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Ipsilateral Coordination Deficits and Central Processing Requirements Associated With Coordination as a Function of Aging

Department of Kinesiology, Group Biomedical Sciences, K.U. Leuven, Belgium.

Address correspondence to S. Heuninckx, Motor Control Laboratory, Department of Kinesiology, K.U. Leuven, Tervuursevest 101, 3001 Leuven, Belgium. E-mail: Sofie.Heuninckx{at}FABER.KULEUVEN.BE

	Abstract

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Young and elderly participants performed concurrent ipsilateral hand–foot movements either isodirectionally or nonisodirectionally. We determined performance by measuring the maximal cycling frequency at which the coordination pattern could be performed successfully (CF_max). We also determined attentional costs by means of a dual-task paradigm. Findings revealed that CF_max was significantly lower in the elderly than in the young participants for the nonisodirectional mode, whereas we observed no differences for the isodirectional mode. Under dual-task conditions, coordination deteriorated in the elderly group only. However, when we equated levels of task difficulty, differences between the groups disappeared. Furthermore, attentional costs did not differ between isodirectional and nonisodirectional movements. This indicates that age-related coordination deficits were not primarily evoked by reduced attentional resources or control in elderly persons.

IT IS well documented that normal aging is accompanied by a deterioration in motor performance (Seidler & Stelmach, 1995; Spirduso, 1982; Welford, 1984). For simple tasks such as key presses and card sorting, age-related increases in simple reaction time and movement execution time have been extensively described (for review, see Welford, 1988). In contrast to the observed changes in unimanual movements, little is known about the effects of aging on interlimb coordination, that is, the spatiotemporal organization of more than one limb or segment.

The study of interlimb coordination patterns has resulted in the identification of two elementary and naturally preferred modes of movement coordination—the in-phase and antiphase pattern (Kelso, 1984; Swinnen, 2002; Swinnen, Jardin, Meulenbroek, Dounskaia, & Hofkens-Van Den Brandt, 1997; Turvey, 1990). With respect to bilateral finger, hand, or arm movements, in-phase coordination refers to the simultaneous contraction of homologous muscles, that is, flexing or extending the arms or wrists simultaneously. Antiphase coordination refers to the simultaneous activation of nonhomologous muscle groups, that is, flexing one limb segment while extending the other. With respect to movements of the ipsilateral limbs (e.g., hand–foot coordination on the same side of the body), in-phase and antiphase movements are defined with respect to movement direction in external space. In particular, in-phase refers to limb motions in the same direction (isodirectional) and antiphase to motions in opposite directions (nonisodirectional; Baldissera, Cavallari, Marini, & Tassone, 1991; Kelso & Jeka, 1992; Serrien & Swinnen, 1997; Swinnen et al., 1997). These two coordination modes are not equally difficult (Baldissera, Cavallari, & Civaschi, 1982; Baldissera et al., 1991; Carson, Goodman, Kelso, & Elliott, 1995; Kelso & Jeka, 1992; Serrien, Swinnen, & Stelmach, 2000; Swinnen et al., 1997). Whereas isodirectional movements can be performed easily without much effort, the nonisodirectional coordination mode is much more difficult and is produced with a lower degree of phase accuracy and stability. Furthermore, with increasing cycling frequency, participants often undergo an involuntary transition from the nonisodirectional to the more stable isodirectional pattern at a critical frequency (Baldissera et al., 1982, 1991; Carson et al., 1995).

When Greene and Williams (1996) determined this critical frequency (i.e., the maximal frequency at which the coordination pattern was still performed correctly) for a cyclical bimanual coordination task, they found that older participants exhibited involuntary transitions from the more difficult antiphase to the easier in-phase coordination pattern at a significantly lower critical frequency than the younger participants. In contrast, Serrien and colleagues (2000) did not observe age-related deficits during in-phase and antiphase coordination of the homologous limbs at a single low frequency (1.00 Hz). However, ipsilateral coordination was affected, particularly when the limbs were moved according to the nonisodirectional mode.

Whereas age-related deficits in interlimb coordination have occasionally been studied, their underlying mechanisms are still unknown. In general, aging is accompanied by a decline of sensory (Skinner, Barrack, & Cook, 1984; Stelmach & Sirica, 1986) and motor (Welford, 1988) functions. However, the focus has recently shifted to age-related changes in attentional resources, their regulation, or both, which might also affect coordination performance in the elderly population. More specifically, investigations of the attentional cost associated with gait by means of dual-task paradigms have revealed that this common task requires a greater portion of attentional resources in older as compared with younger adults (for review, see Woollacott & Shumway-Cook, 2002). Similar results were also yielded during stance (Woollacott & Shumway-Cook, 2002), which involves comparable postural but less invasive coordination demands. Therefore, it remains to be tested whether age-related deficits in attentional resources or control also directly affect interlimb coordination.

Interlimb coordination represents a prototype of a complex task in which cognitive processing and attentional regulation play a prominent role (Temprado, Monno, Laurent, & Zanone, 1999). Attention is considered to be an intervening variable in the voluntary stabilization of coordination patterns (Monno, Chardenon, Temprado, Zanone, & Laurent, 2000). However, it is still not known to what extent older adults exhibit differences in attentional requirements to control and stabilize coordination patterns, as compared with their younger counterparts.

In the present study we addressed the ability of elderly versus young participants to produce ipsilateral coordination patterns with the right wrist and foot and according to the isodirectional and nonisodirectional coordination modes. Our first aim was to determine the maximal speed at which younger and older adults could perform these modes. Therefore, cycling frequency was stepwise increased, until the participants failed to produce the required coordination mode. In the second part of the experiment we addressed the central–attentional cost associated with producing the two basic ipsilateral coordination patterns in younger and older adults by means of a dual-task paradigm in which these patterns were performed together with a secondary attention task.

On the basis of the aforementioned studies, our first hypothesis was that the upper frequency levels at which ipsilateral coordination modes are performed successfully would be lower in older as compared with younger adults. Moreover, we expected this age-related difference to be more pronounced in the "difficult" nonisodirectional than in the "easier" isodirectional mode. Second, we hypothesized that ipsilateral coordination patterns, executed in parallel with a secondary attention task, would be performed less accurately and with lower stability in older as compared with younger adults. Again, we predicted this effect to be more prevalent in the nonisodirectional as compared with the isodirectional coordination mode.

	METHODS

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Participants
Thirty healthy volunteers participated in the study, including 15 young adults (age, M = 24.5 years; range, 22–31 years; 9 women and 6 men) and 15 older adults (age, M = 64 years; range, 61–72 years; 3 women and 12 men). All were right handed as assessed by the Edinburgh Handedness Inventory (Oldfield, 1971) and had no history of neurological disease or skeletomotor dysfunction. We assessed general cognitive function in all participants by using the Mini-Mental State Examination (Folstein, Folstein, & McHugh, 1975). All participants scored within normal limits (score 26). Participants were informed about the experimental procedures, and they provided written informed consent. The study design was approved by the local Ethics Committee of Biomedical Research at K.U. Leuven and was performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki.

Apparatus
Each participant lay supine on a flat surface. The lower legs were supported by a cushion to ensure free ankle rotation. The right arm was extended along the trunk, and the distal part of the arm was supported to enable free movements of the wrist. The wrist and foot were positioned in a wrist–hand and ankle–foot device (orthosis), respectively. Movements were restricted to the sagittal plane. The frictionless axis of the orthosis was aligned with the anatomical axis of the joint such that movements were not hindered. Angular displacements of the joints were registered by means of high precision shaft encoders (HP, 2048 pulses per revolution, sampled at 100 Hz), fixed to the movement axis of the orthosis. A PC recorded data and signaled the start and end of the trials, by controlling the onset and offset of an electronic metronome. All movements were metronome paced whereby a complete movement cycle was completed on every second beat (one beat was given for peak flexion, and one for peak extension). Participants watched a computer screen through a mirror system that was placed in front of them. During the first part of the experiment, they were instructed to fix their eyes at the black computer screen, whereas figures were projected on this screen during the second part of the experiment. Limb movements were prevented from vision at all times.

Procedure
Frequency limits
The first part of the experiment was designed to detect the maximal cycling frequencies at which young and elderly participants were able to perform the two basic ipsilateral coordination patterns successfully. Participants performed cyclical flexion–extension movements of the right wrist and ankle in the sagittal plane, either according to the isodirectional (ISODIR) or nonisodirectional (NONISODIR) coordination mode. Both limb segments were moved in the same direction during ISODIR coordination and in opposite directions during NONISODIR coordination. The duration of each trial was 12 s. Both coordination patterns were performed in accordance to a metronome-paced cycling frequency. To ensure successful performance at the lowest frequency levels in both age groups, we set the initial cycling frequency at 0.75 Hz for the ISODIR coordination mode and at 0.50 Hz for the NONISODIR mode. Movement frequency was increased in steps of 0.25 Hz following every second trial and up to the level at which participants failed to produce the required coordination mode. Rest periods were provided in between trials to avoid muscle fatigue. To ensure correct performance, experimenters trained the older adults the day before the experimental session (15–30 trials at varying frequencies) and again 15 min prior to the experimental session. The younger adults were only involved in a 15-min practice session prior to the experimental session.

Attention and dual-task conditions
In the second part of the experiment we addressed age-related changes in central cost to maintain and stabilize the previously described coordination patterns by means of a dual-task paradigm. Participants performed an attention task separately as well as together with the coordination pattern (dual task).

Attention task
Participants were shown a meaningless line figure on the computer screen in front of them and were instructed to memorize this target figure. Subsequently, a series of similar line figures was presented, of which some were exactly the same as the target figure (see Figure 1). The line figures followed one another with a random interval that ranged between 500 and 1,250 ms. The target figure appeared between two and seven times within the 12-s trial, which corresponded to a probability of 30%. The participants were instructed to count how often the target figure appeared within the series and to report it to the experimenter at the end of the trial. This attention task was chosen because it is not rhythmic and does not require a motor response. The task extracts from the available pool of mental resources without interfering with the motor system; that is, it does not cause structural interference (Kahneman, 1973). Four series were performed, two with the metronome beating at 1.00 Hz and two at 1.50 Hz. Two series (one at 1.00 Hz and one at 1.50 Hz) were performed prior to and two following the dual-task conditions. The metronome was used for equalization of the single- and dual-task conditions, as we discuss next.

View larger version (10K): [in this window] [in a new window]   Figure 1. Line figures used during the secondary task. Target refers to the image that had to be memorized. Parts A–F are the distractor images

Data Analysis and Measures
Relative phase measures
We assessed the quality of interlimb coordination by means of relative phase measures. We subtracted the phase angles of each limb segment from each other according to the following formula: = _w – _f = tan^–1[(dX_w/dt)/X_w – tan^–1[(dX_f/dt)/X_f], whereby w and f denote wrist and foot, respectively. Here _w refers to the phase of the wrist movement at each sample, X_w is the position of the wrist after rescaling to the interval [–1, 1] for each cycle of oscillation, and dX_w/dt is the normalized instantaneous velocity. Following computation of the continuous estimate of relative phase, we determined the absolute deviation from the target relative phase (AE; either 0° or 180°) for each data point and averaged across the whole trial to obtain a continuous measure of relative phase accuracy. We used the standard deviation of relative phase as a measure of coordinative stability.

Spatial measures
The spatial measure consisted of the absolute value of the peak-to-peak amplitude for each individual cycle, averaged across each trial. We computed mean and standard deviation values.

Determination of maximal cycling frequency
We determined maximal performance frequency limits for each individual separately. This referred to the maximal frequency at which the AE and standard deviation of the relative phase did not exceed 50° and 30°, respectively, indicating that the required pattern was no longer performed correctly. We then used the obtained individual frequency values for subsequent comparisons of younger and elderly participants.

Number of errors in the attention task
In addition to the previous measurements, we also counted errors made in the attention task performed either separately (ATT) or in combination with the coordination modes (ISODIR-ATT, NONISODIR-ATT). Error scores referred to the absolute deviation between the reported and actual number of shown target figures. We averaged the error scores across the two trials of each frequency and coordination mode.

Statistical Analyses
We performed a statistical analysis by means of an analysis of variance (ANOVA) with repeated measures. If applicable, we performed Tukey's post hoc analysis.

	RESULTS

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Analysis of the Maximal Cycling Frequency
The mean AE and standard deviation of the relative phase with respect to both coordination modes and across the different frequencies are shown in Figure 2. The data represent the means of only those participants who were still able to perform the coordination mode correctly at each frequency level (AE and standard deviation of relative phase below 50° and 30°, respectively). The number of successful participants is indicated above each group's curve. Considering the ISODIR coordination mode, we find it evident that the AE did not differ much between the older and younger groups across frequency levels (see Figure 2A). However, when the cycling frequency rose above 1.50 Hz, the number of elderly participants who could correctly perform the coordination mode dropped systematically. In contrast, the majority of young participants performed well up to a frequency of 2.00 Hz. Considering the NONISODIR coordination mode, we find that the AE was always higher for the elderly than for the younger participants, with the exception of the lowest frequencies (0.50 Hz and 0.75 Hz; see Figure 2B). In addition, at the frequency level of 1.00 Hz, two elderly participants already failed to perform the NONISODIR coordination mode correctly (i.e., they reverted to the ISODIR coordination mode). At the frequency level of 1.50 Hz, only two elderly participants performed the required pattern properly, whereas all 15 young participants were still successful. When we look at the standard deviation measures, we obtain similar findings; that is, elderly participants generally performed coordination patterns with a higher variability than younger participants (see Figures 2C and 2D).

View larger version (27K): [in this window] [in a new window]   Figure 2. Mean absolute error (AE) of relative phase during (A) isodirectional and (B) nonisodirectional coordination and mean standard deviation of relative phase during (C) isodirectional and (D) nonisodirectional coordination for younger and older adults. The number of participants who are capable of performing the task correctly is indicated for each frequency level above both groups' curves

View larger version (13K): [in this window] [in a new window]   Figure 3. Mean maximal cycling frequencies during isodirectional and nonisodirectional coordination modes for younger and older adults. Vertical bars denote standard errors

View larger version (16K): [in this window] [in a new window]   Figure 4. Mean error scores on the attention task during single- and dual-task conditions for younger (Y) and older (O) adults. Vertical bars denote standard errors

Accuracy
The main effects of group, F(1, 28) = 7.4, p <.05, and loading, F(1, 28) = 12.6, p <.01, were significant. These effects can most appropriately be interpreted in view of the significant Group x Loading interaction, F(1, 28) = 8.8, p <.01 (see Figure 5A). A post hoc analysis clarified that relative phase accuracy was similar across single (M = 19.5°) and dual (M = 20.3°) tasks for the younger adults (p >.05), whereas AE was lower during single-task (M = 27.6°) as compared with dual-task (M = 36.6°) coordination conditions in the older adults (p <.05). Surprisingly, we found no significant difference between the isodirectional (M = 25.7°) and nonisodirectional (M = 31.2°) coordination mode under dual-task conditions, F(1, 28) = 0.79, p >.05.

View larger version (9K): [in this window] [in a new window]   Figure 5. Mean absolute error (AE) of relative phase of the coordination modes performed in the absence (single) and presence (dual) of the attention task for younger and older adults, at the same "absolute" frequency (A) and at comparable levels of task difficulty for both age groups (B). Vertical bars denote standard errors

Comparable levels of NONISODIR task difficulty for both age groups
The aforementioned reported findings might be misleading because the older participants performed the coordination patterns "relatively" closer to their maximal cycling frequency (CF_max) than the younger group, that is, at a higher difficulty level. For the NONISODIR coordination mode, 1.00 Hz corresponded to 85% of the mean CF_max of the older participants (1.15 Hz) and 1.50 Hz corresponded to 85% of the mean CF_max of the younger participants (1.77 Hz). Therefore, we compared the performance of older versus younger participants for a normalized frequency level (85% of CF_max), ensuring that both groups performed the task at the same level of difficulty. Again, we studied accuracy and stability by using a 2 x 2 (Group x Loading) repeated measures ANOVA. We excluded the data from one older participant because of failure to perform the NONISODIR coordination mode at 1.00 Hz.

Accuracy
We obtained only a significant main effect of loading, F(1, 27) = 6.6, p <.05 (M = 24.5° for single task, M = 33.2° for dual task). This normalization procedure thus resulted in the disappearance of the significant group effect (see Figure 5B).

Stability
An analysis of standard deviation scores showed a significant main effect of loading, F(1, 27) = 6.7, p <.05 (M = 19.6° for single task, M = 24.2° for dual task). Similar to the AE, the group effect disappeared when we normalized for CF_max.

Spatial measures
We analyzed mean amplitude by using a 2 x 2 x 2 x 2 (Group x Limb x Mode x Loading) ANOVA with repeated measures on the latter three factors. We obtained a significant main effect of limb, F(1, 28) = 12.2, p <.01 (M = 31.9° for hand, M = 27.0° for foot), and mode, F(1, 28) = 15.7, p <.001 (M = 27.7° for ISODIR, M = 31.2° for NONISODIR). We also observed a significant main effect of limb for the standard deviation of amplitude, F(1, 28) = 102.0, p <.0001 (M = 2.7° for hand, M = 1.4° for foot). For both measures, we observed no differences between groups and between single- and dual-task conditions (p >.05).

	DISCUSSION

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The present study addressed age-related changes during the production of isodirectional and nonisodirectional coordination patterns with the ipsilateral limbs. Two major findings were revealed. First, the maximal cycling frequency (CF_max)for nonisodirectional coordination was significantly lower in the older participants than in the younger participants, whereas no differences between groups were observed for the isodirectional mode. Second, performing the coordination modes under dual-task conditions resulted in a significant decrease of accuracy and stability in elderly participants, whereas the dual-task condition only influenced phasing stability in younger participants. Following equation of difficulty levels (relative to CF_max), differences between groups disappeared. Remarkably, both isodirectional and nonisodirectional coordination modes were affected by the secondary task to the same extent within the frequency levels that were tested.

Younger participants accomplished the synchronization of the upper and lower limbs up to 2.52 Hz for the isodirectional and 1.77 Hz for the nonisodirectional mode, whereas CF_max in older participants was 2.26 Hz and 1.15 Hz, respectively. The CF_max of the isodirectional mode did not differ significantly between both groups. In contrast, loss of coordination was observed during nonisodirectional coordination. More specifically, phase transitions from the nonisodirectional to the isodirectional coordination pattern occurred at lower cycling frequencies in the older than in the younger adults. Thus, manipulation of cycling frequency had a higher impact on the performance of the nonisodirectional mode in older than younger participants. These results complement findings by Greene and Williams (1996). In their phase transition experiment, whereby cycling frequency was systematically increased for a bimanual (i.e., hand–hand) coordination task, older participants exhibited abrupt transitions from the antiphase to the in-phase mode at significantly lower frequencies than the younger participants. Furthermore, our results complement findings of Wishart, Lee, Murdoch, and Hodges (2000), who found that older adults performed as well as younger adults during bimanual in-phase coordination across all frequency levels (0.5–2.0 Hz) whereas age differences emerged most prominently during the antiphase pattern at the higher levels (1.5 and 2.0 Hz). Our findings demonstrated a decrease in the maximal cycling frequency at which the older adults were able to perform the nonisodirectional coordination mode successfully.

In the second part of our experiment, we addressed whether the aforementioned age-related performance decreases were due to reduced attentional resources or control. Therefore, we investigated age-related changes in the central–attentional cost, allocated by the central nervous system to maintain and stabilize the coordination patterns, using a dual-task paradigm. The results showed that older participants were significantly less successful in performing the attention task as compared with their younger counterparts, irrespective of whether it was performed in isolation or together with the coordination tasks. This is consistent with previous evidence of impaired working memory and attentional mechanisms in older adults (for reviews, see Craik & Salthouse, 2000), presumably as a consequence of several age-related structural and physiological changes in the brain (Grady, 2000).

Whereas a study using a dual-task combination of walking and memorization revealed that older adults prioritized the sensorimotor over the memory task to avoid loss of balance, resulting in a performance decrease on the memory task (Li, Lindenberger, Freund, & Baltes, 2001), we observed that the effect of the dual-task condition was mainly reflected in performance on the coordination task (that did not require stringent balance requirements). Performing the coordination patterns together with the attention task caused a decrease in phasing accuracy and stability, which was more pronounced in the older than in the younger participants. These results are in agreement with previous evidence of impaired performance of older adults in dual tasks (for review, see McDowd, Vercruyssen, & Birren, 1991). However, it is well known that the performance deterioration in dual tasks is less pronounced if a distractor task is executed in parallel with a simple as compared with a difficult task (Crossley & Hiscock, 1992; McDowd & Craik, 1988). Moreover, this influence of task difficulty on dual-task performance is higher in elderly than in young adults. This prompted us to conduct an alternative analysis of the data, which we discuss next.

In our study, both groups performed the coordination tasks together with the secondary task at a cycling frequency of 1.00 Hz. However, particularly for the nonisodirectional pattern, the required frequency was much closer to CF_max in the older than in the younger participants. This implies that task difficulty level differed between groups. When we corrected for CF_max,the age-related effect disappeared. This is in agreement with previous findings whereby age differences in divided attention ability disappeared when researchers corrected for single-task performance levels (Somberg & Salthouse, 1982). However, Salthouse, Rogan, and Prill (1984) and McDowd and Craik (1988) reported age-related differences in dual-task performances even after they corrected for individual differences in single-task performances. Subsequently, it is suggested that the extent to which one observes age-related differences in dual-task performances despite such corrections depends on the relative complexity of the component tasks (for an extensive discussion, see McDowd et al., 1991).

Surprisingly, our dual-task results did not show performance differences between the isodirectional and nonisodirectional coordination modes, in contrast with previous reports by Monno and colleagues (2000) and Temprado and colleagues (1999) on bimanual coordination whereby attentional requirements were found to be higher during the "more difficult" antiphase than the in-phase coordination mode in young adults. However, the latter study made use of a secondary task that required a motor response, that is, a manual response to an auditory signal. Consequently, these findings could be contaminated by structural interference as a result of shared sensory or motor processing resources for both tasks such that conclusions about attentional costs should be drawn with care. Conversely, we chose a secondary task that did not require a motor response, such that dual-task performance more likely reflected central processing costs. In particular, the simultaneous execution of both tasks influenced the isodirectional and nonisodirectional coordination patterns to the same extent in the elderly participants. Consequently, our data tentatively suggest that reduced attentional resources–control was not the main mechanism for the observed age-related deterioration during nonisodirectional coordination. Other mechanisms might play a more prominent role, as we discuss next.

It has been argued that performance of the isodirectional mode only requires limited feedback control, whereas the nonisodirectional coordination mode relies more profoundly on the processing of kinesthetic feedback afferences (Baldissera et al., 1991). This differential use of kinesthetic feedback has been underscored by studies in which sensory input was disturbed by passive movements of a third segment. Phasing accuracy and stability of the nonisodirectional mode degraded more profoundly as compared with the isodirectional movements, supporting the hypothesis that both coordination modes involve different processing requirements (Serrien, Li, Steyvers, Debaere, & Swinnen, 2001; Swinnen, Dounskaia, Verschueren, Serrien, & Daelman, 1995). Higher sensorimotor demands for the nonisodirectional coordination mode were also supported by brain imaging data on the ipsilateral coordination of the wrist and foot, demonstrating that nonisodirectional wrist and foot movements were associated with increased activation in the supplementary motor area (SMA) relative to isodirectional movements (Debaere et al., 2001). It has been shown that the SMA contributes to the central processing of kinesthetic feedback during movement production (Mima et al., 1999a,1999b; Naito, Ehrsson, Geyer, Zilles, & Roland, 1999; Radovanovic et al., 2002; Weiller et al., 1996) and that it plays a principal role in the control of interlimb coordination (for review, see Wenderoth, Debaere, & Swinnen, 2004). In addition, it has been argued that sensory processing abilities deteriorate with aging (Skinner et al., 1984; Stelmach et al., 1986). Elderly persons show a significant decrease in both cutaneous vibratory and joint sensations (Diener, Dichgans, Guschlbauer, & Mau, 1984; Skinner et al., 1984). Therefore, it might be argued that age-related proprioceptive processing deficits compromise motor functions in which this source of sensory information is of critical importance. Recent brain imaging data have shown that, during performance of repetitive finger or wrist movements, the aging brain recruits additional sensorimotor regions, presumably to compensate for these deficits (Hutchinson et al., 2002; Mattay et al., 2002). Therefore, our current working hypothesis is that age-related deficits in complex coordination tasks might be a consequence of decreased proprioceptive processing capabilities in elderly individuals.

In summary, our findings suggest that the age-related deterioration of interlimb coordination is not primarily caused by a deficit in attentional resources or control. Instead, it seems more likely that elderly persons are increasingly impaired when coordination tasks require a high degree of processing of proprioceptive information.

	Acknowledgments

 
Support for this study was provided through a grant from the Research Council of K.U. Leuven, Belgium (Contract OT/03/61) and the Flanders Fund for Scientific Research (Projects G.0460.04 & G.0105.00). S. Heuninckx is funded by a fellowship from F.W.O. Vlaanderen.

	Footnotes

 
Decision Editor: Margie E. Lachman, PhD

Received for publication October 21, 2003. Accepted for publication April 15, 2004.

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