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Reliability of Ankle Isometric, Isotonic, and Isokinetic Strength and Power Testing in Older Women

  1. Sandra C. Webber and
  2. Michelle M. Porter
  1. S.C. Webber, MSc, BMR(PT), is a PhD candidate, Department of Physiology, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada.
  2. M.M. Porter, BPHE, MSc, PhD, is Professor, Faculty of Kinesiology and Recreation Management and the Department of Physiology, University of Manitoba, 207 Max Bell Centre, Winnipeg, Manitoba, Canada R3T 2N2.
  1. Address all correspondence to Dr Porter at: portermm{at}ms.umanitoba.ca.

Abstract

Background Ankle strength (force-generating capacity) and power (work produced per unit of time or product of strength and speed) capabilities influence physical function (eg, walking, balance) in older adults. Although strength and power parameters frequently are measured with dynamometers, few studies have examined the reliability of measurements of different types of contractions.

Objective The purpose of this study was to examine relative and absolute intrarater reliability of isometric, isotonic, and isokinetic ankle measures in older women.

Design This was a prospective, descriptive methodological study.

Methods The following dorsiflexion (DF) and plantar-flexion (PF) measures were assessed twice (7 days apart) by the same examiner in 30 older women (mean age=73.3 years, SD=4.7): isometric peak torque and rate of torque development (RTD), isotonic peak velocity, average acceleration and peak power, and isokinetic peak torque and peak power (30°/s and 90°/s). Several statistical methods were used to examine relative and absolute reliability.

Results Intraclass correlation coefficients (ICCs) for the DF tests (ICC=.76–.97) were generally higher than ICCs for matched PF tests (ICC=.58–.93). Measures of absolute reliability (eg, coefficient of variation of the typical error [CVTE]) also demonstrated more reliable values for DF tests (5%–18%) compared with PF tests (7%–37%). Isotonic peak velocity tests at minimal loads were associated with the lowest CVTE and ratio limits of agreement values for both DF (5% and 14%, respectively) and PF (7% and 18%, respectively). Isometric RTD variables were the least reliable (CVTE=16%–37%).

Limitations This study was limited to a relatively homogeneous sample of older women.

Conclusions Test-retest reliability was adequate for determining changes at the group level for all strength and power variables except isometric RTD. Minimal detectable change scores were determined to assist clinicians in assessing meaningful change over time in ankle strength and power measurements within individuals.

Loss of neuromuscular mass, strength (force-generating capacity), and power (work produced per unit of time or product of strength and speed) are closely associated with functional decline, loss of independence, and mortality in older adults.1 Some studies suggest that the rate of loss of neuromuscular power exceeds the rate of loss of strength with age.2,3 Cross-sectional studies also have demonstrated that functional capabilities such as the ability to get up and down from a chair, climb stairs, and walk quickly may be more closely associated with power than strength4–6 and that loss of power may be more related to the etiology of falls.7 In order to determine the true nature of the relationships among changes in strength and power with age and changes in function, reliable testing techniques are required.

Isokinetic dynamometers frequently are used to assess neuromuscular function because they provide detailed torque, velocity, and position data with high mechanical reliability.8 Although researchers have investigated the reliability of dynamometer strength assessment in older adults, the focus has largely been on the knee.9–14 However, the distal leg muscles exhibit reductions in strength and power with aging15,16 and are important for walking,17,18 maintaining balance, avoiding falls,7,19 and braking a vehicle. Greater limitations in ankle strength (as opposed to knee strength) have been associated with falls in nursing home residents.20,21 Dorsiflexion (DF) power has been found to be closely associated with function in community-dwelling older women in terms of their ability to get up and down from a chair and climb stairs.4 Plantar-flexion (PF) strength has been shown to be positively related to both habitual gait speed and fast gait speed.4,18 Despite the important role that ankle function plays in mobility, there have been only a few evaluations of the reliability of ankle strength protocols11,12,22 and only one assessment concerned with reliability associated with measures of ankle power in this population.11

Because power is defined as work (force × distance) divided by time, it is influenced by both strength and speed. Peak or average power (watts=newton-meters × radians/s) can be measured using either the isokinetic or isotonic mode on a dynamometer. Other indirect measures that may be associated with the ability of the neuromuscular system to generate force or torque rapidly can be evaluated using either the isometric mode (rate of torque development [RTD]) or the isotonic mode (velocity, acceleration).2,16,23,24 Hartmann et al11 reported reliability scores for ankle average isokinetic power tests, but reliability of isotonic and isometric measures related to power has not been investigated previously in older adults. Preliminary findings suggest that isokinetic evaluations of power may not be as reliable as strength measures9,11 and that RTD averaged over a specified range (eg, from 30% to 60% of peak torque) may yield more consistent results compared with peak RTD.24 Further research is needed to compare reliability of strength and power measures and to determine which measures are associated with lower levels of measurement error for use in research and clinical situations.

Physical therapists and other health care providers need to be able to properly interpret measurement change to determine the relative effectiveness of different interventions. Test-retest studies provide information about relative reliability, that is, the degree to which repeated measurements reveal consistent ranking of individuals' scores within a group. Measures of absolute reliability describe individual variability and measurement error and, therefore, are important in determining levels for clinically significant change. Although the literature suggests that power may be more important than strength in terms of function in older adults, very little is known about the reliability of different measures of power or power-related variables (eg, velocity during isotonic movements). The objective of this study was to determine the relative and absolute intrarater reliability for ankle strength and power measurements obtained using isometric, isotonic, and isokinetic tests on a dynamometer in older women.

Method

Participants

Thirty older women (mean age= 73.3 years, SD=4.7) were recruited to take part in this study. This sample size was consistent with those traditionally chosen for studying dynamometer measures in older adults.9–12,14,25 A convenience sample consisting of women who had expressed an interest or participated in previous research in our laboratory was used. However, none of the women had been tested previously on an isokinetic dynamometer or participated in an exercise-related study. Exclusion criteria included acute or unstable chronic disease and neurological or musculoskeletal impairment that would interfere with testing. In addition to the 30 women who participated, another 22 women were approached to be involved in the study but either did not meet the inclusion and exclusion criteria or were not interested in participating. Participant characteristics are presented in Table 1. A physician's signed Physical Activity Readiness Medical Examination (PARmed-X) form was required when potential contraindications to exercise (eg, history of cardiac disease, uncontrolled hypertension, hernia, detached retina) were identified. All testing took place in a university research laboratory over the summer of 2008. At the initial evaluation session, participants provided their written informed consent.

View this table:
Table 1.

Participant Characteristics

The median number of comorbidities reported by participants was 2 (range=0–3). Arthritis (18), hypertension (14), previous cancer diagnosis (5), and diabetes (4) were most commonly reported. Seven participants reported that they had fallen in the past year, and 1 participant regularly used a cane. The median number of medications prescribed was 1 (range=0–3). None of the participants changed their medications during their testing week.

Procedure

Upon admission to the study, participants completed a health/demographic questionnaire. All other tests were conducted twice, by the same physical therapist, exactly 1 week apart, at the same time of day. Information on the isokinetic dynamometer setup from session 1 was used for session 2, but the examiner was blinded to the results from session 1 until after session 2 was conducted. Resting blood pressure, heart rate, body mass, and height were measured using standard procedures. Active range of motion was measured for both DF and PF (right ankle) with the participant in a seated position with the knee supported in extension. Two measurements were taken in each direction and then averaged to determine active range of motion. A universal goniometer was used for the measurements, with the axis of the goniometer aligned with the lateral malleolus of the ankle, the proximal arm aligned with the head of the fibula, and the distal arm parallel to the lateral border of the fifth metatarsal. Ankle range of motion was measured to ensure that participants had adequate range of motion to tolerate the starting positions used for dynamometer testing. No participants were excluded because they lacked sufficient range of motion.

Dynamometer Tests

Dorsiflexion and PF torque, position, and velocity were measured using a Biodex System 3 Pro dynamometer.* The mechanical reliability of this dynamometer has been shown to be excellent.8 Calibration of the dynamometer was verified each day prior to testing. Participants warmed up by walking for 4 minutes on a treadmill before being seated on the Biodex dynamometer (right lateral malleolus aligned with the axis of rotation, right knee flexed 45°–55°, trunk reclined 5° from vertical). Only the dominant leg (defined as the leg that would be used to kick a ball) was tested. All participants reported right leg dominance. Each participant kept her arms folded across her chest during testing, and belts provided stabilization around the waist and over the right thigh. The end limits of range of motion were set at 10 degrees of DF and 30 degrees of PF for all tests. Participants performed isometric tests, concentric isotonic tests, and concentric isokinetic tests, always in that order, with DF contractions preceding PF contractions. Standardized, consistent verbal encouragement was provided for all tests using a script.

Isometric Tests

Following 3 practice trials, 3 maximal voluntary isometric contractions were performed for DF (at 25° of PF) and then for PF (at 0°). Test angles were chosen based on previous literature2,12,16,26 to correspond approximately to the angles at which maximum isometric torques can be produced. Participants were strongly encouraged to contract “fast” and to hold each contraction for 3 to 5 seconds. They were given 90 seconds of rest between trials.

Isotonic Tests

The dynamometer then was switched to the isotonic mode. The DF and PF movements were each performed against 2 set resistance levels: (1) a minimal resistance level (DF=1 N·m, PF=15 N·m) and (2) a load equal to 50% of isometric peak torque. These resistance levels represent the boundaries of those previously published for the dorsiflexors.2 Isotonic DF trials were initiated from 30 degrees of PF, and PF trials were initiated from 10 degrees of DF. Again, participants were strongly encouraged to move “fast.” Two practice trials preceded 5 test trials for each of the 4 conditions (DF and PF, 2 loads each). Thirty seconds of rest was provided between all repetitions.

Isokinetic Tests

Maximal-effort isokinetic concentric DF and PF tests were performed at 30°/s and 90°/s (in that order). These velocities were chosen because they are within the range of velocities typically studied for measurements about the ankle in older adults.4,25,27,28 In addition, restricting the highest velocity to 90°/s allowed for constant velocity to be maintained over approximately one third (12°) of the total 40-degree range of motion excursion (once acceleration and deceleration were accounted for). The passive mode on the dynamometer was used for these constant velocity tests because it provided a passive return to the start position after each concentric contraction; therefore, all concentric DF contractions were completed before PF testing began (ie, DF and PF contractions were not performed immediately back-to-back). Furthermore, many participants would not be able to generate enough DF torque to overcome the torque related to the combined mass of the foot and footplate to initiate DF movement. Matching concentric contractions with the onset of passive movement avoided this difficulty. Participants were given 3 to 5 submaximal practice trials for familiarization before 5 test trials were conducted for each movement, at each velocity. A 2-minute rest period was provided between velocities. One participant was unable to generate torque in the DF direction at the higher velocity; therefore, DF peak torque and peak power values were recorded at 90°/s for 29 participants.

Data Analysis

Biodex data were collected at a frequency of 100 Hz and exported for analyses in SigmaPlot (version 11.0).† All variables used in the analyses were means of the repetitions performed (isometric measures=mean of 3 repetitions, isotonic and isokinetic measures=mean of 5 repetitions). Because all scores inherently include some random error (which either adds to or subtracts from the true score), using mean scores may reduce the magnitude of the error component contributing to the total score.29

For each isometric contraction, peak torque (in newton-meters) was identified and RTD (in newton-meters per second) was calculated by 2 methods. Change in newton-meters/time was first determined from 0% to 50% of peak torque and then from 40% to 80% of peak torque (Fig. 1). Calculating RTD over a specified range has been shown to be more reliable than determining peak RTD.24 These specific ranges were chosen to allow comparison of RTD reliability between relatively steep sections of the isometric torque curve (0%–50% of peak torque) and less steep sections (40%–80% of peak torque).

Figure 1.
Figure 1.

Torque recorded during one repetition of isometric dorsiflexion for one representative participant. Rate of torque development was calculated as the change in newton-meters/change in time from 0% to 50% of peak torque and from 40% to 80% of peak torque. Torque is designated as negative in the dorsiflexion direction.

For each isotonic contraction, peak velocity (in degrees per second), average acceleration (peak velocity/time to reach peak velocity, measured in degrees per second squared), and peak power (watts= newton-meters × radians/s) were determined (Fig. 2). Although the dynamometer was set to the isotonic mode for these tests, torque is not held absolutely constant throughout the range of motion on this setting (Fig. 2). As has been noted previously,30,31 the sampling rate (100 Hz) of the dynamometer does not permit adjustments in speed to occur fast enough to result in a continuous torque level. For each isokinetic contraction, peak torque (in newton-meters) and peak power (watts=N·m × radians/s) were analyzed.

Figure 2.
Figure 2.

Torque, angle, velocity, and power measurements during one repetition of isotonic plantar flexion (against 50% of isometric peak torque) for one representative participant.

Statistical analyses were conducted using SPSS (version 15.0)‡ and SigmaPlot. Means and standard deviations were calculated for each variable tested at time 1 and time 2. Paired t tests were conducted to look for significant bias between test sessions (P<.05). The intraclass correlation coefficient (ICC [2,k]) was used to evaluate both systematic and random errors that may affect relative test-retest reliability.32 Specifically, ICC [2,3] was used for all isometric measures because they were based on the mean of 3 repetitions, and ICC [2,5] was used for all isokinetic and isotonic measures that were scored as the mean of 5 repetitions. Normality of the difference scores was assessed using the Shapiro-Wilk test. Data were checked visually with Bland-Altman plots for the presence of heteroscedasticity, and Pearson correlation coefficients were calculated between absolute differences and the means of the 2 tests.

Measures of absolute reliability were expressed using standard error of the measurement (SEM), coefficients of variation of the typical error (CVTE), limits of agreement (LOA), ratio limits of agreement (RLOA), and the minimal detectable change (MDC). Absolute reliability describes within-subject variation and the degree to which observed scores will vary with repeated measurements.33 Generally, CVTE and RLOA are used to describe heteroscedastic data, and SEM and LOA are used to describe homoscedastic data.34 The majority of the strength and power variables studied did not demonstrate heteroscedasticity (greater measurement error when measured values were larger) and, therefore, could be adequately described using SEM and LOA.34 However, because a few variables demonstrated a positive relationship between the degree of measurement error and the magnitude of the measured value, CVTE and RLOA statistics also are included.

The SEM was determined as the square root of the residual mean square error term from the analysis of variance table.35 The SEM describes (in units of the actual measure) the limits for change required to indicate a real increase or decrease for a group of individuals following some sort of intervention.25 Whereas SEM values express typical error in original units, CVTE expresses typical error as a percentage, making it useful for comparing reliability among different measures and across different studies. Typical error was calculated as the standard deviation of the differences scores between sessions, divided by Embedded Image.33 Coefficient of variation of the typical error is defined as typical error divided by the mean of all trials from both sessions, multiplied by 100.36 The LOA was calculated as the systematic bias: (mean difference between 2 test sessions) ± the random error component (1.96 × standard deviation of the difference between the 2 test sessions), which is identical to systematic bias ± MDC9534 (MDC95=1.96 × Embedded Image × SEM37,38). The MDC95 values provide information about the confidence limits associated with measurement error so that, for example, it can be stated with 95% confidence that an individual's change score that exceeds the LOA represents a true change. The MDC95 values also were expressed as a percentage in order to allow for comparisons among measures and across studies (RLOA=MDC95/mean of all observations × 100).

Role of the Funding Source

Ms Webber was supported by a Canadian Institutes of Health Research, Institute of Aging fellowship.

Results

Means and standard deviations for the isometric, isotonic, and isokinetic strength and power variables are presented in Table 2. There were no significant differences between session 1 and session 2 for almost all of the variables; however, PF isometric torque and RTD increased (P<.05). In addition, changes in DF isotonic average acceleration (1-N·m load) and PF isokinetic peak power at 30°/s were very close to being statistically significant (P=.05 and P=.06, respectively).

View this table:
Table 2.

Means and Standard Deviations for Isometric, Isotonic, and Isokinetic Tests

Table 3 reports the reliability data for all DF and PF tests. The ICC values for DF tests (ICC=.76–.97) were higher (signed rank test, P<.001) than ICC values for matched PF tests (ICC=.58–.93), with the exception of 2 isotonic values (peak power against minimal load and peak velocity against 50% of maximum isometric load). Measures of absolute reliability (CVTE) also demonstrated more reliable values for all DF tests (5%–18%) compared with PF tests (7%–37%), except for the same 2 isotonic measures (signed rank test, P<.001). Isotonic peak velocity tests at minimal loads were associated with the lowest CVTE and RLOA values for both DF (5% and 14%, respectively) and PF (7% and 18%, respectively). Isometric RTD0%–50% and RTD40%–80% demonstrated the highest levels of variability between test sessions for both DF (CVTE=16% and 18%, respectively, and RLOA=45% and 50%, respectively) and PF (CVTE=35% and 37%, respectively, and RLOA=97% and 104%, respectively). The MDC values, considered to be the minimal amount of change in an outcome measure that can be measured for an individual that is not due to systematic or chance variation in measurement,37,39 are included for all variables. Specifically, MDC95 values indicate that a person can be 95% confident in the true nature of changes that exceed these levels. The LOA were equal to the systematic bias ± the MDC95 value.

View this table:
Table 3.

Relative and Absolute Reliability Scores for Isometric, Isotonic, and Isokinetic Testsa

Bland-Altman plots (individual participant differences plotted against the mean for both test sessions) were created for all outcome variables to look for systematic bias, outliers, and the presence of heteroscedasticity. Pearson correlation coefficients were not significant for heteroscedasticity for 23 of the 26 tests, but isotonic PF peak velocity (against 50% of isometric peak torque), PF RTD0%–50%, and PF RTD40%–80% did demonstrate significant positive correlations (r=.45–.65, P≤.01). Reliability statistics associated with the RTD and peak isotonic velocity (against 50% of isometric peak torque) variables were relatively poor; therefore, other strength or power variables should be chosen in test-retest situations. No data transformations were conducted.

Discussion

This study was conducted to establish relative and absolute reliability scores for isometric, isotonic, and isokinetic strength- and power-related measures about the ankle in older women. Although the reliability of some of these measures (eg, isokinetic tests) has been investigated previously, other parameters (eg, isotonic values) have been reported infrequently in the literature with no associated reliability information provided. Results demonstrated that isometric, isotonic, and isokinetic measures of strength and power were associated with good relative reliability (all ICCs>.75, with the exception of PF RTD)29 and measures of absolute reliability were similar to previously published results involving both younger and older individuals.11,24,25,36

Virtually all isometric, isotonic, and isokinetic DF and PF measures demonstrated good relative reliability, indicating that these measures generally exhibited consistency for repeated measurements at the group level. With the exception of PF RTD0%–50% (ICC [2,k]=.63) and PF RTD40%–80% (ICC [2,k]=.58), all ICC values exceeded .75, and more than half of the values reached .90 or greater. The 95% confidence intervals associated with the ICCs (Tab. 3) provide a more thorough understanding of the reliability of these measurements. In the majority of cases (18/26), the lower confidence interval did not fall below 0.70; however, at the worst extreme, PF RTD measurements demonstrated lower confidence limits of 0.20 and 0.12, indicating very poor test-retest reliability.

In terms of strength, women in this study obtained DF and PF isokinetic peak torque values similar to those previously reported.11,12,16,40 The ICC values associated with isokinetic DF and PF peak torque and peak power (ICC=.85–.97) also were very similar to those reported in a previous study (ICC=.92–.98) of older women and men tested at 60°/s.11 The current study is the first to report reliability statistics associated with isometric and isotonic tests about the ankle in older women; therefore, no comparisons of these variables could be made.

Clinically, SEM values (expressed in absolute units) and CVTE values (expressed as a percentage) can be used to determine whether significant change has occurred in a group over time. The SEM and CVTE results were similar to those reported in other ankle strength and power studies for isokinetic parameters11,25,36 and isometric RTD.24 The CVTE values for DF and PF isometric peak torque were relatively low in this study (6% and 12%, respectively), whereas CVTE results were slightly higher for isokinetic peak torque and peak power results, ranging from 9% to 14%.

Clinicians can use MDC95 and RLOA values to determine whether true change has occurred over time in individual patients. Based on our results, changes in isokinetic peak torques in individual patients would need to exceed the following thresholds to exceed measurement error: 4.2 N·m (DF=30°/s), 3.2 N·m (DF=90°/s), 24.2 N·m (PF=30°/s), and 24.7 N·m (PF=90°/s). Isometric peak torques would need to exceed 3.5 N·m (DF) and 25.4 N·m (PF). The MDC95, LOA (systematic bias ± MDC95), and RLOA results in this study were similar to those previously reported.11,12,25 Small differences among study results may be attributed to differences in the participants (eg, sex, age), the raters, or the test protocol itself (eg, test velocities, measurement of peak versus average power, and participant positioning).

The older women in our study reached peak isotonic velocities of 275°/s for PF and 161°/s for DF when testing was conducted against minimal loads. Only one previous study has reported peak isotonic velocities about the ankle in older adults.2 The current study adds to the literature by providing detailed information about relative and absolute reliability associated with different isotonic parameters that have been measured infrequently to date. In the present study, isotonic peak velocity and average acceleration were associated with low CVTE values when the load was minimal (CVTE for DF=5% and 6% for peak velocity and average acceleration, respectively, CVTE for PF=7% and 10%, respectively) and slightly higher CVTE values (DF=13% and 11%, respectively, and PF=12% and 15%, respectively) when the load was equal to 50% of isometric peak torque. Isotonic peak velocity measured against low loads was associated with less variation compared with other isotonic and isokinetic variables. Further research involving other joint movements and different populations is needed to determine whether peak velocity is consistently more reliable than other more traditionally measured parameters. This information may be important clinically, as the isotonic setting allows for evaluation of contractions in which velocity is not constrained, and results, therefore, may be more functionally relevant compared with isokinetic tests.

In all but 2 instances (isotonic peak power against minimal load and isotonic peak velocity against 50% of maximum isometric load), DF scores demonstrated better reliability compared with PF scores. This result is in agreement with the findings reported by Hartmann et al.11 In both studies, participants were positioned with the knee flexed for PF tests. Although the upper body and thigh were well stabilized with straps, it is conceivable that attempts to extend the knee or hip may have occurred during PF movements, adding variability to these PF measurements that did not occur with DF movements. Reliability of ankle PF measures may be improved with different positioning during testing (eg, hip in neutral and knee extended with the individual in a prone position).

It has been suggested that from a functional perspective, increases in RTD may represent one of the most important adaptations that occurs in response to resistance training in older adults.41 That is, the ability to generate moderate forces quickly may be more important than being able to generate high forces, especially when quick action is required (eg, to regain balance and avoid a fall). Improvements in RTD are likely associated with a greater capacity to generate power. Although RTD may be a functionally important variable, our study demonstrated that it had the lowest absolute reliability of all the power-related variables studied about the ankle. The PF results were especially variable. Positioning used in this study (sitting with the hip and knee partially flexed) and the longer duration associated with isometric testing (3 seconds) likely contributed to some of this variability (greater potential contributions of hip or knee extension accompanying isometric PF attempts). It should be noted that isokinetic dynamometers may not be as reliable for isometric tests as other devices that are inherently more stable (eg, custom-made isometric rigs). It is recommended that the reliability of measurements of RTD about the ankle be examined in future studies using different joint and body positions and possibly using different types of strength testing equipment.

In this study, a familiarization session on the dynamometer was not provided before the 2 test sessions. This lack of a familiarization session may represent a limitation of the study if learning had an effect on the scores during the second testing session. However, familiarization sessions are rarely provided in clinical situations and may not always be feasible in research circumstances because of time constraints, associated costs, and availability of equipment. For these reasons, we elected to omit a familiarization session. Measured levels of systematic bias were minimal for most variables, indicating that there was no substantial learning effect.

This study involved a relatively homogeneous sample of community-dwelling older women. Future studies should continue to investigate the reliability of strength and power measurements attained using different modes on the dynamometer about different joints and in other segments of the older population (eg, older men, individuals who are more frail).

Conclusions

Interpreting and setting threshold levels for acceptable reliability results depends on the particular testing circumstance.29 In this study, many variables demonstrated good ICC results and CVTE values in the range of 6% to 13%, which are comparable to previous strength and power assessments in younger and older people.9,10,25,36 These levels are likely adequate to determine gross changes in strength- and power-related parameters among groups over the course of a training period; but ideally, more reliable measures would provide greater confidence in interpreting clinically meaningful change within individuals. Further research is needed to examine the reliability of isotonic variables that have been studied infrequently using dynamometers. These measures may prove to be more reliable and relevant to function in older adults than the more commonly reported isometric and isokinetic strength and power parameters. In the meantime, MDC95 scores have been presented for all DF and PF isometric, isotonic, and isokinetic variables to provide meaningful thresholds for clinicians and researchers to identify changes in individuals beyond those expected by measurement error.

Footnotes

  • Both authors provided concept/idea/research design, data collection, and project management. Ms Webber provided writing and data analysis. Dr Porter provided fund procurement, facilities/equipment, and consultation (including review of manuscript before submission).

  • Ethical approval for this study was granted by the Education/Nursing Research Ethics Board of the University of Manitoba.

  • Some of the results specific to isotonic tests were presented orally at the Canadian Society for Exercise Physiology meeting; November 11–14, 2009; Vancouver, British Columbia, Canada.

  • Ms Webber was supported by a Canadian Institutes of Health Research, Institute of Aging fellowship.

  • * Biodex Medical Systems Inc, 20 Ramsey Rd, Shirley, NY 11967.

  • † Systat Software Inc, 1735 Technology Dr, Ste 430, San Jose, CA 95110.

  • ‡ SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.

  • Received November 30, 2009.
  • Accepted April 10, 2010.

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