalf muscles, using an echo time of 14ms and a repetition timeof 800 ms. The order for thigh and calf measurements wasmaintained across imaging sessions. A phantom was used toperiodically determine pixel size stability and showed nochange during the study. Using the same method, we havereported a test–retest reliability of 2.3% (26).

For each muscle region, the outlined area (pixels) wasplotted against position (mm). After appropriate adjustmentsto compare identical regions, volume was obtained by add-ing the number of pixels under each area curve and con-verting to cubic centimeters. Typically, this precluded usingthe entire scanned region because of slight positioning errorsbetween repeat scans during the study (26).

Quantitative computed tomography. Quantitativecomputed tomography (QCT) scanning of the hip and spinewas performed before and after training using a LightspeedUltra 8-slice CT scanner (General Electric) at a local hospital(Clear Lake Regional Medical Center). A trained and ex-perienced hospital technician acquired the desired imagesunder the supervision of a member of the research staff. Thesubject wore standard medical scrubs and removed all metal(such as jewelry) before the technician positioned the subjectin the CT scanner. The volumetric spine CT scan was per-formed first. A phantom was placed under the subject’slower back and was scanned simultaneously to calibrate theCT Hounsfield units to equivalent concentrations of calciumhydroxyapatite. As part of the scanning protocol, an initiallateral-view scout scan was acquired to determine the loca-tion of the L1 and L2 vertebrae. The actual L1–L2 scan wasthen acquired, using 5 mm above the L1 superior end plateand 5 mm below the L2 inferior end plate as the outer limitsof the scan region. The slice thickness was 2.5 mm.

The subject was then repositioned for the hip (femur) scan.The phantom was placed under the patient in the scan regionso that the head end of the phantom aligned with the iliac crestof the subject. A scout scan was acquired to define the hipscanning volume. The hip scan limits were set at 1 cm superior

to the superior aspect of the femoral head and at 5 mm inferiorto the inferior aspect of the lesser trochanter. The hip scan wasacquired using a slice thickness of 2.5 mm. Femoral neck,trochanter, total femur, and spine volumetric BMD (vBMD)were obtained using methods described previously (16). Liet al. (21) have reported a precision of 4.5%, 0.6%, 0.8%, and2.0%, respectively, for these measurements. Posttraining QCTmeasures were performed in the week after 1-RM measure-ment to avoid scheduling conflicts.

DXA. Pretraining and posttraining BMD (gIcmj2) wasmeasured using DXA for each of three body scanning sites:whole body, spine, and hip. To ensure the reliability of theDXA measurements, all pretraining and posttraining scanswere conducted and analyzed by the same operator. Mea-surements were obtained in triplicate for each site using a fan-beam x-ray densitometer (Discovery; Hologic, Inc., Bedford,MA). All measures of areal BMD (aBMD) were obtainedusing previously reported techniques with a precision of 1%for the whole body, 1.4% for the lumbar spine, and 1.5% forthe femoral neck (27). Whole-body lean mass and leg leanmass were calculated using previously published methodswith a precision of 0.9% and 1.3%, respectively (27). DXAmeasures were performed before any strength measures tominimize any effects of fluid shifts.

Bone and muscle biomarkers. Urine samples werecollected as separate voids into individual bottles during aperiod of 24 h. The volume of each was determined, and a24-h pool was prepared. After total volume and pH mea-surements were made, aliquots were removed, processed, andfrozen at j70-C until batch analysis. Urine samples wereanalyzed for collagen cross-links using commercially avail-able kits (Pyrilinks and Pyrilinks-D (Quidel, Inc., Santa Clara,CA) or Osteomark ELISA Kit (Ostex International, Inc.,Seattle,WA)). Serum and urinary calciumwere determined byatomic absorption spectrophotometry. Urinary creatinine andphosphorus were determined spectrophotometrically. Urinary3-methyl histidine (3-MH) was analyzed on a Hitachi L-8800Amino Acid Analyzer (Hitachi Instruments Incorporated,Danbury, CT). Biochemical tests were completed using pre-viously published methods (28,29,35).

Fasting (910 h) blood samples were collected in themorning before any exercise for that week. Samples werestored at j70-C and assayed after all subjects had com-pleted the study. Circulating bone- and calcium-related fac-tors (such as intact parathyroid hormone (PTH) and vitaminD metabolites) were assessed in serum. Bone-specific serumalkaline phosphatase (BSAP) and osteocalcin, which aremarkers of bone formation, were also measured. All bio-chemical tests were completed using previously publishedmethods (28,29,35). Serum and urinary cortisol (DiaSorin,Stillwater, MN) and insulin-like growth factor (IGF-1; Di-agnostic Systems Laboratories, Webster, TX) were analyzedby radioimmunoassay. Growth hormone and testosterone(both free testosterone and total testosterone) were alsoanalyzed by radioimmunoassay (Diagnostic ProductsCorp., Los Angeles, CA).


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Statistical analyses. A power analysis was performedto determine the required number of subjects to achievestatistical significance based on expected increases in mus-cle strength, the primary dependent variable of interest.Assuming that the mean increase in strength would be sim-ilar to that observed in the previous training study in ourlaboratory (26) and that all assumptions of the test were met,eight subjects per group were needed to detect a differencebetween training groups with an > of 0.05 and a power of0.80. To model the distribution of men and women in theastronaut corps (15), three additional female subjects wereincluded in each study group. All data were analyzed using atwo-way repeated-measures ANOVA. The Tukey post hocanalysis was performed when the ANOVA revealed signif-icant differences. The criterion for statistical significancewas set a priori at P e 0.05. All data are presented as themean T SE, unless otherwise noted.

RESULTS

Exercise training. There were no statistically signifi-cant differences between groups concerning height, bodymass, or age, and training had no effect on body mass ineither group (Table 1). Overall subject compliance acrossthe 16 wk of training for the FW and ARED groups was97% and 98% of the prescribed exercise sessions, respec-tively. If a subject was unavailable for exercise training asprescribed, a makeup session was scheduled in most cases.In the event a full week of training was missed, the trainingschedule was altered to include up to 4 dIwkj1 of trainingimmediately before and/or after a missed week of training;all others were considered missed sessions. The groups didnot differ significantly in the percentage of exercise pre-scription completed, repetitions completed per workout, orpeak resistance lifted during the training period (Table 2).

Muscle strength. FW SQ strength increased duringtraining in both groups (Fig. 5), and a significant group �time interaction occurred. In the FW group, FW SQ strengthincreased from pretraining to midtraining (31.9% T 4.9%),midtraining to posttraining (12.9% T 1.9%), and pretrainingto posttraining (48.9% T 6.1%). In the ARED group, FW SQstrength increased only from pretraining to midtraining(28.2% T 4.3%) and from pretraining to posttraining (31.2% T3.8%). However, FW SQ strength was not significantly dif-ferent between groups at any time point. Similarly, AREDSQ strength increased in both groups during training (Fig. 5),and a significant group � time interaction occurred. In theFW group, ARED SQ strength increased from pretrainingto midtraining (28.6% T 4.5%), midtraining to posttraining

(9.7% T 1.9%), and pretraining to posttraining (41.1% T5.3%). In the ARED group, ARED SQ strength increasedonly from pretraining to midtraining (20.2 T 2.9%) and frompretraining to posttraining (26.3% T 3.0%). However, therewas no significant difference between groups at any timepoint for ARED SQ strength.

FW HR strength increased in both the ARED and FWgroups, but there was no group � time interaction. FW HRstrength (Fig. 5) increased from pretraining to midtraining

TABLE 1. Age, height, and body weight (mean T SE).

Group Age (yr) Height (cm)PretrainingBW (kg)

MidtrainingBW (kg)

PosttrainingBW (kg)

ARED 35.7 T 2.1 171.2 T 3.2 78.5 T 4.1 78.8 T 4.0 78.9 T 4.0FW 31.8 T 1.2 169.9 T 2.3 73.5 T 3.7 74.5 T 3.9 74.6 T 4.0

The training period was 16 wk; midtraining was 8 or 9 wk.BW, body weight.

TABLE 2. Percentage of overall prescription completed, repetitions completed perworkout, and peak load lifted during the training period (mean T SE).

Exercise Group% PrescriptionCompleted

Repetitions Completedper Workout

Peak LoadLifted (kg)

SQ FW 100.0 T 0.1 42 T 0.1 61.9 T 7.5ARED 99.9 T 0.1 42 T 0.1 58.1 T 4.1

HR FW 99.9 T 0.2 42 T 0.1 116.1 T 8.6ARED 99.5 T 0.3 42 T 0.2 115.8 T 10.8

DL FW 99.6 T 0.2 42 T 0.1 79.4 T 9.0ARED 99.8 T 0.1 42 T 0.1 85.4 T 6.3

FIGURE 5—SQ, HR, and DL 1-RM strength (mean T SE) of the FWand ARED training groups during FW and ARED testing at pre-training, midtraining, and posttraining. *Significantly greater than atpretraining. †Significantly greater than at midtraining.


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(ARED = 14.2% T 2.4%, FW = 7.9% T 2.0%) and frompretraining to posttraining (ARED = 18.0% T 1.6%, FW =12.3% T 2.4%). ARED HR strength also increased in bothgroups, with a significant group � time interaction. TheARED training group increased ARED HR strength frompretraining to midtraining (22.3% T 2.7%), midtraining toposttraining (8.4% T 2.6%), and pretraining to posttraining(32.7% T 4.6%), whereas the FW training group increasedAREDHR strength frommidtraining to posttraining (11.3% T2.5%) and from pretraining to posttraining (13.8% T 4.2%).However, no significant difference occurred between groupsin ARED HR strength at any time.

There was a significant main effect of time for FWDL strength but no group � time interaction. Both groupsincreased FW DL strength (Fig. 5), measured using the FWhardware, from pretraining to midtraining (ARED = 13.1% T2.1%, FW = 13.0% T 2.7%), from midtraining to post-training (ARED = 9.1% T 2.6%, FW = 9.0% T 2.1%), andfrom pretraining to posttraining (ARED = 23.2% T 2.8%,FW = 23.3% T 4.4%). When testing was conducted with theARED, both groups increased in ARED DL strength frompretraining to midtraining (ARED = 15.1% T 3.3%, FW =11.0% T 3.0%) and from pretraining to posttraining (ARED =20.0% T 3.0%, FW = 18.1% T 3.7%).

VJ height. VJ height increased in both groups, and therewas a significant group � time interaction (Fig. 6). The FWtraining group increased in VJ height from pretraining tomidtraining (11.5% T 2.1%) and from pretraining to post-training (14.3% T 2.7%); the ARED training group increasedfrom pretraining to posttraining (8.4% T 2.1%) only. How-ever, there was no significant difference between groups atany time point for VJ height.

MRI. Thigh muscle volume (Fig. 7) increased in bothgroups from pretraining to posttraining (ARED = 7.1% T1.2%, FW = 9.8% T 0.9%). Lower leg muscle volume in-creased from pretraining to posttraining in the FW group(FW = 3.0% T 1.1%, P e 0.01) and tended to increase

(P = 0.09) in the ARED group (ARED = 2.1% T 0.7%).There were no significant differences between groups forthigh or lower leg muscle volume.

Lean tissue mass. Whole-body lean mass (Table 3)increased significantly in both groups from pretraining tomidtraining (ARED = 2.4% T 0.5%, FW = 2.5% T 0.6%) andfrom pretraining to posttraining (ARED = 2.6% T 0.7%, FW =2.5% T 0.7%), and leg lean mass increased significantly frompretraining to midtraining (ARED = 3.2% T 0.7%, FW =3.6% T 0.9%) and from pretraining to posttraining (ARED =4.8% T 0.7%, FW = 3.9% T 1.1%). There were no significantdifferences between groups for either measure.

QCT. Lumbar spine trabecular vBMD increased in bothgroups from pretraining to posttraining (ARED = 12.3% T3.1%, FW = 8.9% T 2.4%; Table 4), and there was no dif-ference between groups. Trabecular vBMD of the total fe-mur, femoral neck, and trochanter did not change significantlyfrom the pretraining period in either group.

BMD. Both groups showed an increase in lumbar spineaBMD from pretraining to posttraining (Table 5). A group �time interaction occurred for greater trochanteric aBMD. Inthe FW group, greater trochanter aBMD increased frompretraining to midtraining and tended to increase from pre-training to posttraining (P = 0.07), but no significant dif-ference occurred in the ARED group. Training had no effecton total hip or femoral neck BMD. There was no significantdifference between groups at any time point for any of theBMD variables.

Bone and muscle biomarkers. A main effect oftraining occurred in IGF-1, serum cortisol, PTH, BSAP,osteocalcin, N-telopeptide (NTX; expressed either as per dayor per creatinine), and helical peptide (per day or per creat-inine; Tables 6 and 7); however, post hoc analysis was un-able to detect specific pairwise differences. No significant

FIGURE 6—VJ height (mean T SE) of the FW and ARED traininggroups pretraining, midtraining, and posttraining. *Significantly dif-ferent from pretraining.

FIGURE 7—Volume of thigh and lower leg muscles (mean T SE) of theFW and ARED training groups pretraining and posttraining. *Signif-icantly greater than pretraining.

TABLE 3. Whole-body and leg lean mass (mean T SE) pretraining, midtraining,and posttraining.

Location GroupPretrainingLM (kg)

MidtrainingLM (kg)

PosttrainingLM (kg)

% ChangeMid

% ChangePost

Whole body ARED 53.3 T 3.4 54.5 T 3.4a 54.5 T 3.2a 2.4 T 0.5 2.6 T 0.7FW 53.0 T 3.6 54.3 T 3.6a 54.2 T 3.5a 2.5 T 0.6 2.5 T 0.7

Leg ARED 17.7 T 1.2 18.3 T 1.2a 18.5 T 1.1a 3.1 T 0.6 4.6 T 0.8FW 18.3 T 1.3 18.9 T 1.4a 19.0 T 1.3a 3.4 T 0.8 4.1 T 1.1

a Significant difference from pretraining.LM, lean mass.


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group� time interaction occurred for any of the biochemicalmarkers. Data for growth hormone are not reported becauseapproximately two-thirds of the data points were below thelowest standard on the kit (0.1 ngImLj1). Similarly, data forfree and total testosterone are not reported for the femalesubjects.

DISCUSSION

High-intensity resistance exercise attenuates bed rest–induced musculoskeletal deconditioning (6,5,12,27) andhas been recommended as a countermeasure for use byastronauts in microgravity (3). The iRED, NASA’s first-generation resistance exercise hardware on the ISS, fails tomaintain muscle strength and BMD during long-durationspaceflight (16,31). The ARED was designed to addressseveral of the key limitations of the iRED (such as peakresistance, eccentric-to-concentric force ratio, and inertialforces) with the intent of improving overall countermeasureeffectiveness. Although some measures had large disparitiesin percent change between ARED and FW training at vari-ous time points, the primary finding of the present study wasthat after 16 wk, ARED training resulted in adaptations thatwere not statistically different from those resulting from FWtraining.

Muscle strength and power. When evaluating theeffect of two different exercise devices on muscle strengthand power, the principle of training specificity indicates thatstrength gains should be less when training and testing areperformed using different exercise devices (13). Pipes (24)evaluated two different strength training devices andreported large increases in strength when testing and trainingwere performed on the same hardware, but they reportedlittle to no change in strength when tested with the othersystem. Because we were evaluating two different devices,we speculated that we might find differences in strength

gains between ARED- and FW-trained groups when testingwas performed using the other modality. Interestingly, wefound no statistical difference between ARED and FWtraining regardless of which device was used for testing.These results suggest that ARED training mimics FWenough to elicit similar training adaptations in musclestrength and power.

Although both groups increased muscle strength andpower, some device-dependent differences occurred duringresistance exercise training. Specifically, the FW group im-proved FW and ARED SQ strength at a greater rate frommidtraining to posttraining than the ARED group, and theFW group increased VJ height (an indicator of lower bodypower) at a greater rate than the ARED group. Given thatthe exercise prescription (resistance, volume, frequency, andchoice of exercises) for the two groups was virtually identical(Table 2), differences in strength and power adaptations maybe due to subtle differences between the exercise hardware.Because the ARED provides constant resistance throughoutthe range of motion and an eccentric-to-concentric force ofapproximately 90% (unpublished NASA engineering analy-sis), we speculate that differences in adaptation rate were dueto variations in inertial resistance characteristics and/or exer-cise kinematics.

Subtle device-dependent differences in inertial character-istics may have a sizeable impact on muscle strength andpower. Rapidly decelerating a mass before the concentricmotion occurs activates the stretch reflex, resulting in greaterpeak accelerations, greater peak forces (8,23), and a strongerconcentric action (30). The ARED flywheels were designedto mimic the inertia of FW up to 91 kg (200 lb). Approxi-mately 41% of all ARED exercises were performed at re-sistances greater than 91 kg (200 lb), potentially providinglower forces at heavier resistances compared with FW. Overtime, the reduced forces might affect chronic strength andpower adaptations to training.

Although no direct measures were obtained in this study,anecdotal evidence indicates that biomechanical differencesexist between devices. During SQ, ARED subjects reportedthat they felt as if the ARED bar was ‘‘forcing them forward,’’and test operators reported a corresponding increase in hipflexion during the ARED SQ. Increased forward torso leanhas been associated with reduced forces at the knee (14,34),resulting in reduced forces imparted to the thigh musculatureand a reduced training stimulus (10). During HR, the AREDrestricted only lateral movements, thus requiring subjects to

TABLE 4. Trabecular vBMD (mean T SE) before and after 16 wk of training.

Location GroupPretraining(gIcmj3)

Posttraining(gIcmj3) Change (%)

Total femur FW 0.146 T 0.01 0.144 T 0.01 j0.9 T 0.9ARED 0.142 T 0.01 0.142 T 0.01 0.5 T 1.2

Femoral neck FW 0.140 T 0.02 0.137 T 0.02 j1.3 T 1.9ARED 0.150 T 0.01 0.150 T 0.01 j0.5 T 1.9

Trochanter FW 0.146 T 0.01 0.145 T 0.01 j0.7 T 0.8ARED 0.140 T 0.01 0.141 T 0.01 0.6 T 1.3

Spine FW 0.185 T 0.01 0.200 T 0.01a 8.9 T 2.4ARED 0.182 T 0.01 0.204 T 0.01a 12.3 T 3.1

a Significant difference from pretraining.

TABLE 5. aBMD (mean T SE) before, during, and after 16 wk of training.

Location Group Pretraining Midtraining Posttraining % Change Mid % Change Post

Femoral neck ARED 0.846 T 0.052 0.842 T 0.051 0.850 T 0.053 j0.3 T 0.4 0.4 T 0.6FW 0.858 T 0.024 0.863 T 0.021 0.869 T 0.023 0.7 T 0.5 1.3 T 0.5

Greater trochanter ARED 0.749 T 0.045 0.745 T 0.045 0.751 T 0.045 j0.5 T 0.4 0.4 T 0.4FW 0.718 T 0.030 0.729 T 0.031a 0.727 T 0.030 1.5 T 0.4 1.3 T 0.4

Total hip ARED 1.008 T 0.048 1.006 T 0.048 1.007 T 0.047 j0.3 T 0.3 j0.1 T 0.3FW 0.976 T 0.035 0.984 T 0.035 0.980 T 0.034 0.8 T 0.1 0.4 T 0.3

Total lumbar spine ARED 1.073 T 0.042 1.088 T 0.043 1.091 T 0.042a 1.4 T 0.4 1.8 T 0.6FW 1.020 T 0.020 1.035 T 0.023 1.046 T 0.022a 1.5 T 0.5 2.5 T 0.9

a Significant difference from pretraining.


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balance the resistance in the sagittal plane. Exercising whileattempting to maintain balance results in an increased effortto maintain stability, reducing force production (1,7) andpotentially resulting in device-specific adaptations. In con-trast, DL form did not seem to differ between devices, andthis may explain the lack of statistical difference between thetraining groups.

BMD. Long-duration spaceflight reduces aBMD(18,19,26)and vBMD, with the decrease in vBMD occurring more intrabecular than in cortical bone (16,17,32). With limitedprevious information describing the effect of resistance ex-

ercise on vBMD, our results indicate that as little as 16 wk ofFW or ARED training can increase lumbar spine trabec-ular vBMD. These results agree with the aBMD results in thecurrent study and a previous investigation from our laboratory(26). Although no changes in hip BMD were seen aftertraining, also previously observed (26), this does not precludethe possibility that ARED resistance exercise could mitigateBMD loss in a microgravity environment. Shackelford et al.(27) maintained total hip aBMD using high-intensity resis-tance exercise after 17 wk of bed rest. In addition, our studymight have been too short to detect changes in hip BMD

TABLE 6. Bone and muscle biomarkers (mean T SE) in blood serum.

Group Pretraining Training Week 4 Midtraining Posttraining % Change Pretraining to Posttraining

1,25 Vit D (pmolILj1) FW 140 T 16 152 T 18 131 T 16 147 T 18 12.6 T 15.1ARED 154 T 23 153 T 22 149 T 15 169 T 16 30 T 17.8

25 Vit D (nmolILj1) FW 68.8 T 5.8 62.9 T 6.1 63.5 T 3.7 61.3 T 5.3 j8.9 T 6.2ARED 54.3 T 4.1 56.3 T 3.5 58.5 T 4.8 56.8 T 4.7 6.8 T 8.3

BSAP (UILj1)a FW 32.6 T 2.8 31.6 T 3.3 32.9 T 3.1 37.3 T 3.8 14.5 T 5.4ARED 37.2 T 2.2 39.1 T 3.7 38.9 T 3.1 41.2 T 2.7 11.5 T 5.5

Calcium (mgIdLj1) FW 9.6 T 0.1 9.7 T 0.1 9.6 T 0.1 9.5 T 0.1 j0.6 T 1.7ARED 9.6 T 0.1 9.7 T 0.1 9.5 T 0.1 9.6 T 0.1 0.7 T 1.2

Osteocalcin (ngImLj1)a FW 9.2 T 0.9 10.5 T 0.5 9.7 T 1.0 10.5 T 0.8 24.1 T 17.9ARED 8.8 T 0.7 10.1 T 0.7 9.0 T 0.9 10.4 T 0.8 21.3 T 9.2

Cortisol (KgIdLj1)a FW 22.7 T 2.7 25.4 T 4.1 21.8 T 2.8 21.5 T 3.1 j5.8 T 7.2ARED 27.6 T 3.4 32.6 T 5.2 26.5 T 3.0 25.8 T 3.0 j4.6 T 6.3

PTH (pgImLj1)a FW 48.8 T 6.6 43.9 T 5.8 43.5 T 4.9 51.7 T 5.4 14.8 T 8.2ARED 49.6 T 6.7 43.2 T 4.6 52.4 T 5.9 54.4 T 5.7 18.7 T 17.3

IGF-1 (ngImLj1)a FW 236 T 30 225 T 35 174 T 26 217 T 27 j2.2 T 10.4ARED 207 T 23 233 T 18 194 T 30 246 T 27 25.6 T 17.9

Free testosterone (pgImLj1) FW 10.9 T 1.1 11.2 T 0.9 10.5 T 2.0 8.9 T 1.2 j15.6 T 11.7ARED 8.8 T 0.7 8.3 T 0.9 10.6 T 1.0 8.8 T 0.7 2.1 T 3.9

Total testosterone (ngIdLj1) FW 537 T 42 546 T 41 522 T 81 515 T 50 j1.5 T 11.5ARED 540 T 50 516 T 63 568 T 48 542 T 53 0.5 T 3.5

a Main effect of time.1,25 Vit D, 1,25-dihydroxyvitamin D; 25 Vit D, 25-hydroxyvitamin D.

TABLE 7. Bone and muscle biomarkers (mean T SE) in urine.

Group Pretraining Training Week 4 Midtraining Posttraining % Change Pretraining to Posttraining

pH FW 6.1 T 0.1 6.2 T 0.1 5.8 T 0.1 6.0 T 0.1 j2.7 T 2.3ARED 6.2 T 0.2 6.1 T 0.1 6.2 T 0.1 6.2 T 0.1 0.3 T 3.0

Volume (mL) FW 1771 T 250 1468 T 212 1297 T 209 1833 T 306 2.9 T 7.8ARED 1787 T 287 1668 T 22 1927 T 284 1915 T 223 28.1 T 23.0

3-MH (KmolIdj1) FW 267 T 37 285 T 28 242 T 30 285 T 43 12.3 T 13.7ARED 297 T 32 245 T 23 247 T 22 234 T 16 j7.0 T 9.0

Calcium (mgIdj1) FW 165 T 40 180 T 41 184 T 43 190 T 43 18.3 T 12.2ARED 1567 T 26 164 T 26 156 T 19 149 T 21 8.3 T 16.2

Creatinine (mgIdj1) FW 1729 T 165 1782 T 136 1740 T 134 1829 T 170 9.4 T 8.1ARED 1754 T 167 1717 T 134 1815 T 114 1694 T 159 0.5 T 7.2

Phosphorus (mgIdj1) FW 833 T 139 889 T 141 774 T 131 816 T 143 8.0 T 22.8ARED 907 T 114 620 T 78 680 T 98 738 T 129 j13.9 T 11.7

Cortisol (KgIdj1) FW 64 T 10 69 T 7 66 T 12 82 T 15 34.8 T 17.7ARED 68 T 9 87 T 12 81 T 9 82 T 12 25.3 T 12.1

PYD (nmolIdj1) FW 284 T 30 267 T 28 277 T 27 267 T 29 j4.1 T 5.9ARED 216 T 21 203 T 19 235 T 27 235 T 35 7.9 T 9.7

PYD (nmolImmolj1 creatinine) FW 19.0 T 1.6 17.0 T 1.3 18.2 T 1.3 16.9 T 1.6 j7.6 T 9.6ARED 14.6 T 1.3 13.5 T 0.8 14.6 T 1.3 15.4 T 1.2 8.9 T 9.0

DPD (nmolIdj1) FW 70 T 8 68 T 6 70 T 6 71 T 6 7.3 T 9.1ARED 59 T 8 58 T 10 62 T 9 64 T 11 7.5 T 8.3

DPD (nmolImmolj1 creatinine) FW 4.6 T 0.4 4.4 T 0.4 4.6 T 0.3 4.5 T 0.3 0.9 T 8.4ARED 3.9 T 0.4 3.7 T 0.4 3.8 T 0.4 4.1 T 0.5 7.2 T 4.0

NTX (nmolIdj1)a FW 444 T 62 524 T 70 530 T 60 579 T 51 45.9 T 16.9ARED 385 T 48 388 T 56 404 T 50 440 T 59 20.9 T 14.9

NTX (nmolImmolj1 creatinine)a FW 28.4 T 2.2 33.2 T 3.6 33.9 T 2.2 36.6 T 2.1 32.9 T 9.7ARED 25.4 T 2.6 25.5 T 2.9 25.9 T 2.4 28.6 T 2.2 18.6 T 9.8

Helical peptide (KgIdj1)a FW 673 T 92 653 T 99 735 T 122 863 T 141 60.4 T 21.2ARED 746 T 94 884 T 93 868 T 94 1063 T 103 37.2 T 21.3

Helical peptide (KgImmolj1 creatinine)a FW 48.8 T 5.1 56.5 T 5.4 55.8 T 4.2 67.1 T 6.0 47.6 T 15.4ARED 43.1 T 4.1 41.9 T 4.5 44.5 T 5.8 55.8 T 7.5 30.4 T 13.7

a Main effect of time.DPD, deoxypyridinoline; PYD, pyridinoline.


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because 6–12 months of resistance training seems to be nec-essary to detect changes in hip aBMD (25,33).

Spaceflight. Spaceflight imposes considerable obstaclesto developing in-flight exercise hardware. First, exercisehardware must be vibration-isolated because vibrations dueto exercise resonate throughout the ISS, potentially resultingin damage and a reduced life span of the vehicle (e.g., thevibrations can extend to the long, protruding solar panelsand impart substantial torque on their attachments). Second,exercise hardware must require minimal-to-no externalelectrical power because power is in limited supply duringspaceflight. Third, exercise hardware must provide a com-plement of exercises to sufficiently load all major musclegroups and joints, especially the lower body and spine. Fi-nally, exercise hardware must be reliable and easily main-tainable for a 15-yr period with minimal need to providereplacement parts. For these reasons, designing an exer-cise device to mimic the resistance characteristics of FW(i.e., providing high levels of resistance, constant force, aneccentric-to-concentric force close to 100%, and an inertialcomponent) while adhering to the constraints imposed byspaceflight was a considerable challenge.

The current study demonstrates an increase in musclestrength, muscle size, and BMD after 16 wk of training inambulatory subjects. However, these results must be treatedwith caution when extending them to spaceflight becauseincreasing muscle strength, muscle volume, and BMD inambulatory subjects does not translate directly to an abilityto prevent loss during unloading. Schneider et al. (26) re-ported that iRED training increased muscle strength and sizein ambulatory subjects, with no change in BMD. However,astronauts continue to experience measurable decreases inmuscle strength, muscle mass, and BMD after long-durationspaceflight, despite using the iRED (16,31). With thislimitation in mind, our data indicate that the ARED has agreater likelihood than the iRED of being a successful coun-termeasure device on the ISS because the resistance char-

acteristics during ARED exercise closely resemble thosedemonstrated to provide effective protection against muscleatrophy and bone loss during bed rest (27). Future data fromcrewmembers participating in spaceflight missions will allowus to test this hypothesis.

CONCLUSIONS

The ARED was designed to address the limitations of theiRED and serve as the next generation of in-flight resistanceexercise hardware. We evaluated the musculoskeletal adap-tations to 16 wk of training using the ARED and comparedthese results with those obtained when training with FW. Inambulatory subjects, ARED training resulted in increasessimilar to those with FW training for all of the variablesmeasured, the only exceptions being a greater rate of in-crease in SQ strength from midtraining to posttraining and agreater rate of increase in VJ height in the FW group. Thesedevice-dependent differences might relate to the inability ofthe ARED flywheels to mimic inertial characteristics of FWthroughout the full range of resistances, or they could relateto differences in the biomechanics of exercise between thetwo devices. Given these findings, and considering the ef-fectiveness of FW training at mitigating bed rest–induceddeconditioning, we expect that ARED training will be amore effective countermeasure than iRED training againstmusculoskeletal deconditioning during spaceflight.

Funding for this work was provided by the Exercise Counter-measures Project at NASA Johnson Space Center.

The authors thank the subjects for their enthusiastic participationin this training study; Jason Bentley, Roxanne Nash, and Mark Leachfor their assistance with exercise training and testing; Scott A. Smith,Mary Jane Maddocks, and Dr. Harlan Evans for their contributions toMRI and DXA data collection; Dr. Alan Feiveson for his support of thestatistical analyses; and Meghan Everett and Dr. Carwyn Sharp fortheir review of this article.

The results of the present study do not constitute endorsementby the American College of Sports Medicine.

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Musculoskeletal adaptations to training with the advanced resistive exercise device

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