Abstract
Background The construct validity and reliability of the short form of the Wolf Motor Function Test (S-WMFT) in people with subacute stroke and chronic stroke (S-WMFT subacute stroke and chronic stroke versions) have not been investigated.
Objective The purpose of this study was to investigate the dimensionality, item difficulty hierarchy, differential item functioning (DIF), and reliability of the S-WMFT subacute stroke and chronic stroke versions in people with mild to moderate upper-extremity (UE) dysfunction.
Design This was a secondary study in which data collected from randomized controlled trials were used.
Methods Data were collected at baseline from 97 people with chronic stroke (>12 months after stroke) and 75 people with subacute stroke (3–9 months after stroke) at 3 medical centers in Taiwan. Test structure, hierarchical properties, DIF, and reliability were assessed with Rasch analysis.
Results The test structure for both versions was unidimensional. No DIF relevant to sex, age, or stroke location (hemispheric laterality) was detected. The tasks of moving a hand to a box and moving a hand to a table in the S-WMFT for subacute stroke showed a significantly high correlation. The reliability coefficients for both versions were approximately .90.
Limitations The findings were limited to people with stroke and mild to moderate impairment of UE function.
Conclusions The S-WMFT subacute stroke and chronic stroke versions are useful tools for assessing UE function in different subgroups of people with stroke and show evidence of construct validity and reliability. A high correlation between the tasks of moving a hand to a box and moving a hand to a table in the S-WMFT for subacute stroke suggests that the removal of 1 of these 2 items is warranted.
More than 80% of people who have had a stroke have impaired function of the upper extremity (UE),1 and many do not regain full functional abilities of their arms and hands.2 An important issue in addressing these deficits after stroke and improving UE function after rehabilitation therapy is to identify reliable and valid outcome measures for examining UE motor function. The Wolf Motor Function Test (WMFT), which is used to measure the recovery of UE motor function after stroke, is such a test.3,4
The WMFT was originally designed to evaluate UE function in people with chronic stroke5 and was modified to examine the effects of constraint-induced movement therapy in the Extremity Constraint-Induced Therapy Evaluation (EXCITE) study.6,7 Researchers further extended its use to study the effects of electrical stimulation–assisted therapy8 and bilateral arm training.9
The current WMFT consists of 15 performance tasks and has a performance time scale for evaluating the time to complete a task and a 6-point functional ability scale (FAS) for evaluating the quality of UE function during a task. In previous research, classical test theory was extensively used to study the measurement properties of the WMFT. The findings indicated that the predictive and concurrent validity, reliability, and responsiveness of the performance time scale and the FAS were adequate to excellent in people after stroke.10–14
Rasch analysis has several potential advantages over classical test theory in assessing UE function when an ordinal scale is used. In classical test theory, a total score is the sum of the scores for all items on an ordinal scale. Because the true distances between the items and between the rating categories of the items are unknown,15 the simple summation of scores can lead to imprecise conclusions about differences between people as well as about change.16 To address this limitation, Rasch analysis has been increasingly used in rehabilitation research to create sound measures.
Rasch analysis transforms ordinal scores into interval data, which may yield more accurate estimates of test item information in parametric analyses. Also, Rasch analysis can explore the construct validity of a UE measure to determine whether the tasks reflect a single construct: the functional ability of the UE.17,18 The estimated hierarchy of item difficulty (from easy to difficult to perform) advances contemporary expectations of deficit, repair, and recovery after stroke. The targeting of item difficulty to UE motor ability specifies tasks that people can and cannot perform, quantifies motor impairment by locating an individual's ability to function along a continuum of UE motor function, and informs progress in the recovery of functional status.
Rasch analysis also enables the examination of differential item functioning (DIF). Differential item functioning is an indicator of biased items that have resulted in people from different subgroups within a population, but with similar UE motor function, having different responses to certain types of UE tasks. When DIF is present, task performance is affected by multiple factors that are not the focus of study, and the instrument may not be accurate for assessing UE motor function. Therefore, DIF analyses potentially can improve knowledge of whether observed differences in scores across groups represent a measurement problem, a true difference in UE function, or both, and can provide evidence to support the validity of the WMFT.19
Woodbury et al20 conducted a Rasch analysis to validate the theoretic basis of the WMFT FAS. Their findings suggested that all items in the WMFT measure a single latent trait, UE motor function, and that the item difficulty hierarchy is consistent with the original item difficulty expectations. However, researchers have not documented any findings in DIF analyses to indicate, after controlling for people's UE functional abilities, whether the relationships between item responses and UE motor function measured by the WMFT differ across groups with regard to demographic and stroke-related characteristics.
To improve the efficiency of administration, Bogard et al21 evaluated the relative contributions of the 15 WMFT timed performance tasks to the overall change in the WMFT total score in EXCITE study participants and proposed 2 short forms of the WMFT (S-WMFT), one for people with subacute stroke (3–9 months after stroke)17,18,22 and one for people with chronic stroke (12 months or longer after stroke). The 2 versions share 4 common tasks (moving a hand to a box, lifting a can, lifting a pencil, and folding a towel), and each version has 2 distinct tasks (extending elbow 28 cm on tabletop [0.4536-kg (1-lb) weight] and turning a key in a lock for the chronic stroke version and moving a hand to a table and reaching and retrieving for the subacute stroke version).
Bogard et al21 developed the S-WMFT to reflect differences in the progress of motor abilities in people at different stages of recovery after stroke.23 People show relatively fast recovery for performing simple arm movements early after stroke (eg, reaching) but slow recovery for performing complex movement tasks.24 At 1 year after stroke, people gain small improvements in movement skills and are aware of how these small changes may be related to functional gains in daily activities.25 These findings imply that various test items may be required to evaluate motor function in people at different stages of recovery.
In 1 investigation,26 the validity of the S-WMFT subacute stroke version was studied, but no research to date has documented the measurement properties of the S-WMFT chronic stroke version. The performance time scale study of the S-WMFT subacute stroke version revealed comparable responsiveness and concurrent validity but slightly higher predictive validity relative to those of the WMFT. These findings indicated the clinical utility of the S-WMFT subacute stroke version in outcome evaluation and promise for further validation. However, no investigations of the test hierarchy, DIF, or test structure of the S-WMFT subacute stroke or chronic stroke version have been performed.
Although previous studies investigated the measurement properties of the WMFT, the measurement properties of the S-WMFT may not be similar to those of the original scale. The removal of test items may jeopardize 1 or more important aspects of the performance of the original test and, as a result, the psychometric properties of a short form may be different from those of the original.27 Because the measurement properties of the WMFT may not be generalized to the S-WMFT, further study of the hierarchical properties of the S-WMFT in people with subacute or chronic stroke is warranted.
The goal of the present study was to extend the previous findings for the S-WMFT and fill the knowledge gap about the measurement properties of the S-WMFT. Because the 2 versions of the S-WMFT are targeted to people with stroke at different stages of recovery (subacute and chronic), we examined the psychometric properties of the 2 versions for their targeted populations, including the item difficulty hierarchy, test structure, targeting, and reliability of the S-WMFT FAS. In addition, we investigated DIF to understand whether the test structure is the same for people with different clinical and demographic characteristics. The Rasch rating model was used for the study.
Method
Participants
Ninety-seven people with stroke at the chronic stage were recruited to study the psychometric properties of the S-WMFT chronic stroke version, whereas data from 75 people with subacute stroke were used to investigate the measurement properties of the S-WMFT subacute stroke version. The sample size required to achieve stable item calibrations with dichotomous data ranges from 64 to 144 for an accuracy of ±0.5 logit at the 95% confidence interval; polytomous observations require a smaller sample size.28 Thus, the sample sizes in the present study (97 for people with chronic stroke and 75 for people with subacute stroke) were sufficient to achieve stable item calibrations. The inclusion criteria were first-ever stroke with onset between 3 and 9 months prior (subacute stroke group) or 12 months prior or longer (chronic stroke group), ability to understand the study and respond to questions (score of ≥22 on the Mini-Mental State Examination),29 demonstration of Brunnstrom stage III or higher for the proximal part of the affected arm,30 and no excessive spasticity at any joint of the arm (score of ≤2 on the Modified Ashworth Scale).31 Excluded were people with physician-determined major medical problems, such as poor physical condition.
Participants were enrolled in an ongoing randomized controlled trial investigating the effects of motor rehabilitation after stroke. All participants provided written informed consent.
Outcome Measure
The S-WMFT chronic stroke and subacute stroke versions have 6 items. They share 4 common tasks, and each has 2 distinct tasks. Table 1 shows the contents of the 2 versions of the S-WMFT. For the present study, we used only FAS item ratings and not item performance time. The FAS is a 6-point scale ranging from 0 (no use) to 5 (normal). Detailed descriptions of the FAS ratings are given in Table 2.
Item Statistics for the Chronic Stroke and Subacute Stroke Versions of the Streamlined Wolf Motor Function Testa
Functional Ability Scale
Procedure
Three certified and trained occupational therapists administered the WMFT to participants before and after the rehabilitation programs. The administration took an average of 20 minutes. Every participant was instructed to execute all of the test tasks with the affected arm and, if necessary, with help from the unaffected arm.
Data Analysis
Because the focus of the present study was the S-WMFT, we analyzed only the baseline scores for certain tasks. For the subacute stroke group, these tasks were moving a hand to a table, moving a hand to a box, reaching and retrieving, lifting a can, lifting a pencil, and folding a towel; for the chronic stroke group, these tasks were extending elbow 28 cm on tabletop (0.4536-kg weight), moving a hand to a box, lifting a can, lifting a pencil, turning a key in a lock, and folding a towel. We examined the appropriateness of rating categories (ie, rating scale diagnostics), test structure, item difficulty hierarchy, DIF, and reliability using Winsteps software (version 3.70.0.5).32
Rating scale diagnostics were examined to ensure that people with a lower level of UE motor function received predominantly lower ratings and that people with a higher level of UE motor function received predominantly higher ratings. The criteria were as follows33: there were at least 10 responses per rating category; the average participant's motor ability in each rating category increased as the rating value increased; and the outfit mean square, a type of goodness-of-fit statistic, of each rating category was less than 2. If a rating category failed to meet these criteria, then collapsing the rating category would be considered.
To assess construct validity, we investigated dimensionality, DIF, and item difficulty hierarchy. Rasch principal components analysis of the residuals was used to test against the hypothesis of unidimensionality. The variance in UE motor function explained by the S-WMFT was analyzed. Unidimensionality was supported when the variance explained by the first dimension exceeded 50% and an eigenvalue of the unexplained variance in the first residual factor was less than 2.32 Item difficulty and person UE function were simultaneously evaluated with Rasch analysis. We expected that less difficult tasks would be more likely to be accomplished by all participants than more difficult tasks and that participants with a low level of UE function would be more likely to do poorly on difficult items than participants with a high level of UE function.
The mean square and standardized z score fit statistics were used together to examine whether an item deviated significantly from the expectation of the Rasch model (misfit). Two types of unexpected ratings were summarized: responses close to the difficulty of an item (infit) and responses far from the difficulty of an item (outfit). On the basis of recommendations in the Winsteps software, an item with a mean square of greater than 1.5 and a standardized z score outside the range of −2.0 to 2.0 was considered to be misfit.32
The item-person map indicating the relationship between item difficulty and person UE motor function was examined. An ideal instrument is capable of targeting a wide range of people's UE motor function; that is, the mean person UE motor function should be relatively close to the mean item difficulty, and the item difficulty range should cover a substantial range of people's UE motor function.34 Differential item functioning analyses were conducted to examine whether responses to items were influenced by demographic or clinical characteristics after controlling for person UE motor function. The 3 factors chosen in the present study were sex, age (≥65 years old or <65 years old), and hemispheric laterality. The DIF contrast was the difference in item difficulty between 2 groups and should have been at least 0.5 logit to be noticeable.32 A DIF contrast of 0.5 or higher with a probability of less than .05 was considered to be significant.32
Finally, correlations between items and reliability were assessed. A correlation of greater than .90 indicated that 2 tasks were highly related. Reliability was examined with the index of person separation, person reliability, and the Cronbach alpha. Person separation specified the number of significant strata into which the samples of participants were divided by UE motor function. A person separation value of greater than 1.5 indicated that the S-WMFT could distinguish people into at least 2 groups with different levels of motor function.35 The Winsteps person reliability was algebraically distinct but conceptually similar to the traditional internal consistency that is usually measured with the Cronbach alpha.36 For clinical application, a value of 0.7 represented an acceptable level of reliability, a value of 0.8 represented a good level, and a value of 0.9 represented an excellent level.37
Role of the Funding Source
This project was supported, in part, by the National Health Research Institutes (NHRI-EX99-9920PI and NHRI-EX100-10010PI), the National Sciences Council (NSC 97-2314-B-002-008-MY3 and NSC 99-2314-B-182-014-MY3), and the Healthy Ageing Research Center at Chang Gung University (EMRPD1A0891) in Taiwan.
Results
The median ages of the participants in the chronic stroke and subacute stroke groups were 56.6 and 55.6 years, respectively. In the chronic stroke group, 64 of 97 participants were men, and the mean time since stroke onset was 20 months. In the subacute stroke group, 58 of 75 participants were men, and the mean time since stroke onset was 7 months. Table 3 shows the demographic and clinical characteristics of the participants.
Demographic and Clinical Characteristics of the Participantsa
Rating Scale Diagnostics
Except for the rating category “0,” the FAS met all of the criteria. The category “0” was given 9 times in the chronic stroke group and 7 times in the subacute stroke group. The subacute stroke group had disordered average person measures. Participants with subacute stroke and a rating of 0 on the FAS had –1.86 logits of motor ability across any item, but participants with a rating of 1 had −1.88 logits. Thus, we collapsed the original category “0” with the category “1.” Reanalysis showed that the new 5-point rating scale met all of the essential criteria and functioned properly. The recoded data were used in the subsequent analyses. (The revised rating categories are shown in Tab. 2.)
Dimensionality
Rasch principal components analysis supported the assumption of unidimensionality. We found that 72.4% and 70.8% of the variance could be explained by the Rasch dimension in the S-WMFT chronic stroke and subacute stroke versions, respectively. The eigenvalues of the first contrast residuals were less than 2 in both versions. Rasch indexes suggested adequate fit of all tasks (Tab. 1). We concluded that the S-WMFT chronic stroke and subacute stroke versions were unidimensional.
DIF
No significant DIF by sex, age, and hemiplegia laterality was detected in either version (P>.05). The item difficulty of each task for the women was not significantly different from that for the men. Every item had the same difficulty for the older (≥65 years old) and younger (<65 years old) groups of participants. The difficulty of individual items for participants with right-side lesions was equivalent to that for participants with left-side lesions.
Item Difficulty Hierarchy
For the S-WMFT chronic stroke version, the item difficulty hierarchy from low to high was moving a hand to a box, extending elbow 28 cm on tabletop (0.4536-kg weight), lifting a pencil, folding a towel, lifting a can, and turning a key in a lock (Fig. 1). The average UE function was −0.31 logit (SD=2.57). For the S-WMFT subacute stroke version, lifting a pencil was the most difficult task, folding a towel and lifting a can were next, and reaching and retrieving was the easiest task (Fig. 2). The average UE function was 0.86 logit (SD=2.36). These findings suggest that the S-WMFT subacute stroke version did not target UE motor function as well as the S-WMFT chronic stroke version did.
Item difficulty hierarchy of the streamlined Wolf Motor Function Test for participants with chronic stroke. The numbers at the left are logits. Plots of items along the center line are based on average difficulty. The most able people and the most difficult items are at the top, and vice versa. M=mean, S=standard deviation, T=2 standard deviations, X=participant's upper-extremity motor function.
Item difficulty hierarchy of the streamlined Wolf Motor Function Test for participants with subacute stroke. The numbers at the left are logits. Plots of items along the center line are based on average difficulty. The most able people and the most difficult items are at the top, and vice versa. M=mean, S=standard deviation, T=2 standard deviations, X=participant's upper-extremity motor function.
Item Correlations and Test Reliability
In the 2 versions of the S-WMFT, all but 1 pair of items had correlations between .37 and .74, and the correlation between the tasks of moving a hand to a table and moving a hand to a box in the S-WMFT subacute stroke version was .93. This correlation indicated that these items may measure similar contents of UE motor function. The investigation of residual correlations between pairs of items revealed that all but 1 pair of items had correlations of less than .30. The tasks of moving a hand to a table and moving a hand to a box in the S-WMFT subacute stroke version were highly locally dependent, with a residual correlation of .70. These 2 items shared more than 50% of their random variance, suggesting that only 1 was needed32 for the S-WMFT subacute stroke version.
The person separation values in the S-WMFT chronic stroke and subacute stroke versions were 3.19 and 2.78, the person reliability values were .91 and .89, and the Cronbach alpha values were .91 and .91, respectively. People with chronic stroke could be divided into 4.59 strata/groups, and people with subacute stroke could be divided into 4.04 strata/groups.35
Discussion
The present study is the first to examine the test structure, DIF, item difficulty hierarchy, item characteristics, and reliability of the S-WMFT FAS for people with chronic stroke and people with subacute stroke. The 2 versions of the S-WMFT were unidimensional, as is the original WMFT. The results of DIF analyses and the expected item difficulty hierarchy further supported the construct validity. Both versions had high reliability and were capable of differentiating our samples into 4 distinct groups on the basis of UE motor ability. Evidence from the present study indicated differences between the 2 versions in the difficulty of the 4 common items. When the mean person ability was compared with the mean item difficulty, the average item difficulty in the S-WMFT subacute stroke version was lower than that in the S-WMFT chronic stroke version. The S-WMFT subacute stroke version may have a redundant item.
The results of the present study supported the unidimensionality of both S-WMFT versions, implying that all of the items in the S-WMFT consistently measure UE motor function in people with stroke. In accordance with the findings of Woodbury et al,20 Rasch principal components analysis of residuals supported unidimensionality. We further validated the construct validity by investigating DIF in the S-WMFT chronic stroke and subacute stroke versions. No DIF relevant to sex, age, or hemiplegia laterality was detected; that is, the rating scores for individual tasks in the S-WMFT were determined by a participant's UE motor ability, not by other factors, such as sex, age, or hemiplegia laterality. These findings indicate that both S-WMFT versions may exclusively measure motor ability.
The Rasch analysis–derived item difficulty hierarchy depicted how the items in the S-WMFT chronic stroke and subacute stroke versions related to one another, and the findings were consistent with the motor control literature. Researchers have reported that increasing the amount of precision required for the action and the number of performance steps amplifies task difficulty. We found that the easiest items in both versions of the S-WMFT (eg, moving a hand to a box in the S-WMFT chronic stroke version and reaching and retrieving in the S-WMFT subacute stroke version) required the control of primarily 1 joint and involved 1-step actions, whereas the most difficult items required the control and coordination of multiple joints and involved multistep actions. The Rasch analysis–derived S-WMFT item difficulty hierarchy provided evidence of the construct validity of the 2 versions of the S-WMFT.
Among the 4 tasks common to the 2 S-WMFT versions, the most difficult task in the chronic stroke version was lifting a can; folding a towel, lifting a pencil, and moving a hand to a box were next. People in the chronic stage of stroke regain some fine motor ability (eg, pinching) but still exhibit spasticity or the late development of spasticity 1 year after stroke.38,39 Without sufficient recovery of volitional control of finger and thumb extension, lifting a can may be more difficult for people with chronic stroke than lifting a pencil or folding a towel. In the subacute stroke version, the item difficulty hierarchy of the 4 common tasks was lifting a pencil, folding a towel, lifting a can, and moving a hand to a box. Previous studies suggested that people need to regain voluntary control of wrist and finger extension at the early stage after stroke to exhibit improved dexterity.40,41 As a result, tasks related to dexterity, such as lifting a pencil and folding a towel, are considered to be more difficult than tasks related to voluntary control of wrist and finger extension, such as lifting a can.
The order of difficulty for lifting a pencil and lifting a can was reversed in the 2 S-WMFT versions. However, it was impossible to directly compare the results obtained with the 2 versions because the item difficulty was estimated for 2 different samples with different scales. The item difficulty estimates were sample dependent, and further transformations were required for comparisons. Future studies can make additional efforts to identify items with similar difficulty estimates across samples by use of DIF analysis, to anchor the “common items” in a separate calibration for each version and, finally, to compare the hierarchies for the groups.
The closeness of the average UE motor function and the average item difficulty suggested that both versions of the S-WMFT performed well in targeting UE motor ability in people with stroke. The gaps in the item hierarchies seemed to be large in Figures 1 and 2; adding more difficult items (eg, stacking checkers) and easier items (eg, moving a forearm to a table) might extend construct coverage. However, the gaps did not diminish the sensitivity of both versions of the S-WMFT to changes in UE motor ability. Because there were 5 points in the revised rating scale and 4 steps in the item calibrations, each point scored for an individual task could capture different levels of UE motor function in people with stroke. Items in both versions of the S-WMFT covered a substantial range of UE motor function. Sensitivity for detecting changes in ability provides useful information when clinical practitioners evaluate individual changes in UE motor function over time.
All items in the S-WMFT chronic stroke version showed biserial correlations higher than .7, contrary to the findings of Morris et al,10 who reported a low item-total correlation for extending elbow 28 cm on tabletop (0.4536-kg weight) in the EXCITE study. The difference may be a result of methodology. In the present study, we used Rasch analysis to transform the WMFT FAS scores from ordinal data to interval data, possibly yielding unbiased estimates to support the adequate fit of items in the S-WMFT chronic stroke version. In contrast, Morris et al10 used classical test theory, which does not have the described advantage.
An indication of the redundancy of items was found in the S-WMFT subacute stroke version. The tasks of moving a hand to a box and moving a hand to a table showed a significantly high correlation, indicating that these 2 items provide similar information about UE function. When either of the 2 tasks was removed from data analyses, the unidimensionality held, and the item difficulty hierarchy of the remaining 5 tasks remained the same. The person reliability and person separation values dropped slightly, from .89 to .88 and from 2.78 to 2.69, respectively. The removal of 1 of the 2 tasks did not seem to adversely affect the construct validity or reliability of the S-WMFT subacute stroke version; thus, the removal was warranted.
The S-WMFT chronic stroke and subacute stroke versions had the precision to distinguish people with different levels of UE function after stroke and exhibited high person separation and person reliability values.15 Although the person separation values of the 2 versions of the S-WMFT were slightly lower than that of the WMFT,20 the 2 versions could be used to divide our samples into 4 groups according to the level of UE function. These findings are consistent with those of a previous study of the S-WMFT in patients with subacute stroke,26 which found that the S-WMFT was sensitive and able to distinguish people.15,42
Study Limitations and Future Research
Of note, the rating “0” revealed a low frequency of use in the S-WMFT for people with subacute stroke (2%) and people with chronic stroke (1%), consistent with the results found for the WMFT.21 In the present study and the study of Woodbury et al,20 the participants were people with stroke and mild to moderate impairment of motor ability. Future research might include people with severe deficits in motor ability to examine the appropriateness of the rating “0.”
Other limitations might influence the generalizability of the results of the present study. First, future research with larger sample sizes is needed to validate our findings. Second, we did not conduct a longitudinal study to track the within-participant progress of motor ability after stroke. Additional research is needed to validate the findings for changes in the item difficulty hierarchy to understand the recovery process.
Conclusion
The S-WMFT chronic stroke and subacute stroke versions are unidimensional measurement scales that are efficiently administered and appropriately targeted for the assessment of UE motor function in people who have had a stroke. The S-WMFT subacute stroke version did not target the UE motor function of people with subacute stroke as well as the S-WMFT chronic stroke version did for people with chronic stroke. The S-WMFT subacute stroke version with 1 of 2 tasks (moving a hand to a box or moving a hand to a table) removed might still maintain similar Rasch analysis–derived validity and reliability. We conclude that the S-WMFT chronic stroke and subacute stroke versions are useful outcome measures for evaluating motor deficits during recovery or during a treatment course in people who have mild to moderate impairment after stroke and are at different stages of recovery.
Footnotes
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Dr Hui-fang Chen, Dr Wu, Dr Lin, and Dr Hsieh-ching Chen provided concept/idea/research design. Dr Hui-fang Chen, Dr Wu, and Dr Lin provided writing. Dr Wu, Dr Lin, and Dr Hsieh-ching Chen provided data collection. Dr Hui-fang Chen provided data analysis. Dr Wu and Dr Lin provided project management, fund procurement, and institutional liaisons. Dr C.P-C. Chen and Dr C. Chen provided participants and facilities/equipment. All authors provided consultation (including review of manuscript before submission).
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Institutional review board approval for this study was obtained from the study sites.
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This project was supported, in part, by the National Health Research Institutes (NHRI-EX99-9920PI and NHRI-EX100-10010PI), the National Sciences Council (NSC 97-2314-B-002-008-MY3 and NSC 99-2314-B-182-014-MY3), and the Healthy Ageing Research Center at Chang Gung University (EMRPD1A0891) in Taiwan.
- Received May 27, 2011.
- Accepted April 27, 2012.
- © 2012 American Physical Therapy Association
References
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