Postural adjustments and bearing angle use in interceptive actions

RESEARCH ARTICLE

Ambreen Chohan Æ Geert J. P. Savelsbergh

Paulien van Kampen Æ Marline Wind

Martine H. G. Verheul

Postural adjustments and bearing angle use in interceptive actions

Received: 19 May 2005 / Accepted: 29 September 2005 / Published online: 10 January 2006� Springer-Verlag 2006

Abstract The experiment investigates the effect of ballvelocity and walking direction on the adherence to thebearing angle (BA) strategy in adults. Adult participants(N=12) approached a moving ball in order to manuallyintercept it at a predefined target area. Results revealedthat during locomotion the BA strategy was imple-mented, but on reaching the point of interception, thisstrategy broke down and the BA strategy of the wristcompensated for the movement requirements relative tothe ball velocity and approach angle. Larger deviationsfrom the BA occurred when the angle of approach wasdecreased and when the ball velocity increased. Whenthe BA strategy was adhered to, postural adjustmentswere reduced. Increased movements occurred in aproximal–distal direction with an increasing approachangle and a faster ball velocity.

Keywords Bearing Angle strategy Æ Human ÆInterception Æ Locomotion Æ Posture

Introduction

Interception and object avoidance are complex percep-tual-motor responses to external stimuli, in which themovement is shaped to accommodate the future(Montagne et al. 1999). Everyday activities may involvecoupling visual information with a particular action inorder to successfully catch a ball, grasp a cup or on alarger scale cross a busy road or walk in a crowd. With

respect to information about object position and orien-tation (Laurent et al. 1994), optical variables have beenformalised and evaluated empirically in the regulation oftiming of grasping movements (Caljouw et al. 2004;Peper et al. 1994; Laurent et al. 1996). It is not clear,however, which of these variables are exploited when thehand and arm are free to move for interception of amoving object and the approaching object follows aspatially defined path.

An attractive candidate optic variable in the task ofintercepting a moving object is the bearing angle (BA).The BA has been used by sailors for many years incollision avoidance (Le Brun 2002). In object intercep-tion BA is subtended at the point of observation by thecurrent position of the ball and the direction of dis-placement (Chardenon et al. 2004). Recent research inadults suggests that using the head as the angle centre,the angular position of the ball with respect to theinterception point remains close to constant (Lenoiret al. 1999, 2002). This strategy implies that only oneinformation source is required in order to facilitateinterception, and a person may maintain the constantbearing angle (CBA) strategy on approach, in order tointercept at the right time (Lenoir et al. 1999). Compli-ance to this strategy, however, has not been fully ex-plored in relation to an actual interceptive action.

Participants adjust to horizontal properties of angu-lar bearing during interception of a moving object alonga V-shaped track whilst riding a tricycle (Lenoir et al.1999, 2002). The BA scarcely changed during approachand velocity adaptations were made in order to suc-cessfully complete the ‘catch’. It was therefore proposedthat a regulation is achieved through adherence to theCBA strategy. The present paper aims to analyse themanual interception of an object travelling at a constantvelocity along a defined trajectory. It intends to inves-tigate how participants regulate movement, in relationto the ball velocity and angle of approach, in order tointercept at the right time.

Recently, it has been shown that constraining postureor movement during a prehension or interception task

A. Chohan (&) Æ G. J. P. Savelsbergh Æ M. H. G. VerheulInstitute for Biophysical and Clinical Research into HumanMovement, Manchester Metropolitan University, Hassall Road,Alsager, ST7 2HL Cheshire, UKE-mail: [email protected].: +44-161-2475504Fax: +44-161-2476375

P. van Kampen Æ M. Wind Æ G. J. P. SavelsberghInstitute for Fundamental and Clinical Human Movement Science,Vrije Universiteit, Amsterdam, The Netherlands

Exp Brain Res (2006) 171: 47–55DOI 10.1007/s00221-005-0239-z

significantly affects perceptual-motor organisation(Davids et al. 2000; Steenbergen et al. 1995; Savelsberghet al. 2005). For example, seating skilled and novicecatchers affects their interceptive actions differently(Savelsbergh et al. 2005). In previous BA studies, whenlooking at extrinsic factors in relation to intercepting amoving object, most have neglected active prehensileaction during or post self-motion (Chardenon et al.2004, 2005), some also controlling or stabilising theparticipants, walking velocity using a treadmill. Stabil-ising walking velocity in such a way may artificiallyconstrain the task and therefore restrict strategy imple-mentation, making it difficult to relate results directly toeveryday activities. The present study looks to extendthe previous research (Chardenon et al. 2004; Lenoiret al. 1999, 2002) by allowing more strategy implemen-tation by participants. Participants can freely adjustvelocity on approach and adjust temporal parametersduring the grasp of the object to be intercepted. Only theangle of approach and the interception point are defined.The CBA hypothesis is therefore applied to a realmanual interceptive task as opposed to a virtual envi-ronment task. As participants are forced to decelerate atthe point of interception to prevent collision with thetrack, in order to intercept successfully it is expected thatthe CBA strategy will transfer from central (at the head)during locomotion to the distal limb (wrist) in order tograsp.

The present study looks at the role of the CBAstrategy in a prehension task, and its effect on couplingprehension with postural adjustments. It is expected thatmanipulations of walking direction and ball velocity giverise not only to a different response in terms of the CBAstrategy, but also to different displacement kinematics(Chardenon et al. 2004; Lenoir et al. 1999, 2002). Inaddition, the present paper looks to depict a relationshipnot only between the BA strategy and the change in ballvelocity or a participant’s angle of approach, but alsobetween the postural and temporal response in inter-ceptive behaviour. It is anticipated that in accordancewith the previous research a larger BA will result in agreater deviation from the CBA strategy (Chardenonet al. 2004; Lenoir et al. 2002). It is also expected thatincreasing ball velocity will lead to greater deviationfrom the CBA strategy (Chardenon et al. 2004).

When manually intercepting a moving object, the BAof the head and reaching limb will be almost equal onapproach. However, when reaching, the hand movesaway from the body and adopts a different trajectory.When walking perpendicular to the object to be inter-cepted, more postural adjustments are possible and theimmediate visual field is larger. When walking from asmaller approach angle for example, at a larger BA withthe object, large head movements are required to see theball and the BA strategy is more likely to break downand fewer postural adjustments are possible.

The CBA strategy may explain a participant’s re-sponse when approaching an object to be intercepted;however the effects of BA information on postural and

reaching adjustments involved in manual interception ofa ball have not yet been addressed. When approachingan object to be intercepted, it is expected that the par-ticipants make postural and reaching adjustments as theball velocity increases to allow greater range of motionin order to successfully intercept (Verheul et al. 2003).Based on the previous research it is also hypothesisedthat the peak velocity of the wrist will increase relative tothe increase in ball velocity (Dubrowski and Carnahan2001). In the present study, the angle of approach shouldhave no effect in this case as temporal variables areinfluenced by temporal–spatial information provided bythe target to be intercepted.

Methods

Participants

Twelve healthy adults (8 females, 4 males; aged22.2±1.3 years) participated in this experiment on avoluntary basis. The experiment was approved by theLocal Research Ethics Committee and a written in-formed consent in accordance with the Declaration ofHelsinki (1964) was obtained from each participantprior to the experiment. Participants performed theexperiment with their preferred hand where the righthand was used in every case. All participants had normalor corrected to normal vision and were not familiar withthe hypothesis of the experiment.

Materials

The object to be intercepted was a tennis ball (ø 60 mm)on a small platform, moving along a track (length 3 m,height 0.79 m, interception area 0.2 m) driven by a 12 Vmotorized conveyor belt. The track was operatedthrough LabView in forward or reverse mode and wascapable of producing constant velocity. A light sensorattached at the beginning of the track triggered record-ing via the motion analysis software OPTOTRAK�(Northern digital Inc., Canada) as the ball passed thesensor. Two OPTOTRAK� camera units (data sam-pling frequency: 200 Hz) were strategically placed at thebeginning of the track (Fig. 1).

Light emitting diode (LED) markers were placed onthe ball and the right wrist (styloid process), elbow(olecranon process), shoulder (external face of theacromion process), hip (most lateral point of the iliaccrest) and head (above the right ear on a baseball cap).The two camera units recorded the three-dimensionalposition co-ordinates of these LED markers.

Two walking lines of 4 m length were marked outalong the floor at 90� (perpendicular to the track) and atan angle of 45� with the track. Participants started fromthe end of the line and were instructed to start walking atthe moment the ball started moving and to interceptthe ball within the marked area of the track with their

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preferred hand. Running was not allowed in the task atanytime, but the participants were free to adjust theirwalking velocity should they require. The ball ap-proached at three different velocities (VB) 0.45, 0.65, and0.85 m/s. The velocities remained constant along themovement trajectory. Participants were required tointercept 5 balls in each of the 6 conditions (3 ballvelocities · 2 walking directions), making a total of 30trials. Pilot tests using a random sample of participants(not used in the presented study) established velocities atwhich all subjects were able to reach the track in time tosuccessfully grasp the ball.

Participants were instructed to intercept the ballwithin the marked area (20 cm) of the track with theirpreferred hand. On approach, in order to distinguish thestart of reach, participants were asked to cover the hipmarker with their hand, releasing only to reach for andintercept the ball when they required. Prior to testing, theexperimenter demonstrated the direction of locomotionand explained the goal objective to intercept the ball atthe given point. The demonstration did not involve themoving object, to ensure the participants could notsimply imitate a strategy performed by the experimenter.

The dependent variables for analysis were categorisedinto three sections; BA variables, temporal variables andpostural involvement in interception.

Bearing angle variables

Constant bearing angle (CBA)

The OPTOTRAK data were used to calculate thebearing angles between (1) the head and the ball and (2)the wrist and the ball in order to determine the strategy

used relative to change in walking direction and ballvelocity. As participants were instructed to start walkingwhen the ball started to move and the ball had a con-stant velocity after that, constant BA strategy is definedby sustaining the initial BA throughout the trial. In or-der to analyse dissimilarity between CBA and the mea-sured BA, the CBA was calculated as the averagestarting BA from each walking direction. For a walkingdirection of 90� the BA of the head and wrist was 55�,and for an approach angle of 45� it was 88�.

Bearing angle head and wrist (BAH and BAW)

The actual BA was calculated using the following for-mulae at each time interval of 0.05 s;

BAH ¼ Tan�1heady � ballyheadx � ballx

� �; ð1Þ

BAW ¼ Tan�1wristy � ballywristx � ballx

� �; ð2Þ

*x = direction of the ball along the track;y = perpendicular to the track and ball.

Deviation from constant bearing angle (dCBA)

From the start of locomotion to grasp the dCBA wascalculated using the following two formulae.

dCBAH = absolute CBA - BAHð Þ½ � ð3ÞdCBAW = absolute CBA - BAWð Þ½ � ð4Þ

Duration of the trials varied with ball velocity, sodCBAH was also analysed by dividing the trajectory into

Fig. 1 Experimental design(birds eye view). Schematicrepresentation of theexperimental set-up, indicatingthe two walking directions andthe direction of the ball relativeto the participant. The set-upincluded 2 Optotrak� beams, a12 V motorised conveyor beltcreating the track for the ball totravel on at constant velocityand an integrated light sensorto trigger motion capture. a andb indicate the bearing angle

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quartiles and looking at the deviation that occurred ateach comparable point in a trajectory. In the deviationfrom the CBA, the trajectory was divided into quartilesand the dCBA at 25, 50 and 75% were analysed as thestart, and endpoints (0 and 100%) of the movementtrajectory were pre-defined by the task conditions (ballmovement initiation and grasp).

Temporal aspects of interception

Time to contact (ttc)

The time taken from start of reach to interception(grasp). The start of the reach was determined byappearance of the hip marker. The time of grasp wasindicated by the disappearance of the ball marker.

Peak wrist velocity (PVW) during reach

Calculated as the maximum incremental displacement ofthe wrist marker during the reach, the peak velocity ofthe wrist (PVW) occurred during the reach and waspreceded by an acceleration phase (TA) from the start ofthe reach to PVW. This was followed by a decelerationphase (TD) as the hand moved into closer proximity ofthe target object and decelerates in order to accomplishprecision in interception.

Postural involvement

Data from the LED markers were also used to determinepossible postural adjustment strategies employed. El-bow, shoulder and trunk angles at the start and end ofthe reach phase were calculated, after which the differ-ences between the calculated angles at the start and endof the interceptive action (i.e. at reach initiation andgrasp) were determined as an indication of elbowmovement, shoulder movement and approximation oftrunk movement (see Fig. 2).

Data Analysis

Data were collected at 200 Hz and filtered using a sec-ond order Butterworth filter with a cut-off frequency of10 Hz. Data were statistically analysed using repeatedmeasures analysis of variance (3-way ANOVA fordeviation in CBA (dCBA): 2 directions · 3 ball velocities· 3 quartiles; 2-way ANOVA: 2 directions · 3 ballvelocities for other variables) followed by post-hoc pairwise comparisons with Bonferroni corrections (signifi-cance level a=0.05). When using repeated measuresANOVA, violation of Mauchley’s sphericity assumptionwas accounted for, using the Greenhouse–Geisser cor-rection if required to avoid a type one error and makethe test more conservative, but no significances wererecorded.

Results

Bearing angle strategy

Deviation of BAH from the constant bearing angle(dCBAH)

The BAH displayed a relatively horizontal trajectory,deviating in accordance to walking direction and ballvelocity as the target was in closer proximity (Fig. 3a, b).A significant effect of ball velocity on dCBAH

[F(2,18)=5.97; P<0.01] was found. Post-hoc pair wisecomparisons revealed that dCBAH was significantlydifferent for the lowest than for the highest velocity(P<0.01). A further significant effect of direction ondCBAH [F (1,9)=42.26; P<0.001] indicated that thedeviation from CBA is significantly larger in the 45�walking condition (Fig. 3b). This was the case especiallyat 75% of the approach, as indicated by the significantinteraction between walking direction and quartile[F(2,18)=5.23; P<0.05]. There were no other significantinteractions.

Bearing Angle Wrist (BAW)

On examination of the BA strategy of the wrist ongrasping; where dCBAH increased, dCBAW decreasedand vice versa (Fig. 3c, d). Results for dCBAW indicatedsignificant effects of direction [F(1,9)=8.57;P<0.05] andball velocity [F(2,18)=6.426; P<0.01]. Post-hoc analysisindicated a significant difference between the slower0.45 m/s ball and the two faster balls (P<0.05), con-firming the hypothesis that an increase in ball velocityled to increasing deviation from the CBA strategy.

Velocity and timing of interception

Time to Contact (TTC)

The time taken from reach to grasp decreased signifi-cantly as ball velocity increased [Table 1; F(2,18)=8.04;P<0.01]. Post-hoc pair wise comparisons revealed asignificant difference between ball velocities 0.45 and0.85 m/s. No significant effect of direction was foundand no interaction between direction and ball velocitywas found for TTC.

Peak Velocity (PVW)

Ball velocity had a highly significant effect on PVW

[F(2,18)=185.17; P<0.001]. Increasing ball velocity ledto increasing peak velocity of the wrist (Table 1). Post-hoc tests indicated that all differences between ballvelocities were highly significant (all P<0.001). No sig-nificant effect was found for direction and no significantinteractions were revealed.

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Acceleration and Deceleration time (TA and TD)

Results indicated no significant effects of ball velocity ordirection of travel (Table 1) on acceleration and decel-eration time of the wrist. No significant interactionswere present.

Postural Involvement

Postural involvement during interceptive action couldexplain the strategy used by participants in accordanceto defined task constraints such as ball velocity and thedirection of approach.

Elbow movement (EM)

Elbow movement increased significantly with increasingball velocity [F(2,18)=3.63; P<0.05]. Post-hoc com-

parisons however showed no significant differences. Theeffect of direction clearly indicated a significant outcome[F(1,9)=8.18; P<0.05]. When walking from 90�, par-ticipants adopted an elbow extension strategy fromreach to interception for increase in ball velocity,whereas when walking from 45�, elbow angle was re-duced significantly from reach initiation to interception(Table 1). No interactions were denoted through statis-tical analysis.

Shoulder movement(SM)

Participants displayed increasing shoulder movement forincreasing ball velocity [Table 1; F(2,18)=13.15;P<0.001]. Significant differences with the fastest ball(i.e. between 0.85 and 0.45 m/s, and between 0.85 and0.65 m/s) were visible through post-hoc pair wise com-parisons (both P<0.05). This suggests shoulder move-ment (SM) was recruited significantly more when

Fig. 2 Calculation of postural adjustments. Schematic diagramsand equations indicating the calculation of postural angles (elbow,shoulder and trunk) using the positional coordinates of the joints

calculated via the motion analysis (Optotrak �) system. Movementis defined as total difference in postural angle from reach initiationto grasp

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intercepting the faster balls than when intercepting slowballs. Walking direction did not affect how far theshoulder flexion-extension during interception. No sig-nificant interaction between ball velocities and walkingdirection was found.

Trunk movement (TM)

Trunk movement increased as ball velocity increasedand when walking from 90� compared to walking from45� (Table 1). Statistical analysis indicated that bothdirection [F(1,9)=9.38; P<0.05] and ball velocity[F(2,18)=8.39; P<0.01] had a significant effect on trunkmovement, though there was no significant interactionbetween the two. Post hoc tests indicate significant dif-ferences (P<0.05) in trunk movement between the slowball and the two faster balls, indicating that participantsclearly distinguish between intercepting the slowest ballcompared to the two faster balls.

Total Movement

The total movement was defined as the combinedmovement of the elbow, shoulder and trunk, i.e. a simpleaddition of the above mentioned movement ranges, so asto combine the movement in the skeletal system as awhole. This technique was used in order to establish theextent to which joints are held more rigidly duringinterception. Joint rigidity was indicated by a lower valuefor total movement and freeing of postural involvementwas indicated through more postural adjustments.

The effect of walking direction was clearly visiblethrough looking at the total movement in either direc-tion for increasing ball velocity (Table 1). This indicatedthat more postural adjustments were made withincreasing ball velocity for the larger angle of approach.

Statistical analysis indicated a significant effect of ballvelocity [F (2,18)=18.14; P<0.001]. Post-hoc pair wisecomparison indicated significant differences between thethree ball velocities (P<0.05). A significant effect ofwalking direction [F (1,9)=9.64;P<0.05] on total pos-tural involvement was also noted.

Discussion

The primary theoretical framework of this study focusedon postural and timing adjustments made when applyingthe CBA strategy to a real interceptive task, whilstmanipulating ball velocity and participant walkingdirection.

In summary, it may be drawn from this experimentthat in an interceptive task, when approaching from anangle, individuals complied with the BA strategy upuntil the point of interception, at which point the bear-ing angle of the wrist essentially took over. A largerapproach angle coincided with greater deviation of thehead from the CBA at interception, but a lower devia-tion from the CBA strategy for the wrist. A smallerangle of approach caused greater compliance to theCBA strategy during locomotion but greater deviationon interception.

In concordance with current theory (Steenbergenet al. 1995), it seemed that the smaller angle of approachdoes lead to greater deviation from CBA strategy. Theextent of the deviation could also be dependent on ballvelocity. There was a significant difference between theslowest and fastest ball velocity. Though previous liter-ature has addressed the degree of CBA complianceduring the approach phase in interceptive actions, thestrategy employed during the actual interceptive actionhas not been clearly established. For this reason the BAstrategy of the wrist was analysed.

Table 1 Dependent Variable Data Mean and standard deviation (between brackets) data for calculated dependent variables for eachcombination of walking direction and respective ball velocity.

Dependent Variable Walking direction 90� Walking direction 45�

0.45 m/s 0.65 m/s 0.85 m/s 0.45 m/s 0.65 m/s 0.85 m/s

(Units = Degrees)dCBAH: 25% 6.868 (1.1) 4.048 (1.1) 5.291 (1.1) 1.098 (0.5) 1.208 (0.8) 1.32 (0.68)50% 8.262 (1.3) 4.291 (1.3) 7.515(1.5) 5.238 (1.3) 1.807 (2.4) 3.30 (2.62)75% 13.64 (1.4) 9.294 (2.2) 10.17 (1.9) 13.35 (8.2) 6.49 (3.8) 5.3 (4.63)dCBAW: 25% 3.032 (0.8) 3.986 (0.8) 4.202 (0.8) 2.33 (0.9) 3.13 (0.94) 3.45 (0.99)50% 3.780 (1.6) 5.32 (1.4) 6.55 (1.7) 2.64 (16.0) 3.82 (5.1) 5.33 (5.44)75% 7.219 (1.7) 10.13 (2.4) 11.87 (1.9) 4.46 (15.4) 6.31 (7.3) 8.59 (7.75)Elbow movement (EM) 6.34 (13) 8.07 (20) 13.8 (14) �4.69 (11) �6.75 (9) �0.46 (5)Shoulder movement (SM) 48.275(13) 56.463 (16) 66.05 (14) 46.51 (14) 54.66 (14) 63.8 (18)Trunk movement (TM) 5.98 (4) 9.33 (4) 12 (5) 5.18 (2) 7.24 (3) 7.15 (3)Total movement 60.59 73.86 91.85 46.99 55.15 70.49(units = ms)Time to contact (TTC) 500 (133) 459 (81) 439 (88) 525 (121) 426 (75) 404 (116)Acceleration time (TA) 127 (109) 35 (200) 41 (255) 86 (431) �38 (376) 118 (148)Deceleration time (TD) 373 (106) 424 (201) 398 (216) 439 (509) 464 (347) 286 (150)(units = ms)Peak wrist velocity (PVW) 1390 (147) 1870 (264) 2400 (251) 1260 (256) 1770 (233) 2280 (246)

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Bearing Angle

The experiment proposed to extend previous research(Chardenon et al. 2004; Lenoir et al. 1999, 2002) byanalysing the effects of manual interception of a realtarget following locomotion rather than use of a tread-mill, tricycle or restricting analysis to the head. Resultsindicated a clear difference in BA strategy used by par-ticipants walking from the two prescribed directions thatmay be explained by the visual information being re-ceived. Although this has not been experimentally veri-fied, it could be that walking perpendicular to themoving object, participants used their peripheral visionto keep the target in sight at all times. It may be sug-gested that head movements, in order to maximizespatial and temporal precision in grasping at the giveninterception point may be largely unnecessary, whichpreserves energy (Vereijken et al. 1992). The deviationfrom CBA, for increase in ball velocity shown for the 90�condition may indicate a profound relationship in

strategy implementation, in that a decision could beinstantly formed and adhered to, relative to visualinformation. Walking from 45�, various hand trajecto-ries may be implemented in order to successfully inter-cept at the right time and place. This may not be entirelybased on the visual information provided by the targetdisplacement rate, however, as the ball velocity anddirection of approach have no significant interactionwith the dCBAH quartile.

It was hypothesised that the CBA strategy brokedown or essentially transferred to the wrist on reachinitiation; accounting for the larger deviation at inter-ception, previously documented (Lenoir et al. 1999,2002; Steenbergen et al. 1995). The results of thisexperiment suggest that in comparison to the dCBAH,the deviation of the wrist had a somewhat opposing ef-fect, in that a larger approach angle led to the deviationof the wrist to be lower during grasping. This suggests acombined BA strategy in both head and wrist shownthrough the calculated difference in dCBA (Fig. 3d).

Fig. 3 Bearing Angle Analysis. a Bearing Angle of the Head fromstart of locomotion to grasp. Results of a typical participant’s BAH

for varying ball velocities when walking from 90�. CBA constantbearing angle, i.e. required BA; other trajectories display the BAactually used. b Percentage course of the deviation of the head fromthe CBA (dCBAH), at quartile points (0, 25, 50 and 75%, 100%) ofthe trajectory. Differences between the CBA and the actual bearingangle of the head from start of locomotion to grasp of target ballare indicated c Percentage course of dCBAW at quartile points (0,25, 50 and 75%, 100%) of the trajectory, indicating difference

between the CBA and the actual bearing angle of the wrist fromstart of locomotion to grasp of target ball. The position of the wristwas partly controlled, in that participants were instructed to keepthe hand on the marker until they needed to reach for the target. Asthe hand is free to move through any path during the reachingphase of the movement, variability of the bearing angle of the wristmay decipher the interception strategy. d Difference betweendCBAH and dCBAW for each walking direction and ball velocityat each percentile of the trial, indicating relative adherence to thebearing angle strategy up until 75% of the trial

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This indicated that when BAH adhered more to the CBAstrategy, dCBAW increased relative to ball velocity andvice versa.

Temporal aspects of interception

In accordance with previous literature (Verheul et al.2003; Dubrowski and Carnahan 2001) wrist velocityappeared to be coupled to ball velocity in that a higherwrist velocity was used for faster balls in order tosuccessfully intercept. Reaching time decreased withincreasing ball velocity, supporting the CBA strategyimplementation suggested earlier. Individuals used thevisual information provided by optical displacement ofthe ball along the trajectory. Participants visually dis-tinguished the ball speeds and adjusted timing ofinterception accordingly; the direction of walk had noeffect.

Postural involvement in interceptive action

The results for trunk movement suggest that trunkflexion was the first adjustment made during interceptivebehaviour in each trial. Since out of the three (elbow,shoulder and trunk), the trunk has by far the largestmass, it defines postural stability; its low range ofmotion making overall movement more energy efficient.This is due to the related stability defined by skeletalstructure of the spine. As the trunk has limited range ofmotion, it could be controlled easily in order to provideenergy efficiency and control. The extent of trunkmovement is defined by the velocity of the ball,increasing relative to increase in ball velocity, displacingthe centre of mass in order reduce the amount of workinvolved in bringing the hand in close proximity to theball (time to contact).

In synchronisation with trunk movement, shoulderand elbow movement also increased with increasing ballvelocity, indicating freeing of movement (Vereijken et al.1992) in the reaching limb as the velocity of the targetobject increases. It remains probable that this energyefficient strategy (Sparrow and Irizarry-Lopez 1987)could be seen to increase the success in task outcome, asthe range of possible movement trajectories of the armincreased. Results suggest increase in elbow, trunk andshoulder movement relative to ball velocity; their extentof involvement appears to adhere to a proximal–distalcontrol strategy. The strategy indicates that control isachieved by first freezing movement closest to the centreof mass and freeing those more distal, in order to controlmovement stability.

As there was no combined effect of direction of ap-proach and ball velocity on the BA strategy employed, itmay be suggested in line with previous literature thatindeed on approach, individuals require only oneinformation source in order to intercept successfully(Lenoir et al. 1999). This source of information is indi-

cated by the degree of compliance to the BA strategy.Though this is not entirely verified experimentally,where immediate visual information is reduced, whenapproaching from a smaller angle, the CBA strategy isadhered to, to a larger extent up until the point ofinterception or reach initiation. It is this point where theBA of the wrist takes over in order to complete theinterception successfully compensating by deviatingmore from the CBA strategy.

The emergence of this proposed strategy suggests animmediate link with postural involvement in movement,in that the direction of approach dictates the degree ofpostural adjustments involved in interception. When theindividuals adhered more to the CBA strategy on ap-proach, the postural adjustments involved in intercep-tion were reduced. This indicates an effect of visualinformation provided through direction of locomotion,where greater use of peripheral visual informationessentially allows for more postural involvement andfreeing of movement synchronisations during intercep-tion.

With respect to the regulation of the CBA strategy,the distinct difference in response to slow and fast ballsmay govern locomotion and the combined hand and ballinvolvement during interception. The tight coupling ofperception and action facilitates such a behavioural re-sponse, as adults are able to predict the success of usinga particular strategy against another. As ball velocityincreased, there was more postural involvement and lessrestriction to the number of trajectories the hand couldtake when homing in on the target object. The infor-mation provided by the change in ball velocity (i.e.through looming angle) could act as the perceptual keyto producing effective interception, in that its greatesteffects occurred on the timing of the actual interceptionand postural involvement.

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