Biomechanic Modeling of Sit-to-Stand to Upright Posture for Mobility Assessment of Persons With Chronic Stroke

Article Outline

• Abstract
• Methods
  • Strength and Motor Performance Tests
  • The STS-to-UP Test
  • Data Analysis
    • The TIP model output parameters
    • The postural parameters
  • Statistical Analysis
• Results
  • Strength and Motor Performance Tests
  • The STS-to-UP Test
    • The STS phase
    • The pUP phase
    • The UP phase
• Discussion
• Conclusions
• Acknowledgments
• References
• Copyright

Abstract 

Mazzà C, Stanhope SJ, Taviani A, Cappozzo A. Biomechanic modeling of sit-to-stand to upright posture for mobility assessment of persons with chronic stroke.

Objective

To test the suitability of using biomechanic measures associated with a minimum measured input model (MMIM) approach to assess mobility of people with chronic stroke during the execution of a sit-to-stand (STS) to upright posture motor task.

Design

Single group, observational.

Setting

Institutional settings in the United States and Italy.

Participants

Twenty-nine subjects with chronic unilateral lower-limb impairments and resultant mobility limitations secondary to stroke.

Interventions

Not applicable.

Main Outcome Measures

Manual measurement of lower-limb strength; performance-based tests including repeated chair standing, walking speed, and standing balance; and ground reactions measured with a force platform during STS and upright posture. The ground reactions were fed to a telescopic inverted pendulum model of the musculoskeletal system. Parameters representing the model outputs were compared with performance-based and strength measures to assess, respectively, motor ability and impairment-related changes in subjects’ motor strategies.

Results

The parameters derived from the model effectively differentiated between motor strategies associated with different performance-based scores, and allowed the identification of relevant difficulties encountered in STS execution. These difficulties could be associated with different strength scores. This was also true for subjects scoring the maximum in both performance-based and strength tests.

Conclusions

The MMIM is a relatively inexpensive and noninvasive approach that enhances mobility assessment of hemiparetic subjects with different motor ability levels. It provides information that correlates well with performance-based and strength scores and, in addition, it allows for subject-specific motor strategy identification.

Key Words:  Disability evaluation , Lower extremity , Motor skills disorders , Rehabilitation , Stroke

THE QUANTITATIVE ASSESSMENT of a person’s function is of paramount importance when the effectiveness of rehabilitation interventions is being evaluated. For this purpose, various tests have been proposed with which to assess mobility limitation, including observing a patient performing selected motor tasks. These tests provide semiquantitative or quantitative information based on predefined scales. Among these scales, the Barthel Index¹ and the motor component of the FIM instrument² are the 2 most recommended for measuring poststroke physical limitations, having been widely validated and globally used.³ Both tests are economical and are well accepted by patients, however, they both have important limitations. Both scales have ceiling effects⁴ that represent significant drawbacks, especially when patients with chronic conditions are being assessed. In fact, patients may use compensatory motor strategies, which are not quantified by either of these tests, to obtain higher, even maximum, scores on both scales. A short questionnaire, the Stroke Impact Scale (SIS), has been recently proposed⁵ for use with stroke patients to overcome such ceiling effect. This questionnaire includes complex activities such as stair climbing, walking at different speeds, and carrying heavy objects. The SIS test captures the broad range of poststroke mobility limitations and can detect the effects of mild residual impairments.

Researchers and practitioners, however, may have difficulties in interpreting the clinical meaning of the changes in the summary scores of the above-mentioned tests.³ These difficulties can be attributed to the fact that these scores provide insufficient information in terms of identifying residual physical impairments and describing their role in determining mobility limitations. The latter information is intrinsically associated with the motor strategy a subject adopts to perform a given task. A description of the factors that act together in determining a motor strategy can be obtained using a biomechanic analysis. A human movement analysis laboratory in its complete configuration, in fact, could certainly provide objective and reliable data that would allow pursuit of this objective.⁶, ⁷ This approach, however, may be extremely demanding on both the subject and the experimenter, and its high cost and the competence level required may make it unsuitable for broad use in a clinical setting. For these reasons, there is limited availability of effective quantitative methods for assessing mobility limitations that are associated with motor strategy identification, and that combine clinical applicability with data objectivity, particularly for patients with chronic conditions. Our purpose in this study was to contribute to the solution of this problem, with special reference to people who have had a stroke.

The proposed method requires the measurement of the forces exchanged by the person with the ground, using a single forceplate. The approach has been thoroughly described in previous articles⁸, ⁹, ¹⁰ and is based on a relatively inexpensive instrumentation that permits subjects to perform in an unencumbered manner. A mathematical model of the musculoskeletal system (minimum measured input model [MMIM]) receives these forces as input and estimates basic aspects of the subject’s movement pattern and relevant muscular activity during a selected motor task. The specific MMIM we used in this study was the telescopic inverted pendulum (TIP) model.⁹ The selected motor task consisted of a sit-to-stand (STS) task followed by the maintenance of steady upright posture (UP) (hereafter STS-to-UP). Between these 2 phases of the motor task, a transitory phase can be identified during which body sway is reduced to a minimum in preparation for UP (pUP).¹¹ Each of these phases may provide specific information suitable for assessing mobility. STS has already been investigated in stroke subjects for both evaluation and rehabilitation purposes. Cheng et al¹² highlighted the main characteristics of the motor task and the correlation of these characteristics with falls and with the effect of specific rehabilitation programs.¹³ Subjects were asked to rise from a chair, stand for 30 seconds, and return to a seated position. Fallers needed a significantly longer time than nonfallers to rise from the chair and to stabilize sway after attaining UP. Similarly, Chou et al¹⁴ investigated the effects of STS functional training on gait performance and used the vertical component of the ground reaction and the center of pressure (COP) displacement to assess these effects. The UP test is widely used in clinical practice. Because decreased postural stability is a common problem for patients with hemiparesis secondary to stroke, instrumented and noninstrumented balance tests have been proposed to evaluate both dynamic and static posture control.¹⁵, ¹⁶, ¹⁷, ¹⁸, ¹⁹

Our purpose in this study was to evaluate the combined utility of an STS-to-UP task and the TIP model to enhance identification of motor strategies, residual impairments, and mobility limitations in patients presenting a range of chronic stroke severities. The output of the TIP model is expected to provide an objective and reliable description of the STS-to-UP motor strategy adopted by the subject with stroke residual effects and to provide a set of parameters that can be used to quantitatively assess mobility limitation. The information provided by the proposed biomechanic method was compared with that provided by summary scores obtained from selected strength and motor performance tests.

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Methods 

The study involved a sample of 29 subjects (20 men, 9 women; age, 65±8y; height, 1.67±0.08m; mass, 82.1±13.6kg) who had chronic unilateral lower-limb impairments and resultant mobility limitations secondary to stroke for at least 1 year before participating in the study. Subjects were excluded if they were not medically stable; had movement disorders caused by other neuropathies; had cognitive impairment precluding communication or comprehension; were unable to ambulate at home without aids; or were unable to rise from a chair with folded arms or to stand for 60 seconds.

This study was conducted under a protocol approved by the institutional review board at the U.S. National Institutes of Health. Each participant gave written informed consent. All measurements were performed on the same day.

Strength and Motor Performance Tests 

Assesssment of the subjects began with manual muscle testing of lower-limb strength with respect to 12 functions²⁰: hip flexion, abduction, and internal and external rotation; knee flexion and extension; ankle eversion, inversion, dorsiflexion, and plantarflexion; and toe flexion and extension. These evaluations were performed by an expert physiotherapist on both the affected and the nonaffected limb, with the subject lying supine. Strength (impairment level) was assessed by assigning a score between 0 and 10 for each measure: 0, no contraction felt in the muscle; 1, contraction felt, but no movement produced; 2, partial range, gravity minimized; 3, complete range, gravity minimized; 4, full range, but releases; 5, holds position, antigravity; 6, holds against slight pressure; 7, holds against slight to moderate pressure; 8, holds against moderate pressure; 9, holds against moderate to strong pressure; and 10, holds against strong pressure. A summary lower-limb strength score was then computed from the sum of the scores associated with the above-listed muscle groups and ranged from 0 (highest impairment) to 120 (lowest impairment).

A subject’s mobility limitation level was then determined using a battery of performance tests²¹ that included motor tasks that were consistent with those selected for the biomechanic test, that is, standing balance and repeated chair standing and, in addition, spontaneous walking. For the standing tests, participants were asked to maintain the side-by-side, semitandem, and tandem positions for 10 seconds each. For standing balance, participants scored 1 if they could hold a side-by-side stance for 10 seconds but were unable to hold a semitandem stance for 10 seconds, scored 2 if they held a semitandem stance for 10 seconds but were unable to hold a full tandem stance for more than 2 seconds, scored 3 if they held the full tandem stance for 3 to 9 seconds, and scored 4 if they held the full tandem stance for 10 seconds. The repeated chair standing test was performed with a standard chair with a seat height of .45m. Subjects were free to choose the position of their bare feet and were instructed to “stand and sit 5 times consecutively as fast as possible after a verbal ‘go’ signal, with your upper extremities folded on your chest.” The time from the go signal to the end of the fifth stand was recorded with a stopwatch and scored (repeated chair standing) from 1 to 4, where 1 for a time of 16.7 seconds or more; 2 for a time of 16.6 to 13.7 seconds; 3 for a time of 13.6 to 11.2 seconds; and 4 for a time 11.1 seconds or less. For the walking tests, participants were asked to walk at their self-selected normal speed, starting from a standing position, and were timed over a 2.4-m distance. Scores (walking speed) were based on the faster of 2 walks: a score of 1 was given for a time of 5.7 seconds or more (≤.43m/s); 2 for a time of 4.1 to 5.6 seconds (.44–.60m/s); 3 for a time of 3.2 to 4.0 seconds (.61–.77m/s); and 4 for a time of 3.1 seconds or less (≥.78m/s).

For all tests, a score of 0 was given if subjects could not perform the task. A summary score of the performance tests, ranging from 0 (lowest mobility level) to 12 (highest mobility level), was generated by summing the scores for standing balance, walking speed, and repeated chair standing. The reliability of the performance-based score, as obtained using an internal consistency test, was good (Cronbach α=.76).²¹

The STS-to-UP Test 

Body mass, stature, and thigh, shank, and foot length data were recorded for each subject. An adjustable-height seat was located over a 6-component forceplate,^a the surface of which was enlarged with a metal baseplate (.60×.90m²) and lined with graph paper. Subjects were asked to sit on the seat, after its height was adjusted at knee height,²² with their bare feet freely positioned on the metal baseplate. The positions of bilateral landmarks (heel, malleoli, fifth metatarsal head, big toe) were measured relative to the forceplate axes, with the aid of the graph paper, and the foot prints were traced to ensure precise repositioning. Initial knee and ankle angles were also measured with a goniometer. Subjects were asked to rise from the seat and stand still with their feet kept flat on the ground and their arms folded across their chest, while looking at a visual target positioned at standing eye height. A practice session was allowed. The STS task was performed at 2 different self-selected speeds, defined as normal and maximum speeds. Five trials were performed for each speed, following a randomly established order. Subjects were given rest periods between each trial. Forceplate signals were recorded for 60 seconds (sample frequency, 100 samples/s) and a “go” signal was verbally given to the subjects immediately after the beginning of data acquisition.

Data Analysis 

The motor task was divided into the 3 previously defined phases (STS, pUP, UP). The times when the STS task (t_STS), the pUP phase (t_pUP), and the UP phase (t_UP) started were determined using the forceplate signals and 3 previously devised algorithms.¹¹ STS and pUP durations (T) were then determined as T_STS=t_pUP–t_STS and T_pUP=t_UP–t_pUP, respectively. The duration of the UP phase was set to 40 seconds.

The TIP model (fig 1) is composed of a telescopic link that joins a stationary hinge, located in a point corresponding to the center of the base of support, to the center of mass (COM) of the moving portion of the body.⁹ In the interval preceding seat unloading, only the head-arms-torso (HAT) system moves and therefore is accounted for in the model. After the loss of contact with the seat, the TIP model represents the whole body. During seat unloading, the model is not applicable because the subject’s base of support, and thus the model hinge, cannot be considered to be stationary. Beginning (t_bu) and ending (t_so), where bu is beginning of unloading and so is seat-off, of this seat-unloading phase was determined from the forceplate signals.²³

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• Fig 1. 

  Schematic representation of the 2 TIP models employed for the description of STS: TIP1 (left), before seat unloading; and TIP2 (right), after seat-off. The sign convention for the actuators is also indicated. Abbreviations: FA, frontal actuator; HAT, head-arms-torso; LA, linear actuator; SA, sagittal actuator; WB, whole body.

The link may vary its length by the effect of a force (linear) actuator that models the muscles responsible for the elevation of the COM. The orientation in space of the link is controlled by 2 (rotational) actuators that generate a couple about the mediolateral (ML; sagittal actuator [SA]) and anteroposterior (AP) axes (frontal actuator [FA]) of the hinge, and model the muscular function mostly aimed at controlling the AP and ML location of the COM relative to the base of support.

The recorded ground reaction forces, together with the measured anthropometric quantities and initial joint angles, were fed to the equations that describe both kinematics and kinetics of the TIP model.⁹ As a result, the muscle equivalent actuators kinematic (linear and angular displacement, velocity) and kinetic variables (force, couple, power) versus time were obtained. For the purpose of statistical analysis, these time functions were described using the set of parameters (maxima, minima, relevant time of occurrence) that, in previous studies, proved to be effective in motor strategy description and discrimination.²⁴

Mobility, as associated with a subject’s motor strategy, can be assessed by investigating the timing and relative importance of the musculoskeletal functions a person adopts when executing the STS task. We conducted this investigation with reference to the kinematics of the muscle-equivalent actuators of the TIP and, in particular, using the parameters listed in table 1. Impairment was assessed by investigating the capacity of the lower-limb muscles to generate and sustain the body segment movements during the STS phase that follows the seat-off. Relevant information was extracted from the kinetics of the whole body TIP actuators. In particular, we used the parameters listed in table 1.

Table 1. Description of the TIP Model and Associated Parameters

NOTE. Subscripts 1 and 2 indicate before (TIP1) and after seat-off (TIP2), respectively. The instantaneous length of the link was normalized to its value during quiet sitting (before seat-off) and quiet standing (after seat-off). Force data were normalized to body mass, and couples and powers to the product of body mass and stature.

Abbreviations: FA, frontal actuator; LA, linear actuator; SA, sagittal actuator.

The COP trajectory in the plane of support during the pUP and UP phases was computed from the measured ground reaction loads. The root mean square (RMS) distance from the mean to the instantaneous location of the COP, in both the ML and AP directions, and the sway path (length of the COP trajectory)²⁵ were used to assess balance control during both the pUP phase and the UP phase. The values of these parameters were normalized with respect to the area of the base of support to account for differences in foot dimension and position.²⁶

Statistical Analysis 

The TIP parameters and the postural parameters for all trials represented the data set on which statistical analysis was performed. We did a reliability analysis using the intraclass correlation coefficient (ICC_2,k, with k equals 5). We used the Spearman rank correlation coefficient (ρ) to assess the correlation of the parameters with the performance-based and strength scores. A 2-factor analysis of variance was performed: a functional status 2-level factor was defined as low if the performance-based score was 6 or less, and high if the performance-based score was greater than 6 and was considered as a between-subject factor; speed was considered as a within-subject factor (2 levels: normal speed, maximum speed). Significance level was set at P equal to .05.

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Results 

Strength and Motor Performance Tests 

The strength score varied between 68 and 120 for the unaffected limb (mean, 116±10), and between 21 and 118 for the affected limb (mean, 82±35). The performance-based score varied between 2 and 12, with a mean value of 7±3 (standing balance, 3±1; walking speed, 2±1; repeated chair standing, 2±1); 13 subjects were accordingly included in the low functional status group and 16 were included in the high functional status group.

The STS-to-UP Test 

All subjects were able to perform the STS-to-UP test at both normal speed and maximum speed. No significant correlation was found between the area of the base of support chosen by the subjects and the performance-based and strength scores. T_STS correlated with the performance-based score (normal speed, ρ=–.59; maximum speed, ρ=–.56), and was shorter for the subjects in the low function group (3.29±1.24s vs 2.21±0.74s, P=.008).

The mean values and standard deviations of the TIP parameters are reported in table 2 for both normal speed and maximum speed trials. Correlation coefficients between the TIP parameters and the performance-based and strength scores are reported in table 3 to highlight the capacity of the model parameters to describe both mobility limitation and lower limb impairment. All the TIP parameters were highly reliable (ICC range, .76–.99) at both STS speeds.

Table 2. TIP Parameters for the 2 Task Speeds

NOTE. Values are mean ± standard deviation.

Abbreviations: See table 1; NC, no change.

Table 3. Spearman Rank Correlation Coefficients Between the TIP Parameters and the Performance Test and Strength Scores for Normal Speed (NS) and Maximum Speed (MS) Trials

NOTE. Correlations between the parameters are reported when significant (P<.05).

Abbreviation: NS, not significant.

The lateral rotations of the HAT and of the whole body, as measured by the frontal actuator rotation, were directed toward the unaffected side of the body, and the maximum absolute values of the frontal actuator angular displacement, both before (φ_FA1) and after (φ_FA2) seat-off, were associated with the greatest level of lower-limb impairment. As indicated in figure 2, a threshold strength value of about 60 seems to exist, beyond which all subjects exhibited negligible rotations in the frontal plane as reported in the literature for healthy subjects.²⁶ No significant differences were found between normal speed and maximum speed values.

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• Fig 2. 

  Frontal actuator maximum rotations, representing (A) the HAT (φ_FA1) and (B) the whole body (φ_FA1) movements, are reported as a function of the strength scores recorded for each subject. These values have been reported for both normal speed (NS) and maximum speed (MS) trials.

The relative timing between the sagittal actuator rotation and the linear actuator displacement also contributes to the STS motor strategy description. Two different stereotypical patterns were observed: one in which the subject displaces the whole body COM vertically (raises) and frontward toward the base of support (gains equilibrium) in a simultaneous fashion, and another in which the COM moves first forward and then upward.²⁶ The length (l₂) of the link of the whole body TIP model, as measured when the COM is vertical relative to the base of support, normalized with respect to its maximal value, allowed discrimination between the 2 above-mentioned strategies. The higher the value of l₂, the closer the strategy is to the former stereotype. We found that the l₂ value was significantly lower in the normal speed trials than in the maximum speed trials (79%±6% vs 83%±6%, P<.001).

Another important factor in the motor strategy is the forward rotational acceleration and the associated rotational momentum gained by the HAT at the beginning of the STS (0–t_ω). The TIP parameters that account for this factor are the peak velocity ω₁, and the peak couple (C₁) and power (SP₁) that cause this velocity to be attained. These parameters, obtained at maximum speed, correlated with the performance-based score. We found a significant interaction between speed and functional status for both C₁ (P=.048) and SP₁ (P=.014), but no significant differences in these parameters could be attributed to the functional status level.

For all subjects, at the beginning of seat unloading the higher the speed, the lower the HAT flexion (lower φ_SAbu). For the maximum speed, however, φ_SAbu correlated negatively with performance-based scores, which means that low mobility level is associated with a more flexed HAT than the high mobility level. No significant differences, however, were found between low- and high-function groups. HAT linear velocity peak values (v_bu) correlated with the performance-based score, and were higher for the subjects with the high functional status (normal speed, .28±.15m/s vs .37±.11m/s; maximum speed, .28±.15m/s vs .40±.16m/s; P=.039, with no interaction between speed and functional status).

To take STS to completion, sufficient forward momentum of the whole body needs to be generated after the seat-off. The values of the angular velocity recorded at seat-off (ω_so) were used to assess this factor. The ω_so values correlated strongly with the performance-based score, and larger values of this parameter were associated with a high functional status (normal speed, .38±.18rad/s vs .61±.13rad/s; maximum speed, .52±.21rad/s vs .78±.16rad/s; P=.001, with no interaction between speed and functional status).

Differences in motor strategies can also be related to the generation of the whole body linear acceleration that is required to attain the final standing position. Velocities, forces, and powers of the TIP model 2 actuators can be used to assess the above differences. Higher and delayed velocity peaks (v₂) were recorded in maximum speed trials. The v₂ values correlated positively with performance-based and strength scores. Subjects with high functional status reached higher v₂ than did those with the low functional status (normal speed, .41±.13m/s vs .47±.11m/s; maximum speed, .47±.15m/s vs .62±.13m/s; P=.026). No interaction was recorded between speed and functional status. The force of the linear actuator (F2) correlated with the strength score, and this correlation was stronger during the maximum speed condition. Moreover, F2 values correlated with the performance-based scores at maximum speed, and were lower for the low functional status subjects (normal speed, 10.6±1.1N/kg vs 11.1±0.5N/kg; maximum speed, 11.3±0.6N/kg vs 11.9±0.7N/kg; P=.035), with no significant interaction between the 2 factors. Higher forces were accompanied by higher power (LP₂): LP₂ values, were, in fact, higher at maximum speed, and correlated with the performance-based score and, at maximum speed, with the strength score. Subjects with low functional status exerted less power than their counterparts (LP normal speed, 2.1±0.9W·kg^–1·m^–1 vs 2.7±0.7W·kg^–1·m^–1; LP maximum speed, 2.8±1.0W·kg^–1·m^–1 vs 3.7±0.8W·kg^–1·m^–1; P=.031, with no interaction with the speed).

Finally, the generation of the sagittal actuator angular forward rotation that characterizes the last part of the STS, was described by the sagittal actuator angular velocity values (ω_tv). These values correlated with both performance-based and strength scores, but no significant differences were found between low- and high-functional status groups.

T_pUP was 4.5±1.7 seconds for the normal speed trials and 5.3±2.3 seconds for the maximum speed trials, and the difference between the 2 experimental conditions was not significant. These values were noticeably higher and more variable than those reported in the literature⁸ for young healthy subjects (normal speed, 2.7±0.5s; maximum speed, 2.7±0.8s). In the normal speed condition, the mean T_pUP recorded for each subject correlated with his/her performance-based score (ρ=.36, P<.001), but no difference was found in association with functional status.

Postural parameters in the pUP were highly reliable (ICC_2,5>0.7) and no significant differences were recorded between normal speed and maximum speed trials. In this transitory movement, the residual effects of the propulsion associated with the STS maneuver were expected to result in body oscillations mainly directed in the AP direction compared with the ML direction. Because of the presence of the hemiparesis, however, no significant differences were found between the parameters computed in the ML and in the AP directions.

Standing balance and performance-based scores correlated with both RMS (ρ=–.54) and sway path (ρ=–.57) in the ML direction. Moreover, subjects with lower functional status had more difficulties in controlling their postural stability, as shown by the differences recorded in the ML direction between the low- and high-level groups (RMS, 9±4mm vs 5±2mm; P=.023; sway path, 214±108mm vs 119±43mm; P=.008).

All parameters extracted from the COP during UP were highly reliable (ICC_2,5>0.8) and no residual effects of the STS movement were detectable, as shown by the fact that none of these parameters varied significantly between normal speed and maximum speed trials.

Contrary to what is reported in the literature for healthy subjects,²⁴, ²⁵ oscillations in the AP direction were not higher than those in the ML direction (RMS, 4±3mm vs 5±3mm; sway path, 438±282mm vs 569±327mm). In the ML direction, these parameters correlated significantly with the balance score (RMS, ρ=–.53; sway path, ρ=–.67). Moreover, these values are much higher than those reported for healthy subjects, and this dependence from subjects’ functional status was supported by the differences between low- and high-level groups (RMS, 6±4mm vs 3±1mm; P=.012; sway path, 595±341mm vs 332±169mm; P=.011).

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Discussion 

Our objective in this study was to test the suitability of a biomechanic approach to enhance the assessment of mobility of people with chronic stroke conditions, using a combined motor task and a model of the musculoskeletal system for the interpretation of ground reaction forces recorded during the task. Results obtained with the model were described with purposely selected parameters and were compared with independent measures of subjects’ lower-limb strength and motor ability. In particular, we investigated the capacity of the model to identify and quantify differences in the motor strategy, as associated with speed changes, impairment, and functional ability level, and to detect asymmetries in the movement, as associated with the stroke residual effects.

By describing body forward and lateral rotations and vertical displacement, and relevant angular and linear velocities, the TIP model effectively identified the motor strategies associated with different functional status levels, as assessed with a battery of motor performance tests. Similarly, the description of the forces, couples, and powers associated with the lower-limb actions permitted us to identify the different movement execution strategies associated with the residual lower-limb strength scores. These results were obtained despite the fact that the musculoskeletal system was represented in a very schematic fashion, and behaved more consistently when the subjects were asked to perform the more demanding task of rising from the chair at their maximum speed. The correlation analysis returned values that accounted for a small proportion of the variance in the data. It must be stressed, however, that use of the TIP was expected to provide complementary information with respect to performance-based and strength scores, and was not expected to be a substitute for these assessment tools.

The STS duration values reported in the literature²⁴ for a similarly aged, healthy elderly subject group are lower than those of the patients involved in this study, with a higher difference between the 2 groups in the normal speed condition. The values recorded for the stroke group in the maximum speed trials were similar to those reported for the healthy subjects in normal speed trials. Moreover, all subjects were able to increase the speed of execution of the STS task, but this behavior was most evident for the subjects with the highest mobility level, which confirms results reported in the literature.¹², ²⁷ As described by the model, the motor strategy that subjects adopted to accomplish this goal was that of changing the relative timing of body COM elevation and forward rotation.

As a consequence of the reduced lower-limb residual strength, the most impaired subjects accomplished the STS task at lower velocities and exerted lower couples, forces, and powers. The values of these parameters were also lower than those reported in the literature for healthy young and elderly subjects.²⁴ These subjects accomplished the STS maneuver by leaning toward the unaffected side of the body. This motor strategy is clearly attributable to the asymmetries in the frontal plane, typically present in stroke patients because of residual impairments.²⁷, ²⁸

The analysis of the preparation phase and maintenance of the UP permitted us to assess balance control. In particular, the postural parameters could help identify asymmetric dynamic and static posture, which has been indicated as one of the prevalent locomotor deficits of stroke-related hemiparesis.²⁹, ³⁰ As expected, subjects with the highest mobility level were better able to control body lateral and frontal oscillations during both the pUP and the UP tasks, and significantly impaired subjects had the most difficulties in controlling the oscillations in the ML direction.

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Conclusions 

The MMIM approach provided additional interpretative value with respect to performance test and strength measures and made possible subject-specific motor strategy identification. Information obtained from the force platform signals, using both the TIP model and the postural parameters, appears to be reliable and makes possible the identification and evaluation of different motor strategies associated with mobility limitations and impairment-related movement alterations. Moreover, the MMIM model was suitable for assessing subjects with the highest level of mobility, and may be used to guide treatment and goal setting for therapists, just as the current activity scales do for a lower level of mobility.

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Acknowledgments 

We gratefully acknowledge the collaboration of Francesco Benvenuti, Timothy Brindle, Kelly Nelson, Susan O’Connell, and Karen Siegel. The Physical Disabilities Branch is a collaboration between the National Institute of Child Health and Human Development and the Warren G. Magnuson Clinical Center, National Institutes of Health (NIH). The opinions presented in this report reflect the views of the authors and not those of NIH or the US Public Health Service.

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References 

1. Mahoney FI , Barthel DW . Functional evaluation (the Barthel index) . Md State Med J . 1965;14:61–65
   • View In Article
   • MEDLINE
2. Kidd D , Stewart G , Baldry J , et al.   The Functional Independence Measure (a comparative validity and reliability study) . Disabil Rehabil . 1995;17:10–14
   • View In Article
   • MEDLINE
   • CrossRef
3. Kwon S , Hartzema AG , Duncan PW , Min-Lai S . Disability measures in stroke (relationship among the Barthel Index, the Functional Independence Measure, and the Modified Rankin Scale) . Stroke . 2004;35:918–923
   • View In Article
   • CrossRef
4. Wallace D , Duncan PW , Lai SM . Comparison of the responsiveness of the Barthel Index and the motor component of the Functional Independence Measure in stroke (the impact of using different methods for measuring responsiveness) . J Clin Epidemiol . 2002;55:922–928
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (81 KB)
   • CrossRef
5. Duncan PW , Lai SM , Bode RK , Perera S , DeRosa J . Stroke Impact Scale–16 (a brief assessment of physical function) . Neurology . 2003;60:291–296
   • View In Article
6. Cappozzo A , Figura F , Marchetti M . The interplay of muscular and external forces in human ambulation . J Biomech . 1976;9:35–43
   • View In Article
   • MEDLINE
   • CrossRef
7. Simon SR . Quantification of human motion (gait analysis-benefits and limitations to its application to clinical problems) . J Biomech . 2004;37:1869–1880
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (575 KB)
   • CrossRef
8. Cappozzo A . Minimum measured-input models for the assessment of motor ability . J Biomech . 2002;35:437–446
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (320 KB)
   • CrossRef
9. Papa E , Cappozzo A . A telescopic inverted-pendulum model of the musculo-skeletal system and its use for the analysis of the sit-to-stand motor task . J Biomech . 1999;32:1205–1212
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (384 KB)
   • CrossRef
10. Mazzà C , Cappozzo A . An optimization algorithm for human joint angle time-history generation using external force data . Ann Biomed Eng . 2004;32:764–772
    • View In Article
    • MEDLINE
    • CrossRef
11. Mazzà C , Zok M , Della Croce U . Sequencing sit-to-stand and upright posture for mobility limitation assessment (determination of the timing of the task phases from force platform data) . Gait Posture . 2005;21:425–431
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (168 KB)
    • CrossRef
12. Cheng PT , Liaw MY , Wong MK , Tang FT , Lee MY , Lin PS . The sit-to-stand movement in stroke patients and its correlation with falling . Arch Phys Med Rehabil . 1998;79:1043–1046
    • View In Article
    • Abstract
    • Full-Text PDF (529 KB)
    • CrossRef
13. Cheng PT , Wu SH , Liaw MY , Wong AM , Tang FT . Symmetrical body-weight distribution training in stroke patients and its effect on fall prevention . Arch Phys Med Rehabil . 2001;82:1650–1654
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (57 KB)
    • CrossRef
14. Chou SW , Wong AM , Leong CP , Hong WS , Tang FT , Lin TH . Postural control during sit-to-stand and gait in stroke patients . Am J Phys Med Rehabil . 2003;82:42–47
    • View In Article
    • MEDLINE
    • CrossRef
15. Nashner LM , Peters JF . Dynamic posturography in the diagnosis and management of dizziness and balance disorders . Neurol Clin . 1990;8:331–349
    • View In Article
    • MEDLINE
16. Bohannon RW , Leary KM . Standing balance and function over the course of acute rehabilitation . Arch Phys Med Rehabil . 1995;76:994–996
    • View In Article
    • Abstract
    • Full-Text PDF (324 KB)
    • CrossRef
17. Black FO . What can posturography tell us about vestibular function? . Ann N Y Acad Sci . 2001;942:446–464
    • View In Article
    • MEDLINE
    • CrossRef
18. Garland SJ , Willems DA , Ivanova TD , Miller KJ . Recovery of standing balance and functional mobility after stroke . Arch Phys Med Rehabil . 2003;84:1753–1759
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (148 KB)
    • CrossRef
19. Ikai T , Kamikubo T , Takehara I , Nishi M , Miyano S . Dynamic postural control in patients with hemiparesis . Am J Phys Med Rehabil . 2003;82:463–469
    • View In Article
    • MEDLINE
    • CrossRef
20. Kendall FP , McCreary EK , Provance PG . Muscles (testing and function) . Baltimore: Williams & Wilkins; 1993;
    • View In Article
21. Guralnik JM , Simonsick EM , Ferrucci L , et al.   A short physical performance battery assessing lower extremity function (association with self-reported disability and prediction of mortality and nursing home admission) . J Gerontol . 1994;49:M85–M94
    • View In Article
    • MEDLINE
22. Mazzà C , Benvenuti F , Bimbi C , Stanhope SJ . Association between subject functional status, seat height, and movement strategy in sit-to-stand performance . J Am Geriatr Soc . 2004;52:1750–1754
    • View In Article
    • MEDLINE
    • CrossRef
23. Kralj A , Jaeger RJ , Munich M . Analysis of standing up and sitting down in humans (definitions and normative data presentation) . J Biomech . 1990;23:1123–1138
    • View In Article
    • MEDLINE
    • CrossRef
24. Papa E , Cappozzo A . Sit-to-stand motor strategies investigated in able-bodied young and elderly subjects . J Biomech . 2000;33:1113–1122
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (368 KB)
    • CrossRef
25. Prieto TE , Myklebust JB , Hoffmann RG , Lovett EG , Myklebust BM . Measures of postural steadiness (differences between healthy young and elderly adults) . IEEE Trans Biomed Eng . 1996;43:956–966
    • View In Article
    • MEDLINE
    • CrossRef
26. Chiari L , Rocchi L , Cappello A . Stabilometric parameters are affected by anthropometry and foot placement . Clin Biomech (Bristol, Avon) . 2002;17:666–677
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (384 KB)
    • CrossRef
27. Engardt M , Olsson E . Body weight-bearing while rising and sitting down in patients with stroke . Scand J Rehabil Med . 1992;24:67–74
    • View In Article
    • MEDLINE
28. Hesse S , Schauer M , Petersen M , Jahnke M . Sit-to-stand manoeuvre in hemiparetic patients before and after a 4-week rehabilitation programme . Scand J Rehabil Med . 1998;30:81–86
    • View In Article
    • MEDLINE
    • CrossRef
29. Winstein CJ , Gardner ER , McNeal DR , Barto PS , Nicholson DE . Standing balance training (effect on balance and locomotion in hemiparetic adults) . Arch Phys Med Rehabil . 1989;70:755–762
    • View In Article
    • MEDLINE
30. Nichols DS . Balance retraining after stroke using force platform biofeedback . Phys Ther . 1997;77:553–558
    • View In Article
    • MEDLINE

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