Comparison of upper-extremity balance tasks and force platform testing in persons with hemiparesis.

Key Words: Balance, Force platform, Function, Hemiparesis, Measurement.

The manifestations of stroke are variable and can consist of motor deficits, sensory disturbances, perceptual impairments, functional limitations, and balance difficulties.[1] Normal standing balance has been defined as the ability to maintain the body's center of gravity over its base of support with limited postural sway.[2,3] Both anticipatory and compensatory postural adjustments are required for normal standing balance.[4] Anticipatory postural movements that occur prior to self-generated perturbations of balance or in preparation for external perturbations of balance are controlled by feedforward mechanisms.[1,5-7] Compensatory adjustments that accompany the disturbances of equilibrium rely on visual, somatosensory, and vestibular feedback mechanisms.[1,4-7] In addition, maintenance of an upright posture is dependent on coordinated motor activity.[2,6,8] Communication between sensory system inputs and motor system outputs is believed to be required for normal standing balance.[7]

In an attempt to measure standing balance, a large number of evaluation tools have been developed. Tools that assess an individual s ability to maintain balance during self-generated or external perturbations are thought by some researchers [9,10] to be more useful because control of the body during movement is the essence of practical standing balance. The quantification of standing balance in persons with stroke has taken several forms, including measuring performances on self-generated balance tasks[9,10] and on force platform units capable of producing balance-threatening external perturbations.[11]

The Functional Reach Test[9] (FRT) and the arm raise and arm reach tasks described by Goldie et al[10] are examples of upper-extremity balance tasks involving self-generated perturbations of movement using the nonparetic limb. These upper-extremity tasks are based on current concepts of balance control. Numerous studies involving individuals without balance impairments have demonstrated that anticipatory postural adjustments occur prior to balance-disturbing voluntary upper-extremity movements during standing.[12-19] This anticipatory postural activity is hypothesized to keep the body s center of gravity within its base of support by counterbalancing upcoming perturbations generated by voluntary arm movements.[9,12-15] For example, the backward and downward acceleration of the body s center of gravity during forward flexion of the arm in standing is compensated for by anticipatory postural adjustments that precede the prime mover by 40 to 60 milliseconds and that produce an upward and forward acceleration.[20,21]

Horak et al[22] found that subjects with hemiparesis were unable to raise their nonparetic arm as rapidly as subjects without hemiparesis because of the greater latency of anticipatory postural activity on the contralateral hemiparetic side of the body relative to deltoid prime mover activity. Individuals with hemiparesis lacked vital postural stabilization on the paretic side.[22] Voluntary upper-extremity movements in standing, therefore, appear to require normal anticipatory, as well as compensatory, postural adjustments.[9] The FRT attempts to measure a person's ability to adapt to a single self-generated arm movement and to adjust to a self-imposed forward displacement of the center of mass.[9] In contrast, the arm raise and arm reach tasks described by Goldie et al[10] assess the ability to perform repeated self-paced movements using the nonparetic upper extremity during timed trials.

Many commercial units are available for the evaluation of standing balance. The Balance System[TM](*) is a force platform used to measure balance in response to externally imposed perturbations. According to the manufacturer,[11] this device can be used to measure a person's center of balance, weight distribution, and postural sway in response to linear or angular platform displacements during a timed period. Many investigators have examined postural responses associated with force platform perturbations in subjects without known neurological impairment[23-27] and in subjects with hemiparesis.[8,25,27] There is, however, still a lack of a documented relationship between self-generated upper-extremity balance tasks and force platform measures that depend on responses to external perturbations.

The influence of age on balance is well described.[9,28-35] Many researchers[28,31-33] have reported age-associated increases in postural sway. In addition, slower arm movements[34,35] and age-related declines in anticipatory postural activity for self-paced[34] and reaction-time arm movements[35] have been observed in elderly individuals without known neurological impairment. The FRT has also been shown to be age sensitive[9,29,30] as well as positively influenced by height.[9,30] We believe that researchers investigating the relationships between such measures as postural sway, functional reach, and balance-disturbing arm movements should consider the influences of age and height.

The objective of our study was to determine whether, in individuals with hemiparesis, performances of self-generated upper-extremity balance tasks are correlated with responses to external perturbations on a force platform. Upper-extremity balance tasks are inexpensive, practical, and clinically accessible tests.[9,10] In contrast, the Balance System[TM] is a more complex and quantitative balance tool, which is not readily available in all clinical settings. In the event that a strong correlation is found, performance on one test may be used to reliably predict performance on the other test. Because of the large number of standing balance tests available to physical therapists, we contend that relationships among these various balance tools must be investigated.

Similar postural strategies are believed to be used for the maintenance of balance in expectation of self-generated upper-extremity movements and in response to externally imposed perturbations.[6,19,22] Better balance is associated with a lower amount of postural sway (improved stability),[2,28,36-39] greater postural symmetry,[2,26,40,41] and improved functional skills.[42-45] We therefore hypothesized that (1) a negative correlation would exist between self-generated upper-extremity balance tasks and postural sway in response to external perturbations on the Balance System[TM], (2) a positive correlation would exist between self-generated upper-extremity balance tasks and a measure of symmetry of weight distribution in response to external perturbations on the Balance System[TM], and (3) a positive correlation would exist among various self-generated upper-extremity balance tasks.

Method

Subjects

A sample of convenience, consisting of 20 subjects with hemiparesis, was recruited from The Ohio State University Hospitals (Columbus, Ohio). Criteria for inclusion into the study were that the subjects (1) be within 12 months of their first and only unilateral cerebrovascular accident, (2) be able to follow simple commands, (3) report no orthopedic, vestibular, or additional neurologic disease, (4) be able to stand for at least 60 seconds without assistance, (5) report no diplopia or visual-field defects, (6) have normal passive range of motion of the nonparetic upper extremity and both lower extremities, as examined by the researcher, and (7) be pain free in areas that may limit their ability to reach with the nonparetic arm, stand, and balance. To evaluate these criteria, each subject completed a questionnaire and a single researcher physically examined each participant. The subjects ranged in age from 35 to 79 years ([bar]X=57.9, SD= 13.6), and all subjects were within 28 weeks of their cerebrovascular accident at the time of testing ([bar]X=8.8, SD=6.8) (Tab. 1). Nine subjects ambulated without the use of an assistive device, and 11 subjects required the use of a straight cane, quad cane, or walker. In this latter group, 5 subjects required the additional support of an ankle-foot orthosis (AFO) or an elastic wrap during ambulation to maintain ankle dorsiflexion on the paretic side. In accordance with university policy and to protect the rights of the subjects, informed consent was received from all subjects prior to participation in this study.

Table 1.
Subject Demographics

               No. of
               Subjects    [bar]X    SD       Range

Hemiparesis
  Left          12
  Right          8
Gender
  Male          12
  Female         8

Age (y)                    57.9      13.6       35-79
Height (m)                  1.71      0.09    1.52-1.85

No of weeks                 8.8       6.8        2-28
  after stroke

Instrumentation

The FRT, developed by Duncan et al,[9] was one of the self-generated upper-extremity balance tasks used in this study. The subject assumes a comfortable stance and stands perpendicular to a wall where a yardstick is horizontally affixed at the level of the subject's acromion. With a fisted hand, the subject extends an arm along the yardstick. The position of the third metacarpal is recorded prior to and after a maximum forward reach along the yardstick. Scoring consists of the mean difference (in centimeters) between the two measurements for three successful trials of reaching.[9] This balance measure yields measurements that are both reliable (K Amis, W Lockridge, unpublished research, 1995)[9] and valid,[9,29,46] and it has been used extensively with various patient populations.[47-51] In a sample of 16 persons with cerebrovascular accidents between the ages of 37 and 87 years, Amis and Lockridge (unpublished research, 1995) found same-day functional reach measures to have intrarater reliability with an intraclass correlation coefficient (ICC[1,1]) value of .92.

The arm raise and arm reach tasks described by Goldie et al[10] require the use of a stopwatch, a tape measure, masking tape, and an adjustable standard walker. The stopwatch is needed to time the trials. The tape measure is needed to form a floor grid, demarcated by masking tape, for the alignment of both feet in the stance positions. In addition, the masking tape is used as targets on a rail, wall, or standard walker. These self-generated upper-extremity balance tasks consist of the maximum number of arm raises or arm reaches on the nonparetic side during 60-second periods. Each task is performed twice in both parallel and step stance positions. Scoring consists of the mean frequency of response of both trials for each stance position.[10] These balance tests also yield reliable[10,52] and valid[52] measurements of postural control in standing in individuals with hemiparesis.

The force platform testing was performed using the Balance System[TM]. This device consists of a platform that is supposed to be capable of angular rotation and linear translation. Four independent electronic pressure transducers located in two footplates are used for measurement, and an adjustable safety harness and adaptable grab bars provide safety for this computer-driven device.[11] The Balance System[TM], according to its manufacturer,[11] is capable of computing a person's center of balance, percentage of weight distribution, and amount (expressed as a percentage) of postural sway (sway index). The center of balance, or the location of the person's center of gravity in relation to the supporting area, represents the percentage of change in body-weight distribution away from the geometric center of balance. This information is presented on paper as x and y coordinates (COBx and COBy), and it is computed from the vertical distribution of force over the footplates. The system is also supposed to be able to measure the percentage of weight on the toes and the heel of the person's left foot or right foot.[11] These data can be used to determine the symmetry of weight distribution for the duration of each trial. The sway index refers to the amount of postural sway relative to the mean center of balance during a timed trial. Postural sway is computed using the standard deviations of the coordinates during the timed trial, and it is calculated by the mean square method. A high sway index is supposed to be related to a large amount of sway and diminished balance, whereas a low sway index is said to occur when there is little postural sway and better balance,[11] but evidence for these claims has not been published in peer-reviewed form.

At 100% of its capacity, the Balance System[TM] is supposed to be able to produce linear translations consisting of forward and backward displacements of a 1.9-cm (3/4-in) distance from the center position that occur at a speed of 2.54 cm (1 in)/0.8 s.[11] In contrast, the angular rotations are supposed to cause displacements of 4 degrees in the toes-up and toes-down positions at a speed of 2[degrees]/s. All descriptions of the unit's performance characteristics are based on the manufacturer's information,[11] as these performance characteristics were not verified in our study. The reliability and validity of measurements obtained with the Balance System[TM] have been discussed previously.[53-55]

There is moderate to high reliability for measurements of postural sway in individuals with hemiparesis in standing on stable and moving platforms (ICC[1,1]=.58 and .74, respectively).[53] Similarly, Levine et al[54] reported ICCs (3,1) of .75, .65, and .80, respectively, for postural sway for a static platform, linear translation, and angular rotation conditions in subjects with hemiplegia standing on the Balance System[TM]. In addition, the reliability of measurements of the percentage of change in body-weight distribution in a sitting position on this unit was very strong for subjects with no known balance impairment (ICC[2,1]=.86-.96) and moderate to very high for subjects with hemiparesis for stable and leaning conditions (ICC[2,1]=.30-.75 and .53-.95, respectively).[55] Although the Balance System[TM] is only poorly to moderately correlated with the Clinical Test of Sensory Interaction in Balance (CTSIB)[53] and the Functional Independence Measure (FIM),[55] it demonstrates greater discriminative validity than the CTSIB for postural sway in a standing position when comparing performances of subjects with no known balance impairment and subjects with hemiparesis.[53] The Balance System[TM] has been described elsewhere in great detail.[56]

Testing

All measurements were obtained by one researcher during a single session. The tests were administered in random order. Testing was conducted in a quiet and nondistracting environment.

For the purpose of our study, functional reach testing was performed with each subject in a standardized parallel stance position (feet 10 cm apart). This modification was made to ensure consistency of stance width with the tasks described by Goldie et al.[10] The subject stood perpendicular to a wall where a yardstick was horizontally adjusted at the level of the acromion on the nonparetic side.[9] Good postural alignment, without excessive shoulder protraction or retraction, was ensured prior to each reach. The researcher observed the subject's scapula with the subject in a standing position and repositioned it into proper alignment if necessary. The subject was instructed to reach as far forward (as possible) without losing ... balance or taking a step and without touching the wall.[9] The trial was otherwise thrown out and repeated.[9] The reaching technique was not controlled, and, to ensure safety, the subject was supervised by the researcher.[9] There were no practice trials. The mean functional reach measurement was calculated for three successful trials.[9]

Measurements for the arm raise and arm reach tasks were obtained using the methods described by Goldie et al.[10] Each task was performed twice, first in the parallel stance position (feet 10 cm apart) and then in the step stance position (paretic leg forward, heel to toe, feet 10 cm apart). The subject stood perpendicular to a wall, with the medial border of the inner nonparetic foot 25 cm from the wall. A standardized floor grid, demarcated by masking tape, was used for all subjects. The researcher counted aloud during the 60-second trials and monitored the subject for safety. The paretic foot was stabilized at the heel by the researcher s own foot during step stance testing. In the event that the stance position was not maintained, the trial was eliminated and repeated immediately.[10] There were no practice trials.

Targets made of masking tape delineated the starting and end positions of each movement used in the tasks described by Goldie et al.[10] During the arm raise task, the subject lifted and lowered the nonparetic arm between two targets on the wall as frequently as possible during 60 seconds. The lower target was identified by asking the subject to raise the arm forward, without leaning at the trunk, to a point on the wall at hip level. The upper target was placed on the wall at the subject s shoulder level directly above the lower target. The mean frequency of response for both trials was calculated for each stance position. The arm reach task required the use of the testing arrangement displayed in the Figure. The subject touched a side target on the wall and reached for a front target on a standard walker with the nonparetic upper extremity as many times as possible during a timed 60-second period. The subject was instructed to visually follow the reaching maneuver during testing. The mean frequency of response for both trials was also computed for each stance position for the arm reach task.[10]

Each subject also underwent testing with eyes open on the Balance System[TM]. The unit was calibrated prior to each testing session. The same standardized grid was reproduced on the system's platform, and the subject was tested first in the parallel stance position and then in the step stance position. The subject was tested under three different conditions for each stance position: stable platform, linear translation, and angular perturbation. To become familiar with the Balance System[TM], the subject was always tested first on the stable condition. This condition consisted of standing on the stationary platform for the timed duration. Randomization of the perturbations was conducted to prevent any potential effects of order. Testing was carried out at 50% of the unit's capacity because full-capacity testing was thought to be too challenging for our subjects. The linear translations, therefore, involved forward and backward displacements of a 1.9-cm (3/4-in) distance from the center position, which occurred at a speed of 1.27 cm (1/2 in)/0.8 s.[11] Angular rotations consisted of displacements of 4 degrees in the toes-up and toes-down positions, which occurred at a speed of 1 [degrees]/s.[11] Prior to each trial, the subject was warned of the upcoming condition yet was unaware of the initial platform direction (eg, forward versus backward). The subject was told that the platform would move forward and backward or tilt up and down. A single trial lasting 10 seconds was performed for each condition. In the event that the subject was unable to complete the first trial, a repeat trial was performed. The subject was eliminated from the study, however, if the second attempt was unsuccessful. The Balance System[TM] computed the subject's percentage of weight distribution on the paretic side and amount of postural sway (expressed as a percentage) for the duration of each testing condition.[11]

Data Analysis

Pearson product-moment correlation coefficients were used to investigate the relationships between the numerous variables (maximum functional reach, frequency of arm raise and arm reach response, postural sway, and symmetry of weight distribution). Scores for symmetry of weight distribution were calculated by taking the absolute values of the percentage of weight on the paretic side as computed by the Balance System[TM] minus 50%. We considered these absolute scores to be indicative of postural symmetry and not representative of whether too much or too little weight was shifted onto the paretic limb for various platform conditions. The lower the score, the more symmetrically the subject stood on the Balance System[TM]. Height and age were taken into account as possible confounding variables using partial correlation coefficients. The level of significance was set a priori at an alpha level of .05. Correlations were deemed to be low if below .40, moderate if between .40 and .65, and strong if above .65.

Results

A total of 21 subjects agreed to participate in this study. Twenty subjects were able to successfully perform the tests in the study's protocol. Only one subject, a woman, had to be eliminated from the study because she was unable to complete a testing condition on the Balance System[TM] on her second attempt. Means and standard deviations for all variables obtained are shown in Tables 2 and 3.

Table 2.
Means and Standard Deviations for the Upper-Extremity
Tasks (N=20)

          FRT(a)   Arm Raise (repetitions/min)
          (cm)     PS(b)                SS(c)

[bar]X    25.05    52.25                56.23
SD         8.37    15.56                15.66

          Arm Reach (repetitions/min)
          PS                   SS

[bar]X    37.53              38.55
SD         8.23               9.45

(a) FRT = Functional Reach Test.[9]
(b) PS=parallel stance: feet 10 cm apart.
(c) SS=step stance: paretic leg forward, heel to
toe, feet 10 cm apart.
Table 3.
Means and Standard Deviations far the Balance System[TM]
for All Force Platform Variables Measured (N=20)

                                      Platform Condition
                                      Stable
Variable                              [bar]X      SD

Postural sway(a)
  PS(b)                                5.46      3.01
  SS(c)                                9.08      4.04
Weight on paretic sided
  PS                                  42.70     11.30
  SS                                  34.12      8.72
Symmetry of weight distribution(e)
  PS                                  10.20     8.62
  SS                                  15.99     8.52

                                       Angular Rotation
                                      [bar]X      SD

Postural sway(a)
  PS(b)                               23.89      8.90
  SS(c)                               19.01      5.06
Weight on paretic sided
  PS                                  46.11      9.63
  SS                                  36.59     10.10
Symmetry of weight distribution(e)
  PS                                   7.88      6.58
  SS                                  15.00      7.39

                                      Linear Translation
                                      [bar]X      SD

Postural sway(a)
  PS(b)                               11.89      5.48
  SS(c)                               13.88      3.86
Weight on paretic sided
  PS                                  44.64     10.51
  SS                                  33.81      9.78
Symmetry of weight distribution(e)
  PS                                   9.00      7.45
  SS                                  17.63      6.68

(a) Expressed as a percentage and computed by the system using the mean square method.

(b) PS=parallel stance: feet 10 cm apart.

(c) SS=step stance: paretic leg forward, heel to toe, feet 10 cm apart.

(d) Expressed as a percentage.

(e) Expressed as a percentage and calculated using the formula: [absolute value (% weight on paretic side--50%)].

Relationship Between Upper-Extremity Tasks and Postural Sway

We first examined the relationship between the self-generated upper-extremity tasks and postural sway by condition on the Balance System[TM]. No relationship was found, and the correlations ranged from a negligible value of .00 to a low value of -.29. Height and age also were not correlated with postural sway for the six tested conditions on the Balance System[TM].

Relationship Between Upper-Extremity Tasks and Postural Symmetry

We next investigated the relationship between the self-generated upper-extremity tasks and postural symmetry by condition on the Balance System[TM]. The FRT was moderately correlated with measures of postural symmetry in the parallel stance position on the Balance System[TM] for the stable platform (r=.66, P[is less than] .001), angular rotation (r=.49, P[is less than].05), and linear translation (r=.54, P[is less than].01) (Tab. 4). These relationships between the FRT and measures of postural symmetry were stronger after controlling for age by using partial correlation coefficients (stable platform: r=.78; angular rotation: r=.66; linear translation: r=.66; P [is less than or equal to] .001). Height and age were not found to correlate with symmetry of weight distribution for the six tested conditions on the Balance System[TM].

Table 4.
Correlation Coefficients Between the Upper-Extremity Tasks
and the Balance System[TM] by Condition for Symmetry of
Weight Distribution (N=20)

                                   Arm Raise
                        FRT(a)     PS(b)      SS(c)
PS
 Platform stable       .66(***)     .06        .06
 Angular rotation      .49(*)       .02        .00
 Linear translation    .54(**)      .06        .06
SS
 Platform stable       .24         -.35       -.41(d)
 Angular rotation      .05         -.35       -.42(d)
 Linear translation    .05         -.45(d)     .52(e)

                         Arm Reach
                         PS        SS
PS
 Platform stable         .17       .36
 Angular rotation        .03       .18
 Linear translation      .11       .28
SS
 Platform stable        -.36      -.25
 Angular rotation       -.23      -.18
 Linear translation     -.40(d)   -.34

(a) FRT=Functional Reach Test.[9] Single asterisk (*) indicates P <.05, double asterisk (**) indicates P <.01, triple asterisk (***) indicates P <.001.

(b) PS=parallel stance: feet 10 cm apart.

(c) SS=step stance paretic leg forward, heel to toe, feet 10 cut apart.

(d) Not claimed to be significant at P <.05, as opposite of predicted direction.

(e) Not claimed to be significant at P<.01, as opposite of predicted direction.

Relationship Between Functional Reach and Arm Raise and Arm Reach Tasks

Finally, we examined the relationship between the FRT and the arm raise and arm reach tasks described by Goldie et al.[10] The FRT was moderately correlated with the arm raise task in the parallel and step stance positions (r=.43, P[is less than].05) as well as with the arm reach task in the step stance position (r=.44, P[is less than].05) (Tab. 5). Age was negatively correlated with the arm raise task in both stance positions (r= -.43, P[is less than].05). Age, however, was not correlated with the FRT (r= -.22, P[is less than].18).

Table 5. Correlation Coefficients(a) Between the Functional Reach Test(9) (FRT) and the Arm Raise and Arm Reach Tasks (N=20)

                 FRT

                                Partial(c)      Partial(d)
                Simple(b)       (Height)        (Age)

Arm raise
  PS(e)         43(*)            43(*)           .38
  SS(f)         43(*)            43(*)           .37

Arm reach
  PS           .30               .30             .28
  SS           .44(*)            .45(*)          .42(*)

(a) Asterisk indicates P<.05.

(b) Pearson product-moment correlation coefficients.

(c) Partial correlation coefficients controlling for height.

(d) Partial correlation coefficients controlling for age.

(e) PS=parallel stance: feet 10 cm apart.

(f) SS=step stance: paretic leg forward, heel to toe, feet 10 cm apart.

Discussion

Relationship Between Upper-Extremity Tasks and Postural Sway

Negative correlations were expected between the scores on the self-generated upper-extremity tasks and the amount of postural sway by condition on the Balance System[TM]. Analysis of the subjects' performances, however, revealed low and nonsignificant relationships. These findings are inconsistent with the results of several studies that have compared functional balance scores and laboratory balance measures.[36,37,39,57] In subjects with traumatic brain injuries, Lehmann et al[37] found that static postural sway (mean displacement) was moderately to highly correlated with walking speed and distance on a balance beam (r= -.51 to -.72), as well as with the Sickness Impact Profile functional index (r=.72). The Get Up and Go Test, another balance tool, had a moderate relationship with the mean postural sway path measure in elderly subjects during quiet standing.[57] The Berg Balance Scale[36,39] was moderately correlated (r= -.55) with speed and amplitude measures of spontaneous postural sway and more modestly correlated (r= -.38) with similar measures of postural sway during pseudorandom perturbations in an elderly population.

Our findings, however, are similar to the results of a study by Thapa et al,[30] which showed that the FRT was not correlated with elliptical area (r=.09) or mean speed (r=.15) of postural sway during quiet standing in elderly nursing home residents. Measures of balance such as timed chair stands, mobility maneuvers, and timed walk were also poorly correlated with these biomechanical measures of postural sway.[30] The relationship between increased postural sway and reduced balance ability may not be straightforward. Maki and colleagues[31,38] hypothesized that less stable subjects may have a greater tendency to increase the stiffness of their postural system and limit their postural sway during balance-threatening conditions. In addition, caution is needed when comparing various studies using force platform measures of postural sway because different variables are discussed in the literature (eg, root mean square amplitude, elliptical area, mean speed, mean frequency, mean sway path, range of sway).[30-32,37,39,57] Based on our findings, however, it appears that the amount of postural sway during quiet standing and in response to external perturbations on the Balance System[TM] may have little relation to the postural control necessary for some self-generated upper-extremity balance tasks.

Our subjects performances on the FRT ([bar]X=>25.05 cm) indicated that this group was at risk of falling.[46] According to Duncan et al,[46] if the FRT measurement is greater than 15.24 cm (6 in) but less than 25.4 cm (10 in), the odds ratio associated with two falls is 2.00. This association found between the FRT and risk of recurrent falls was independent of age, depression, and cognition.[46] Although Overstall and colleagues[28,33] found that fallers have increased postural sway, there was no relationship between the FRT and measures of postural sway in our study.

Relationship Between Upper-Extremity Tasks and Postural Symmetry

Positive correlations were expected between the self-generated upper-extremity tasks and symmetry of weight distribution by condition on the Balance System[TM]. After controlling for age, the FRT was correlated with the stable and moving conditions in parallel stance on the force platform unit. Although this relationship was in the predicted positive direction, its strength just barely reached significance (as defined by a correlation value of .70). In our study, we used a standardized stance width of 10 cm for all testing methods. Although the FRT is typically administered with the subject assuming a comfortable and relaxed stance,[9,29,46,58] a standardized stance width of 10 cm was chosen to ensure consistency with the tasks described by Goldie et al.[10] Because foot position reportedly affects balance,[59] perhaps our findings would have been different if the subjects had been instructed to assume their natural stance. The present findings, however, still appear to suggest that the FRT not only evaluates a person s ability to adapt to self: generated center-of-mass displacements[9] but also relates to a person s capability to balance symmetrically during quiet standing and in response to external perturbations. Such a relationship between the FRT and measures of postural symmetry in people with cerebrovascular accidents has not been previously documented in the literature.

The arm raise task in step stance was moderately yet negatively correlated with stable and moving (linear and angular) platform conditions in step stance on the Balance System[TM]. Although this relationship was noted at probability values of less than .05, these correlations were deemed to be statistically nonsignificant because they were not in the predicted or anticipated direction. Decreased weight on the paretic lower extremity in a standing position contributes to postural unsteadiness.[2,40] Based on our results, however, standing asymmetrically (reduced or excessive weight on the paretic side) on the Balance System[TM] in step stance was related to better performance on the self-generated arm raise task in step stance. Two factors may have contributed to these findings. The subjects may have placed too much weight on the nonparetic leg positioned posteriorly in step stance. According to Kirby et al,[59] the step stance position reportedly encourages individuals to bear more weight on the posterior foot. In addition, our subjects may have adopted compensatory strategies and favored the nonparetic posteriorly positioned leg. Hocherman et al[60] found that individuals with hemiparesis who placed more weight on the nonparetic side actually performed better on activities of daily living that utilized the lower trunk and legs. Arguably, such compensatory strategies may also be related to better performances on some self-generated upper-extremity activities.

The subjects may also have placed too much weight on the paretic forward leg. To determine whether this was the case, we visually inspected the scores for percentage of weight on the paretic side. We found that subjects placed excessive weight on the paretic side in only 10% of the platform conditions in step stance on the Balance System[TM]. Based on our results, it therefore appears that individuals who assumed more asymmetrical postures and who placed excessive weight on the nonparetic posteriorly positioned leg in step stance in response to external perturbations performed better on the self-generated arm raise task in step stance.

Relationship Between Functional Reach and Arm Raise and Arm Reach Tasks

The FRT has been found to correlate with age in individuals without known balance impairment,[9] community-dwelling persons,[29] and elderly nursing home residents.[30] Our study, however, suggests that the FRT does not appear to be age sensitive in individuals with hemiparesis.

The positive correlations between the FRT and the arm raise and arm reach tasks involving the nonparetic upper extremity were in the predicted direction, yet they were weaker than expected. Numerous investigators[36,39,61] have identified very strong relationships between different functional scales. The moderate correlations that we found are nevertheless consistent with those identified in elderly nursing home residents[30] and in a heterogeneous group of male veterans undergoing inpatient rehabilitation.[58] Those two studies[30,58] demonstrated only modest relationships between the FRT and other physical performance measures such as timed walk, mobility skills, timed chair stands, and the FIM. As proposed by Weiner et al,[58] such physical performance tests and the arm raise and arm reach tasks used in our study may actually rely heavily on strength, endurance, and flexibility. In contrast, the FRT may prove to be a quicker measure of dynamic standing balance, which is less reliant on strength and endurance than other physical performance tests.[58]

Limitations of the Study

The importance and the generalization of our findings must be viewed with caution due to the small sample size. Modifications in the testing procedures may inadvertently have influenced the validity and reliability of the measurements and therefore our results. Instead of assuming a natural stance, subjects stood with a standardized stance width of 10 cm on the Balance System[TM] and for the FRT. In addition, no practice trials were given for the different scales. These alterations were made to ensure consistency of stance width across the different tests and to prevent any possible learning effects. Learning effects have previously been identified on the Balance System[TM] (L Wegener, unpublished research, 1994). For practical reasons, the lower and side markers for the arm raise and arm reach tasks, respectively, consisted of a wall target instead of a rail target. Due to the large number of force platform characteristics described in the literature, comparison and generalization of our findings may be difficult and limited. It is possible that the relationships among the different balance tests were not linear. Therefore, the actual correlations may potentially be significantly and clinically stronger than those found in our study. Finally, potential sources of error in all clinical testing typically include factors related to the subject (fatigue, fear, attention, motivation, understanding), the tester (observations, clarity of instructions), and the testing location (distractions, positioning of equipment).[10] Although we attempted to minimize these factors, some factors may inadvertently have influenced the testing sessions and our results.

Conclusion

No relationship was identified between the self-generated upper-extremity tasks and measures of postural sway in response to external perturbations on the Balance System[TM]. Either the biomechanical measure of postural sway is perhaps assessing a different component of balance, or it has little to do with the postural control needed for self-generated balance tasks.[38] In contrast, the measure of symmetry of weight distribution in response to force platform perturbations was found to be more closely related to the self-generated upper-extremity tasks. The strong relationship between postural symmetry in parallel stance and the FRT, after controlling for age, warrants clinical consideration and further investigation. With a relative degree of certainty, therapists may use one test and reliably predict patients' performances on the other test. Finally, the moderate strength of the relationship found between the FRT and the arm raise and arm reach tasks limits its clinical relevance and suggests that the tests may be evaluating different aspects of balance. The FRT may be a more challenging measure that assesses an individual's ability to perform a self generated forward weight shift to the limit of stability. In contrast, the repetitive self-generated arm raise and arm reach tasks may rely more heavily on a more limited form of balance control as well as strength and endurance. Future research should further investigate the multiple dimensions of functional and force platform balance measures with larger sample sizes. Comparison of balance performances by side of lesion in individuals with hemiparesis is also needed.

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