Journal of Strength and Conditioning Research, 2006, 20(4), 915-918i j 2006 National Strength & Conditioning Association

ISOMETRIC SQUAT FORCE OUTPUT AND MUSCLEACTIVITY IN STABLE AND UNSTABLE CONDITIONS

JEFFREY M, MCBRIDE, PRUE CORMIE, AND RUSSELL DEANE

Neuromuscular Lahoratory, Department of Health, Leisure and Exercise Science, Appalachian State University,Boone, North Carolina 28607.

ABSTRACT. McBride, J.M., P. Cormie, and R. Deane. Isometricsquat force output and muscle activity in stable and unstableconditions. J. Strength Cond. Res. 20(4):915-918. 2006.—Thepurpose of this study was to assess the effect of stable vs. un-stable conditions on force output and muscle activity during anisometric squat. Nine men involved in recreational resistancetraining participated in the investigation by completing a singletesting session. Within this session subjects performed isometricsquats either while standing directly on the force plate (stablecondition, S) or while standing on inflatable balls placed on topof the force plate (unstable condition, U). Electromyograpby(EMG) was recorded during both conditions from the vastus la-teralis (VL), vastus medialis (VM), biceps femoris (BF), and me-dial gastrocnemius (G) muscles. Results indicated peak force(PF) and rate of force development (RFD) were significantly low-er, 45.6% and 40.5% respectively, in the U vs. S condition ip <0.05). Average integi-ated EMG values for the VL and VM weresignificantly higher in tbe S vs. U condition. VL and VM muscleactivity was 37.3% and 34.4% less in U in comparison to S. Nosignificant differences were observed in muscle activity of theBF or G between U and S. The primary finding in tbis investi-gation is that isometric squatting in an unstable condition sig-nificantly reduces peak force, rate of force development, and ag-onist muscle activity with no change in antagonist or synergistmuscle activity. In terms of providing a stimulus for strengthgain no discemable benefit of performing a resistance exercisein an unstable condition was observed in the current study.

KEY WORDS, instability, strength, power, resistance training

INTRODUCTION

he influence of unstable resistance training onpossible physiologic adaptations has been re-cently reviewed (2). However, limited studies

^ have investigated the effect of instability dur-ing exercise and its influence on muscle activity and forceoutput (2, 4, 5). Several investigations have examinedmuscle activity in unilateral vs. bilateral exercises whichcreates a certain level of instability (4, 5, 8, 11). It is oftenassumed that unstable environments create an increasedlevel of muscle activation, especially in antagonist or syn-ergist muscles. However, few data exist that actually sub-stantiate this theory.

A standard type of resistance training is typically usedto increase muscle strength (7). More importantly, the re-lationship between the intensity of training and gains instrength has been highlighted. It has been reported thatintensities of less than 80% of 1 repetition maximum(IRM) strength result in decreases in electromyographic(EMG) activity of involved muscles, with a subsequentdecrease in muscle strength (6, 7). As highlighted by An-derson and Behm (2), increased EMG activity of associ-ated antagonist and synergist muscle in unstable exercis-es is not necessarily reflected in overall muscle force out-

put. This was demonstrated in an investigation by Behmet al. (4) which reported that antagonist muscle activitywas greater in an unbalanced resistance exercise. How-ever, the force output was diminished by 20.29r. Thus,instability may result in increased activity of associatedmuscles but does not result in any appreciable gains inmeasured force output.

The muscle activity levels and kinetic variables asso-ciated with stable vs. unstable resistance exercise are, asmentioned above, limited. One recent investigation utiliz-ing a dynamic bench press reported that force output inan unstable condition was 59.6% less than the force pro-duced during the stable condition (1). Muscle activity ofthe pectoralis major, deltoids, and triceps was not signif-icantly different hetween the stable and unstable condi-tions. The unstable condition was effective in decreasingforce output considerably; bowever, it was not effective inincreasing activity of either the prime mover muscle orassociated stabilizer and synergist muscles. A similar re-sult was also reported in a study involving dynamic legextensions (4). In this study force output in tbe unstablecondition was reduced by 20.2%. However, antagonist andsynergist muscle activity did increase by 29.1% and30.3%, respectively. It appears that the majority of thisadditional muscle activity most likely is contributing tojoint stability and does not effectively contribute to over-all force output in the movement.

The purpose ofthe current investigation was to assessthe effect of stable vs. unstable conditions on force outputand muscle activity in the lower body. This study utilizeda multi-joint structural exercise, the isometric squat, toexamine this phenomenon. An isometric action was cho-sen to increase the validity of the muscle activity mea-surements through EMG. If force output decreases ap-preciably in a resistance exercise performed in an unsta-ble condition, it is questioned if this level of activation issufficient to adequately stimulate strength gain in thecontext of a longitudinal training study.

METHODS

Experimental Approach to the Problem

The subjects completed 1 testing session. Within this ses-sion subjects performed isometric squats either whilestanding directly on the force plate (stable condition, S)or while standing on inflatable balls placed on top oftheforce plate (unstable condition, U) (Figure 1). EMG wasrecorded during both conditions from the vastus lateralis(VL), vastus medialis (VM), biceps femoris (BF), and me-dial gastrocnemius (G) muscles. Following a 5-minute bi-cycle ergometer warm-up, the subject was exposed to eacbcondition in a randomized fashion. Subjects were allowed

915

916 MCBRIDE, CORMIE, AND DEANE

FIGURE I. Isometric squats in unstable condition.

4 trials for each condition. Adequate rest (3 minutes) wasallowed between each effort. Twenty-five minutes of restwere allowed between conditions and another 5-minutebicycle ergometer warm-up was completed before the sec-ond bout of squats.

SubjectsThis study involved 9 athletic male college students (age,22.4 ± 2.7 years; height, 175.61 ± 5.34 cm; weight, 85.5± 19.14 kg; percent body fat, 15.06 ± 5.39%). Subjectswere involved in resistance training and some type of rec-reational sporting activities. The volunteers were notifiedabout the potential risks involved and gave their writteninformed consent, approved by the Institutional ReviewBoard at Appalachian State University.

ProceduresIsometric squat. The isometric squat was performed byhaving the subject stand on a force platform (BP6001200;AMTI, Watertown, MA) under a fixed bar position at a100" knee angle (10) while performing a maximal isomet-ric contraction for 3 seconds. An isometric action was cho-sen to increase the fidelity of the muscle activity mea-surements obtained. The S condition was performed whilestanding directly on the force plate. The U condition wasperformed while standing on inflatable balance disks (cir-cumference 90 cm). Utilization of the inflated balancedisks on the force plate was validated in the laboratoryto ensure that a force exerted against the disks was equalto that same force applied directly to the force plate (in-

600

500

400 -

300

200

100

200 300

Stable Force (N)

400 500

FIGURE 2. Linear regression of force applied directly to theforce plate (stable force) and force applied through a balancedisk (unstable force). Intraclass correlation coefficient R'^ =1.0.

terclass correlation coefficient between S and U condi-tions R'^ = 1.0; Figure 2). In addition, Barnett et al. (3)demonstrated that no differences existed in force outputbetween cushioned or uncushioned shoes measured viaan in-shoe transducer and a force plate. Thus, any changein force-time related measurements during U or S was arepresentation of true changes in muscle force output. Aminimum of a 3-second isometric contraction was also uti-lized to ensure that in the unstable condition the inflat-able disks were maximally compressed and the maximalforce had reached a plateau before recording peak force(PF) values used for comparison between conditions.

The bar height was adjusted so that the knee anglewas 100° for each condition. Each individual's bar heightand foot placement remained constant during the testingsession and remained constant under each condition. Thevertical force-time curve was recorded using a shieldedBNC adapter chassis (BNC-2090; National Instruments,Austin, TX) and an analog-to-digital (A/D) card (NI PCI-6014; National Instruments) at 1000 Hz. LabVIEW (ver-sion 7.1; National Instruments) was used for recordingand analyzing the data. PF of the whole 3-second con-traction and average rate of force development (RFD) forthe first 400 ms of the force-time curve were calculated.

Electromyography. The EMG of the VL, VM, BF, andG muscles was completed at 1,000 Hz using a telemetrytransmitter (8-channel, 12-bit analog-to-digital converter;Noraxon USA Inc., Scottsdale, AZ). A disposable surfaceelectrode (2-cm interelectrode distance, 1-cm circular con-ductive area; Noraxon USA) was attached to the skin overthe belly of each measured muscle, distal to the motorpoint, and parallel to the direction of muscle fibers. Theamplified myoelectric signal, recorded during each of thetrials in both S and U was detected by the receiver-am-plifier (Telemyo 900, gain ^ 2,000, differential input im-pedance - 10 Mil, bandwidth frequency 10-500 Hz, com-mon mode rejection ratio = 85 dB; Noraxon USA) andthen sent to an A/D card (KPCMCIA-12AI-C; Keithley,Cleveland, OH) and analyzed using MyoResearch soft-ware (version 4.0; Noraxon USA). The signal was fullwave rectified and filtered (6-pole Butterworth, notch fil-ter 60 Hz, band pass filter 10-200 Hz). The integratedvalue (|xV-s) was calculated and then averaged over the3-second isometric contraction (|xV).

ISOMETRIC SQUAT FORCE 917

TABLE 1. Mean values ± SDs of force and electromyographyvariables.*

Variable

ForcePF(N)R F D ( N R - ' )

AvglEMGVMttJiV)VL(M,V)BF {(xV)G(|i.V)

Stable

2186.95 ± 377.34+2689.32 ± 804.801-

182.35 ± 63.28t206.72 ± 66.56t38.90 ± 23.7129.07 ± 17.74

Unstable

1189.68 ± 427.101599.11 ± 675.43

119.56 ± 54.53129.65 ± 53.8357.61 ± 33.1928.53 ± 18.56

•'' PF = peak force; RFD •-= rate offeree development; AvglEMG= average integrated electromyography; VM = vastus medialis;VL = vastus lateralis; BF = biceps femoris; G = medial gas-trocnemjus.

T Significant (p s 0.05) difference between stable and unstableconditions.

Stable Unstable

FIGURE 3. Mean maximal isometric squat force and rate offorce development (RFD) in stable and unstable conditions. *Significant (p < 0.05) difference between stable and unstableconditions.

Statistical AnalysesA general linear model multivariate analysis with a Bon-ferroni post hoc test was used for data analyses. The cri-terion ot (alpha) level was set at p < 0.05. All statisticalanalyses were performed through the use of SPSS statis-tical software (version 11.0; SPSS, Inc., Chicago, IL}.

RESULTS

Peak force and RFD were significantly higher in the S vs.U condition (Figure 2, Table 1) ip < 0.05). Peak force andRFD were 45.6% and 40.5%, respectively, less in U incomparison to S. Average integrated EMG values for theVL and VM were significantly higher in the S vs. U con-dition (Figure 3, Table 1). Vastus lateralis and VM muscleactivity was 37.3% and 34.4%, respectively, less in U thanS. No significant differences were observed in muscle ac-tivity of the BF or G between U and S.

DISCUSSION

The primary finding in this investigation is that isometricsquatting in an unstable condition significantly reducespeak force. In addition, unstable isometric squatting does

not result in a significant increase in the muscle activityof agonist, antagonist, or synergist muscles. Furthermore,the agonist muscle activity was significantly lower duringthe unstable condition. The results of this study are con-sistent with one other investigation (1) that observed a59.6% reduction in force during a dynamic bench press inan unstable condition. This study also noted no signifi-cant increase in agonist or synergist muscles in the un-stable condition as well. Another investigation reportedthat force in an unstable condition during dynamic kneeextensions was reduced by 20.2%' (4). However, unlikethis investigation antagonist and synergist muscle activ-ity did increase hy 29.1%' and 30.3%, respectively.

The force output in the current study during the un-stable condition was 54.4% of the stable condition. If thestable condition is taken as 100% of maximal strength,then the stimulus for strength gain (54.4%) in the unsta-ble condition would not meet the intensity requirementset by Hakkinen et al. (7) for appreciable strength gainas a result of resistance training. Hakkinen et al. (7) re-ported that training with intensities less than 80% ofmaximal strength resulted in decreased muscle activityand subsequent loss of strength with training. Therefore,the effectiveness of resistance training in an unstable en-vironment is questionable. A consideration must also bemade regarding the level of muscle activity present dur-ing unstable training. If increased activation was a by-product of unstable training then a consideration for itsusage could be made. However, in the current study in-stability resulted in a significant decrease in activity ofthe agonist muscles and no significant change in antag-onist or synergist muscles. Therefore, the noted benefit ofincreased muscle activation with instability seems to beunfounded as well. One study investigating the effective-ness of strength training, balance training, and combinedtraining found no increase in muscle strength after bal-ance training (9). It is recognized that in the current in-vestigation an isometric test was utilized and thus therelationship to dynamic test could be questioned. How-ever, strong statistically significant correlations havebeen reported between structural isometric testing (mid-thigh isometric pull) and dynamic lifting performance{10).

No discernable benefit of performing a resistance ex-ercise in an unstable condition could be surmised fromthe observation made during an isometric squat in thecurrent study. Instability resulted in decreased force out-put, decreased rate of force development and a decreasein agonist muscle activity. No change in antagonist orsynergist muscle activity was observed. It remains un-clear as to what positive influence resistance training inan unstable condition would have.

PRACTICAL APPLICATIONS

Although the use of equipment to simulate unstable en-vironments in resistance training has received much at-tention and consequent use in recent years, the efficacyof this mode of training has been undetermined. Perform-ing exercises with instability does not appear to elicit aresponse intense enough to provide an adequate stimulusfor strength gain. In addition, the mode may not be ef-fective in stressing the neuromuscular system to a great-er extent than traditional resistance training methods. Asmentioned previously, the use of an isometric test in thecurrent investigation brings into question the application

918 MCBRIDE, CORMIE, AND DEANE

of this data to dynamic activities. However, data existthat significantly correlate structural isometric tests todynamic resistance training performance (10). Therefore,the results of this investigation indicate no discernablebenefit of performing a resistance exercise in an unstablecondition.

REFERENCES

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Address correspondence to Dr. Jeffrey M. McBride,mcbridejm@appstate.edu.