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AGE, WEIGHT, AND THE FRONT ABDOMINALPOWER TEST AS PREDICTORS OF ISOKINETIC

TRUNK STRENGTH AND WORK INYOUNG MEN AND WOMEN

PATRICK M. COWLEY, SHARON FITZGERALD, KYLE SOTTUNG, AND THOMAS SWENSEN

Department of Exercise and Sport Sciences, Ithaca College, Ithaca, New York

ABSTRACT

Cowley, PM, Fitzgerald, S, Sottung, K, and Swensen, T. Age,

weight, and the front abdominal power test as predictors of

isokinetic trunk strength and work in young men and women.

J Strength Cond Res 23(3): 915925, 2009First we testedthe reliability of two new field tests of core stability (plank to

fatigue test [PFT] and front abdominal power test [FAPT]), as

well as established measures of core stability (isokinetic trunk

extension and flexion strength [TES and TFS] and work [TEW

and TFW]) over 3 days in 8 young men and women (24.06 3.1

years). The TES, TFS, TFW, and FAPT were highly reliable,

TEW was moderately reliable, and PFT were unreliable for use

during a single testing session. Next, we determined if age,

weight, and the data from the reliable field test (FAPT) were

predictive of TES, TEW, TFS, and TFW in 50 young men and

women (19.0 6 1.2 years). The FAPT was the only significant

predictor of TES and TEW in young women, explaining 16 and15% of the variance in trunk performance, respectively. Weight

was the only significant predictor of TFS and TFW in young

women, explaining 28 and 14% of the variance in trunk

performance, respectively. In young men, weight was the only

significant predictor of TES, TEW, TFS, and TFW, and

explained 27, 35, 42, and 33%, respectively, of the variance

in trunk performance. In conclusion, the ability of weight and the

FAPT to predict TES, TEW, TFS, and TFW was more frequent

in young men than women. Additionally, because the FAPT

requires few pieces of equipment, is fast to administer, and

predicts isokinetic TES and TEW in young women, it can be

used to provide a field-based estimate of isokinetic TES andTEW in women without history of back or lower-extremity injury.

KEY WORDS athletes, core stability, field test, lower-extremity

injury, reliability

INTRODUCTION

The ability of the lumbopelvic skeletal structures

and musculature to withstand compressive forceon the spine and return the body to equilibrium

after perturbation, as well as to effectively transmitforces generated by the limbs to their intended action, is

increasingly recognized as an essential component in the

successful performance of activities of daily living and athletic

pursuits (1,18,30,36). These processes, referred to as core

stability, involve the complex interplay between the

abdominal, hip, and spine musculature.Recent data show that poor core stability is a risk factor for

back and lower extremity injury in athletes; this is particularly

true for female athletes, who are more susceptible to knee

injuries than male athletes (2,7,10,15,22,2729,37,38). To

reduce the risk of injury, athletes are strongly encouraged to

add core stability exercises to their strength and conditioning

program (1,5,11,25,30,34).Given the well-established linkbetween core instability and

back and lower-extremity injury, it is paramount for sports

medicine professionals to assess core stability to identify those

at the greatest risk of injury. Thecomplex interplay among the

core musculature, however, makes it difficult to fully assess

core stability with a single test. For example, core instability

could result from insufficient muscular strength, power,

endurance, coordination, or some combination of deficiencies

in these or other variables. Common tests of core stability

include isometric measures of strength and endurance, iso-

kinetic measures of strength and work, and isoinertial tests,

such as the field test of trunk flexor endurance recommended

by the American College of Sports Medicine and NationalStrength and Conditioning Association (8,13,16,17,26,32).

These tests have limitations. Isometric tests, for example,

only assess core stability at one muscle length, whereas

isokinetic tests require expensive and immovable machines.

Presently, isokinetic testing of trunk extensor and flexor

strength is the standard measure of core stability in clinical

sports medicine, primarily because it is reliable (8,16). Data

from isokinetic tests are used to assess injury or reinjury risk

and to track preoperative and postoperative rehabilitation

Address correspondence to Patrick M. Cowley, [email protected].

23(3)/915925

Journal of Strength and Conditioning Research 2009 National Strength and Conditioning Association

VOLUME 23 | NUMBER 3 | MAY 2009 | 915

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status (9,35). Ultimately, there is a need for robust field teststhat require little or no equipment, are fast to administer, and

assess the various aspects of core stability. Data from thesetests should correlate well with isokinetic trunk flexion and

extension strength; they should also help one accuratelypredict isokinetic measures of trunk performance, thereby

allowing comparisons to the body of normative data.We are aware of only one study that has examined the

relationship between easily administered field tests of corestability and isokinetic trunk performance. In this study,

3 timed sit-up field tests [(a) Kraus-Weber, American Allianceof Health, (b) Robertson, and Physical Education, Recreation,

and Dance and (c) (AAHPERD) sit-up protocols] were com-pared withisokinetictrunk performance in young women andmen (12). There was a significant but weak correlation (r =0.320.42) between the tests and isokinetic trunk perfor-mance, indicating that timed sit-up field tests are not validmeasures of isokinetic trunk performance when used

individually. Because multiple regression analyses were not

performed, it is not known if results from these field tests,perhaps combined with selected anthropometric data from thesubjects, can be used to predict isokinetic trunk performance.

The first purpose of this study was to design and test the

reliability of 2 new field tests of core stability: plank to fatiguetest (PFT) and front abdominal power test (FAPT). Thesetests were designed to measure the endurance and power of

the core musculature. In conjunction, we examined thereliability of isokinetic trunk extension and flexion strength

andwork. It wasexpected that the field tests would be reliableand correlate well with isokinetic measures, but not oneanother. Our second purpose was to determine if the results

from those field tests that were reliable could be used, along

with selected anthropometric measures, to accurately predictisokinetic trunk flexion and extension strength and work in

a group of young men and women.

METHODS

Experimental Approach to the Problem

A preliminarystudy was conducted to test the reliabilityof the

selected measures of core stability in a small set of subjects.This was an a priori decision to ensure the testing protocolswere reliable. Tests of maximal reciprocal concentric iso-

kinetic trunk flexion and extension strength (TFS and TES,respectively) and work (TFW and TEW, respectively), P FT,and the FAPT were performed in a single testing session on

3 separate days. Testing sessions 1 and 2 were separated by7 days, whereas testing sessions 2 and 3 were separated by

2 days. The tests were performed in either of 2 orders: (a)isokinetic trunk flexion and extension tests, PFT, and theFAPT, and (b) the reverse order. Half of the participants were

assigned to thefirst order of tests and half to thereverse order.The order of tests was the same for each testing session.

We examined the ability of the field tests that were reliable,

along with the subjects age and weight, to predict isokineticTES, TFS, TFW, and TEW by having subjects perform the

aforementioned tests in a single testing session. The random-ization procedure used in the main study was different than in

the preliminary study. For the main study, 24 different ordercombinations were generated, and 1 to 2 participants were

assigned to each order combination.The participants were asked to refrain from exercise,

alcohol consumption, and over-the-counter medications onthe day of testing and not to consume any food or caffeine30 minutes before the testing session. An exercise and nutri-tion history questionnaire was used to confirm the absence of

said restrictions.

Subjects

For the preliminary study, 8 subjects (5 women and 3 men)

participated. The mean 6 SD for age, height, and weight ofthe women was 24.4 6 4.0 years, 172.2 6 6.6 cm, and 67.5 610.2 kg and 23.3 6 0.58 years, 184.6 6 6.4 cm, and 87.3 613.7 kg for the men. For the main study, 50 subjects(31 women and 19 men) participated. The mean 6 SD for

age, height, and weight for the women in this subject cohortwas 19.5 6 1.4 years, 163.2 6 6.8 cm, and 61.8 6 8.8 kg;corresponding values for the men were 19.2 6 0.8 years,

181.16 9.3 cm, and 86.66 10.6 kg. Most of the subjects werephysically active, participating in resistance or aerobic exer-cise training programs 37 days per week. Each subject

completed a medical history questionnaire to ensure theabsence of any current or previous back, hip, or leg injury that

would interfere with testing procedures. The InstitutionalReview Board at Ithaca College approved the proceduresused in this study, and all subjects gave informed consentbefore participation.

Procedures

Isokinetic Trunk Flexion and Extension Strength and Work.Isokinetic trunk flexion and extension peak torque and work

were determined using a Cybex trunk extension and flexiondynamometer (Lumex Inc., Ronkonkoma, N.Y.) accordingto the protocol of Karatas et al. (16). The subjects were

positioned in the dynamometer according the manufacturesrecommendations. Briefly, the axis of rotation of the machine

was aligned with the intersection of the midaxillary line andlumbosacral junction. The leg and calf pads were positionedto stabilize the lower body in 15 degrees of knee flexion. The

scapular and chest pads were secured so the participant hada stable pad to exert force against. The range of motion wasset while the subject was in the vertical standing position at

the anatomical reference position of 0 degrees. This alsoserved as the initial starting position. The total range of

motion was preset to 110 degrees, from 95 degrees of trunkflexion to 15 degrees of trunk extension. Maximal concentrictrunk flexion and extension contractions were performed

reciprocally at 60 degrees per second. Initially, subjectsperformed 4 practice trunk flexion and extension contrac-tions at minimal exertion. Afterwards, they performed 5

maximal trunk flexion and extension contractions throughthe preset range of motion while receiving strong verbal

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encouragement from the investigators. The peak torqueattained during the maximal concentric trunk flexion and

extension contractions was taken as a measure of strength,and the average work performed for the 5 repetitions was

taken as a measure of work.

Plank to Fatigue Test

The prone plank, performed with the toes and forearms incontact with the floor, is often used as an isometric exercise in

core stability training programs (11). The prone plank teststhe ability of the subject to maintain a neutral back position.It is performed by first positioning the elbows directlyunderneath the shoulders so the upper arms are perpendic-

ular to the floor. The hips are positioned so that a straight linecan be drawn from the shoulders to the ankles through the

hips. The PFT test required subjects to hold the prone plankposition for as long as possible; they were allowed to place

their feet against a wall to help stabilize themselves, and 10%of their body weight was set on the upper gluteus region to

expedite fatigue (Figure 1). To ensure that the subjectsmaintained the plank position throughout the test, certain

measures were taken. First, they completed a 15-secondunloaded practice trial plank in between two 90-cm-tall

vertical rods to determine the hip height they would berequired to maintain during the test; 1 rod was adjacent to

each hip. Plank height was recorded by placing clamps oneach vertical rod. After the 15-second trial, a back-supportbelt was attached to the subject. The belt was fitted with a 20-cm section of horizontally cut metal tubing with a diameter

of approximately 2 cm. The metal half pipe cradled a 90-cm-long wooden dowel that was laid across the lower back. The

height of each end of this dowel was to be maintained at the

level of the clamps on the vertical rods during the test. A

rubber mat was placed underneath the elbows for comfort.The experimental trial commenced when the additional

weight was placed upon the subject. Verbal feedbackregarding deviation from the starting position was given to

each subject, and an inability to correct the deviation signifiedthe end of the test. Subjects were verbally motivated

throughout the test but were not informed of the elapsedtime. Time to task failure was recorded in seconds.

Front Abdominal Power Test

The FAPT was modified from Cowley and Swensen (6).

Briefly, an exercise mat was laid on the floor parallel to anopen area free of objects. Subjects were instructed to lay withtheir back on the mat, arms along the sides, and feet shoulder

width apart. Knees were then bent to 90 degrees, at whichpoint the tips of the feet were aligned with the end of the mat.

The feet were then secured to the ground by an E-Z curl bar(Champion Barbell, Dallas, Tex) weighted with two 20.5-kgplates. Subjects then raised their arms over their head by

flexing the shoulder; elbows and wrists were extended withthe hands supinated and thumbs from the left and right handstouching. A 2-kg medicine ball was placed in the supinated

hands, which then cradled the ball. From here, the subjectwas instructed to keep the shoulders, elbows, and wrists

locked in this position with the medicine ball securelycradled in the hands. The subject was then instructed toperform an explosive concentric contraction of the abdom-

inal and hip flexor muscles while using the arms as a lever toproject the medicine ball. The feet and buttocks remained incontact with the floor. The medicine ball was released out of

the hands when they were over the subjects knees. Thedistance the medicine ball was projected was recorded and

measured from the tip of the feet to where the medicine balllanded. The subject was given 3 practice trials and thenperformed 6 trials of the FAPTwith a rest period of 2 minutes

between trials. The average of 6 trials was used in the dataanalyses.

Statistical Analyses

For the preliminary study, a number of reliability analyseswere performed. To detect for learning effect in the measure-ment protocol, an analysis of variance with repeated measures

was performed with each testing session (session 1, 2, and 3)as the within-subject factor. The relative reliability of the datawas determined by calculating intraclass correlation coeffi-

cients. For TES, TFS, TEW, TFW, and PFT, a two-wayrandom effects model with single-measure reliability intra-

class correlation coefficient was calculated, and a two-wayrandom effects model with average-measure reliability intra-class correlation coefficient was calculated for the FAPT. The

absolute reliability of the data was determined using thelimits-of-agreement method, SEM, and coefficient of varia-tion where applicable. First, Bland-Altman plots were gener-

ated to examine the correlation (i.e., R2) between the absolutedifferences and the mean values for each test to detect for the

Figure 1. The plank to fatigue test (PFT) was performed by first

positioning the elbows directly underneath the shoulders so the upper

arms were perpendicular to the floor. The subjects held the prone plank

position for as long as possible; they were allowed to place their feet

against a wall to help stabilize themselves, and 10% of their body weight

was set on the upper gluteus region to expedite fatigue.


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presence or absence of heteroscedasticity in the data (4). Ifthe R2 was between 0 and 0.1, the data were consideredhomoscedastic; accordingly, the SEM, limits-of-agreementmethod random error, and systematic bias were calculated.

The SEMwas calculated using the square root of the meansquare error term from the analysis of variance model. If the

R2 was .0.1, the data were considered heteroscedastic;accordingly, the coefficient of variation and limits-of-agreement method ratio were calculated after transformingthe data using a natural logarithm. The coefficient of

variation was calculated for each participant by dividingthe SDof the 2 testing sessions by the mean of the 2 testingsessions, then multiplying by 100; the mean coefficient ofvariation is reported. We deemed that the test-retestmeasurement error for TES, TEW, TFS, TFW, PFT, and

FAPT would be acceptable for use during a single testingsession if: (a) intraclass correlation coefficient exceeded 0.85,(b) the limits-of-agreement method random error and

systematic bias were less than 20 and 5% of the sample

mean between the testing days, respectively, or the limits-of-agreement method ratio was less than 20%, and (c) the SEMwas less than 10% of the sample mean between the testingdays or the coefficient of variation was less than 10%.

For the main study, a matrix of Pearson correlationcoefficients (R) was generated to assess the direction andstrength of association among selected anthropometric

variables and any measure of core stability with acceptablesingle-test reliability. Backward elimination multiple re-

gression was performed with TES, TFS, TEW, or TFWentered as the dependent variable and age, weight, and anymeasure of core stability with acceptable single-test reliability

as a predictor variable. The alpha-to-remove was set at 0.15.

Separate analyses were performed for men and women.From these analyses, the coefficient of determination (R2)was determined; it represents the percentage of explainedvariance in the respective trunk isokinetic variable and

indicates the predictive ability of the regression models. Thestandard error of estimate (SEE) was also calculated; itindicates the prediction accuracy of the regression models.

To evaluate the quality of the regression models producedfrom our data set, we ran a number of diagnostic tests to look

for the following: (a) the influence of outliers or unusualobservations, (b) nonlinearity between the dependent vari-

able and predictor variable(s), and (c) normality andhomoscedasticity of the residuals. The examination of

outliers or unusual observations was performed by evaluatingpartial regression plots and by identifying observations witha standardized residual of.3, a Cooks D value of.0.13 forthe female analyses and 0.21 for the male analyses, and

a standardized DFBETA value.0.36 for the females analysesand 0.46 for the male analyses; values are based on sample

size (n = 50). If an observation was identified by theaforementioned criteria, it was considered an outlier orunusual observation, and it was removed from the analysis.

Nonlinearity between the dependent variable and predictorvariable(s) was examined by evaluating partial regressionplots to see if the linear model provided the best fit.

Normality of residuals was tested using the Shapiro-Wilk test

of normality. To examine if the variance of the residuals washomoscedastic, we evaluated the scatter plot of thestandardized residual and predicted values. Finally, we usedthe nonparametric bootstrap method to determine the bias-

corrected 95% confidence intervals (CI) and SEof the R2 andSEEfor the final regression models. To do this, we resampledthe data 1000 times to simulate the sampling distribution of

R2 and SEE that would arise from repeatedly sampling thepopulation (39). The 95% CI and SE of the R2 and SEEgenerated from the bootstrap distribution estimate theprecision of the final regression models (31).

Data are presented as mean 6 SDunless otherwise stated.The alpha was set at p# 0.05. SPSS 13.0 (SP SS, Chicago, Ill)

and STATA 9.2 (STATA, College Station, Tex) statisticalpackages were used for data analysis.

RESULTS

Reliability Analyses

For the preliminary study, the mean 6 SD for TES, TFS,TEW, TFW, PFT, and FAPT are provided in Table 1. There

TABLE 1. Mean 6 SD for isokinetic trunk extension strength, trunk extension work, trunk flexion strength, trunk flexionwork, plank to fatigue test, and the front abdominal power test for testing session 1, 2, and 3; n = 8.

Variable Testing session 1 Testing session 2 Testing session 3

Trunk extension strength (Nm21) 176 6 70.1 184 6 81.6 193 6 91.6Trunk extension work (J) 126 6 63.1 128 6 72.9 132 6 74.3Trunk flexion strength (Nm21) 182 6 54.0 179 6 61.1 184 6 56.2Trunk flexion work (J) 147 6 53.3 142 6 58.2 147 6 56.8Plank to fatigue test (s) 94 6 42.4 111 6 46.2* 116 6 41.0Front abdominal power test (m) 1.53 6 0.5 1.63 6 0.5 1.49 6 0.4

*Significantly higher for testing session 2 compared with testing session 1 (p , 0.05).




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was no difference in the mean between the testing sessionsfor TES, TFS, TEW, TFW, and FAPT. There was a signifi-

cant difference between the mean for the PFT betweentesting session 1 and 2 (946 15.0 vs. 1116 16.3 seconds; p=0.01). The intraclass correlations coefficients and 95% CI forTES, TFS, TEW, TFW, PFT, and FAPT are provided inTable 2. The intraclass correlation coefficients indicate high

reliability for all tests. The results of the Bland-Altman plotanalyses and the applicable reliability statistics are providedin Table 3. Based on the criteria we deemed acceptable for

test-retest measurement error, the TES, TFS, TFW, andFAPT were shown to possess high reliability for use during

a single testing session, whereas TEW was moderatelyreliable, and the PFT was not reliable.

Correlation and Regression AnalysesFor the main study, the mean 6 SD for the dependentvariables (TES, TFS, TEW, and TFS) and potential predictor

variables (age, weight, andFAPT) are provided in Table 4.

The men were heavier than thewomen (p, 0.05), and they alsoexhibited greater TES, TEW,TFS, and TFW (p, 0.05). The

mean distance on the FAPTwasgreater for the men than thewomen (p, 0.05).

The matrix of Pearson corre-

lation coefficients is provided inTable 5. For women, weight was

significantly correlated with theFAPT (R = 0.41), TFS (R =0.43), and TFW (R = 0.37).In addition, the FAPT wassignificantly correlated with

TES (R = 0.40), TFW (R = 0.39), and TFS (R = 0.33). Formen, weight was significantly correlated with TES (R= 0.52),

TEW (R= 0.59), TFS (R= 0.65), and TFW (R= 0.58).The results of the regression analyses are provided in Table

6. For women, the only significant predictor of TES andTEW was the FAPT, which explained 16 and 15% of the

variance in TES and TEW, respectively. The SEE for therespective models was 35 Nm21 and 22 Nm21. The onlysignificant predictor of TFS and TFW in women was weight,

explaining 28 and 14% of the variance in TFS and TFW,respectively. The SEE for the respective models was 19Nm21 and 16 Nm21. For men, the only significant predictorof TES, TEW, TFS, and TFW was weight, which explained27, 35, 42, and 33% of the variance in TES, TEW, TFS, and

TFW, respectively. The SEE for the respective models was

40 Nm21, 28 N

m21, 20 N

m21, and 19 N

m21. Finally, age

was not a significant predictor of TES, TEW, TFS, and TFW

in either sex.

TABLE 2. Intraclass correlation coefficient and 95% confidence interval for isokinetictrunk extension strength, trunk extension work, trunk flexion strength, trunk flexionwork, plank to fatigue test, and the front abdominal power test over the 3 testingsessions (n = 8).

VariableIntraclass correlation coefficient

(95% confidence interval)

Trunk extension strength (Nm21) 0.93 (0.800.99)Trunk extension work (J) 0.95 (0.860.99)Trunk flexion strength (Nm21) 0.97 (0.900.99)Trunk flexion work (J) 0.98 (0.930.99)Plank to fatigue test (s) 0.85 (0.610.97)Front abdominal power test (m) 0.95 (0.830.99)

TABLE 3. Results of the Bland-Altman plot analyses and applicable reliability statistics (limits-of-agreement [LOA], SEM,and coefficient of variation [CV]) for isokinetic trunk extension strength (TES), trunk extension work (TEW), trunk flexionstrength (TFS), trunk flexion work (TFW), plank to fatigue test (PFT), and the front abdominal power test (FAPT) over the3 testing sessions (n = 8).

Testing session

TES (Nm21) TEW (J) TFS (Nm21) TFW (J) PFT (s) FAPT (m)

Statistic 1 vs. 2 2 vs. 3 1 vs. 2 2 vs. 3 1 vs. 2 2 vs. 3 1 vs. 2 2 vs. 3 1 vs. 2 2 vs. 3 1 vs. 2 2 vs. 3

Heteroscedastic? YES YES YES YES YES NO YES YES NO NO YES YESRandom error 33 28 36Systematic bias 5 16.5 5.9LOA ratio (%) 15 17 23 13 9 15 17 15 17SEM 12 10 13CV (%) 9 9 12 7 6 9 9 9 9



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The diagnostic tests indicated that these regression modelsprovided an acceptable description of the data because noviolations of the assumptions were observed. Only one outlieror unusual observation was identified during the regression

analysis for women with TFS serving as the dependentvariable. The observation was omitted from the analysis

because the standardized residual was .4, Cooks D valuewas .0.13, and the standardized DFBETA value was .0.36.

For the final models, the partialregression plots of the depen-

dent and predictor variablesindicated the linear model pro-

vided the best fit for the data.The Shapiro-Wilk test of nor-

mality indicated the residualswere normally distributed (p.0.05). Finally, the scatter plot ofthe standardized residual and

predicted values suggested ho-moscedasticity for all models.

The bootstrap bias-corrected95% CI and SE for R2 and theSEE are provided in Table 6.The results indicate the truevalue of the SE of R2 in thepopulation for the TES, TEW,

TFS,andTFWregression models

ranges from 0.11 to 0.12 forwomen and 0.16 to 0.19 formen. The true value ofR2 in thepopulation for the TES, TEW,

TFS, and TFW regression models likely falls between 0.001and 0.51 in women and 0.003 and 0.77 in men. Further, thetrue value of the SEof the SEEin the population for the TES,TEW, TFS, and TFW regression models ranges from 1.8 to3.1 Nm21 in women and 3.2 to 5.3 Nm21 in men. The true

value of the SEEin the population for the TES, TEW, TFS,and TFW regression models likely falls between 13.3 and41.6 Nm21 in women and 13.2 and 50.0 Nm21 in men.

TABLE 4. Mean 6 SD for front abdominal power test, isokinetic trunk extensionstrength, trunk extension work, trunk flexion strength, and trunk flexion work.

Sex Variable Mean 6 SD

Women (n = 31) Age (y) 19.0 6 1.4Weight (kg) 61.8 6 8.8Front abdominal power test (m) 0.87 6 0.2Trunk extension strength (Nm21) 120.8 6 37.5Trunk extension work (J) 75.2 6 23.8Trunk flexion strength (Nm21) 139.5 6 28.0Trunk flexion work (J) 91.1 6 17.0

Men (n = 19) Age (y) 19.0 6 0.8Weight (kg) 86.6 6 10.6*Front abdominal power test (m) 1.3 6 0.3*Trunk extension strength (Nm21) 267.5 6 45.0*Trunk extension work (J) 178.7 6 33.1*Trunk flexion strength (Nm21) 247.5 6 25.8*Trunk flexion work (J) 181.1 6 22.1*

*Significantly higher for men than women (p , 0.05).

TABLE 5. Pearson correlation coefficients between age, weight, front abdominal power test (FAPT), isokinetic trunkextension strength (TES), trunk extension work (TEW), trunk flexion strength (TFS), and trunk flexion work (TFW) for menand women (n = 50).

Sex Age Weight FAPT TES TEW TFS TFW

Women Age 1.00Weight 20.09 1.00FAPT 20.24 0.41* 1.00TES 20.05 0.10 0.40* 1.00TEW 20.10 0.11 0.39* 0.95* 1.00TFS 20.14 0.43* 0.33* 0.51* 0.48* 1.00

TFW 20.03 0.37* 0.27 0.50* 0.52* 0.76* 1.00Men Age 1.00

Weight 0.00 1.00FAPT 0.38 20.33 1.00TES 0.22 0.52* 20.12 1.00TEW 0.11 0.59* 20.13 0.94* 1.00TFS 0.17 0.65* 20.22 0.46* 0.58* 1.00TFW 0.16 0.58* 20.07 0.55* 20.13 0.89* 1.00

*Correlation is significantly higher than 0.




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DISCUSSION

The purpose of this study was to test the reliability of 2 new

field tests of core stability: PFT and FAPT. We alsoreexamined the reliability of isokinetic TES and TFS, which

are the current standard measures used to assess core stability.In conjunction, we tested the reliability of isokinetic TFWand

TEW. Afterwards, we examined the relationship among thevarious tests and then assessed if the field tests, along withselected anthropometric data, were predictive of isokineticTES, TEW, TFS, and TFW.

Reliability of Selected Measures of Core Stability

It is important to know the reliability or consistency of ameasure because it allows one to effectively interpret the

outcome of a training program. For example, is a change instrength, either from injury or rehabilitation, real or is it anartifact of the measurement error associated with the test used

to assess the change? We found TES, TFS, TFW, and the

FAPT were highly reliable during 3 testing sessions in a smallgroup of young men and women without any history of back,

hip, or leg injury. In contrast, TEW was only moderatelyreliable, whereas the PFT was not reliable. Therefore, mea-

sures of TES, TFS, TEW, and TFW using a dynamometerand the FAPT can be used during a single testing session toobtain reliable data. This conclusion is drawn from the

process we used.First, we used an analysis of variance with repeated mea-

sures to look for a learning effect in the measurement proto-

col. We found a small nonsignificant change in TES, TEW,TFS, TFW, and FAPT between the first and second and

second and third testing sessions. This finding indicates that

our measurement protocol limited possible learning effects forthese tests. There was, however, a significant increase in per-

formance on the PFT between the first and second testingsession of 18% but not between the second and third testingsession (change in performance of only 5%), which indicates

a learning effect in the measurement protocol between thefirst and second testing sessions. Based on these data, we do

not recommend using the P FT when only one testing sessionis used.

After looking for possible learning effects, we determined

the relative reliability of each measure by calculating itsintraclass correlation coefficient. The intraclass correlationcoefficients for TES, TFS, TEW, TFW, FAPT, and PFTwere

0.93, 0.95, 0.97, 0.98, 0.95, and 0.85, respectively. These datashow that all tests had excellent test-retest reliability.

Although the use of the intraclass correlation coefficient isrecommended as a measure of reliability, it cannot be theonlystatistical measure of reliabilitybecause it is affectedby sample

heterogeneity. Therefore, it is recommended that absolutemeasures of reliability, such as the SEM, coefficient ofvariation, and limits-of-agreement method be used in

conjunction with intraclass correlation coefficients to fullyassess the reliability of a test.

TABLE

6.

Regressionpre

dictionequationsformenandwomenwithisokinetictrunkextensionstrength(TE

S),trunkextensionwork(TEW),trunkfle

xionstrength

(TFS),ortrunkflexionwork(TFW)servingasthedependentvariableandpotentialpredictorvariablesofag

e,weight,andfrontabdominalpowertest(FAPT).The

p-value,R

2

forthemodel,SEofR2

fromthebootstrapdistribution,

bias-corrected95%confidenceinterval(CI)ofR2

fromthebootstrapdistribution,s

tandarderror

ofestimate(SEE)forthe

model,SEoftheSEEfromthebootstrap

distribution,andbias-corrected95%CIo

ftheSEEfromthebootstrapdistribution

areprovided.

Sex

N

Regressionpredictionequations

P

R2

SE

of

R2

R2

9

5%CI

SEE

SEofthe

SEE

SEE

95%CI

Women

31

TES=FAPT(m)77.4

88+53.4

10

0.0

3

0.1

6

0.1

2

0.0

070.4

3

35.0

3.1

30.441.6

31

T

EW

=FAPT(m)48.5

45+32.9

67

0.0

3

0.1

5

0.1

2

0.0

040.4

3

22.3

2.4

18.327.8

30

TFS=weight(kg)1.2

90+56.5

64

0.0

0

0.2

8

0.1

2

0.0

30.5

1

19.3

2.5

14.524.2

31

T

FW

=weight(kg)0.7

01+47.7

40

0.0

4

0.1

4

0.1

1

0.0

010.4

0

16.1

1.8

13.320.8

Men

19

TES=weight(kg)2.2

02+76.7

97

0.0

2

0.2

7

0.1

6

0.0

030.5

8

39.6

5.3

29.250.0

19

T

EW

=weight(kg)1.8

53+18.2

28

0.0

1

0.3

5

0.1

6

0.0

50.6

5

27.5

4.1

20.837.0

19

TFS=weight(kg)1.5

88+110.0

39

0.0

0

0.4

2

0.1

9

0.0

30.7

7

20.2

3.2

15.026.5

19

T

FW

=weight(kg)1.2

05+76.7

89

0.0

1

0.3

3

0.1

8

0.0

20.6

8

18.6

3.2

13.225.3



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The appropriate measure of absolute reliability depends onthe presence of heteroscedasticity in the data set. Hetero-

scedasticity is an important issue in sports medicine researchand practice because its presence indicates that the amount of

random error of a test increases as the score on the testimproves. If data are heteroscedastic, an appropriate measure

of absolute reliability is the coefficient of variation, whereasthe SEMis used with homoscedastic data (3). Bland-Altmanplots indicated that TES, TEW, TFW, and FAPT data wereheteroscedastic, whereas the PFT data were homoscedastic.

The TFS data were heteroscedastic between the first andsecond testing sessions but homoscedastic between second

and third testing sessions. The appropriate reliability statisticsare displayed in Table 3. Interpreting these statistics isstraightforward; for example, because the SEM for TFSbetween the first and second testing sessions was 12 N m21,the typical score for TFS will vary by 612 Nm21 on anygiven day. Similarly, because the coefficient of variation for

TES is 9% between the first and second testing sessions, 68%

of the difference between repeated TES tests lies within 9% ofthe mean of the data.

The limits-of-agreement method can also be used to assessthe absolute reliability of both homoscedastic and hetero-

scedastic data. The major advantage of using the limits-of-agreement method as a measure of absolute reliability is that itincludes 95% of the variability (or error) in the test, whereas

the SEMand coefficient of variation account for only 68% ofthe associated measurement error (3). For example, the

systematic bias and random error for the PFT between thefirst and second testing sessions was 16.5 and 28 seconds,respectively. This indicates the expected systematic bias is

positive, and the range of random error between 2 tests

is 628 seconds with 95% probability. Interpreting the limits-of-agreement method ratio is also straightforward. For

example, because the limits-of-agreement method ratiofor the FAPT between the second and third testing sessions

was 17%, the expected difference between any two testscaused by the measurement error will be no more than 617%in new individuals from the studied population (3).

It is important to note that, whereas the limits-of-agreement method is recommended by some statisticians(4), others suggest it is too stringent to help determine if

a change in an individuals score is real or an artifact ofmeasurement error (14). For this reason, we reported both

the SEMand coefficient of variation, which are less stringent

then limits of agreement method.We deemed that the test-retest measurement error for TES,

TEW, TFS, TFW, FAPT, and PFT would be acceptable for

use during a single testing session if: (a) the intraclasscorrelation coefficient was.0.85, (b) the limits-of-agreement

method random error and systematic bias were ,20 and 5%of the sample mean between the testing days, respectively, or

the limits-of-agreement method ratio was ,20% (dependingon the presence or absence of heteroscedasticity), and (c)SEMwas,10% of the sample mean between the testing days

or the coefficient of variation was ,10% (depending on thepresence or absence of heteroscedasticity). According to the

criteria, we found that TES, TFS, TFW, and the FAPT werehighly reliable across a span of the 3 testing sessions, which

indicates they can be used when only 1 testing session is used.For TEW, the limits-of-agreement method ratio and the

coefficient of variation between the first and second testingsessions were slightly higher than our established cutoffs butstill acceptable; hence, TEW is moderately reliable whenonly 1 testing session is used. The PFT did not meet the

established criteria for high reliability when only 1 testingsession is used. Future investigators may wish to modify the

test or measurement protocol to enhance data reliability.Our findings that isokinetic reciprocal concentric trunk

flexion and extension peak torque measurements made at

60 degrees per second are highly reliable using a dynamom-eter support the work of Karatas et al. (16), who used ameasurement protocol similar to ours. They found intraclass

correlation coefficients ranging from 0.89 to 0.95 for TFS and

0.80 to 0.92 for TES during 3 testing sessions, each separatedby 48 hours, in 15 healthy subjects. Our data also extend thefindings of Karatas et al. (16) by providing measures ofabsolute reliability, such as the SEM, coefficient of variation,and limits-of-agreement method. These measures providetangible values for measurement errors that one is likely tofind when assessing TES and TFS with an isokinetic

dynamometer, which enables one to better determine if achange in performance is real or simply an artifact of test

measurement error. The intraclass correlation coefficient, ameasure of relative reliability that is highly affected bysample heterogeneity, does not provide this pertinent

information.

Others have found tests of isokinetic trunk flexion andextension strength to be reliable and unreliable using different

protocols than our own (8,17,23). Collectively, the data fromthe various studies examining TES and TFS show that the

measurement protocol used in this study and by Karatas et al.(16) should be used to measure isokinetic trunk flexion andextension strength in healthy participants.

Our findings also indicate the FAPT is a highly reliable fieldtest of core stability. These data are consistent with thefindings from Cowley and Swensen (6), who showed that

FAPTwas highly reliable in 24 young women across 2 testingsessions separated by 24 hours. We modified the test by

Cowley and Swensen (6) by anchoring the subjects feet to

the ground and using an average of 6 trials as opposed to only3 trials. Cowley and Swensen (6) reported an intraclass

correlation coefficient for the FAPTof 0.95, aSEMof 0.24 m,and a random error using the limits-of-agreement method of0.68 m. These data agree with the findings from the present

study, where the intraclass correlation coefficient for theFAPT was 0.95, the coefficient of variation was 9% (0.14 m),

and the limits-of-agreement method ratio was 15% (0.24 m).Collectively, the data from both studies indicate that theFAPT is a highly reliable field test of core stability.




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Sex Differences in Selected Measures of Core Stability

The men had greater TES, TEW, TFS, and TFW than the

women, findings that support the literature (8,12). Men alsoperformed better on the FAPT. This finding has not beenpreviously reported. A likely explanation for the sex differ-

ences in the aforementioned variables is that young men have

greater fat-free or muscle mass than young women (20,33).Our reasoning is only speculative, given that we did notmeasure body composition in our subject cohort.

Correlation Analyses of Selected Measures of Core Stability

A matrix of Pearson correlation coefficients was generated toassess the direction and strength of association among

selected anthropometric variables and any measure of corestability with acceptable single-test reliability. Age was not

significantly correlated with weight, FAPT, TES, TEW, TFS,and TFS in men and women. This is not surprising, con-sidering our subjects were young, with no previous history of

low back pain or injury.

For men, there was a significant positive correlationbetween weight and TES, TEW, TFS, and TFW (R =0.520.65). These data are similar to findings from Mannionet al. (24), who showed weight is positively correlated with

isometric trunk extensor strength in men (R = 0.68). Incontrast, there was no relationship between the FAPT andTES, TEW, TFS, and TFW, suggesting that these tests mea-

sure different aspects of core stability in young men. Thisfinding confirmed expectations because each measure of corestability is highly specific, and measures of isokinetic trunk

strength are weakly correlated with timed sit-up field tests(12,19,21). Finally, the FAPT was not correlated with weight

in men.

For women, there was a significant positive correlationbetween weight and TFS and TFW (R = 0.43 and 0.37,respectively) but not TES and TEW (R = 0.10 and 0.11,respectively). Our TES data on women are inconsistent withfindings from Mannion et al. (24), who showed weight is

positively correlated with isometric trunk extensor strengthin women (age range, 1842 years; R= 0.64). There was alsoa significant positive relationship between the FAPT, TES,TEW, and TFS (R= 0.330.40) but not TFW (R= 0.27) inwomen. In contrast to the men, there was a significant posi-

tive correlation between the FAPT and weight in the youngwomen.

Combined, the correlation data indicate there is a differen-

tial relationship between weight and TES and TEW betweenthe sexes because weight is more related to TES and TEW

in men than women. Additionally, there is a differentialrelationship between the FAPT and TES, TEW, and TFSbetween the sexes because the FAPT is more related to TES,

TEW, and TFS in young women than men. Thus, therelationship between weight and isokinetic trunk perfor-mance is apparently stronger in young men than women,

whereas the relationship between the FAPT and isokinetictrunk performance seen in young women is absent in men.

Predictors of Isokinetic Trunk Flexion and Extension

Strength and Work

We sought to determine if the PFTand FAPTwere predictiveof isokinetic trunk flexion and extension strength and workcapacity in a group of young men and women. We were only

able to include 1 field test in our regression analyses because

thePFT wasnot reliable foruse during a single testing session.We used age, weight, and the FAPT to predict isokinetic trunkperformance in the hope that this type of prediction equationcould be used by sports medicine professionals working in the

field to adequately predict isokinetic trunk performance ina short amount of time with few pieces of equipment, therebyenabling them to make comparisons to thebody of normative

data for such measures in the literature.Results from the regression analyses were consistent with

the correlation analyses and indicated that in young men,weight was the only significant predictor of TES, TEW, TFS,and TFW, explaining 27, 35, 42, and 33% of the variance,

respectively. In young women, weight was the only significant

predictor of TFS and TFW, explaining 28 and 14% of thevariance, respectively, whereas the FAPT was the only

significant predictor of TES and TEW, explaining 16 and 15%of the variance, respectively. This latter finding was un-

expected because the primary body movement during theFAPTis trunk flexion, whereas TES and TEWprimarily stressthe trunk extensors. Given that the FAPT is a dynamic

movement in which the armsare extended overhead, perhapsthe test required more trunk extensor activity in the womenthan the men to help stabilize the body.

Overall, the ability to predict TES, TEW, TFS, and TFWfrom weight and the FAPT is less likely in young women than

men because the different regression models explained only

1428% of the variability in TES, TEW, TFS, and TFW foryoung women, compared to 3342% in young men. The pre-

dictive accuracy of our regression models ranged from 16 to35 Nm21 in women and 19 to 40 Nm21 in men. Of specialnote is that the FAPT was the only significant predictor of

TES and TEW in young women. This finding has practicalsignificance because the FAPT can be administered fairly

quickly to provide a field based estimation of TES and TEWin young women using our prediction equation.

To provide an estimate of the predictive ability and

accuracy of our sample regression models in the population atlarge, we used the nonparametric bootstrap method to gen-erate bias-corrected 95% CI and SEfor R2 and SEEfrom our

subject sample (31). The true value of the SE of R2

in thepopulation for the TES, TEW, TFS, and TFW regression

models ranged from 0.11 to 0.12 in women and 0.16 to 0.19 inmen. Hence, we are 95% confident the true value ofR2 in thepopulation for the TES, TEW, TFS, and TFW regression

models is somewhere between 0.001 and 0.51 in women and0.003 and 0.77 in men. Similarly, the true value of the SEofthe SEE in the population for the TES, TEW, TFS, andTFW regression models ranged from 1.8 to 3.1 N m21 forwomen and 3.2 to 5.3 Nm21 for men. Thus, we are 95%



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confident the true value ofSEEin the population for the TES,TEW, TFS, and TFW regression models is likely between

13.3 and 41.6 Nm21 in women and 13.2 to 50.0 Nm21 in men.Ultimately, to fully validate our regression models, additional

population samples (i.e., competitive athletes) must beexamined. Additional potential carriers that correlate with

isokinetic trunk performance could also be added to improvethe predictive ability and accuracy of the equations.

In conclusion, the major findings of this study are: (a) TES,TFS, TFW, and the FAPTare highly reliable, whereas TEWis

moderately reliable for use during a single testing session inyoung men and women, (b) the PFT is not reliable, and future

investigators may wish to develop better measurementprotocols for this test, (c) the FAPT was the only significantpredictor of TES and TEW in young women, (d) weight was

the only significant predictor of TFS and TFW in youngwomen and theonly significant predictor of TES, TEW, TFS,and TFW in young men, and (e) the ability of weight and the

FAPT to predict TES, TEW, TFS, and TFW was higher in

young men than women. Finally, because the FAPT requiresfew pieces of equipment, is fast to administer, and predictsisokinetic TES and TEW in young women, it may be used toprovide a field-based estimate of isokinetic TES and TEW in

young women without history of back or lower-extremityinjury.

PRACTICAL APPLICATIONS

There is a need for field tests that are reliable, require little or

no equipment, are fast to administer, and assess the variousaspects of core stability. In addition, it would be ideal if these

tests could accurately predict isokinetic trunk flexion and

extension strength, thereby allowing comparisons to the bodyof normative data. The FAPT is a reliable field test of core

stability that requires a 2-kg medicine ball and tape measure.Performing 6 trials of the FAPTduring a single testing sessioncan provide reliable data for this test. It is important, however,

to determine the reliability of this test in your laboratory orfacility before implementation. An example of the usefulness

of our reliability statistics follows: assume performance on theFAPTwas 1.5 m before the implementation of a core stabilitytraining program and improved to 1.8 m afterwards. Is the

0.3-m increase in performance an actual improvement or justan artifact of test measurement error? Given that the 0.3-mchange is greater than the expected error using the coefficient

of variation (0.14 m) and limits-of-agreement ratio (0.24 m),performance has likely increased. The results of the FAPTcan

also be used to predict isokinetic trunk extension strength andwork capacity in young women with no previous history ofback or lower-extremity injury. An example of the usefulness

of our regression equation follows: assume performance onthe FAPT for a young woman was 1.25 m. What is thepredicted isokinetic strength for this woman? Multiply the

FAPT distance by 77.488, and then add 53.41; the predictedisokinetic trunk extension strength for this woman is

150.3 Nm21. We are 95% confident the predicted value willbe 150 6 30 to 42 Nm21.

ACKNOWLEDGMENTS

The authors would like to express their appreciation to the

individuals who participated in this study.

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