PHYS THER
Vol. 87, No. 3, March 2007, pp. 326-336
DOI: 10.2522/ptj.20050303
Deficits in Surface Force Production During Seated Reaching in People After Stroke
Darcy S Reisman and
John P Scholz
DS Reisman, PT, PhD, is Assistant Professor, Department of Physical Therapy and Interdisciplinary Biomechanics and Movement Science Program, University of Delaware, 322 McKinly Laboratory, Newark, DE 19716 (USA)
JP Scholz, PT, PhD, is Professor, Department of Physical Therapy and Interdisciplinary Biomechanics and Movement Science Program, University of Delaware
Address all correspondence to Dr Reisman at: dreisman{at}udel.edu
Submitted September 26, 2005;
Accepted October 11, 2006
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Abstract
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Background and Purpose: In order to design effective treatment strategies for the rehabilitation of reaching after stroke, it is necessary to understand the underlying deficits. Although the kinematic aspects of reaching after stroke have been studied frequently, little attention has been paid to the surface force production underlying this behavior. The purpose of this study was to investigate surface force production and its coordination with arm movement during seated reaching in a group of people with hemiparesis.
Subjects: Seven people with mild right hemiparesis after stroke and 7 people who were neurologically healthy participated.
Methods: Subjects performed seated reaching at 160% their normal speed toward ipsilateral and contralateral targets placed 160% beyond arm reach. Surface forces beneath the seat and feet and 3-dimensional hand movement and joint motions of the upper extremity and trunk were recorded.
Results: A weight shift from seat to feet occurred earlier whereas the onset of medial-lateral seat force was delayed and smaller in magnitude in people with hemiparesis.
Discussion and Conclusion: The results suggest that the normal magnitude and timing of surface force production during reaching beyond arm’s length are altered in people with even mild hemiparesis after stroke, particularly during reaching toward the hemiparetic side.
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Introduction
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To develop the most effective and efficient rehabilitation strategies to help people regain maximal function and community participation after stroke, a thorough understanding of the pathokinesiology underlying movement dysfunction is required. For people who have survived stroke, attention has been paid primarily to the pathokinesiology of reaching toward targets that are within arm’s length.1–6 Less attention has been paid to dysfunction associated with reaching beyond arm’s length,3,7,8 despite the fact that this type of reaching is important for many functional activities. Moreover, previous studies3,7,8 focused primarily on the kinematic aspects of the movement, which represent only one component of reaching. In the present study, we investigated the production of forces by the buttocks and feet against the seat and floor (surface force production), respectively, as an important contributor to reaching beyond arm’s length.
The relationship between surface force production and arm movement in subjects who are healthy was identified previously.9,10 Prior to reach onset, surface forces are initiated in the direction opposite the reach direction (eg, down, back, and to the left for a forward reach to the right). These forces provide momentum toward the target.9 The distance to the target and therefore the amount of trunk motion required to reach the target influence the timing of such forces.9 Because surface force production provides momentum toward a target,9 learning to control these forces is critical for people with hemiparesis after stroke, who often have difficulty reaching beyond arm’s length because of weakness or deficits in range of motion. In other tasks, people who have survived stroke have been shown to have difficulty timing surface forces appropriately with the focal movement.11,12 The present study provides an important first step in understanding deficits in surface force production that contribute to reaching dysfunction after stroke.
The purpose of this study was to investigate surface force production relative to arm movement during reaching beyond arm’s length in a group of people who had chronic, mild hemiparesis and who performed the reaching task with the hemiparetic arm. On the basis of evidence from other tasks, we hypothesized that, compared with control subjects without hemiparesis, subjects with hemiparesis would demonstrate a delay in the timing of surface forces with respect to the focal movement and a decreased magnitude of surface force production. We further hypothesized that these deficits would occur primarily during reaching toward the hemiparetic side.
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Method
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Participants
Seven people (64.4±13.5 [
±SD] years of age) with right hemiparesis after a first unilateral, left hemispheric stroke and 7 people who were neurologically healthy (64.8±11.4 years of age) participated in this study. Control subjects were matched by age and sex to the subjects with hemiparesis and had no musculoskeletal problems affecting either arm or the trunk. All subjects were right-hand dominant, including, before their stroke, the subjects with hemiparesis. All subjects gave approved written consent before participating. The subjects with hemiparesis were recruited as a sample of convenience through referrals from local physical therapy clinics and stroke support groups.
Clinical Evaluation
Potential participants were screened and excluded if they scored less than 20 on the Mini-Mental State Examination13; scored less than 40 of 56 on the Berg Balance Scale14; had more than 8 errors on the Motor Free Visual Perception Test; had a cerebellar lesion, as determined by magnetic resonance imaging or computed tomography; or had other neurologic, musculoskeletal, or vascular conditions affecting arm, trunk, or pelvic movement.
All participants also were administered the arm portion of the modified Fugl-Meyer Scale15 and the Fugl-Meyer Scale16 and the sensation, proprioception, and pain assessment portions of the Fugl-Meyer Scale.16 Scores on the Orpington Prognostic Scale revealed that all participants had deficits associated with a minor stroke according to this scale (score of <3.2).17 Demographic and clinical data are summarized in Table 1.
Reaching Task
Subjects reached to targets placed at a distance that was 160% their functional arm length in both the contralateral and ipsilateral work spaces (30° to the left and right of midline). Functional arm length was defined as the distance from the acromion of the shoulder to the metacarpophalangeal joint of the index finger; this distance was obtained during test trials in which subjects, with their trunks restrained, were asked to reach straight forward as far as possible with the hemiparetic hand while raising that arm as close to a 90 degree elevation as possible.
In the starting position, subjects sat on an adjustable-height bench with the hips and knees flexed to 90 degrees and approximately 50% of the length of the thigh being supported. The seat rested on a Bertec* force platform measuring 60x90 cm. The subjects’ feet rested on an identical force platform in front of the other platform (Fig. 1). For all tasks, subjects grasped a wooden dowel with a 3.5-cm diameter and with a magnet attached to the bottom. The dowel and arm rested in a small trough that allowed for control of the initial arm position. The subjects’ trunk starting position was controlled by a series of rods that lightly touched the subjects’ upper trunk and pelvis when the correct position was obtained. The target object, a lightweight disk, was placed on a tripod at knee height.
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Figure 1. Experimental setup. (A and B) Devices used to standardize trunk starting position. (C) Trough used to support arm in standardized starting position. (D and E) One force platform was under the seat (D), and the second platform was under the feet (E).
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The subjects’ preferred reaching speeds were determined by asking them to reach with the hemiparetic arm to the contralateral target at a comfortable speed. Then, to ensure consistent speeds across trials, a task that was otherwise difficult for the subjects, a metronome was adjusted so that the interval between 2 pulses matched 160% the preferred movement time. Subjects were instructed to initiate their reach on any metronome beat of their choice, to retrieve the object on the next beat, and to return to the starting position by the third consecutive beat. This relative movement time was chosen on the basis of pilot testing with 3 people without disabilities and 3 people with hemiparesis. These people were asked to reach at increasingly faster percentages of their preferred speed until they could no longer complete the task. The speed at which all subjects could complete the task was 160% their preferred speed.
The subjects were given multiple practice trials to familiarize themselves with the task. Experimental trials began once the subjects were able to complete 2 trials in a row correctly matching the metronome beats and retrieving the metal disk. Twenty trials, presented in blocks of 5 trials with a random order of target direction for each block, were completed. The subjects were offered rest breaks as needed, and no subject complained of fatigue.
Data Collection
A 6-camera VICON
motion measurement system recorded the motion (120 Hz) of 7 rigid bodies containing reflective, spherical markers placed on the thigh, pelvis, upper trunk, upper shoulder girdle, arm, forearm, and hand as described in our earlier study.3 Spherical markers also were placed at each of the 2 target locations, and their positions were captured and used to create a local coordinate system whose x-axis was aligned with a vector from the position of the hand in the starting location to the target location and whose y-axis and z-axis were orthogonal (following the right-hand rule) to the x-axis and each other. The global coordinates of the arm markers during the experimental trials were rotated into this local coordinate system. The local coordinates of the hand were used to determine movement onset and termination (see next paragraph). The 6 components of the signals from the 2 force platforms were sampled at 960 Hz with a National Instruments
analog-digital converter and time synchronized with the video data by the VICON software.
Data Reduction
Movement onset (or termination) was determined as the time when the velocity of the filtered hand marker exceeded (or returned to) 1% the maximum velocity by use of an automatic algorithm verified by visual inspection. For force and moment calculations, the resting bit values for each component of the force platform signals, measured with the seat and alignment devices in place, were subtracted from the signals obtained during experimental trials to account for the weight of the apparatus. These signals then were converted to the 3 forces Fx, Fy, and Fz and moments Mx, My, and Mz by applying the calibration matrices (scale factors) of each platform. The data were represented as action forces (forces applied by the subject) in newtons.
Each signal was filtered with a bidirectional, second-order Butterworth low-pass filter (20 Hz) in MATLAB.
The resulting surface forces from each force plate were then down-sampled to 120 Hz to align them with the video data. Finally, the baseline surface forces were measured during a 150-millisecond window that began 500 milliseconds before movement onset. The baseline values were subtracted from the forces to examine changes in surface forces relative to the resting condition. The onset of a force event was determined as the time when the first derivative of the force diverged continuously from the baseline.
Data Analysis
Average vertical seat and foot forces for the subjects with hemiparesis and the control subjects are shown in Figure 2. "Unloading" of the seat refers to a decrease in vertical seat force below the baseline force, and "loading" of the feet occurs when the vertical foot force rises above the baseline force. These force transitions occur when the subject shifts his or her weight from the seat to the feet.
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Figure 2. Vertical seat and foot forces during reaching toward contralateral (CL) and ipsilateral (IP) targets. The dashed line represents data averaged across subjects with hemiparesis, and the solid line represents data averaged across age- and sex-matched control subjects. Error bars represent ±1 SEM. The vertical line (t0) represents the onset of hand movement. Zero vertical force represents the baseline, resting force determined in a 150-millisecond window 500 milliseconds prior to movement onset.
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Average medial-lateral (M-L) seat forces for the subjects with hemiparesis and the control subjects are shown in Figure 3. During reaching toward either target, subjects in both groups generated an initial M-L propulsive force at the seat that was in the direction opposite to that of the movement. This finding is consistent with a previous report of seated reaching toward near and distant targets in young people who are healthy.9
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Figure 3. Medial-lateral seat forces during reaching toward contralateral (CL) and ipsilateral (IP) targets. Conventions are as described in the legend to Figure 2.
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The sagittal-plane moment about the center of mass results from the vertical components of reaction forces at both the seat and the feet acting at an anterior-posterior (A-P) distance from the body center of mass and the A-P components of seat and foot forces acting below the body center of mass (see calculations below for dependent variable 2). In this task, the moment is propulsive initially, creating the forward movement toward the target, and then reverses to help slow the movement as the subject approaches the target.
On the basis of the hypothesis that the magnitude and timing of surface force production would differ between subjects who are healthy and subjects with hemiparesis after stroke, the following dependent variables were calculated.
- (1) The relative time of the onset (trel-onset) of the forces under the feet or buttocks with respect to the onset of hand movement was calculated with the following formula:
where tF-dir is the time of onset of the force under the feet or buttocks in a particular direction (ie, A-P, M-L, or vertical), tonset is the time of the onset of hand movement, and 
is the time between the hand movement onset and contact of the magnet with the object (Fig. 4 shows an example).
- (2) The relative time of onset of the sagittal moment about the center of mass of the body with respect to the hand movement was calculated as described above, with the time of force onset being replaced by the time of moment onset. The sagittal-plane moment Msag was calculated with the following formula:
where CMA-P is the A-P position of the center of mass (computed from the kinematic data and estimates of segmental masses and their anatomical locations18), COPA-P is the A-P position of the center of pressure, FZ is the combined vertical seat and foot forces, CMvert is the vertical position of the center of mass (computed from the kinematic data), COPvert is the vertical position of the center of pressure (by definition, the surface of the force plate), and FA-P is the combined A-P seat and foot forces.
- (3) The magnitude of the peak M-L seat propulsive force was determined as the maximum M-L seat force value after the hand movement onset and before the force reversal.
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Figure 4. Example of values used to calculate the relative timing of the force onset (black line) with respect to the hand movement onset (gray line). The onset of the seat force is indicated by tFseat, and the onset of the hand movement is indicated by tonset. The time (t) between the hand movement onset and contact of the magnet with the object is illustrated by the dashed line.
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On the basis of the hypothesis that subjects with hemiparesis after stroke and matched control subjects would differ in the magnitude and timing of surface force production and that this difference would be most pronounced during ipsilateral reaching, the following independent variables were calculated: target work space location (contralateral or ipsilateral) and group (stroke or control).
The Levene test of the homogeneity of variance was completed on each dependent variable for the groups (A-P, vertical, and M-L surface force timing and magnitude). Because of a lack of homogeneity in all dependent variables, nonparametric statistics (Mann-Whitney U test) were used.
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Results
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Reaching Speeds
Reaching speeds were 0.686±0.063 and 0.673±0.037 m/s for the subjects after stroke and the control subjects, respectively. This difference was nonsignificant (P=.684).
Relative Timing of Surface Forces and Hand Movement
The relative timing of the hand movement onset and the onset of surface force production for a particular target direction is shown in Table 2. When the force onset occurred before the hand movement onset, the value for relative timing was negative and vice versa. There were no differences between the groups in the relative timing of the A-P forces with respect to the hand movement onset (Tab. 2).
The relative timing of vertical seat and foot forces differed between the groups. Group data for the relative timing of the vertical forces are shown in Table 2. As shown in Figure 5 (compare Fig. 5A with Figs. 5B, 5C, and 5D), the weight shift from the buttocks to the feet occurred earlier for the subjects with hemiparesis relative to the onset of the hand movement during reaching toward either target (Mann-Whitney U test: P<.05 for both targets and both the seat and the feet) (Tab. 2).
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Figure 5. Average data for control subjects (A) and 3 representative subjects with hemiparesis (B–D) for the relative timing of the onset of seat forces with respect to the hand movement onset. Reaches to both contralateral (CL) and ipsilateral (IP) targets are shown. The larger the negative value, the earlier the force onset with respect to the hand movement onset. Error bars represent SEMs.
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The above-described results were supported by the results for the sagittal-plane moment about the body center of mass (Fig. 6). The subjects with hemiparesis demonstrated an earlier reversal of this moment with respect to the hand movement termination during reaching toward the ipsilateral target (Mann-Whitney U test: P<.02) (Fig. 6).
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Figure 6. Relative timing of the onset of the reversal of the propulsive sagittal-plane moment about the center of mass with respect to the hand movement termination during reaching toward contralateral (CL) and ipsilateral (IP) targets. Average data for each group are presented, and error bars represent SEMs.
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The relative timing of the M-L seat force differed between the groups. Group data for the relative timing of the M-L forces are shown in Table 2. As noted earlier, both groups of subjects generated an initial M-L seat propulsive force opposite the movement direction during reaching toward either target (Fig. 3). Although the timing of the M-L seat force onset with respect to the hand movement onset was not different between the groups during contralateral reaching (Mann-Whitney U test: P=.94) (Tab. 2), most subjects with hemiparesis showed a delayed onset of this force during ipsilateral reaching compared with control subjects, as shown in Figure 7 (Mann-Whitney U test: P<.05) (Tab. 2).
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Figure 7. Mean relative timing of the medial-lateral (M-L) seat forces with respect to the hand movement onset during reaching toward the ipsilateral (IP) target for control subjects (black bar) and individual data for subjects with hemiparesis (blue bars). The larger the negative value, the earlier the force onset with respect to the hand movement onset. Note that some subjects with hemiparesis did not develop medial-lateral forces until after the hand had begun to move (positive values). The error bar represents the SEM.
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The magnitude of the M-L seat force differed between the groups. Compared with the control subjects, subjects with hemiparesis also demonstrated a lower peak M-L seat force during ipsilateral reaching (Mann-Whitney U test: P<.01) (Fig. 8). This difference was not observed during contralateral reaching (Mann-Whitney U test: P=.225) (Fig. 8).
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Figure 8. Average peak medial-lateral (M-L) seat force during reaching toward contralateral (CL) and ipsilateral (IP) targets. Average data for each group are presented, and error bars represent SEMs.
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Discussion
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The results of the present study show that the normal magnitude and timing of surface force production during reaching beyond arm’s length are altered even in people with mild hemiparesis after stroke. These alterations are consistent with previously reported kinematic reaching deficits in the same population performing the same task.3 For example, in the previous study it was shown that peak trunk velocity occurred earlier in people with hemiparesis. This finding is consistent with the earlier weight shift from the seat to the feet and the earlier reversal of the sagittal-plane moment reported here. These data emphasize that normal movement kinematics are related to appropriate surface force production and that knowledge of this relationship is important for understanding the pathokinesiology of seated reaching after stroke.
Relationship Between Kinematic and Surface Force Production Deficits in Subjects After Stroke
The subjects with hemiparesis showed an earlier weight shift from the seat to the feet and an earlier reversal of the sagittal-plane moment compared with the control subjects. The relative loading and unloading of the seat and the feet and the reversal of the sagittal-plane moment are related. Earlier unloading of the seat and loading of the feet would contribute to an earlier reversal of this moment, largely because of the vertical force components. Although adjustments in the timing of the A-P forces could act to counteract the shift in the vertical force onset, apparently this scenario was not the case in the present study, because there were no differences between the groups in the timing of the A-P forces.
People with hemiparesis use greater trunk motion, and the magnitude of the trunk movement early during reaching is greater than that of people who are healthy during reaching beyond arm’s length.7 If the trunk is moving to a greater extent earlier in the movement, then this would correspond to an earlier forward weight shift from the seat to the feet. Indeed, this scenario is what was observed in the subjects with hemiparesis in the present study. In an earlier study,3 we found that during reaching beyond arm’s length, the timing of the peak velocity of the trunk movement occurred earlier in subjects after stroke than in subjects who are healthy. An earlier reversal of the sagittal-plane moment would lead to an earlier deceleration of the movement (including the trunk movement) and likely to an earlier occurrence of the peak velocity of the trunk movement. Thus, the altered arm kinematics observed when people who are long-term survivors of stroke and who have mild impairments reached beyond arm’s length are related at least partly to an altered ability to exert appropriate surface forces.
Changes in M-L Force Timing Were Influenced by Target Work Space Location in Subjects After Stroke
Compared with the control subjects, the subjects with hemiparesis demonstrated a delay in the M-L seat force onset during ipsilateral reaching. This delay probably contributed to the observed lower M-L seat force magnitude during reaching toward this target. With less time to produce the propulsive force because of the delayed onset, it is likely that there was not enough time for the subjects with hemiparesis to build up to a force magnitude similar to that of the control subjects. This idea is supported by the findings that the M-L seat force onset was not delayed and that the magnitude of the force did not differ between the groups during contralateral reaching.
These findings are similar to the results of previous studies that investigated M-L surface force production in subjects with hemiparesis during the performance of other tasks.12,19 In both of those tasks, the balance requirements were substantial, as subjects were required to produce M-L forces while standing. The results of the present study extend the findings to a seated task, with less substantial balance requirements. Thus, it seems that an alteration in the timing of M-L surface force production with movement toward the hemiparetic side is a consistent finding during a variety of tasks with different balance requirements. These results also are consistent with the suggestion of many clinicians that people with hemiparesis are "reluctant" to shift weight toward the hemiparetic side and tend to adopt asymmetric postures favoring the less involved side.20–22
Potential Mechanisms
The findings of this and previous studies suggest that people with hemiparesis exhibit an impaired ability to generate both the appropriate direction and the magnitude of surface forces to accomplish functional tasks efficiently.11,12,23 In the current context, people would need to organize the muscles of the leg, pelvis, and lower spine10,24 to generate an appropriate temporal pattern of forces (amplitude and direction) into the support surface.
For example, appropriately timed contraction of the lower abdominal and gluteal muscles is required to shift the center of pressure under the buttocks before controlled forward or backward trunk movement. It is well known that people who have survived stroke have difficulty with the coordination of muscle activity in preparation for movement.25,26 It is possible that, in this task, deficits in the coordination of muscle activity in advance of the movement led to the alterations in surface force production. If this hypothesis is correct, it reinforces the importance of addressing deficits in the coordination of all muscles involved in the task, including those associated with the generation of forces against the support surface, rather than focusing on coordination of the arm alone.
Variability of Individual Results
Despite the general consistency of the pattern of results in a comparison of the subjects who had survived stroke with the control subjects (Fig. 7), there was considerable variability among the subjects’ responses. Individual variability during the performance of reaching tasks among people who have survived stroke has been found in other studies.3,7,27 In the present study, this variability occurred despite the fact that all subjects who had survived stroke exhibited only mild impairments. It was suggested previously that such variability represents an individual’s unique adaptive response to his or her deficits.27 Future studies should investigate the relationships between different patterns of altered surface force production and other clinical and laboratory measures. Such relationships may lead to a better understanding of deficits in seated reaching in different subgroups of people who have survived stroke.
Limitations of the Present Study
The present study was the first attempt to describe the relationship between deficits in seated surface force production and reaching dysfunction in people who have survived stroke. The survivors who were recruited were mildly impaired, a fact that limits the generalization of the results to this subgroup of survivors. The results will need to be extended to a larger sample so that any relationships among stroke chronicity, levels of impairment, lesion location, and surface force production can be evaluated. Additionally, the present study investigated surface force production for reaching at a fast speed. Previous research25 showed that postural activity related to fast arm movements in subjects who have survived stroke is less variable and more similar to that of subjects who were healthy than is postural activity related to slow or self-paced movements. Therefore, it is possible that even greater deficits in surface force production will be observed if subjects are allowed to move at self-selected speeds. This scenario will need to be investigated in future studies.
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Conclusion
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The findings of the present study provide initial insight into the role of altered surface force production in the pathokinesiology underlying reaching dysfunction in people who are long-term survivors of stroke and who have mild impairments. The results indicate that the kinematic deficits previously observed when people who are long-term survivors of stroke reached beyond arm’s length are related to an impairment of surface force production. The results provide the basis for future studies, which should investigate whether these findings are consistent in people who have survived stroke and who have different levels of impairment and stroke chronicity. If such kinetic deficits are found to be a consistent feature of seated reaching dysfunction after stroke and are related to deficits in movement production, then rehabilitation strategies should address surface force production deficits directly.28
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Footnotes
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Both authors provided concept/idea/research design, writing, and project management. Dr Reisman provided data collection and analysis. Dr Scholz provided fund procurement.
This study was approved by the Human Subjects Review Committee, University of Delaware.
A platform presentation of this research was given at the Combined Sections Meeting of the American Physical Therapy Association; February 12–16, 2003; Tampa, Fla.
This study was funded by National Institutes of Health grant NS-50880 awarded to Dr Scholz. Dr Reisman was supported by the Foundation for Physical Therapy during her doctoral studies, during which this study was conducted.
* Bertec Corp, 6171 Huntley Rd, Suite J, Columbus, OH 43229. 
Oxford Metrics, 9 Spectrum Pointe Dr, Lake Forest, CA 92630
National Instruments Corp, 11500 N Mopac Expressway, Austin, TX 78759-3504.
The MathWorks Inc, 3 Apple Hill Dr, Natick, MA 01760-2098.
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Introduction
Method
