J. Child Psychol. Psychiat. Vol. 29, No. 5, pp. 691-701, 1988 0021-9630/88 $3.00 + 0.00Printed in Great Britain. Pergamon Press pic

© 1988 Association for Child Psychology and Psychiatry.

PATTERNS OF ROTARY PURSUIT PERFORMANCE IN CLUMSYAND NORMAL CHILDREN

RICHARD LORD* and CHARLES HULME*

Abstract—The present study is concerned with the development of motor programs in clumsy children.In order to investigate this, the performance of clumsy and normal children on a rotairy pursuit trackingtask was compared. The performance of the clumsy group was inferior to that of the control groupin terms of time on target, but the pattern of performance across successive trials was broadly similarfor the two groups, suggesting a progression from control by visual feedback to control by motorprograms. It was concluded that the performance of clumsy children on the rotary pursuit task maybe limited more by impaired visual feedback control than by an impairment in the ability to developmotor programs.

Keywords: Clumsy children, rotary pursuit tracking, motor programming, motor impairment

INTRODUCTION

A SUBSTANTIAL PROPORTION of chOdren experience developmental disabilities of varioustypes, involving motor, cognitive and communicative development, which cannotreadily be explained. Across the spectrum of developmental disabilities, the mostfrequent signs leading to medical referral are those related to motor output (Denckla,1984). Children with such problems are commonly referred to as "clumsy". Clumsychildren experience difficulties in performing both gross and fine motor skills, despitenormal intelligence and virtually normal sensation and co-ordination by the standardsof conventional neurological assessment (Cubbay, 1975). Clinical studies of clumsychildren often iterate the idea that clumsiness may be the result of either perceptualdifficulties or of' 'motor organisation'', or both (e.g. Dare & Cordon, 1970; Gubbay,1975). This is reflected in the use by some authors of the adult neurological termsagnosia and apraxia to denote, respectively, perceptual and motor organisation problems(e.g. Walton, Ellis & Court, 1962; Cubbay, 1975). Walton et al, in fact, suggestthat "it is never possible to distinguish completely apraxia from agnosia, for defectsof recognition almost invariably lead to defects of execution".

Previous research has demonstrated wide ranging deficits in the perceptualprocessing of visual information in clumsy children (Hulme, Biggerstaff, Moran &McKinlay, 1982a; Hulme, Smart & Moran, 1982b; Hulme, Smart, Moran &McKinlay, 1984; Lord & Hulme, 1987). Unfortunately, however, the concept of motor

Accepted manuscript received 7 March 1988

* Department of Psychology, University of York, U.K.Requests for reprints to: Dr. Charles Hulme, Department of Psychology, University of York,

Heslington, York YOl 5DD, U.K.

692 R. LORD and C. HULME

organisation is not dearly defined in clinical studies, and there appears to be no attemptreported in this literature to measure motor organisation in clumsy children.Impairment in motor organisation seems simply to be inferred from the inability toperform certain tasks, without addressing possible underlying causes, and as suchappears to be merely a restatement of the diagnosis of clumsiness. Dare and Cordon(1970), however, offer the description of motor organisation as the "ability to buildup the patterns of movement essential for the smooth performance of any motorfunction". If this is interpreted as the ability to develop motor programs, then it isimplied that the processes involved in motor programming may be impaired in clumsychildren. Motor programs are well-learned sequences of movement, which are carriedout without recourse to feedback. As such, they are postulated to explain the abilityto overcome the finite delays resulting from reaction times and refractory periodsinherent in feedback control, and so enable performance of be smoother or morecontinuous in nature.

It is evident that we badly need experimental studies which analyse the motor skillsof clumsy children. Recently, van Dellen (1987) has examined the performance ofdiscrete hand movements to stationary targets in clumsy children. In this task childrenhad to move their hand rapidly from one button to another in response to a signal.Clumsy children showed longer reaction times and made more errors on this taskthan normal children and were more adversely ciffected by increases in task complexity.It was concluded that the clumsy children had difficulty in translating stimulusinformation into an appropriate response code.

We were concerned to look at the development of motor programs in clumsy childrenin a simple task where rapid learning normally takes place. The concept of motorprogramming has been studied experimentally in a variety of ways (see, e.g., Sheridan,1984). One way in which the measurement of motor programming has beenoperationalised in experimental studies is through the use of rotary pursuit trackingtasks, improvement on which is regarded as a measure of the development of motorprograms (Eysenck & Frith, 1977). Rotary pursuit tracking entails following a movingtarget, usually a light source, with a stylus, where performance is measured in termsof the amount of contact between the target and stylus. From an extensive reviewof studies of rotary pursuit tracking, Eysenck and Frith (1977) have concluded thatpursuit tracking involves two components. The first is a "response" consisting ofthe detection and correction of any mis-match between the stylus and the target. Thisis characterised as control by visual feedback. The second component involvessequences of movement which anticipate the movements of the target which, onceinitiated, are carried out without visual feedback. These sequences of movements,based upon previously acquired knowledge concerning the spatial and temporalcharacteristics of the movement of the target, are termed "motor programs" (Keele& Posner, 1968). The use of these motor programs enables the subject to make fewerdetection and correction responses.

In a study by Frith and Frith (1974), the rotary pursuit tracking performance ofa group of Down's syndrome children, who commonly demonstrate extremeclumsiness, was compared with that of groups of normal and autistic children. It wasfound that, in contrast with the normal and autistic children, the Down's childrenfailed to show evidence of improvement, in terms of total time spent on target, following

ROTARY PURSUIT 693

a rest pause. Frith and Frith interpreted this result as evidence for a specific deficitin the development of motor programs in Down's syndrome children. They arguedthat while these children may by unimpaired in tasks involving feedback control, theyeither fail to develop motor programs, or take much longer to do so than normalchildren. Subsequent studies have confirmed the findings of Frith and Frith, althoughthe adequacy of the explanation of their results in terms of a deficit in motorprogramming has been questioned (for a review, see Henderson, 1986).

The present study was designed to investigate whether clumsy children demonstratea difficulty in the development of motor programs. Typically, the pattern ofperformance across repeated trials in normal subjects would be one of negligibleimprovement during periods of continuous practice, with considerable improvementfollowing a rest interval (Eysenck, 1965). The post-rest improvement is referred toas reminiscence, and has been defined by Osgood (1953) as a "temporary improvementin performance, without practice". Reminiscence is measured by calculating thedifference between performance immediately prior to the rest and that after the rest(Rachman & Crassi, 1965). According to Eysenck and Frith, reminiscence is dueto the consolidation of learning, which occurs during the rest interval. In the presentexperiment, it would be expected that the overall performance of the clumsy children,in terms of total time spent on target, would be inferior to that of the normal children,since clumsy children perform more poorly on most motor tasks, possibly for a varietyof reasons. However, if the clumsy children do not show the typical pattern of improvedperformance following rest-intervals, then that may be regarded as evidence of a deficitin motor programming ability. Such a deficit would have considerable implicationsfor the understanding and possible remediation of the motor problems of clumsychildren.

METHOD

SubjectsThere were 19 children (16 boys, 3 girls) in the clumsy group, who had been referred to a Regional

Child Development Centre because of significant motor problems. Clumsiness was confirmed on thebasis of age-appropriate motor tests, but full conventional neurological examination gave no evidenceof overt physical or neurological impairment. The 19 children in the control group were individuallymatched for age and sex.

Details of age and IQ for the two groups are given in Table 1. Verbal IQ was measured by theVocabulary and Similarities sub-scales of the WISC, and performance IQby the Block Design andObject Assembly sub-scales. This short form of the WISC has a validity coefficient of r = 0.943 againstthe full test (Sattler, 1974), and also provides a relatively independent assessment of verbal and spatialabilitv.

TABLE 1.

AgeVIQPIQ

AGES AND VERBAL AND PERFORMANCE I Q S

AND CONTROL GROUPS

ClumsyMean S.D.

9.87106.2683.37

1.3418.3120.89

FOR CLUMSY

ControlMean S.D.

9.82114.58113.21

1.3217.1820.46

694 R. LORD and C. HULME

There was no difference between the ages of the two groups {F[l,36\=0.0l, P=0.9), and no significantdifference between the two groups on verbal IQ(i^l ,36] = 2.08, P = 0.16). The difference on performanceI Q however, was highly significant (F[l,36] = 19.8, P< 0.0001).

Motor measuresIn order to provide an independent assessment of the motor abilities of the clumsy children, a short

battery of tests, measuring fine and gross motor skills, was included.

(A). Balance (BAL). The child was asked to balance for as long as possible on one foot.

(B). Throw-Clap-Catch (TCC). The task was to throw a tennis ball into the air, clap hands variousnumbers of times, and catch the ball. The score given was the number of trials successfully completedout of 15 (Gubbay, 1975).

(C). Skipping (SKIP). The number of consecutive skips with a rope was recorded for the best of threeattempts.

(D). Bead-threading (BEADS). Time taken to thread beads on a string was recorded (Stott, Moyes& Henderson, 1984).

The means and S.D.s for these measures are given in Table 2.

TABLE 2. MEANS AND S.D.S OF MOTOR MEASURES

Clumsy ControlMean S.D. Mean S.D.

TCCBALBEADSSKIP

05.5715.5758.7903.47

03.7613.8314.7209.03

11.3140.9239.8319.10

02.9211.4908.2415.38

27.5337.7423.9914.58

The performance of the Clumsy group was significantly inferior (/"< 0.001) on all four of the motormeasures.

Handwriting. The subjects were asked to write the sentence "The quick brown fox jumped over thel2izy dog", in their normal handwriting, using a pencil, on lined paper, but to write as neatly as they could.

Two teachers rated the handwriting samples, independently, in terms of neatness for the age of thechild. The teachers were asked to assign scores on a scale from 1 to 9, 1 being poor, 9 being goodand a score of 5 representing average performance. The agreement between the two teachers wasmoderately good (Spearman r = 0.7). The hiandwriting of the clumsy children (mean 3.60, S.D. 1.55)was rated, on average, as being less neat than that of the control group (mean 5.58, S.D., 1.38), andthis difference was significant (i^l,36] = 17.21, P < 0.0001).

Thus, the clinical diagnosis of clumsiness was confirmed.

ApparatusA pursuit rotar (MK3/T, Forth Instruments Ltd) was used, in which a radial strip of light, set on

a revolving turntable, serves as the target. Above this, a sheet of light-proof plastic with a clear circle2 cm wide and 27 cm in diameter was placed. This resulted in a 2 cm square patch of light being visible,moving around the track. The speed of revolution of the target could be continuously varied from5 to 50 r.p.m. The stylus was a 16 cm long, pen-like rod with a photo-electric cell at its tip, whichclosed a relay whenever it was over the target patch of light.

The apparatus was able to record the total time on target throughout the trial, and time on targetfor constituent sample periods of 10 sec. In addition, an auditory tone was emitted whenever the styluswas over the target.

ROTARY PURSUIT 695

ProcedureAfter demonstration by the experimenter, the children held the stylus in the preferred hand, in the

manner in which they would hold a pencil to draw with. They were instructed to try to keep the stylus overthe target as much as possible. Three 2 min trials were given, each separated by a 5 min rest pause,with the target speed set at 15 r.p.m. Total time on target and time on target for each 10 sec sampleperiod were recorded for each trial. In addition, the tone emitted by the apparatus whenever the stylusis over the target was recorded, to allow a detailed analysis of the pattern of performance.

RESULTS

In order to analyse the pattern of performance, three measures were obtained: timeon target, hit-rate and mean hit length.

Time on targetIn order to establish that substantial learning does not take place during periods

of continuous practice, as suggested by Eysenck and Frith (1977), performance onthe first period of each trial (Period 1) was compared with that on the last period(Period 12). The time on target for the first and last 10 sec periods for each trialare shown in Table 3.

TABLE 3. MEANS OF TIME ON TARGET (SEC) FOR FIRST AND LAST

1 0 SEC PERIODS, FOR TRIALS 1 - 3 , FOR CLUMSY AND CONTROL GROUPS

PeriodTrialTrialTrial

123

344

1.69.57.84

Clumsy

333

12.50.68.64

677

Control

1.23.06.49

126.206.316.96

Analysis of variance for Trial 1 yielded a significant groups affect (/{l,36] = 14.249,P< 0.001], a non-significant periods effect (i^[l,36] = 0.199, F< 0.658) and a non-significant interaction {I\l ,36j = 0.105, F< 0.747), indicating that the clumsy groupperformed more poorly than the control group, but that the performance of neithergroup changed significantly during the trial.

For Trial 2, the groups effect was again significant (F[l,36] = 12.379, P < 0.001).This time, however, the periods effect was also significant (7̂ 1̂ 1,36] = 7.857, F< 0.008),which actually reflected a decline in performance levels by both groups. The interactionwas not significant (F[l,36] = 0.055, P < 0.816).

The pattern for Trial 3 was the same as for Trial 2: a significant groups effect(/'[1,36] = 21.993, F< 0.001), a significant periods effect (F[l,36] = 9.624, F< 0.004)and a non-significant interaction {F[l,36] = 1.407, ? < = 0.243). Again, performancedeclined over the trial as a whole. Thus, neither group can be said to have learnedappreciably during the actual periods of continuous practice, and for Trials 2 and3 there was a decline in performance for both groups. This gradual decrease inperformance is termed the post-rest down-swing, and is ascribed by Eysenck and Frith(1977) to the destruction of partially consolidated material.

696 R. LORD and C. HULME

Another measure of interest is the total time on target for each of the three trials.The group means and standard deviations are given in Table 4 below.

A split-plot analysis of variance was performed for the total time scores, whichyielded a significant groups effect (JP[1,36] = 15.695, F< 0.001), a significant trialseffect (F[2,72] = 31.917, F< 0.001) and a non-significant interaction {F[2,72] = 1.716,F< 0.187). The clumsy group seem to have been on target significantly less thanthe control group, and performance of both groups differed across trials. Inspectionof the means in Table 4 suggests that this is largely due to an improvement inperformance of both groups on Trial 2 compared to Trial 1.

TABLE 4. MEANS AND S . D . S OF TOTAL TIME ON TARGET FOR TRIALS

1 TO 3 SEC, FOR CLUMSY AND CONTROL GROUPS, MAX = 1 2 0 SEC

TrialTrialTrial

123

ClumsyMean

42.9050.3251.14

S.

222425

D.

.83

.16

.27

ControlMean

70.6381.2884.25

S.

262521

D.

.68

.48

.41

In order to look at improvement following rest intervals, i.e. reminiscence, split-plot analyses of variance were computed for pre-rest performance (Period 12) andpost-rest performance (Period 1), for the 5 min rest intervals following Trial 1 andTrial 2. For the first rest interval, there was a significant groups effect (F[l ,36] = 11.220,P < 0.002), as would be expected. The trials effect was also significant(jF[l,36] = 17.355, P < 0.001), which in the absence of a significant interaction(F[1,36] = 0.207, F= 0.651) indicates that the performance of both groups improvedfollowing the rest pause.

For the second rest interval, the picture was essentially the same: a significamt groupseffect (/^[1,36] = 13.908, P < 0.001), a significant trials effect (i^[l,36] = 19.514,P < 0.001) and a non-significant interaction (P[l,36] = 0.002, P = 0.963). Again, theperformance of both groups improved following the rest interval.

Thus, although the performance of the clumsy group overgdl was inferior to thatof the control group on these measures, the pattern of performance was essentiallythe same for both groups.

Hit rateTo investigate the pattern of performance further, hit-rate, that is, the number

of times the stylus was held over the target, was counted for each of the three triads.Group means and standard deviations are given in Table 5 below.

TABLE 5. MEANS AND S . D . S OF HIT-RATES FOR TRIALS 1 TO 3 FOR

CLUMSY AND CONTROL GROUPS

TrialTrialTrial

123

ClumsyMean

86.9486.5283.57

S.

241723

D.

.19

.32

.57

ControlMean

73.0065.3660.31

S.

242721

D.

.03

.31

.17

ROTARY PURSUIT 697

Split-plot analysis of variance of the hit-rate scores gave a significant groups effect(P[l,36] =8.439, P < 0.006), a significant trials effect (P[2,72] = 3.769, P < 0.028)and a non-significant interaction {F[2,72\ = 1.396, P= 0.254). The clumsy group scoredsignificantly more hits than the control group, and hit-rate declined over trials forboth groups, as performance improved.

Mean hit lengthNext, mean hit length was computed for each of the three trials. Group means

and standard deviations are shown in Table 6 below.

TABLE 6. MEANS AND S . D . S OF MEAN HIT LENGTHS FOR TRIALS 1

TO 3 (SEC) FOR CLUMSY AND CONTROL GROUPS

Clumsy ControlMean S.D. Mean S.D.

Trial 1 0.48 0.26 1.26 1.27Trial 2 0.59 0.32 1.83 1.74Trial 3 0.62 0.35 1.76 1.22

A split-plot analysis of variance of mean hit length scores gave a significant groupseffect (F[l,36] = 10.927, P < 0.002), a significant trials effect (F[2,72] = 8.832,P< 0.001) and a significant interaction (F[2,72]= 3.476, P= 0.036). Mean hit lengthfor the clumsy group, although lower than for the control group, shows a steadyimprovement across trials, while for the control group, a large increase in mean hitlength in Trial 2 is followed by a slight decrease in Trial 3. Possibly the control grouphave reached their asymptote during Trial 2, leaving little room for improvement insubsequent trials.

In order to examine the differences between performance on Trials 1, 2 and 3 foreach of the two groups, Tukey's HSD was computed, giving HSD = 0.129. Thisindicates that for the clumsy group the differences between successive trials were notsignificant, but the mean hit length for Trial 3 was significantly greater than thatfor Trial 1. For the control group there was a significant increase in mean hit lengthfor Trial 2 compared to Trial 1, but the decrease for Trial 3 was not significant. Thusboth groups showed significant increases in mean hit length, and in view of theinteraction between trials and groups, it appears that this increase was greater inthe control group.

Although there was considerable overlap between the time on target scores for thetwo groups (clumsy group 7.0-88.5 sec, control group 19.5-112.3 sec) theperformance of the clumsy children was inferior to their control counterparts in allbut two cases, the first of whom showed a decline in time on target across the threetrials. There was similar overlap for hits (clumsy group 32-125, control group 17-115)and mean hit length (clumsy group 0.19-1.39 sec, control group 0.34-6.58 sec), thoughonly one clumsy child achieved greater mean hit length than his control.

698 R. LORD and C. HULME

DISCUSSION

According to Eysenck and Frith (1977), to be successful during rotary pursuittracking the subject must observe whether the stylus is in contact with the target,and make appropriate corrective movements. These detection amd correction responsesmust necessarily be intermittent, because of limitations to the system imposed byfinite reaction times and refractory periods. In between these responses, movementsare controlled by motor programs. Consequently, there are three major determinantsof the efficiency of tracking performance. These are the ability to detect errors, theability to make appropriate corrective movements and the ability to develop motorprograms. In the present study, the detection of a mis-match between the stylus andtarget was greatly facilitated by the presence of an auditory tone whenever the styluswas in contact with the target. With respect to corrective movements, Eysenck andFrith have argued that the relationship between the perceived error and the appropriatecorrective movement is a very direct one, which corresponds closely to the well-learnedact of reaching for an object. Thus, they maintain that this ability will change littleduring practice. Eysenck and Thompson (1966) have presented evidence to supportthe contention that the learning (as opposed to the performance) of rotary pursuittracking does not depend substantially on detection and correction responses. Theyfound that a distractor task introduced in the second of three practice periods reducedthe rate at which subjects made corrective responses, and thus total time on targetcorrespondingly decreased, in proportion to the degree of distraction. However, inthe third session, without distraction, there was clear evidence of reminiscence.

This means that improvement in performance depends largely on the ability todevelop predetermined sequences of movement, which requires the subject to predictthe movement of the target, and then convert this knowledge into appropriate handmovements. With learning, these sequences should become longer and more accurate.Frith (1971) has shown that learning of this type is principally responsible for thephenomenon of reminiscence. He found that in variable speed tracking, the greatestreminiscence occurred for the fastest component of the track, where control mustdepend on motor programs. Eysenck and Frith (1977) suggest that a rest is necessaryfor knowledge about the movements of the target to be translated into the appropriatesequence of movement, or, in other words, for consolidation to take place. Analternative explanation, that the reminiscence is due to release from inhibition builtup during pre-rest practice, has been discredited by the finding (Rachman & Grassi,1965) that reminiscence was eliminated in groups who performed an interpolatedtask (reversed cue pursuit) immediately after initial practice, disrupting consolidation,but was evident in a control group, despite a rest period of 4 hr, during which timeany inhibition would have dissipated.

The involvement of inhibition is further ruled out in this experiment by the factthat there was no decline in performance, in terms of time on target for 10 sec sampleperiods, for either group in Trial 1, which a build-up of inhibition would produce;but for Trials 2 and 3, reminiscence was followed by post-rest down-swing, whichis consistent with the destruction of partially consolidated material.

The patterns of performance for the clumsy and control groups may be consideredwithin the framework of Eysenck and Frith's (1977) analysis. As expected, the overall

ROTARY PURSUIT 699

performance of the clumsy group, in terms of time on target, was inferior to thatof the controls. In the first trial both groups will be operating under control by visualfeedback, since no suitable motor programs are available. It is not surprising, therefore,that the clumsy group achieve less time on target, not least because they have beenshown to perform poorly on tasks requiring the use of visual feedback, such as drawingand tracing (Lord & Hulme, 1988), although other factors may be involved. Thus,the baseline performance of the clumsy group, as expected, is lower than that of thecontrol group. Of miore interest, however, is the pattern of performance acrosssubsequent trials. This is best considered by looking at the performance of the twogroups separately, since, while the control group was fairly homogeneous with respectto changes in performance over trials, the clumsy group showed some degree ofheterogeneity.

Looking first at the control group in terms of total time on target, a markedimprovement for Trial 2 was followed by a more modest increase on Trial 3, suggestinga progression from dependence upon control by feedback to increasing developmentof motor programs. This is supported by a reduction in response rate across trialswith a concomitant increase in mean hit length. The relatively large number of hitsand shorter mean hit length on Trial 1 implies that the control children are operatingunder control by feedback at this stage, but in subsequent trials increased total timeon target is achieved through fewer but longer contacts with the target. Seventeenof the 19 control children conformed to this pattern, while two children showed noappreciable change in any of the measures across trials. This pattern supports Eysenckand Frith's (1977) contention that the learning of rotary pursuit tracking has littleto do with error detection and correction responses. The fact that performanceimproved as a result of increases in the duration of programmed sequences concurswith Eysenck and Frith's view, but it also appears to be the case that subjects weremaking their responses to errors more quickly on Trial 2.

For the clumsy group, a different picture emerged. Taken as a group, the patternfor total time on target was similar to that of the control group—a substantialimprovement on Trial 2 and a smaller increase on Trial 3. Again, a relatively largenumber of responses of shorter duration implies initial reliance on control by visualfeedback. However, inspection of individual performance suggests that considerablevariability underlies this apparent similarity. Three of the clumsy children actuallyshowed a decline in total time on target across trials. In two cases this was due toa decrease in hits, with mean hit length remaining fairly stable, while in the otherthe reverse pattern occurred. Reasons for deteriorations in performance are not clear.Of the remaining 16 clumsy children, 10 performed in a manner similar to the majorityof the control children—an increase in total time on target was effected through anincrease in mean hit length and a decrease in hit-rate. The other six chUdren, however,increased their total time on target through an increase in hit-rate without showingappreciable changes in the duration of responses. These children's performance seemsto reflect learning through improved error detection and correction responses.

In summary, there is some evidence for the development of motor programs in clumsychildren, given that 10 of the 19 clumsy children achieved improvements in per-formance by means of increases in the length of contact times and decreases in responserate, which are consistent with the development of motor programs. More difficult

700 R. LORD and C. HULME

to explain is the fact that some clumsy children showed deteriorations in performance,while another subgroup showed improvements through increases in hit-rate ratherthan through development of motor programs. It appears that the ability to developmotor programs is not as well developed in these children as in normal children. Theprocesses involved in motor programming are considered to be the decision and effectorprocesses of response selection and response preparation (Sheridan, 1984). Theresponse selection stage involves the selection of a generalised motor program (orschema), following perceptual processing of input information, while responsepreparation entails the specification of appropriate control parameters. Since thedevelopment of the scheme depends on perceptual processing, it would not besurprising, in view of the perceptual impairments demonstrated in clumsy childrenin other studies, if motor programming were less efficient in clumsy children thanin normal children. For example. Lord and Hulme (1987) found that clumsy childrenwere impaired on visual tasks involving spatial information such as length and position.Clearly, if spatial information is coded inaccurately by the visual system of clumsychildren, we should expect them to have difficulty with the tracking task. However,according to this view the ability of some clumsy children to learn this type of taskmight not be limited by an inability to develop programmed sequences of movementsas such, but rather might be limited by their poor visual feedback control denyingthem the opportunity to gain sufficent knowledge of the movement of the target toallow the expression of such ability that they do possess. Indeed, almost all of theclumsy children who did not show evidence motor program development were poorperformers in terms of total time on target. There are a number of reasons why clumsychildren may be impaired in the use of visual feedback control, including problemswith visual detection of errors, selection and execution of corrective responses,appropriate timing of responses and attentional factors. Further research is requiredto separate the effects of these factors.

Acknowledgements—We gratefully acknowledge the assistance of Dr. S. H. Roussounis, ConsultantPaediatrician, and Ms. R. Straw, Senior Occupational Therapist, Regional Child Development Centre,St. James University Hospital, Leeds, and the children who took part in the study, their parents, teachersand schools.

This research was supported by a Medical Research Council Studentship to the first author.

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