Human Movement Science 1 (1982) 57-72,

North-Holland Publishing Company 57

THE COMPONENT AWARENESS AND ACCURACY OF DUAL- AND SINGLE-JOINTED ACTIONS *

Ann HARRISON and Andrew BISHOP

Sheffield University, UK

Harrison, A. and A. Bishop, 1982. The component awareness and accuracy of dual- and single-jointed actions. Human Movement Science 1, 57-72.

The accuracy with which people execute wrist and elbow movements were measured using three

main conditions: (i) sing/e-jointed (wrist or elbow) movements to targets, (ii) dual-jointed (wrist and elbow) movements to targets, and (iii) components of dual-jointed movements to targets, when

the task for the subject was to perform the elbow or wrist constituent of the action in isolation,

without displacing the second joint. Elbow precision was significantly worse under component

than dual conditions, which is compatible with the notion that wrist and elbow activity are

conjugately, rather than independently, programmed when a dual-jointed action is performed. The

pattern of wrist accuracy was divergent, but possible reasons for this were discussed. In all cases,

error was measured in terms of deviation from perfect posture; using this index, the hypothesis that incorporating more moving joints into an action serves to increase movement complexity and

jeopardise precision was tested, but the results were ambiguous. Discussion also centred on the

problems of using performance data to infer changes in motor programming, and the need for

rigorous conceptualisation and research in this area.

Introduction

A major question in the area of human motor control concerns how it is possible for a skilled performer to cope with wide variations in condi-

* The work reported in this paper forms part of the research programme supported by a grant

from the National Fund for Research into Crippling Diseases (grant number 42 1700). Dr. Bishop was supported by a research studentship from the Social Science Research Council. Both are

gratefully acknowledged.

Reprint requests should be addressed to Ann Harrison, Dept. of Community Medicine and

Behavioural Sciences, Faculty of Medicine, Kuwait University, P.O. Box 24923, Kuwait.

58 A. Harrison, A. Bishop / Accuracy ofjointed uctiom

tions without expending excessive amounts of processing capacity. The exact parameters of a task can vary enormously, and with them the pattern of movement needed for an adequate performance; indeed performers are often faced with novel external demands, as happens when a fielder catches a ball coming towards him at an angle he has never encountered before. Internal factors add to this variance: for the precise efferent changes required to produce a given movement will depend on such things as body posture and what other motor activity is current (for details, see Turvey et al. (1978)). If a performer was capable of adequately surveying external and internal conditions and ascertain- ing what pattern of movement would satisfy the goals set. and moreover knew how to effect this given the internal conditions pertaining, then in theory he could prescribe a suitable pattern of efferent changes; and with adequate monitoring and updating, he could ensure a successful outcome. Such a ‘keyboard’ model seems untenable, however, for any but the very simplest of actions because of the excessive amounts of attention, knowledge and computation involved. One possible strategy for reducing executional demands is to distribute responsibility. Bruner (1970) and Connolly (1970) speculated that the motor equivalent of computer programme ‘sub-routines’ exist to be used as programming units, eliminating the need to generate their constituent commands. Turvey and his colleagues (1978) extended this idea, using the term ‘co-ordinative structure’ to describe “a group of muscles, often span- ning several joints that is constrained to act as a unit” (1978: 563). An abstract structural prescription determines the relative activity of con- stituent muscles, and so the type of action the collective performs. Such an arrangement has the potential for enormously reducing executional load, a single command to the co-ordinative structure being sufficient to trigger a whole series of motor events which would otherwise have to be individually programmed. Changing the structural prescription alters the type of action performed, whilst speed and force are controlled by the metric input value; both mechanisms provide for flexibility in the functioning of the collective.

Turvey et al. (1978) describe a number of possible examples of physiological co-ordinative structures, but warn of the difficulties inher- ent in using overt activity to infer efferent programming. Resultant muscle activity depends not only on the efferent input, but also the context in which this is delivered, so making it problematic to infer or rule out co-ordinated efferent programming. Kots and Syrovegin ( 1966)

A. Harrison, A. Bishop / Accurcrcy ofjointed crctions 59

observed people performing arm actions, and noted that they use only a small number of speed ratios (speed of wrist movement: speed of elbow movement), and that the same ratio is sometimes utilised for more than one pattern of movement. Such stereotypy may indicate co-ordinated efferent activity, for people are capable of producing a vast number of ratios, and so mechanical constraints are not a cause. Bishop and Harrison (1977) provided some support for stereotypy in people’s use of speed ratios, but cautioned that this might be a response to the procedural demand that each action be performed many hundreds of times. A possible shortcoming of both the original and the replication study is that subjects were free to produce movements of any speed, size or force provided that they conformed to the general specification given (e.g. simultaneous wrist and elbow flexion, simultaneous wrist extension and elbow flexion). If co-ordinative structures do exist, it seems reasonable to argue that collectives develop and prescriptions are chosen in order to safeguard the performance of functional patterns of movement, and therefore these are more likely to be detected if pre- cisely localised actions are studied.

The present experiment investigated the execution of arm movements directed at external targets, with the twin aims of establishing whether accuracy is affected by the number of moving joints used, and what awareness people have of the constituent joint movements which com- prise a multi-jointed action. Displacements of the wrist and elbow were measured goniometrically; subjects viewed a target, and then with their eyes closed attempted to execute a suitable ‘ballistic’ action. After wrist and elbow angles had been measured, the subject opened his eyes and adjusted the configuration of his arm so as to be precisely on target. The joint angles originally produced were then compared with those needed for a perfect performance. The precision of dual-jointed (wrist and elbow) and single-jointed (wrist or elbow) actions were evaluated in this way. Various researchers (e.g. Brown et al. 1948; Begbie 1959; Beggs 1972) have speculated that increasing the number of joints to be controlled serves to increase movement complexity and jeopardise precison; error in such studies being measured in terms of effective deviation from the target. The present study provided an opportunity to investigate this issue further, looking separately at wrist and elbow performance. Targets for dual-jointed actions were selected on the basis that only one combination of wrist flexion and elbow flexion would bring the arm on target. Component awareness was assessed using the

60 A. Hurrison, A. Bishop / Accurucy of jornred actions

same group of targets; subjects were asked to think of the dual action which would be appropriate, and produce either the wrist or elbow component of this (whichever was demanded) in isolation, without any accompanying displacement of the second joint. The rationale being that if the component movements of a dual action are programmed independently, demanding them in isolation should not greatly affect accuracy; whereas if they are normally programmed as elements of a multi-joint unit, performance is likely to suffer. Performance under single-jointed action conditions provided essential control data, in case component performance is poor because single-jointed movements per se are executed less accurately than dual ones.

Method

Subjects

The group consisted of three 24 yr old male right-handed post- graduates.

Apparatus

Bourne 7335EA 2KSL wire-wound potentiometers (linear 5 1”) fitted with one fixed and one movable shaft were positioned so as to record

Fig. I. Goniometric recording of joint displacement.

A. Harrison, A. Bishop / Accuracy ofjointed actions 61

o TARGET

upper ‘ornl support

Fig. 2. Experimental arrangement.

faithfully changes in elbow and wrist posture, and then securely at- tached (fig. 1). The voltage outputs from these goniometers were dig- italised using an A-D converter, and selected data were stored on a Nova 840 computer. Joint angles were measured to an accuracy of about l/3”. A light-display interface guided the subject to adopt the starting posture specified (* 2’): separate lights indicated whether the wrist and elbow angles were correct or too large or too small. The same system was used to signal any significant (22”) movement of the ‘stationary’ joint during component testing. Plywood splinting was used to fix hand posture, whilst the upper arm was held firmly in a section of padded plastic guttering. A vertical metal rod fitted with a heavy base served as the target; this was moved to designated loci on a perforated horizontal board. The apparatus (fig. 2) was so arranged that when a subject moved his arm horizontally, his finger tips just cleared the top of the target.

Movement types

Three categories of movement were studied - dual-jointed, single-jointed and component; these involved flexions of the wrist and elbow in a horizontal plane (fig. 3). Two single-jointed wrist conditions were in- cluded in case accuracy is affected by distance: the near (SWn) one used targets approximately 20 cm from the subject’s eyes, and he began with his elbow almost fully flexed; the far condition (SWf) utilised targets about 40 cm away, and the starting posture was with the elbow

62 A. Hurrison, A. Bishop / Accurcrcy o/jointed actions

TARGET *\

Fig. 3. The types of movement executed

almost fully extended. For each type of wrist and elbow movement, a representative sample of 25 amplitudes was selected from the range 10” to 85”; these were tested in randomised order.

Procedure

In session 1, the accuracy of SWf, SWn and SE movements were assessed, always in this order. Session 2 concerned dual and component movement accuracy; for each of the 25 targets selected, dual (DE:DW), CE and CW precision were assessed in randomised order. Before any particular block of testing was embarked on, the subject adopted an appropriate starting posture, and wrist and elbow angles were measured (i) and stored. Th e subject used the same starting posture (~2”) for every target in a block, feedback being provided as described above. ‘Non-visual aiming’ (Beggs 1972) was employed: the subject viewed the target for a few seconds, and then with his eyes closed attempted to produce an appropriate movement. It was stressed that movements should be executed smoothly, without hesitation or end-correction. Feedback alerted the experimenter to any significant (k2’) movement of the ‘stationary’ joint during component testing; if this happened, the trial was aborted and repeated. When the subject had completed his

A. Hurrison, A. Bishop / Accuracy ofJointed crciions 63

attempt, wrist and elbow angles were again measured (ii). Then, with his eyes open, the subject adjusted the configuration of his arm so as to be precisely on target, and joint angles (iii) were noted. When DE:DW, CW and CE accuracy were being assessed for a given target, separate assessments of (iii) were taken; this was necessary because even slight movements of the upper arm could alter the precise location of the arm with respect to the target; however, for any one trial, measurements of movement amplitude and error were reliable.

Results

A small amount of data (4%) was lost because of equipment failure, but not enough to warrant concern.

Movement amplitudes

The aim in selecting target loci was to set a comparable range of movement amplitudes for each test condition. As already noted, how- ever, the amplitude of movement (iii - i) required to reach a given target was variable, with shifts in shoulder posture; and so it was necessary to check the amplitudes tested for comparability (fig. 4). Quite a wide variation was found both within and between subjects, the latter being acknowledged by analysing the data for different subjects separately. The real worry with high within subject variance is that if movement amplitude is found to affect accuracy, then a proper com- parison of test conditions may be precluded. Correlations were, there- fore, run to test this relationship. Two measures of error were used: error being the unsigned difference between the ‘target’ and ‘attempt’ joint angles i.e. iii - ii; and percentage error, error expressed as a percentage of the movement amplitude tested i.e. (iii - ii)/(iii - i) X

100%. In only three of the nine cases was there a significant relationship between error and amplitude, and in only one for percentage error (table 1). It, therefore, seems justified to dismiss variations in the amplitudes tested as contributing significantly to any differences in precision which may be found for single, dual and component condi- tions.

64 A. Harrison, A. Bishop / Accuracy of jointed actions

DWICW

-I

DEICE

Fig. 4. Target amplitude histograms for each subject, for each of the different types of movement

studied. (Ordinate= frequency; abscissa= target amplitude (iii - i)( 100 units = approximately 300).)

Table I Spearman’s Rank Correlations between movement amplitude and error and percentage error, for single movements.

Subject N Percentage error Error

RS P RS P

SWn Sl 25 0.042 “..Y. 0.263 n.s.

s2 25 0.245 “.S. 0.519 <O.Ol s3 24 -0.453 ==O.Ol -0.161 U.S.

SWf Sl 24 - 0.090 n.s. 0.222 U.S.

s2 25 -0.166 n.s. 0.70 I n.s. s3 24 0.340 n.s. 0.26 I “..S.

SE Sl 25 0.185 “..S. 0.419 < 0.05

s2 25 0.364 n.s. 0.41 I < 0.05

s3 25 -0.235 n.s. 0.924 n.s.

A. Hurrison, A. Bishop / Accurrrcy ofjoirtted uctions

ZE-OE

--__5J LL-91

z-o

ZE-OE

&A LL-91

z-o

-+9E

____J zc-06

Ll-91

z-o

Cc +9&

-_2d Z&-O&

LL-91

z-o

4 +9&

-d&l ZE-OE

ILL-91

z-o

4 +9c

4 ZE-OE

I

+A Z&-O&

LL-51

z-o OaacDbN

66 A. Harrison, A. Bishop / Accurticy of jotnted uctions

Table 2

Median error scores recorded for different movement conditions.

Wrist

SW DW CW

Elbow

SE DE CE

(1) Error (degrees)

Sl

s2

s3

Mean

1.3

2.0

3.0

2.1

(2) Percentage error (W)

Sl 3.6

s2 5.8

s3 9.9

Mean 6.4

5.0 6.0 1.3 1.7 4.0

4.7 7.0 1.3 2.7 Il.3

3.7 6.0 1.3 1.3 6.3

4.5 6.3 1.3 1.9 1.2

9.3 9.2 3.9

7.8 21.7 2.0

18.2 14.2 4.6

I I.8 15.0 3.5

5.5

5.2

3.3

4.7

21.1

18.6

15.3

18.3

Accuracy

A breakdown of error scores is given in fig. 5 and table 2; the marked tendency was for subjects to undershoot targets. For no subject did SWn and SWf errors differ significantly (Mann-Whitney lJ test, p < 0.05); and so these distributions were combined to simplify analysis, giving a maximum of 50 SW targets.

Table J Wilcoxon matched pairs signed rank tests comparing dual and component movement error and

percentage error scores.

Subject N Percentage error Error

T P T P

DW:CW Sl 23 132.5 U.S. 114.5 11.5.

s2 23 78.0 I1.S. 5 I .o 40.01

s3 24 130.0 I1.S. 95.0 U.S.

DE:CE SI 23 28.0 <O.Ol 22.5 <O.Ol

s2 23 35.0 <O.Ol 27.0 CO.01

s3 24 42.0 co.01 47.0 co.01

Tab

le

4

Man

n-W

hitn

ey

U-t

ests

co

mpa

ring

th

e er

ror

and

perc

enta

ge

erro

r sc

ores

as

soci

ated

w

ith

wri

st

and

elbo

w

mov

emen

ts,

unde

r di

ffer

ent

expe

rim

enta

l

cond

ition

s.

Subj

ect

Nl,

N2

Perc

enta

ge

erro

r E

rror

u z

P u

I P

SE

v.

SW

Sl

25.4

9 57

4.0

0.44

0 n.

s.

523.

5 1.

017

“..Y

.

s2

25.5

0 36

3.0

2.94

5 <O

.Ol

534.

5 1.

066

n.s.

s3

25,4

8 32

0.00

3.

255

<O.O

l 38

6.0

2.48

8 co

.01

DE

v.

DW

S1

23

,23

154.

0 2.

428

co.0

1 80

.5

4.04

2 <O

.Ol

s2

23,2

3 26

4.0

0.01

1 n.

s.

181.

5 1.

823

n.s.

s3

24,2

4 10

2.0

4.08

4 <O

.Ol

151.

0 3.

124

-=O

.Ol

CE

v.

CW

S1

23

,23

199.

0 1.

439

n.s.

20

4.0

1.32

9 “.

S.

s2

23,2

3 22

9.0

0.78

0 n.

s.

216.

0 1.

066

“.A

-.

s3

24.2

4 28

3.5

0.56

3 n.

s.

299.

0 0.

262

“..Y

.

Tab

le

5

Man

n-W

hitn

ey

U-t

ests

co

mpa

ring

th

e er

ror

and

perc

enta

ge

scor

es

asso

ciat

ed

with

si

ngle

an

d du

al

and

com

pone

nt

mov

emen

ts.

(I)

Wris

t SW

v.

DW

Subj

ect

NI,

N2

Sl

49,2

3 s2

50

.23

s3

4X,2

4

Perc

enta

ge

erro

r

u z

370.

5 2.

33 I

52

7.0

0.57

0 45

3.5

I .703

Err

or

h

P u

z 3

P 5.

8 P

CO

.05

284.

0 3.

376

<O.O

l h

n.s.

40

5.0

2.01

9 C

O.0

5 0

519.

5 0.

936

i n.

s.

n.s.

B

SW

v. cw

Sl

49

,23

323.

0 2.

905

<o.o

i 22

5.0

4.08

8

s2

50.2

3 23

5.0

4.03

2 <o

.oi

17R

.O

4.71

4 s3

48

.24

434.

0 1.

930

n.s.

41

4.5

2. I

56

(2)

Elb

ow

SE

v.

DE

Sl

s2

s3

25,2

3 23

9.0

0.99

1

25,2

3 15

2.0

2.19

6

25,2

4 28

2.5

0.58

2

i7.s

.

co.0

1

n.s.

<O

.Ol

KO

.01

<O

.OI

2X5.

0 0.

052

73.0

4.

42X

296.

0 0.

320

SE

v.

CE

Si

25

,23

103.

0 3.

808

s2

25.2

3 63

.0

4.63

3

s3

25,2

4 54

.5

5.00

6

132.

0 3.

209

73.0

4.

427

65.5

4.

793

<O

.OI

\ L <

O.O

l 2

CO

.05

%

$ 9 n.

s.

k;-

40.0

1 S’

n.s.

z f

<O.O

l 2 2

CO

.01

*

<O.O

l

A. Harrison, A. Bishop / Accurcrcy of jointed actions 69

(I) Dual-jointed: component comparisons In session 2, the same target was used three times, and so a Wilcoxon

matched pairs signed ranks test is appropriate (table3). All subjects executed DE movements significantly more accurately than CE move- ments. Performance of wrist movements under component conditions was likewise consistently worse (table2), but not statistically signifi- cantly so.

(2) Wrist: elbow comparisons Mann-Whitney U tests (table4) were used to compare wrist and

elbow errors (table 2). Precision did not differ under component test conditions. There was a trend for wrist accuracy to be poorer than elbow when single-jointed actions were compared, but significant dif- ferences were not consistently found. Elbow superiority was more marked under dual-jointed testing, and a paired t-test applied to the group’s error data confirmed this (t = 6.392, df = 2, p < 0.05).

(3) Single: dual and single: component comparisons Mann-Whitney U tests (table 5) were used to assess the differences

observed (table 2). There was very little evidence of greater precision under single-jointed than dual-jointed test conditions; but compared with component movements, single-jointed actions were executed with significantly greater precision. It must be noted that sessions 1 and 2 are not fully comparable. In the second, subjects attempted each target three times, and so received additional information about its locus each time (iii) was assessed. This cannot, however, explain the differences found, for it was the session 1 (single-jointed) movements which were performed most accurately.

Discussion

The results show that if a person does not execute both portions of a wrist: elbow action simultaneously then elbow precision suffers. This is not due to the poor accuracy of one joint movements per se, for elbow performance was as good under single-joint conditions as dual. Why then does component testing jeopardise precision? The non-availability of finger location cues seems an unlikely cause. Firstly because every effort was made to ensure that subjects used ‘ballistic’ movements

throughout, and never engaged in end-correction; and secondly because the wrist portions of dual actions proved to be the least accurate, and so it is very dubious whether using such information would enhance elbow precision. Whilst it is tempting simply to blame the ‘artificiality’ or ‘strangeness’ of performing part of an action in isolation, to do so begs the question of why this is so. For a priori it would seem perfectly feasible to have one system controlling the execution and evaluation of wrist components, and a completely separate system regulating elbow components. Even with such an arrangement, the performer may have difficulty identifying which motor program controls which portion of the action, because these may not be classified separately. It has been argued that people typically code muscle activity in terms of functional consequences, such as relocating a limb in space (Russell 1976; Harri- son 1975). If this is so, even though completely separate programs control the elbow and wrist portions of a dual action, these may be classified indistinguishably in terms of the resultant global pattern of movement. On the other hand, if theorists such as Turvey et al. (1978) Bruner ( 1970) and Connolly ( 1970) are correct, and wrist and elbow constituents are controlled as an entity, then component testing will stop the subject using his normal form of programming, and a loss of precision such as was observed would be expected. The evidence provided by the present study is, however, clearly insufficient either to conclude that executional collectives exist, or to distinguish what forms such multi-joint programming units might take.

In contrast to the pattern of elbow errors seen, wrist accuracy was no worse under component test conditions than dual. This appears to have been caused by the poor precision of dual wrist movements, which were executed far less accurately than dual elbow movements. There are sound reasons for believing that selecting an appropriate wrist program is the more difficult task. The wrist physically is a relatively light member attached to a limb of far greater rotational inertia; and so when a dual action is performed, unless the wrist is moved with a fair degree of muscular rigidity, there is always the danger that whiplash effects will interfere with accuracy. In general, elbow movements affect wrist movements far more than wrist movements affect elbow; and so a different pattern of errors for wrist movements is not surprising.

The major advantage claimed for programming constraints is that they lessen the effort involved in completing a task; indeed it is claimed that many actions would be impossibly complex to perform if each of

A. Horrrson, A. Bishop / Accurucy ofjornted actions 11

the muscle groups involved had to be programmed and monitored individually. Presumably whenever it is possible collectives are selected that will bring a high level of precision; but there seems no reason to assume that utilising programming constraints will always improve accuracy. Precision may sometimes be sacrificed in favour of limiting the numbers of collectives, prescriptions or sub-routines used; and some may well be selected for their inherent flexibility or ease of execution rather than their accuracy. In the present study, error was measured in terms of deviation from perfect posture, so that component awareness could be assessed. Using such data, there is no proper way of computing the functional precision of dual actions, in terms of how nearly they reached the targets set, or of comparing the functional effectiveness of simultaneous versus sequential elbow: wrist actions. Various workers (e.g. Begbie 1959) believe that incorporating more moving joints into an action increases movement complexity and so expected error. Langolf et al. (1976) studied information processing rates in a reciprocal tapping task, and found that these varied from 38 bits/set for finger movements to 10 bits/set for whole arm move- ments. On this basis, dual-jointed actions would be expected to be more errorful that single-jointed movements. In terms of the particular error index used, there was no difference in elbow precision; and whilst there is some evidence of such a trend in wrist errors, alternative explanations for it have already been discussed. If DW and DE error scores are combined, the overall precision of dual actions is significantly less than that of single or component movements. The rationale of using a simple summation, however, can be challenged, especially since it does not necessarily correlate with functional precision; and it certainly must not be forgotten that there is very little evidence that elbow precision is disrupted by introducing concurrent wrist activity.

Very real problems were encountered in trying to interpret results in motor programming terms, as the discussion indicates; however, the pattern of elbow errors found is compatible with models which specify conjugate, rather than independent, programming of the wrist and elbow elements of a dual actions. Whether this is in reality the form of organisation used is a basic question in the area of motor skills, and one which will only be answered with rigorous conceptualisation and the development of suitable research paradigms.

12 A. Harmon, A. Bishop / Accuracy of jointed actmns

References

Begbie, G.H., 1959. Accuracy of aiming in linear hand movements. Quarterly Journal of Experi-

mental Psychology 11, 65-75.

Beggs, W.D., 1972. The accuracy of non-visual aiming. Quarterly Journal of Experimental

Psychology 24, 515-523.

Bishop, A. and A. Harrison, 1977. Kots and Syrovegin (1966) - a demonstration of modular units

in motor programming? Journal of Human Movement Studies 3, 99- 109.

Brown, J.S., E.B. Knauft and G. Rosenbaum, 1948. The accuracy of positioning reactions as a

function of their direction and extent. American Journal of Psychology 6 1, 197- 182.

Bruner, J., 1970. ‘The growth and structure of skill’. In: K. Connolly (ed.), Mechanisms of motor

skill development. London and New York: Academic Press.

Connolly, K., 1970. ‘Skill development: problems and plans’. In: K. Connolly (ed.), Mechanisms of

motor skill development. London and New York: Academic Press.

Harrison, A., 1975. ‘Components of neuromuscular control’. In: K.S. Holt (ed.), Movement and

child development. London: S.I.M.P/Heinemann Medical Books.

Kots, Ya M. and A.V. Syrovegin, 1966. Fixed set of variants of interaction of the muscles of two

joints used in the execution of simple voluntary movements. Biophysics 11, 1212-1219.

Langolf, C.D.. D.B. Chaffin and J.A. Foulke, 1976. An investigation of Fitt’s law using a wide

range of movement amplitudes. Journal of Motor Behaviour 8, 113- 128.

Russell, D.G., 1976. ‘Spatial location cues and movement production’. In: G.E. Stelmach (ed.).

Motor control: issues and trends. London and New York: Academic Press.

Turvey, M.L., R.E. Shaw and W. Mace, 1978. ‘Issues in the theory of action: degrees of freedom,

coordinative structures and coalitions’. In: J. Requin (ed.), Attention and performance VII.

Hillsdale. NJ: Lawrence Erlbaum.