Published online October 2, 2006
PEDIATRICS Vol. 118 No. 4 October 2006, pp. e1226-e1236 (doi:10.1542/peds.2005-2768)
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ARTICLE

Development of Hand Function and Precision Grip Control in Individuals With Cerebral Palsy: A 13-Year Follow-up Study

Ann-Christin Eliasson, PhDa, Hans Forssberg, MD, PhDa, Ya-Ching Hung, MEdb and Andrew M. Gordon, PhDb,c

a Department of Woman and Child Health, Karolinska Institute, Stockholm, Sweden
b Department of Biobehavioral Sciences, Teachers College, Columbia University, New York, New York
c Department of Rehabilitation Medicine, College of Physicians and Surgeons, Columbia University, New York, New York


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 APPENDIX: EXPERIMENTAL DETAILS
 REFERENCES
 
OBJECTIVE. Although children with cerebral palsy display large developmental differences in hand function from that of typically developing children by the age of 6 to 10 years, little is known about the developmental processes underlying hand function during subsequent development. In this study we investigated the development of manual dexterity in a timed motor task, the timing and amplitude of fingertip-force application during a precision grasping task, and the relationship between changes in these measures. We applied highly quantitative analytical approaches to determine if the fingertip-force application pattern and trial-to-trial variation of fingertip-force application change during development.

METHODS. Twelve subjects with cerebral palsy (aged 6–8 years) participated in the first data-collection session conducted between 1989 and 1990. Ten of these subjects (5 with hemiplegia and 5 with diplegia, aged 19–21 years) returned between 2002 and 2003. Manual dexterity was measured by using timed tasks of the Jebsen-Taylor test of hand function. Subjects also lifted an object instrumented with force transducers while we measured the temporal coordination of fingertip coordination and the path ratio between the grip and vertical load-force trajectory (straightness). We used generalized procrustes analysis to determine if there were changes in shape of the force trajectory and intertrial variability.

RESULTS. The Jebsen-Taylor test times decreased 45% from the first to the second data session. The overall time to complete the grip-lift task decreased 22%, mainly because of a faster transition from grasp to lift. The grip-force/load-force path ratios decreased from 1.7 to 1.35 (1 = straight line). Generalized procrustes analysis indicated a change in the shape and a decrease in variability in shape of the force-ratio path.

CONCLUSIONS. Our results demonstrate that the efficiency in grasping had developed during a 13-year period for this small group of participants with cerebral palsy, which suggests that improvement in hand function occurs over a longer time frame than commonly would be expected.

Key Words: cerebral palsy • children • development • hand function • hemiplegia • diplegia • movement pattern • generalized procrustes analysis

Abbreviations: CP—cerebral palsy • CI—constraint-induced • GPA—generalized procrustes analysis • CV—coefficient of variation • dGF—grip-force rate • dLF—load-force rate • RMS—root-mean-squared

In typically developing children manipulatory actions and performance of fine motor skills develop rapidly during the first years of life with a subsequent refinement occurring throughout childhood.1–4 Although the characteristics of impaired hand function have been described at discrete developmental points in children with cerebral palsy (CP), little is known about the developmental processes underlying hand function in this population. The main focus of research on hand function in CP has been on children with hemiplegia, although many children with diplegia have asymmetrically decreased hand function that is unrelated to handedness.5 The clinical features of hand impairment in CP include slowness, weakness, uncoordinated movements, incomplete finger fractionation and spasticity.6–8 Furthermore, tactile sensibility is impaired in ~50% of children with hemiplegic CP, and mirror movements of varying severity, which also affect hand use, are common.7,9 These impairments in hand function lead to functional limitations in activities of daily living.10

For the last 15 years, considerable attention has been given to prehensile force control during object manipulation in children with CP.11–19 This objective measure of motor behavior has aided our understanding of the mechanisms underlying impairments in hand function in this population (see ref 20). These studies have shown that children with CP exhibit impairments in fingertip-force timing and amplitude control during object manipulation. Children with CP are slower and have prolonged delays between movement phases. Although most children with CP are capable of adjusting their grip (normal) forces to the object's weight and texture using sensory mechanisms, they typically have a decreased ability to scale the force output in advance (ie, anticipatory control) without extensive practice.12,14,16,19 Typically developing children display an invariant "grip–lift synergy" whereby the grip and load (tangential) forces are initiated simultaneously and subsequently increase in parallel by the age of 4 years.1 In contrast, children with CP have a sequential development of grip and load force (ie, an impaired grip–lift synergy), often pushing the object down against the table support and producing large and variable grip forces before the onset of the lifting drive.11 The impaired grip–lift synergy strongly correlates with impairments in manual dexterity.21 To date, the long-term development of prehensile force control in children with CP has not been investigated. The above-mentioned impairments result in large developmental differences in hand function from that of typically developing children by the age of 6 to 10 years. The question is, what happens during subsequent development? Does the neural function continue to develop, or does the hand function become static?

To date, there have been only 2 studies that investigated longitudinal development of other aspects of hand function in children with CP. The first study reported development of the grip pattern and the spontaneous use of the hemiplegic hand in a small group of children with hemiplegic CP.22 The children demonstrated only a weak tendency to adopt more-advanced grip patterns, and the spontaneous use of the hand did not increase over a 10-year period. The second study included a larger sample of children with different types of CP between the age of 16 and 70 months.23 Growth curves over a 10-month period were constructed for each child. Two different types of assessments were used with somewhat different results: the Peabody Developmental Motor Scale and the Quality of Upper Extremity Skills Test (QUEST). Results of the Peabody Motor Scale24 suggested that the children with mild hemiplegic CP had fairly good fine motor development, whereas the children with severe impairments (eg, quadriplegic CP) actually tended to decline already after 3 years of age. Results of the QUEST25 suggested only weak trends in changes of quality of movement, with the trends ending as early as 4 years even for the children with mild hemiplegic CP. These results are in agreement with clinicians' experiences, which commonly suggest progress of hand-skill development during the preschool period, which subsequently stabilizes or even decreases as the children develop. It is unknown if this diminished development means that there is no long-term development of prehensile force control after this age.

Although there is relatively little known about development of hand function in children with CP, there is some evidence that it is possible to elicit short-term improvements with intensive treatment. For example, motor performance and the amount of spontaneous use of the hemiplegic hand may increase after constraint-induced (CI) movement therapy (eg, see refs 26–32). Even intensive contemporary occupational therapy33 or goal-directed training in combination with botulinum toxin34,35 may lead to improved performance. These studies demonstrate that hand function is dynamic, although the long-term effect or subsequent development is unclear. To effectively evaluate the effect of treatment in randomized, controlled trials, it is essential to know how hand function develops in individuals with CP. Our aim for this study was to quantify the long-term development of hand function both clinically and experimentally in this population. To accomplish this, we studied the development of manual dexterity in a timed motor task (Jebsen-Taylor test of hand function). We also used an experimental task measuring the force timing and amplitude control during a precision grasping task. We were also interested in changes in the fingertip-force application patterns (ie, the grip–lift synergy). Up until now such patterns have only been described qualitatively. Thus, for the first time we applied highly quantitative analytical approaches to determine if the pattern and trial-to-trial variation of fingertip-force application change during development.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 APPENDIX: EXPERIMENTAL DETAILS
 REFERENCES
 
Subjects
Twelve children with CP (6 children with diplegia and 6 with hemiplegia) participated in the first data-collection session conducted between 1989 and 1990 (see refs 11 and 12). Of the 12 participants, 10 returned to participate in a second data-collection session between 2002 and 2003 (Table 1). Despite repeated efforts, we were unable to locate 2 of the original participants. In the first session the participants were between the ages of 6 and 8 years, and in the second session they were between 19 and 21 years old. The severity of impaired hand function varied between subjects. The Jebsen-Taylor test of hand function provides an overview of speed and dexterity for each child, and the Manual Ability Classifications System10 was applied to describe their competence in daily life (note that this test classifies the collaborative use of the 2 hands together) (Table 1).


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  		TABLE 1 Descriptive Data of the Participants

 
All participants completed secondary education in regular school programs. Some participants had required a personal aide, whereas others did not receive any special support services. All participants have had access to pediatric rehabilitation programs. Thus, to a varying extent, all have had some contact with occupational and/or physiotherapists. Although it is not possible to retrospectively determine the treatment frequency or details for each participant, the Swedish health system was designed to likely provide somewhat greater services when they were younger, with some continuing services available until the age of 20 years depending on their needs and desire. The treatment of hand function would have been performed in groups for training of fine motor activity, with a home program for stretching and increasing range of motion. Occasionally, they may have had intensive treatment (eg, after hand surgery or to learn certain skills). In fact, 2 of the participants reported having undergone hand surgery (Table 1).

Materials
The Jebsen-Taylor test of hand function36,37 was used to clinically measure changes in hand function. The test serves as a general timed measure of manual performance. Custom objects instrumented with multiaxial force transducers were used to quantify changes in fingertip-force coordination during precision grip (see Appendix for details of both tests).

Procedure
The procedure was identical during both experimental sessions and was performed by the same investigator. The participants sat in a chair in front of an adjustable table. The table was adjusted so that the participant's forearm was approximately parallel to its surface when objects were grasped. Subjects completed the items of the Jebsen-Taylor test while being timed by the experimenter. To quantify fingertip forces during precision grip, the participants were instructed to grasp the instrumented object between their thumb and index finger, although they often used an additional finger for support. The participants with diplegic CP used their dominant hand (the hand used for writing), and participants with hemiplegic CP used the hemiplegic hand. The task was first explained and demonstrated. The participants then were allowed several practice trials to familiarize themselves with the task. The object was lifted 5 cm above the table with ~5 to 10 seconds between each lift. Ten consecutive lifts were performed with the object's weight adjusted to 200 g. The object's weight was then adjusted to 400 g, and after several practice trials, 5 additional lifts were performed. The first 10 trials with the weight of 200 g were used for analyzing the subjects' basic coordination. Five trials of the 200-g lifts and 5 trials of the 400-g lifts were used for analyzing the anticipatory control.

Data Collection and Analysis
Data were stored on a personal computer. A graphics terminal was used interactively to define the temporal and force parameters (Appendix) (see refs 1 and 11). All data from the first session were reanalyzed to ensure that the criteria were identical. To quantify the grip–lift synergy,1 we measured the grip-force/load-force path ratio from the application to the final forces at liftoff (Appendix). To determine if the shape and trial-to-trial variability of the grip–lift synergy change over development, generalized procrustes analysis (GPA),38 introduced to the study of movement,39–41 was used (Appendix).

Statistical comparisons were made by using paired t tests as well as Pearson correlations. Statistical significance was considered at the P < .05 level.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 APPENDIX: EXPERIMENTAL DETAILS
 REFERENCES
 
Clinical Measurements
Figure 1 shows the Jebsen-Taylor performance times for each participant during each data-collection session, with the mean values superimposed (bold). The time to perform the Jebsen-Taylor test decreased from the first to the second session by 45%, on average (P < .01). The largest decrease occurred in 3 participants with hemiplegic CP (hemiplegic participants 2, 4 and 5; see Fig 1). Participants 2 and 5 had both undergone hand surgery >5 years earlier.


Figure 1
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  		FIGURE 1 Time to complete the 6 timed items (writing excluded) of the Jebsen-Taylor test of hand function for the individual participants as well as the mean (bold line) from the first (age 6–8 years) and second (age 19–21 years) data-collection sessions. Faster times correspond to better performance.

 
The results of the various subtests of the Jebsen-Taylor test are shown in Table 2. All of the subtest times decreased during the second session except card turning, although only the reduction for simulated eating was significant (P < .05). Two participants (hemiplegic participants 2 and 4; Fig 1) could not perform the simulated eating task during the first session, and 1 of them still could not perform the task during the second session. Hemiplegic participants 4 and 5 were unable to move the heavy can during the first session but were able to do so by the second session.


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  		TABLE 2 Subtests of Jebsen-Taylor Test

 
Precision Grip: Temporal Parameters
The overall time to complete the grip-lift task (ie, contact to liftoff) decreased 22% (P < .06; see Figs 2 and 3 and Table 3). The decrease was mainly a result of the decreased time of the preload phase during the second session (P < .001). The contact phase and loading phase did not change (P > .05 in both cases; Fig 3), although the total task time and loading phase became more consistent (lower coefficient of variation [CV]) during the second session (P < .05; Table 3). None of the other temporal measures changed in either duration or consistency (Table 3).


Figure 2
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  		FIGURE 2 Representative trials of grasping and lifting the object for one child (hemiplegic participant 4) at 6 years (left) and the same individual at 19 years (right). Traces represent grip force from the index finger and thumb, load force, dGF, dLF, and vertical position. Vertical lines indicate the initiation of grasping at the first finger contact (T0) and the second finger contact (T1) (ie, finger differences). The preload phase is initiated when the object is grasped with both fingers and the grip force starts to increase; the preload phase ends with the onset of positive load force (T3). Thereafter, the forces increase during the loading phase until the load force overcomes the weight of the object and it lifts off (T4). The measured force parameters are indicated by arrows showing negative load-force peak (F1) when the object is pushed toward the table during the preload phase. F2 corresponds to the grip force at onset of positive load force. The peaks of dLF (F3) and dGF (F4) are measured also.

 

Figure 3
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  		FIGURE 3 Temporal measures of grasping for each individual (mean shown in bold line) at age of 6 to 8 and 19 to 21 years. A, Total time until liftoff (finger difference + preload and loading phases); B, finger difference; C, preload phase; D, loading phase. Faster times correspond to better performance.

 

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  		TABLE 3 Temporal and Force Parameters and CV

 
Precision Grip-Force Parameters
The extent to which the participants pushed the object down before lifting (negative load force) was decreased in the second session (P < .05; Fig 4). This parameter also become more consistent (CV: P < .05; Table 3). Likewise, the grip force at load-force onset was reduced at the second session, reflecting less-sequential force generation (P < .05; Fig 4).


Figure 4
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  		FIGURE 4 Force measures of grasping for each individual (mean shown in bold line) at age of 6 to 8 and 19 to 21 years. A, negative load force (values close to 0 represent better performance); B, grip force at positive load-force onset (lower values represent less sequential performance).

 
Changes in the Grip-Force/Load-Force Application Patterns
Figure 5 shows the mean grip-force/load-force trajectory transformation for each subject in each of the 2 data sessions. Note that, as described above, the negative load force and sequential force generation (as seen by the grip force at the onset of positive load force) decreased in nearly all subjects. Consequently, the mean grip-force/load-force path ratio decreased as well (average decrease from 1.69 to 1.36; P < .05). Because the amplitude of negative load force and grip force at the onset of positive load force both contribute to curvilinearity, we performed GPA. A paired t test between the overall and pooled residuals (to test the change in shape of the intrinsic pattern between the 2 sessions) of all the participants revealed significant differences (P < .05), which indicates that the intrinsic pattern of the trajectory had changed and that the change in shape cannot be accounted for by changes in force amplitudes alone. Figure 6 shows the change of mean residuals between the 2 collected sessions for all the participants, which indicates more consistent performance (P < .05).


Figure 5
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  		FIGURE 5 Mean grip-force/load-force trajectories for each subject (A, hemiplegic subjects; B, diplegic subjects) at the ages of 6 to 8 (left) and 19 to 21 (right) years. The x-axis corresponds to load force (LF) (values to the left of the dotted vertical line represent negative load force [pushing the object against the table]), and the y-axis represents grip force (GF) (lower values at the crossing of the dotted vertical line represent less sequential performance). Straighter lines indicate simultaneous increase of the 2 forces. Dn indicates diplegic participant n; Hn, hemiplegic participant n.

 

Figure 6
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  		FIGURE 6 Mean residuals of the GPA for each individual (mean shown in bold line) at the ages of 6 to 8 and 19 to 21 years. Lower values represent more-consistent grip-force/load-force trajectories. Dn indicates diplegic participant n; Hn, hemiplegic participant n.

 
Precision Grip: Anticipation to Weight
To determine if the participants had anticipatory control, the grip-force rates (dGFs) and load-force rates (dLFs) for the lifts with 200 and 400 g were compared. The participants were unable to scale the dGF or dLF during the first session (ie, the force rates were similar irrespective of object weight). In the second session the participants seemed to some extent have developed this capability (Fig 7). The dGF was higher for the heavier weight (P < .05), although the dLF still did not vary according to the object's weight (Fig 7). The peak of the dGF still occurred early in the loading phase, but there was increased consistency of the peak during the second session (P < .05; Table 3).


Figure 7
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  		FIGURE 7 Mean (±SEM) of peak dGF and dLF for lifts with 200 and 400 g at the ages of 6 to 8 and 19 to 21 years. Larger differences in force rates for the 2 weights represent anticipatory forced scaling. a P < .05.

 
Relationship Between Changes in Clinical Performance and Precision Grip
There was a strong correlation between the improvement of scores (difference scores) of the Jebsen-Taylor test and the overall decreased time to complete the precision grip task (r = 0.72; P < .05). There were also significant correlations between the Jebsen-Taylor test changes and changes in specific temporal parameters (finger differences: r = 0.83; loading phase: r = 0.76) but not for any force measures.


   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
APPENDIX: EXPERIMENTAL DETAILS
 REFERENCES
 
Our results demonstrate that the efficiency in grasping had developed during a 13-year period for this small group of participants with CP. Most of the clinical and experimental measures examined showed improvement. The results extend previous studies of hand-function development in CP. These results, and their clinical implications, are described below.

Continued Development of Hand Function
The development of gross motor function in children in CP has only recently been documented.42 Even less is known about development of fine motor skills in these children. Earlier studies suggest that children with CP have limited potential for improvement of movement patterns.21,23 However, fine motor skills, measured by using the Peabody Motor Developmental Scale, could still be learned despite an impaired motor pattern, especially at an early age for children with mild hemiplegic CP.23 Up until this point, it was assumed that only children with mild impairments show significant improvement that seems to diminish over time. The results of this study suggest that improvement continues even after adolescence regardless of the initial severity of hand function.

For typically developing individuals of corresponding ages, Taylor et al36,37 reported a 38% reduction in time to perform the Jebsen-Taylor test (ie, from 45 to 28 seconds). The time decreased 45% for participants with CP in their study. Thus, the decreased time for the participants with CP is likely of clinical significance. However, the individuals with CP still needed an average of 4 times longer to perform the test than typically developing individuals. Thus, the improvements did not result in a normalization in hand function.

Similar to the Jebsen-Taylor test results, the overall time from finger contact to liftoff of the object decreased. However, the decrease was mainly attributed to a faster preload phase, suggesting that the grip-lift movement became less sequential. Looking at the individual participants in Fig 2 it was obvious that they choose different strategies for lifting objects. Some participants spent a longer time for the pregrasp than earlier (ie, contact phase), whereas others were more careful or slow during the loading phase. On average, the loading phase tended to be longer in the second session. The longer loading phase, in which the fingertip forces are carefully applied to the object, is also seen during development of typically developing children.1,11 Overall, between the 2 data sessions there was a 22% decrease in the time to complete the grip-lift task. In a previous study of typically developing individuals,1,11 the decrease in the overall time to complete the task was smaller (11%) between individuals of comparable ages.

Changes in Movement Efficiency
Coordination of grasping and lifting an object is an important aspect of manual ability. Besides being performed quickly, smooth and dexterous movements require the grasping and lifting to occur using an invariant force-coordination pattern in which the grip and load forces are initiated simultaneously and increased in parallel with unimodal force-rate trajectories.1,11 The majority of participants with CP tested at 6 to 8 years had not developed the force-coordination pattern typical for their age but instead produced an immature or pathologic pattern.21 Even as young adults the participants of this study retained a sequential initiation of grasping and lifting and they press the object downward before lifting it up, resulting in large negative load force and high grip force at the onset of lifting force. These movements resulted in a largely curvilinear trajectory of grip-force and load-force development (ie, a poorly coordinated grip–lift synergy). However, all of these force measures had improved, and the force trajectories became straighter and changed patterns (shape) independent of force-amplitude changes as indicated by the GPA. Thus, quantification of the changes seen in Fig 7 showed that the changes in the grip-force–load-force synergy cannot be accounted for by factors such as skin slipperiness, fingerpad size, or strength, because the pattern changed irrespective of the actual forces used. Because this synergy has been shown to be related to manual dexterity,21 the improvement may have partially accounted for overall improvements in dexterity as measured with the Jebsen-Taylor test.

Dexterous coordination of objects also requires that the force coordination be adjusted to the object's weight and texture and other important aspects of the objects physical properties (eg, see refs 43–45). The development of isometric fingertip forces must be scaled (planned) before the initiation of the movement, because sensory information about the object's weight is not immediately available. This is demonstrated by the peak dGFs and dLFs. The peak rates typically occur in the middle of the loading phase, with higher rates of force development for heavier weights. The force rates then decrease to near 0 at liftoff, providing a dampened lift with a stable acceleration independent of the weight.43–45 When the weight of the object is varied but the visual appearance remains constant, adults and older children typically scale the dGF and dLF on the basis of earlier experience of the object's weight.2,43–45 Children with CP could not scale the grip and load force during grasping on the basis of previous experience with an object at the age of 6 to 8 years.12 Thus, they exhibited planning deficits (see also refs 46 and 47). By the second session the participants could scale the dGF but not the dLF. Thus, despite a decreased ability to use such anticipatory control, there were signs of improvement during development.

Another marker of a well-learned or mature force-coordination pattern is small trial-to-trial variation. Commonly, large variation is seen in young but typically developed children for various types of movements such as grasping, reaching, and postural control.1,11,48 As children get older the variation generally decreases. A common characteristic seen in kinetic and kinematic studies of children with CP and children with minor dysfunctions such as attention-deficit/hyperactivity disorder49,50 is large intertrial variation. Therefore, the decreased CV for the total task time, loading phase, negative load force, static grip force, and the derivative of grip force is of interest, because it is an additional indicator of development with more stable performance. Similarly, the GPA indicated a decrease in the variability in the pattern of force application.

Limitations
There may be several confounding variables and methodologic concerns when repeating a study with 13 years in between data sessions. For example, there could be changes in the participants' concentration and ability to focus on the task, which could influence the results. Furthermore, we were unable to determine the extent to which improvements occurred naturally or were a result of specific (or combinations of) treatments. To minimize problems associated with the long interval between testing sessions, we attempted to repeat the procedures as carefully as possible, and all data were reanalyzed with the same criteria. The most obvious difference was the instrument and the size of the contact pads (the new instrument had smaller contact pads). The smaller contact size made it more difficult to grasp and, thus, may bias the result by showing less improvement than actually occurred. Finally, the sample size is small, and it is unknown whether the rate of improvement was constant in between testing sessions or whether the changes were discontinuous or affected by factors such as growth spurts. The results suggest the importance of conducting a prospective study on a larger cohort of participants.

Implications for Treatment
The term "cerebral palsy" has recently been redefined to include movement disorders that cause activity limitations as defined by the World Health Organization (ie, "the execution of a task or action by an individual"51). Grasping and object manipulation are important components of activity performance; thus, interventions that improve these skills may have important functional implications. In this study, manual ability during functional tasks (both clinical and experimental data) improved in terms of both speed and movement efficiency, which suggests that improvement in hand function occurs over a longer time frame than commonly would be expected. This occurred despite the heterogeneity found in this patient population, which suggests that the developmental changes are robust. Thus, the improvement in hand function is not only related to the early development. However, improvement may not have been apparent earlier, because manipulative demands of age-appropriate functional activities increase with development, perhaps excluding these individuals from more tasks. Although we were unable to determine efficacy of any treatment approach used for the participants, the fact that there was improvement indicates that development of hand function is not static. Furthermore, it suggests that hand function may be amenable to treatment. Although there is limited evidence for most treatment approaches,52,53 recent evidence suggests that therapies that focus on intensive practice such as CI therapy are promising (eg, see refs 26–32). Such intensive treatments can also be reflected by changes in cortical activation patterns after both CI therapy and task-oriented training as measured by functional MRI.54 The improvements in hand function during development provide another theoretical basis for providing intensive practice, suggesting functional and structural plasticity in the human brain.


   APPENDIX: EXPERIMENTAL DETAILS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 APPENDIX: EXPERIMENTAL DETAILS
REFERENCES
 
Materials
Jebsen-Taylor Test of Hand Function
This test of hand function consists of 7 tasks: writing; turning cards over; picking up small, commonly encountered objects; simulated eating; stacking checkers; and moving light (250-g) and heavy (500-g) cans. The writing task was excluded, because individuals with hemiplegia would never write with their hemiplegic hand. Each subtest is quantified as the time (in seconds) it takes to accomplish the task with the hemiplegic hand for participants with hemiplegic CP and the dominant hand for participants with diplegic CP. For each of the tasks the participants were first given the opportunity to practice it before executing it as part of the evaluation. The best performance of 2 trials was used for evaluation. If a participant was unable to complete a task or if task completion exceeded 3 minutes, he or she was arbitrarily assigned a score of 180 seconds on that subtest (in accordance with the test guidelines).36

Measurement of Fingertip-Force Coordination During Precision Grip
To quantify fingertip-force coordination during precision grip, 2 grip instruments were used. Both objects were manufactured by Umeå University (Umeå, Sweden) and were a modified version of an object described earlier.1,55 The grip instrument used for the first session was replaced with a new one before the second data session because of normal wear and tear. The grip surfaces were located at the top of the object and covered with sandpaper. Both objects contained a slot in their base in which appropriate masses could be inserted to change the weight of the objects without changing their visual appearance. Each instrument was calibrated and verified before data collection by applying known weights to it. Each instrument had a resolution of <0.1 N.

The instrument used in the first session had parallel contact pads of 35 x 35 mm, 20 mm apart. The grip force (normal force) at each contact surface and the total load force (tangential to the digital pulps) were measured with strain gauge transducers (DC-160 Hz). Vertical movement was recorded by using a camera housing a light-sensitive photoresistor that senses the position of an infrared light-emitting diode attached to the object (DC-110 Hz, resolution <1 mm).11 The instrument used in the second session had smaller contact pads (15 x 15 mm). The grip and load forces from each contact surface were measured separately by using strain gauges (ATI Industrial Automation, Apex, NC). An electromagnetic position-angle sensor (Polhemus Fastrack, Colchester, VT; 0.30-mm resolution) that was mounted on the apparatus measured the vertical position. The signals from the force transducers and position sensors were sampled at 400 and 120 Hz, respectively, into a flexible computer system (SC/Zoom, University of Umeå).

Data Collection and Analysis
Coordination of Fingertip Forces
The contact phase was the temporal delay between the grip of each opposing digit on the contact surfaces (grip force >0.1 N). The preload phase was defined as the duration between establishing contact and the onset of positive load force >0.1 N. The loading phase was the duration of positive load force until the object overcame gravity (ie, the object's weight) and the object was lifted. The dGF (dGF/{Delta}t) and dLF were calculated by using a ±10-point numeric differentiation, and both the amplitude and timing (relative to the onset of the loading phase) of the maximum rates were recorded. The negative load force was measured as the lowest value during the preload phase. The grip force at onset of positive load force was measured as well as the peak grip force. The static grip force was measured while the object was held in the air for 2 seconds, starting 500 milliseconds after peak grip force (the mean grip force during this "static phase" was calculated). In addition, the CV (SD/mean) was taken for each measured variable and used to determine within-subjects variability. The mean and SD of individual means are shown in the text and tables.

Grip-Force/Load-Force Path Ratio
The ratio of grip-force and load-force application was calculated as the ratio of the length of the grip-force/load-force path actually traveled to that of an ideal straight line between zero force and the grip and load force at liftoff (with the latter defined as the gravitational force).A grip-force/load-force path ratio of 1 represents a linear force increase until liftoff (ideal), whereas a grip-force/load-force path ratio >1 represents a curved force trajectory.

GPA
In applying GPA, each configuration (eg, grip-force/load-force trajectory) is represented by a set number of landmarks. After size normalization, GPA derives the mean of a set of configurations (called a "consensus") by translating and rotating the superimposed landmarks using an interactive least-squares procedure. Thus, GPA removes variations caused by the actual force amplitudes (extrinsic variability) and represents the intrinsic variability. Subsequently, root-mean-squared (RMS) residuals are derived, which indicate deviations of a specific configuration's landmarks from the homologous landmarks of the consensus. Variability of a set of configurations around the consensus is measured by using the mean of the residuals.39–41

To perform the analyses, each trial was resampled by selecting 25 equally spaced data points for the grip-load-force path taken from finger contact through liftoff for each trial. This number was selected on the basis of the trials with the smallest number of data points (shortest time). For each participant's collected session, GPA was conducted on the set of 10 trials (trials 1–10). The mean RMS residuals for the 10 trials within each collected session were calculated to indicate the intrinsic shape variability and were compared between the 2 sessions. To verify if the force pattern (grip–lift synergy) during the 2 sessions had different shapes in their intrinsic patterns, an overall mean residual was derived by averaging the mean RMS residuals of both sessions of each subject. Next, a new consensus was obtained for each subject by pooling the 10 configurations from each of the 2 collected sessions and then deriving new mean RMS residuals (pooled residuals) on the basis of the deviation of the 20 configurations from the new consensus. Changes in the intrinsic pattern were evaluated by comparing overall and pooled residuals. If there were no appreciable change in the consensus from one session to the next, then the simple and pooled residuals would not differ (see ref 41). Conversely, if the mean residual distance between the individual trajectories and the consensus changed, the residuals would differ between the individual and pooled analyses.


   ACKNOWLEDGMENTS
 
This work was supported by National Institutes of Health grant HD 40961 from the National Center for Medical Rehabilitation Research (National Institute of Child Health and Human Development); the United Cerebral Palsy Research and Education Foundation; and the Josef and Linnea Karlssons minnesfond.

We are grateful to the participants for volunteering their time.


   FOOTNOTES
 
Accepted Feb 6, 2006.

Address correspondence to Andrew M. Gordon, PhD, Department of Biobehavioral Sciences, Box 199, Teachers College, Columbia University, 525 W 120th St, New York, NY 10027. E-mail: ag275{at}columbia.edu

The authors have indicated they have no financial relationships relevant to this article to disclose.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 APPENDIX: EXPERIMENTAL DETAILS
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