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ACUTE NEUROMUSCULAR AND METABOLIC RESPONSESTO CONCURRENT ENDURANCE AND RESISTANCE

EXERCISEJACOB P. REED, BRIAN K. SCHILLING, AND ZSOLT MURLASITS

Human Performance Laboratories, Department of Health and Sport Sciences, University of Memphis, Memphis, Tennessee

ABSTRACT

Reed, JP, Schilling, BK, and Murlasits, Z. Acute neuromuscular

and metabolic responses to concurrent endurance and

resistance exercise. J Strength Cond Res 27(3): 793801,

2013Metabolic and neurological responses to 4 bouts of

lower-body or upper-body resistance exercise preceded by

cycle ergometry or rest were assessed. Nine resistance-trainedmen (26.7 6 6.6 years) underwent bouts of (a) cycle ergome-

try then bench press, (b) bench press only, (c) cycle ergometry

then back squat, and (d) back squat only. Cycle ergometry was

performed at 75% maximum heart rate for 45 minutes. Bench

press and back squat protocols required 6 sets to volitional

fatigue at 80% 1RM with 2 minutes rest between sets. Signif-

icantly more repetitions were performed during set 1 for back

squat without preceding aerobic exercise (12.6 6 4.5 vs.

10.0 6 3.5, p = 0.000) and cumulatively at set 3 (27.1 6

10.6 vs. 23.1 6 9.2, p = 0.014), and no differences were

noted for bench press repetitions. Inclusion of cycle ergometry

results in impaired back squat, but not bench press, perfor-

mance likely because of a combination of local metabolic

stress and various neuromuscular effects.

KEYWORDS interference effect, aerobic exercise, resistance

training, force

INTRODUCTION

Different sports require athletes to possess variouscomponents of functional ability (e.g., strength,muscle mass, power, and cardiovascular endur-ance) to excel. Unfortunately, enhancing each of

these variables can take a considerable amount of time, withhypertrophic, strength, and cardiovascular gains occurringseveral weeks after initiation of training (10). One way thatstrength and conditioning professionals approach this diffi-culty is through using concurrent endurance and strength

training. Concurrent training is operationally defined asendurance and strength training in immediate successionor with up to 24 hours of recovery separating the 2 exercisemodes. Although concurrent training allows for optimiza-tion of time while attempting to train multiple componentsof functional ability, this method may hinder adaptationscompared with training performed independently (10). Itappears that concurrent training can increase strength,though not to the same extent as strength training alone(15,23,25,27,33), whereas cardiovascular adaptations arenot affected by the inclusion of resistance training(5,12,17,18,25,27).

Concurrent training was first systematically studied30 years ago by Hickson (18), who found that the combina-tion of endurance and strength training led to diminishedincreases in strength compared with strength training alone,giving indication that endurance training hindered strengthgains. These findings gave way to the concept of interfer-ence describing the effect of concurrent training (18).

A number of theories have been developed regarding themechanisms or causes of interference: specificity of trainingor differential adaptation (endocrine and neuromuscularresponses) (12,17,18,20), cell signaling in the form of AMP-activated protein kinase (AMPK) and mammalian target ofrapamycin (mTOR) (2,16,19,24,32), and the acute fatiguetheory (1,5,9,15,20,31).

Determination of the interference effect sparked a newfield of study; however, though more than 30 years havepassed since the initial publication, the available research isequivocal and the exact mechanisms of this interferenceeffect are largely still unknown. Of the aforementionedtheories, it is of our interest to assess interference inducedby acute fatigue. Sporer and Wenger (30) analyzed the effectsof varying recovery periods during a concurrent trainingsession on leg and bench press repetitions using high-intensity (3 sets of six 3-minute intervals at maximum heartrate [MHR]) and low-intensity (36 minutes at 70% maxi-mum power associated with V_O2max) aerobic exercise bouts.They found that at 4 and 8 hours postexercise, individualscould not replicate the same number of repetitions as in thecontrol protocol, and to do so, 24 hours of rest was neces-sary (30). This finding indicates that acute responses play

Address correspondence to Jacob P. Reed, [email protected].

27(3)/793801

Journal of Strength and Conditioning Research2013 National Strength and Conditioning Association

VOLUME 27 | NUMBER 3 | MARCH 2013 | 793

Co ri ht National Stren th and Conditionin Association Unauthorized re roduction of this article is rohibit

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a role in the interference mechanism. More importantly, theaccumulation of the acute physiological interference, whenapplied over a chronic training program, may lead to sub-optimal training adaptations.

It is possible that interference occurs as a result of localfatigue induced when aerobic exercise precedes resistance

exercise. This has been shown when comparing lower-body(leg press) with upper-body (bench press) strength perfor-mance after lower-body aerobic exercise (9). Bench pressperformance, as measured by repetitions completed, wasnot hindered by the preceding aerobic exercise. Althoughacute fatigue induced by cycle ergometry hinders leg pressperformance, the mechanisms of this effect are not yetknown.

Acute fatigue could occur through neuromuscular mech-anisms, affecting muscle activation, through electromyogra-phy (EMG), and maximal force generation. The amount ofmuscle mass and load used during training determine theeffect and scope of performance-enhancing adaptations.

Prior endurance activity could lead to suboptimal activationand force production and therefore the observed interfer-ence of performance adaptations. Although no acuteresponses regarding the effects of prior aerobic exercise onEMG pattern have been documented, chronic adaptationsof muscle activation have been analyzed after a 22-weekconcurrent training program (15). It was observed that acti-vation of the right and left vastus lateralis had a nonsignifi-cant increase of 29 and 22% for the concurrent traininggroup and 26 and 19%, respectively, in those completingstrength training only. Although these results suggestthat suppressed EMG activity does not occur with concur-rent training, it must be noted that the individual concurrenttraining sessions were separated by 24 hours, which hasshown no acute interference (30). Therefore, it is plausiblethat an acute bout of aerobic exercise immediately occurringbefore a resistance exercise session causes decreased activa-tion of the exercised muscle group (EMG activity) and max-imal force generation, which would in turn lead toperformance decrements.

Gaining insight into the mechanisms that induce fatigueduring concurrent training could provide practitionersinformation on how to program training protocols toeffectively avoid or minimize these detrimental effects. It ispossible that the aforementioned mechanisms of interference

occur as a result of accumulated acute neuromuscular andmetabolic responses, thus causing the compromised strengthand hypertrophic gains, as speculated by current literature(9,11,21,22). Coincidentally, the lack of interference andfatigue associated with lower-body aerobic exercise on a sub-sequent upper-body resistance exercise provides a base ofwhich to assess the differing theories of interference (localvs. systemic). Therefore, it was the intent of this investigationto compare how an acute fatiguing bout of cycle ergometryaffects the metabolic and neuromuscular milieu and subse-quent upper-body (bench press) or lower-body (back squat)

resistance exercise performances. We hypothesized thata preceding cycle exercise would attenuate exercise perfor-mance of the lower body, with no effect on upper-bodyperformance.

METHODS

Experimental Approach to the Problem

This study was designed to compare the acute metabolicand neuromuscular responses to an acute bout of (a)aerobic endurance exercise preceding upper-body strengthexercise, (b) upper-body strength exercise, (c) aerobicendurance exercise preceding lower-body strength exer-cise, and (d) lower-body strength exercise. The approach ofthis design loosely follows that of de Souza et al. (9), in thatboth studies observed the acute effects of concurrent train-ing on subsequent upper-body and lower-body resistanceexercise. However, we used a longer aerobic exercise pro-tocol, separated upper-body and lower-body resistance

exercise sessions by 1 week, incorporated the back squatinstead of leg press, and required different intensity and setsof resistance exercise while also analyzing metabolic andneuromuscular variables associated with acute fatigue.Therefore, based on the limited knowledge of the acuteeffects of interference, we hypothesized that aerobic exer-cise would result in a reduction of repetitions performedduring the back squat but not bench press when comparedwith a control of no aerobic exercise. Finally, we expectedpreceding aerobic exercise to cause an immediate increasein blood lactate, and a decrease in EMG, maximal volun-tary contraction (MVC), and EMG:force for the back squatbut not bench press.

Subjects

Fifteen men (aged 2140 years) were recruited to participate.Of the 15 men recruited, 9 were determined resistance-trained after 1RM testing. The classification of resistancetrained was defined by the ability to back squat 1.5 timesbody weight and bench press 1.2 times body weight and alsohaving participated in a structured exercise training programfor no less than 2.5 hours per week over the 6 months beforetesting. Guidelines such as these allow for a standardizationof training status and also helped to minimize the repeated-bout effect of training. Subject descriptive characteristics arepresented in Table 1. Before participation, each subject was

informed of all procedures, potential risks, and benefitsassociated with the study both verbally and in written formin accordance with the procedures approved by the univer-sity review board for human subjects research. Subjects com-pleted a health history and physical activity assessment, anda detailed description of the procedures, before inclusionwithin the investigation. Upon inclusion, subjects were askedto continue their normal daily routine, including prescribedmedications, diet, sleep, and prior physical activity through-out the duration of the study, but were required to refrainfrom consuming any nutritional supplements before exercise

Acute Responses to Concurrent Exercise

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testing and to abstain from exercise in the 48-hour periodbefore each exercise session.

Procedures

The initial laboratory visit occurred 1 week before the firstexperimental session in the morning after 1 hour of fasting(water was allowed ad libitum). For all following sessions,subjects tested at the same time of day as their initiallaboratory visit. During the subjects initial visit, height andbody weight were measured using conventional equipments.After anthropometric measurements, subjects underwenta 1RM test for both the back squat and the bench press(3). Upon successful determination of inclusion, subjectsreported back to the laboratory 72 hours after the initial visitand underwent a graded exercise test (GXT) using a cycleergometer (Monark 828E; Monark, Vansbro, Sweden) todetermine MHR for use during the aerobic exerciseprescription.

Strength testing began with a general warm-up of5 minutes on a cycle ergometer. Before each separate lift(bench press and back squat), subjects performed a warm-upset of 810 repetitions with a lightweight. The weight wasthen progressively increased for the subsequent trials until nomore than 1 repetition could be completed, according to theguidelines for testing 1RM set forth by the National Strength

and Conditioning Association (3). An acceptable lift for thebench press involved a controlled descent in which the barbriefly touched the chest before concentric contraction.Once the arms were fully lowered, a verbal press commandwas given and the bar pressed upward until achievement offull extension. For a complete lift to be awarded, both feetmaintained contact with the floor at all times with the but-tocks, shoulders, and head in contact with the bench. For theback squat, subjects unracked the weight and walked back,standing with the knee joint fully extended no more than1 m away from the squat rack. The lifter then, under control,

descended until 908 at the knee or lower, at which a verbalup command was given. Once fully standing, the bar wasracked and a successful lift counted. All testing sessions weresupervised by a Certified Strength and ConditioningSpecialist.

A GXT was conducted to determine MHR and maximal

aerobic power output expressed in watts using a cycleergometer (Monark 828E; Monark). After strength testing,participants were instructed not to perform any strenuousphysical tasks during the 48-hour period before the GXT.After warming up at 50 W for 5 minutes, subjects were givena brief period to recover, after which the test began. Initialworkload was set at 30 W and progressively increased by30 W every minute (4,26). At the culmination ofeach minute, MHR was recorded using a heart rate monitor(Polar Electro, Lake Success, NY, USA) and rate of per-ceived exertion (RPE) using the Borg scale of exertion (rangeof 620, with 7 denoting very, very light [rest] and increasinglinearly so that 19 signifies very, very hard with 20 represent-

ing complete exhaustion) (6). The test was terminated whenthe participant could no longer continue because of fatigue(revolutions per minute drop below 50) and the RPE wasrated as $19. Maximum heart rate obtained during testingwas used to calculate the steady-state exercise workload.Participants were allowed to actively cool-down (e.g.,slow-speed cycling) for several minutes until the heart ratefell below 120 b$min21 or stabilized.

Experimental Testing Sessions

Subjects underwent each of the following exercise sessions ina randomized order with 1 week separating each session: (a)lower-body aerobic and upper-body resistance exercise(bench press, AE + BP); (b) upper-body resistance exercise(bench press, BP); (c) lower-body aerobic and lower-bodyresistance exercise (back squat, AE + BS); and (d) lower-body resistance exercise (back squat, BS).

All exercise protocols required subjects to first completea 5-minute warm-up via cycle ergometry. For the aerobicendurance protocols, 45 minutes of cycle ergometry at 75%MHR achieved by the GXT completed during baselinetesting were completed before all resistance exercise. Basedon literature, the average time for aerobic conditioningprotocols during combined aerobic and strength training isbetween 30 and 60 minutes at an average of 75% MHR,

thereby the average exercise interval was set forth at45 minutes (5,8,12,15,17,18,25,27,33).All resistance exercise protocols were preceded by either

a back squat or a bench press isometric MVC, during whichEMG data were also collected. Upon completion of theMVC, subjects performed either an upper-body or lower-body resistance exercise session. Both upper-body andlower-body protocols used a load of 80% of 1RM for 6 setsto voluntary failure. A 2-minute rest was given between eachset. Determination of volume load during resistance exercisewas completed by summing the total number of repetitions

TABLE1. Descriptive characteristics of the 9men.*

Variable Mean 6 SD

Age (y) 26.7 6 6.6

Height (cm) 175.0 6 3.5Body weight (kg) 79.2 6 6.8Bench press 1RM (kg) 117.0 6 22.4Back squat 1RM (kg) 142.2 6 28.3Relative bench press 1.5 6 0.2Relative back squat 1.8 6 0.3Maximum heart rate on

GXT (b$min21)176.2 6 12.1

Max watts on GXT 223.3 6 37.1

*1RM = 1 repetition maximum; GXT = graded exercisetest.

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completed in each of the 6 sets multiplied by load inkilograms.

Blood Collection and Analysis

Blood samples for analysis of whole-blood lactate werecollected via finger stick and immediately analyzed usinga Lactate Plus portable lactate analyzer (Nova Biomedical,Waltham, MA, USA). Collection of lactate occurred beforeexercise initiation for each of the 4 sessions and immediatelyafter cessation of aerobic exercise for a maximum total of 2analyses per session.

Maximal Voluntary Contraction Testing

Back Squat.Isometric back squat was performed in a modifiedsquat rack, where subjects stood on a force plate (Rough

Deck, Rice Lake, WI, USA)with a standardized knee angleof 1208 and braced againsta fixed bar. Subjects wereinstructed upon an auditorycue to push as hard and as fast

as possible for 35 seconds(termination of the contractionoccurred when force hadreached a plateau). A total of3 contractions were completed,with 1 minute separating each.Data were sampled at 1,000 Hzusing a 12-bit analog to digitalboard and filtered at 30 Hzusing a fourth-order low-passButterworth filter with Data-pac 5 software (Run Technolo-gies, Mission Viejo, CA, USA).

Measure of maximum forcewas accessed via the mean force achieved for a 1,000-milli-second period commencing at the beginning of the plateauperiod (force no longer increasing).

Bench Press.Isometric bench press was performed in the samerack as the back squat with the addition of a standard bench.The fixed bar was set for 908 of elbow flexion with the forearmperpendicular to the floor, and the bar in line with the mid-sternum. As in the back squat, subjects were instructed to pushagainst the bench as hard and fast as possible upon an audi-tory cue until a verbal stop command after 35 seconds (when

force had reached a plateau). A total of 3 contractions werecompleted, with 1 minute separating each. Data were sampledand analyzed in the same manner as in the back squat.

Electromyography Protocol

Preparation of the skin forEMG was conducted beforethe general warm-up in allsessions. This included mark-ing of electrode placement viapermanent marker, shaving ofbody hair, and abrasion withsandpaper. After completion

of this initial preparation, the5-minute cycle ergometrywarm-up was performed.After finishing the warm-upor aerobic exercise protocol,a 5-minute period was taken,which allowed for final prepa-ration of bodily surfaces forEMG, including alcohol swabcleansing and electrode place-ment. Electrodes were placed

Figure 1. Bench press (BP) repetitions per individual set. AE = aerobic exercise.

Figure 2. Back squat (BS) repetitions at each individual set. *p , 0.05, BS .AE + BS. AE = aerobic exercise.




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on the vastus lateralis, vastus medialis, and gluteus maximus ofthe upper right leg (back squat) and the right pectoralis major,anterior deltoid, and lateral head of the triceps (bench press).A ground electrode was placed on the patella of the right legfor both squat and bench press EMG. All electrodes wereplaced in accordance with the European Recommendations forSurface Electromyography, 2nd edition (Roessingh Researchand Development, 1999). Once the electrodes had beenplaced, subjects were instructed on the MVC protocol as de-scribed previously. EMG data were collected at 1 kHz, with

the root mean square and electrical activity per unit of force

(EMG:force) calculated duringthe 1,000-millisecond plateauperiod similar to MVC.

Statistical Analyses

Repetitions per set and lactate

were compared using a group(AE + BP, BP, AE + BS, BS) 3time (pre/post) repeated meas-ures analysis of variance(ANOVA). EMG, force, andEMG:force were measured us-ing dependentt-tests with a noaerobic exercise versus aerobicexercise model. All analyseswere performed using SPSS sta-tistical software (SPSS, Inc., Chi-cago, IL, USA). Statisticalsignificance was set at p #

0.05. When the underlyingassumption of sphericity associ-ated with repeated measures

ANOVA was not met, the Greenhouse-Geisser adjustmentwas used as the determinant of statistical significance. Whensignificance was observed, Tukeys honestly significant differ-ence post hoc tests were performed. Data are presented asmean 6SD, with effect sizes for dependentt-tests presentedas v2, with 0.20.4 representing small, 0.50.7 representingmedium, and 0.81.0 representing large differences (31).

RESULTS

Descriptive characteristics of the subjects can be viewed in

Table 1. Individual repetitions per set for bench press andback squat are presented inFigures 1 and 2, respectively.Individual repetitions per setwere not significantly differentbetween bench press protocols(p . 0.05). Aerobic exercisepreceding back squat exercise(AE + BS) resulted in fewerrepetitions during the first setas compared with the BS bout(10.00 6 3.54 vs. 12.56 6 4.50;

p= 0.000). No differences werenoted in each of the subsequentsets (p. 0.05). However, totalrepetitions for the first 3 sets(Figure 3) were significantlyhigher in BS (27.11 6 10.56)than in AE + BS (23.11 69.19;p= 0.014, v2 = 0.33), withno differences between eitherbench press protocol(p= 0.665).

Figure 3. Cumulative repetitions for all protocols at sets 16. *p , 0.05, BS .AE + BS. AE = aerobic exercise;

BP = bench press; BS = back squat.

Figure 4. Electromyographic comparisons for both bench press (BP) protocols. *p , 0.05, AE + BP , BP.

AE = aerobic exercise; RMS = root mean square.


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Lactate was significantly higher than baseline after cycleergometry (2.49 6 1.57 vs. 1.49 6 0.54;p= 0.05). Root meansquare EMG (Figure 4) for the lateral triceps was higher inBP as compared with AE + BP (p = 0.011, v2 = 0.36),whereas MVC (Figure 6) was significantly greater in BPcompared with AE + BP (p= 0.044, v2 = 0.21). Root meansquare EMG (Figure 5) and MVC (Figure 6) were not dif-ferent regardless of the inclusion of aerobic exercise foreither back squat protocol (p. 0.05). Analysis of EMG:forcerevealed a significantly higher value for BP as compared withAE + BP (p = 0.023, v2 = 0.28). Similarly, BS exhibited

a significantly greater EMG:force than AE + BS (p =0.048, v2 = 0.20).

DISCUSSION

We hypothesized that potentialdifferences in the acute metabolicand neuromuscular responsesbetween a no-aerobic-exercisecondition and a moderate-inten-sity-aerobic-exercise conditionperformed before resistance ex-ercise would impact physicalperformance in subsequent re-sistance exercise. Our main find-ing is that moderate-intensityaerobic exercise at 75% MHRfor 45 minutes resulted in a sig-nificant decrease in back squatrepetitions during set 1 (AE +BS , BS) and cumulativelyafter the third set, with no sig-

nificant differences within sub-sequent sets of the back squat.No differences were noted for single-set or cumulative repe-titions between bench press protocols (with and without prioraerobic exercise). We also noted a significant increase in bloodlactate after aerobic exercise. Muscle activity during MVC wassignificantly lower after aerobic exercise in the lateral triceps,with no other differences between muscle groups for eitherbench press or back squat protocol. This decline in muscleactivity for the lateral triceps did result in a slight decline inmaximum force production in the bench press. No differencein maximum force generation was noted between back squatprotocols. Finally, the ratio of maximum force generation to

muscle activity was significantly greater without precedingaerobic exercise for both bench press and back squat.

The fatigue present at initia-tion of the first set, noted bythe decrease in back squatrepetitions, was similar to thatnoted by Sporer and Wenger(30) and Leveritt and Aberne-thy (21), who also observedsignificant differences duringthe first set of lower-body re-sistance exercise after aerobicexercise. Conversely, de Souzaet al. (9) found no differences inthe ability to complete repeti-tions after an aerobic exerciseprotocol. However, it is possi-ble that these differences area result of the type and inten-sity of the exercise used. Ourinvestigation implemented 6sets to failure at 80% 1RMsquats compared with 4 sets

Figure 5. Electromyographic comparisons for both back squat (BS) protocols. AE = aerobic exercise;

RMS = root mean square.

Figure 6. Force characteristics before resistance exercise. *p , 0.05, AE + BP , BP. AE = aerobic exercise;

BP = bench press; BS = back squat.




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to failure at 75% 1RM leg press (30) and 1 set to failure at80% 1RM leg press (9). Leveritt and Abernethy (21) alsoincorporated 80% 1RM squats and noted similar significantdifferences in repetitions to failure, with the differences be-tween the 2 protocols decreasing with the completion ofeach subsequent set. In other words, the groups that com-

pleted aerobic exercise first were more fatigued at the firstset, resulting in fewer repetitions; however, as both groupscontinued to complete the subsequent sets, the differences infatigue became less pronounced. Nonetheless, differenceswere still present between back squat protocols after thecompletion of the third set, resulting in the completion offewer repetitions for the total session when aerobic endur-ance exercise preceded the back squat. However, becauseour goal was to assess the mechanisms of acute fatigue im-mediately after moderate-intensity aerobic exercise, thesedifferences allow quantitative evidence to support that wedid elicit fatigue to a certain degree. To the authors knowl-edge, no direct evidence exists on the cumulative effects of

a small decrease in repetitions from a day-to-day basis. How-ever, based on the review of Schoenfeld (28), this resultinglower training stress might lead to the inability to reach anoverreaching state and therefore hinder performance im-provement. Thus, our findings support those of previous re-search in that lower-body aerobic exercise induces fatigue toan extent that causes a decline in the ability to performrepetitions to failure.

We found that before resistance exercise, blood lactatewas significantly higher than baseline after completion ofcycle ergometry. For the back squat, it seems possible thatthis slight elevation in lactate before resistance exerciseresulted in decreased repetitions at the first set. This couldoccur because intramuscular lactic acid alters the ability ofthe muscle to contract, whereas blood lactate could alsosignify decreased muscle glycogen (7). Lactic acid has beennoted to potentially interfere with the contractile ability ofthe muscle by hindering calcium release from the sarcoplas-mic reticulum and also impairing actin and myosin cross-bridge activity, resulting in decreased force production, butonly in extreme cases of acidosis (7). However, despite theaforementioned possibility, because the absolute differencesin blood lactate before resistance exercise were so small andthe overall lactate response was also minimal, it seems thatthe repetition decrease noted at set 1 of the back squat is

caused by another factor other than an increase in acidosis ofthe leg muscles as noted by whole-blood lactate. Furthersupport for this claim lies in our observation that the lactateresponse to cycle ergometry was the same for thebench press and did not result in a significant difference inrepetitions at any set. However, the reason for decreasedrepetitions could be a local effect of metabolic stress orfatigue within the legs but not the upper body. Elevatedlactate after cycle ergometry most likely reflects metabolicstress of the lower-body musculature involved in the lower-body endurance exercise.

It has been noted using a similar intensity and duration ofcycle ergometry protocol that glycogen was depleted first inthe type I fibers and then proceeding to type II (14). There-fore, it is possible that in our investigation, glycogen contentmay have been reduced during aerobic exercise to an extentthat may have impaired back squat performance. Without

direct measurement of glycogen, this observation is purelyspeculative and in need of future investigation.

It was also observed that muscle activity (through EMG)and maximal force generation (MVC) were significantlydifferent between conditions for the bench press but not theback squat. Bench press EMG was significantly lower in thelateral triceps after cycle ergometry, which was followed bya statistically significant, but small, decrease in force. Thereason for this decline is unclear, but based on the inves-tigators observation, it is feasible that the lateral triceps wassomewhat fatigued because it was used for support as thesubjects were leaning on the handlebar during the 45-minutecycle exercise. However, we cannot completely eliminate cen-

tral fatigue as a mechanism of acute interference because ofthe fact that the activation of the lateral triceps muscle de-creased after aerobic exercise. This alternative explanation isless likely because the activation of the primary muscles in-volved in the bench press exercise (pectoralis major and an-terior deltoid) was not altered. Nevertheless, the small changein muscle activity and isometric force as a result of the pre-ceding cycle exercise did not affect dynamic bench press per-formance. For the back squat, EMG:force was significantlydifferent between conditions; however, the EMG activitywas not significantly different between the 3 analyzedmuscles. It is well known that during prolonged contractionand relaxation, muscle activity will decrease, usually resultingin a decrease in force production (13,29). Although the re-sultant total forces were not different between protocols, itseems that other muscles involved in the back squat contrib-uted to a greater extent after aerobic exercise to produce thesame force as seen without cycle ergometry. Our observationsshow conflicting results for bench press and back squat; thatis, for the bench press, muscle activity was lower for the lateraltriceps resulting in a lower maximum force, but did not cul-minate in fewer completed repetitions. Conversely, neitherback squat maximum force nor muscle activity was differentbetween groups, but fewer repetitions were completed at thefirst set and accumulated third set. This suggests that the in-

terference effect could occur as a result of acute neuromuscu-lar fatigue, but more detail is needed in future research.In conclusion, a moderate-intensity aerobic exercise

session resulted in a significant decrease in repetitions forthe back squat but not the bench press, lending furthersupport that interference induced by same-day concurrenttraining is local to the working muscles. Increased bloodlactate associated with this design is likely not large enoughto elicit a state of acidosis that would impair the contractileability of the muscle. Force-generating capacity and muscleactivity showed that a decrease in maximal force does not


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necessarily result in fewer repetitions in the bench press,whereas the opposite is true for the back squat. Thesefindings indicate that the interference effect as attributed toacute fatigue induced by a moderate-intensity aerobicexercise session occurs as a result of a local metabolic stress,causing a decrease in repetitions for the back squat but not

the bench press. However, we cannot completely eliminatecentral fatigue as a mechanism of acute interference becauseof the fact that bench press maximum isometric force andthe activation of the lateral triceps muscle decreased afteraerobic exercise. Our investigation has brought forth someinsight as to the mechanisms of acute fatigue and theirinfluences on concurrent training. To our knowledge, this isthe first investigation to analyze acute neuromuscularresponses to a concurrent training session. However, moreresearch is needed to determine how metabolic stress causesfatigue and analyze the responses of both AMPK andmTOR, because of the conflicting nature of the adaptationsof which these molecules are associated (2,16,19,24,33).

Finally, methods to bypass the interference effect will allowfor optimization of training time without concomitanthindrances in performance adaptations.

PRACTICAL APPLICATIONS

In designing a concurrent aerobic endurance and resistancetraining program, strength and conditioning coaches shouldbe careful to select an exercise that, within an individualsession, does not cause athletes to become excessivelyfatigued, thereby potentially hindering performance of a sub-sequent exercise in the workout. Applying this principlecould include avoiding lower-body aerobic training beforelower-body resistance exercise, changing the order of

exercises, and separating aerobic and resistance exercisesessions by at least 24 hours.

ACKNOWLEDGMENTS

The authors thank Dr. Richard Bloomer for his time on thefirst authors thesis committee, and Paul N. Whitehead andCourtney L. Collins for their help with data collection.

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