- Email: [email protected]
Age-related trends in locomotor ability and obstacle avoidance
EISEVIER
Human Movement
Science 16 (1997) 507-516
Age-related trends in locomotor ability and obstacle avoidance Kelly M. Pryde *, Eric A. Roy, Aftab E. Patla Department of Kinesiology. University of Waterloo, Waterloo, ON, N2L 3G1, Canada
Abstract Age-related changes in the ability to maneuver through a cluttered environment with control and accuracy were examined in 24 female children. Movement time, errors, and qualitative measures were taken on children’s performance on 3 pathways of increasing complexity: a straight pathway requiring walking only, a winding pathway requiring steering ability, and a winding pathway with obstacles requiring steering and avoidance ability. Children before the age of 11 displayed significantly slower movement times when they were required to avoid obstacles, revealing that the addition of obstacles to a pathway places significantly greater demands on young children. Children under 8 years of age, however, made significantly more errors in the obstacle path and tended more often to take larger than necessary steps over obstacles. The findings are interpreted as a refinement of the essential sensorimotor elements involved in obstacle avoidance before the age of 8 and the more mature integration of these elements after the age of 8. PsycINFO
classification:
Keywords: Age differences;
2330 Human locomotion;
Motor performance
1. Introduction
Obstacle avoidance - maneuvering over and around obstacles in the environment to avoid collision - is an integral part of skilled locomotion and provides * Corresponding 746-6776.
author. E-mail: [email protected];
0167-9457/97/$17.00 Copyright PII SO167-9457(96)00064-4
Tel.:
+ 1 519 885-1211;
0 1997 Elsevier Science B.V. All rights reserved.
Fax: + 1 519
508
K.M. Pryde et al./ Human Movement Science I6 (1997) 507-516
us with a high degree of flexibility for navigating various terrains. Although obstacle avoidance has been studied in a variety of experimental contexts, including manual skill (Bruner, 1970) and cognitive mapping of the environment (Pick and Lockman, 1981>, little is known about its development throughout childhood for locomotor tasks. The present study was designed to investigate age-related changes that occur in the ability to locomote or maneuver through a cluttered environment with control and accuracy. The two sensory modalities most important for locomotion and obstacle avoidance are vision and kinesthesis (Patla et al., 1995). Vision is essential for providing exteroceptive information about the layout of the environment obstacle location and characteristics such as height and width. Vision also provides exproprioceptive information about limb position and orientation as it goes over obstacles. In addition to the two types of information described above, vision also serves a role in postural stability as information from peripheral flow is used to maintain balance. Schmuckler and Gibson (1989) showed that maintaining postural stability and steering through the environment are two discrete functions guided by different aspects of optical flow. They also showed that very ‘inexperienced’ walkers (e.g. 5 months or less walking experience) relative to ‘experienced’ walkers (e.g. between 12 and 24 months walking experience) have difficulty differentiating the information relevant for balance from that for steering. The resultant conflict in visual information causes increases in postural instability when steering through the environment as opposed to when walking along a straight path without having to steer. Although Schmuckler and Gibson (1989) did not find this differentiating process to be complete in their ‘experienced’ walkers, they concluded that it is only with increasing age and walking experience that children are able to distinguish between these two types of visual information and to eventually integrate the two functions and use them with a greater degree of automaticity. Consider next the contributions of the kinesthetic system. Kinesthesis is critical for providing information about the position and movement of the limbs through space. Information determined from the relative joint angles of the leading and trailing limbs provide knowledge about the location of the limb to ensure obstacle clearance. In order to steer oneself through the environment, both visual and kinesthetic information must be integrated and coupled with the motor system to perceive and react to various directional changes. Additional demands are placed on intersensory and sensorimotor integration abilities once objects are added to a locomotor task. In addition to steering oneself along the path the mover is now required to make decisions based on spatial and temporal
K.M. Pryde et al./Human
Movement Science 16 (1997) 507-516
509
information pertaining to the size and location of objects. Timing when to initiate movements to avoid an obstacle based on location information must be coordinated with information about the size of obstacles (i.e. height and width). These properties indicate how high to lift one’s foot to step over on-ground obstacles or how low to bend to successfully avoid overhead obstacles. Patla et al. (1995) have studied the effect of the aging process on obstacle avoidance by examining avoidance strategies in adults and toddlers. Based on the toddlers’ pattern and rate of failure in obstacle avoidance, Patla et al. (1995) point out that interesting signposts exist for the development of a stable avoidance strategy but successful obstacle avoidance must await maturation of the sensory systems and the motor mechanism as well as the coupling between them. Patla et al. (1995) have used these findings to develop a jigsaw puzzle metaphor which summarizes the salient features of obstacle avoidance. As shown in Fig. 1, these features include the visual, kinesthetic and effector system properties which begin as individual pieces during the fast few years of life. During the course of development these pieces or elements become refined until they can be brought together into an integrated whole. It is only when all the pieces of the puzzle or elements of the system can be fully integrated that adult-like strategies can be used for successful obstacle avoidance. This puzzle metaphor suggests that young children with less maneuvering experience and less developed sensory and sensorimotor integration systems may have difficulty modulating their actions to obstacles in the environment. Thus, young children may be more likely to lift their feet too high or bend excessively to avoid obstacles in comparison to older more experienced children who should leave minimal clearance between themselves and objects to produce the most cost efficient movements. The following experiment examined age-related changes that occur in chil-
wGRW_%
DEVELOPMENT eis
\““”
Fig. 1. Jigsaw puzzle metaphor (adapted from Patla et al., 1995).
System
]
510
KM. Plyde et al. /Human Movement Science I6 (1997) 507-516
dren’s ability to guide themselves under conditions of increased pathway complexity. Given the findings of Schmuckler and Gibson (1989) and Patla et al. (1995) we predicted that the increased abilities with age in postural stability and sensorimotor integration along with greater experience in locomotor tasks would enable older children to steer themselves and avoid obstacles with a greater degree of control and automaticity. More specifically, age differences in performance would be relatively small in a simple straight path requiring basic walking abilities and would increase in more complex paths requiring steering and obstacle avoidance. Furthermore, we predicted that younger children with less avoidance experience and less integrated sensorimotor systems would demonstrate overcompensated steps over and/or under obstacles to produce larger safety margins for obstacle clearance.
2. Method 2. I. Subjects The subjects consisted of twenty four female children aged 5 to 16 years with no known physical or mental impairments. The children were volunteers from the Carousel Dance Centre at the University of Waterloo whose parents gave written consent to their child’s involvement in the study. The study was approved and carried out according to the ethical guidelines laid down by the Office of Human Research at the University of Waterloo. The subjects were placed in one of three groups based on their stage of motor development as defined by Gallahue (1982). Group 1 (n = 9) was 5-7 years of age (fundamental motor abilities stage); Group 2 (n = 11) was 8-10 years of age (general movement abilities stage); and Group 3 (n = 4) was 11-16 years of age (specific movement abilities stage) (Gallahue, 1982). 2.2. Apparatus
and experimental
conditions
All children participated in three conditions. The first condition (Straight) involved a straight pathway without obstacles measuring 55 feet in length and 3.5 feet in width. The second condition (Winding) involved a winding U-shaped pathway without obstacles. The pathway measured 55 feet in length and varied in width from 2 feet to 3.5 feet. The third condition (Obstacle) involved the same winding pathway used in the Winding condition with the inclusion of 33 obstacles placed at irregular intervals along the path. The obstacles were made
K.M. Pryde et al. /Human Movement Science 16 (1997) 507-516
511
of soft foam and varied in size, shape, and colour. The various shapes and sizes of the obstacles included: 7 flat rectangular (8”l X 4” w X l”h), 5 rectangular (24”l X 5”~ X S’h), 10 cylindrical (eight 8”diam. X 20”h and two 12” diam. X 12”h used as hanging obstacles), 9 tube-like (4”diam. X 36”1), and 2 block-like (2O”l X 8”~ X 15”h). The on-ground obstacles were arranged in horizontal, vertical, and oblique positions such that the participants were required to maneuver around some obstacles and to step over others. None of the participants were required to step over an obstacle greater than five inches in height. The hanging obstacles hung 55 inches from the floor and were positioned over the centre of the path. A colour video camera was used to record all trials in this condition and was set up in a position such that the performance of all participants was within viewing range. 2.3. Procedure Subjects were tested individually and participated in each of the three conditions in a random order. For each condition, subjects began on a starting line and were instructed to walk at a normal pace along the pathway until they crossed the finish line. Each subject’s walking time from start to finish was timed to the nearest one hundredth of a second. Three trials were collected for each condition. Additional instructions were given in the Winding and Obstacle conditions. In the Winding condition subjects were instructed to ensure that they stayed within the lines of the winding pathway. Any step taken outside the pathway line was recorded as an error. In the Obstacle condition subjects were not only instructed to walk within the lines of the winding pathway but also to ensure they avoided bumping into, knocking over, or stepping on any obstacles. An error in this condition was defined as a step outside of the pathway line and/or any obstacle hits. Because the initial analysis revealed a low number of errors in both the Winding and Obstacle conditions, the different types of errors were not differentiated in this analysis. All errors were recorded during the initial analysis and were later reanalyzed for accuracy from the videotape by two trained observers. A qualitative measure, overcompensating for obstacle clearance, was also recorded in this condition. An overcompensation was defined as any step over an obstacle with toe clearance greater than the length of the foot and any crouch under an obstacle greater than the length of the head from the top of the head to the bottom of the chin. All overcompensations were determined from the videotape.
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3. Results
Inter-rater reliability was assessed for the error and overcompensation measures. A random sample of children was chosen from each group and analyzed by two trained observers. The number of errors and the number of overcompensations observed and recorded by each examiner were then correlated to determine the reliability of inter-rater judgements. The inter-rater correlation of errors was 0.88, p < 0.02. The inter-rater correlation of overcompensations was 0.98, p < 0.001. The dependent variables, movement time, errors, and overcompensations were calculated for each trial. The mean values for the three trials in each condition were examined in a 3 (group) X 3 (condition) analysis of variance. Significance was evaluated at the 0.05 level and post hoc tests were performed using LSD t-tests. 3.1. hkwement time Analyses of movement time revealed slower overall movement times for Groups A (5-7 years) and B (8-10 years) than for Group C (1 l-16 years), F(2,21) = 4.08, p < 0.05 and increases in movement time for increasing complexity of pathways, F(2,42) = 116.31, p < 0.01. An interaction effect between group and condition (see Fig. 2) was also revealed, F(4,42) = 2.93, p < 0.05. Analyses of the simple main effects of group in each condition revealed an age effect only in the obstacle condition. Post hoc analyses revealed that the
35
qsttaight n wtrld!ng n obstacle
30 --
A (5-7 years)
E (8-10 years)
C (11.16years)
Age Gmup
Fig. 2. Interaction
between group and condition for movement time
KM. Pryde et al./ Human Movement Science 16 (1997) 507-516 Table 1 Effect of group on errors and overcompensations
for obstacle condition
Agegroup
Errors
Overcompensations
A (5-7 years) B (8-10 years) C (11-16 years)
1.OO a (0.55) 0.33 b (0.52) 0.33 b (0.27)
5.12 a (2.42) 3.33 a.b (2.07) 2.17 b (2.98)
Means with different superscripts are significantly The standard deviations arc given in parentheses.
513
different at p < 0.05.
youngest groups (A and B), demonstrated than did the oldest group CC>.
significantly
longer movement
times
3.2. Errors ’ Error analyses for the obstacle condition (see Table 1) revealed that Group A (5-7 years) made significantly more errors than Groups B (8- 10 years) and C (11-16 years), F(2,21) = 5.38, p < 0.05. An insufficient number of errors was made on the Winding condition to enable a statistical analysis. 3.3. Over-compensations (see fi. 1) Analyses of overcompensations for the obstacle pathway (see Table 1) revealed that Group A (5-7 years) made significantly more overcompensations than Group C (11-16 years), F(2,21) = 19.77, p < 0.01.
4. Discussion Our findings indicate that there are age-related changes in locomotor ability, although, as we predicted, the effect of these changes is dependent on the nature of the task. Age does have its greatest effect in the more complex pathways. As predicted the differences between the age groups for movement time on the Straight path were small and insignificant. As shown in Fig. 2, the increase in movement time from the Straight path to the Winding path is also approximately the same for all age groups suggesting that the addition of a steering component
’ Analyses of error and overcompensation measures revealed some differences in within-group Further analyses of these differences in variability using the Fm, statistic revealed no significant F,,,,,,(3,10) = 2.07, p > 0.01. errors: Fmro.pP(3,10)= 4.13, p > 0.01; overcompensations:
variability. differences,
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K.M. Pryde et al./Human
Movement Science 16 (1997) 507-516
does not involve sufficient motor control and information processing demands to affect performance. In other words, adding steering requirements did not challenge the locomotor control system of even the youngest children beyond that afforded by the basic walking task. This unexpected finding combined with the fact that no errors were made in these conditions suggests that 5-7-year-olds are able to distinguish the different types of visual information necessary for steering as shown by Schmuckler and Gibson (1989). Recall that Schmuckler and Gibson (1989) found 2-3-year-old children incapable of differentiating the visual information relevant for balance from that for steering which led to decreases in stability. Possibly this differentiating process becomes complete between three and five years of age and therefore the effects of steering would be seen in children of this age group. The addition of avoidance requirements to a steering task does, however, place sufficient demands on motor control and information processing as it has a much larger effect on the movement time of the two younger groups. Although it was not the purpose of the present study to de-confound steering and avoidance abilities, the nature of the age by pathway interaction suggests that adding obstacles alone to the basic walking task (Straight condition) without adding the demands for steering might have the same effect on performance. It is interesting and somewhat surprising that the two younger groups did not demonstrate greater differences in movement time. Analyses of performance in the present study for the Obstacle condition indicate that there were considerable costs paid by the 5-7-year-olds for moving at the same rate as the 8-lo-year-olds in that they made significantly more errors. This apparent trade-off in accuracy suggests that the youngest children should have adopted a slower time to move through the path. In addition to making more errors in the Obstacle path, the 5--7-year-olds also exhibited more overcompensations when stepping over and bending under obstacles, as we predicted. Of particular interest, is the observation that the same movement times caused an increase in errors and overcompensations for the 5-7-year-old children only. One explanation for the greater number of errors and overcompensations in the youngest group comes from the work of Shumway-Cooke and Woollacott ( 1985) which examined the development of postural adjustments in the presence of conflicting sensory information in children aged 15 months to 10 years. Their youngest children, 15-3 1 months, demonstrated exaggerated postural reactions which overcompensated for the externally generated body sway. This finding was consistent with the results of Schmuckler and Gibson’s (1989) study. Of particular interest were the postural patterns of the 4-6-year-old children which were highly variable. Shumway-Cooke and Woollacott (1985) interpreted the
K.M. Pryde et al. /Human Movement Science 16 (1997) 507-516
515
postural patterns of this age group as evidence for a shift away from visual domination of balance control and a greater reliance on kinesthetic information for the maintenance of stability. Finally they found that adult-like postural adjustments were not achieved until 7-10 years of age. If 4-6-year-old children are experiencing difficulties in postural stability due to changes in sensorimotor control, then their fast movement times would create greater perturbations to balance control resulting in more errors. Furthermore, because of the tendency of these children to make more errors they may also adopt a secondary strategy of clearing obstacles with a larger than average safety margin to ensure obstacle clearance in their haste. This compensatory strategy is reflected in the greater number of overcompensations exhibited by the 5-7-year-olds. This explanation is in keeping with the jigsaw puzzle metaphor presented by Patla et al. (1995) suggesting that it is the unrefined elements and immature integration of the sensorimotor system of young children that results in their compensatory strategies. The performance of the 8-lo-year-old group provides evidence for the refinement and more fully developed integration of the sensorimotor elements necessary for adult-like obstacle avoidance as they exhibit fewer errors and overcompensations while moving in the same time. A second explanation that would account for the performance of the 5-7year-olds comes from our hypothesis based on the puzzle metaphor (Patla et al., 1995). This explanation suggests that the greater number of overcompensations exhibited by the youngest children is a direct result of the immature sensorimotor system as opposed to a compensatory strategy. Because the elements of the system necessary for adult-like obstacle avoidance, that is the visual, kinesthetic and effector systems, are in the process of refinement and change as shown by Shumway-Cooke and Woollacott (1985), children before the age of eight years may have difficulty modulating their actions to the size of the obstacles encountered in their path. This theory could be confirmed by requiring young children to visually scale how high they would need to lift their foot to clear various obstacle heights without actually stepping over the obstacles. If in fact children in this age range do have difficulty modulating their actions to visual information about objects in their environment, then they should have significant difficulty performing this task relative to older children. A difficulty in using visual information to modulate their actions combined with a fast movement time would explain an increase in errors since faster movement would afford less time to process the visual information necessary for producing efficient movement. Although the explanations presented cannot be verified on the basis of the methodology of the present study, one way to investigate these ideas is to
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Movement Science 16 (1997) 507-516
control the rate at which children are able to complete the obstacle task. If one or both of these explanations accounts for the increased errors and overcompensations in 5-7-year-olds, then requiring children in this age group to move at a slower rate should improve their performance. In sum, our findings support our predictions about age-related changes in obstacle avoidance and also raise a number of interesting questions regarding the development of this fundamental skill. While mature strategies for steering are achieved at an early age, adult-like strategies for obstacle avoidance do not begin until 8-10 years of age. In keeping with the jigsaw puzzle analogy, these findings suggest that the refinement of the essential properties involved in obstacle avoidance occur before the age of seven years and that these pieces or properties become more firmly integrated between the ages of eight and ten years. Before eight years of age, children show significant difficulty modifying their walking speed and their actions to successfully and efficiently avoid obstacles. It is not until after eight years of age that children begin to show signs of using more adult-like strategies for successful obstacle avoidance. Whether or not the performance of the youngest children can be attributed to postural instability and compensatory strategies or to difficulty modulating their actions based on visual information about the environment must be examined in future research.
References Bruner, J.S., 1970. ‘The growth and structure of skill’. In: K. Connolly (Ed.), Mechanisms of motor skill development. New York Academic Press. Gallahue, D.L., 1982. Developmental movement experiences for children. New York: Wiley. Patla, A.E., S.D. Prentice and L.T. Gobbi, 1995. ‘Visual control of obstacle avoidance during locomotion: Strategies in young children, young and older adults’. In: A.M. Femandez and N. Teasdale (Eds.), Changes in sensorimotor behavior in aging. Pick, H.L. and J.J. Lockman, 1981. ‘From frames of reference to spatial representations’. In: L.S. Liben, A.H. Patterson, and N. Newcombe (Eds.), Spatial representation and behavior across the lifespan. New York: Academic Press. Schmuckler, M.A. and E.J. Gibson, 1989. The effect of imposed optical flow on guided locomotion in young walkers. British Journal of Developmental Psychology, 193-206. Shumway-Cooke, A. and M.J. Woollacott, 1985. The growth of stability: Postural control from a developmental perspective. Journal of Motor Behavior 17, 131-147.
Human Movement
Science 16 (1997) 507-516
Age-related trends in locomotor ability and obstacle avoidance Kelly M. Pryde *, Eric A. Roy, Aftab E. Patla Department of Kinesiology. University of Waterloo, Waterloo, ON, N2L 3G1, Canada
Abstract Age-related changes in the ability to maneuver through a cluttered environment with control and accuracy were examined in 24 female children. Movement time, errors, and qualitative measures were taken on children’s performance on 3 pathways of increasing complexity: a straight pathway requiring walking only, a winding pathway requiring steering ability, and a winding pathway with obstacles requiring steering and avoidance ability. Children before the age of 11 displayed significantly slower movement times when they were required to avoid obstacles, revealing that the addition of obstacles to a pathway places significantly greater demands on young children. Children under 8 years of age, however, made significantly more errors in the obstacle path and tended more often to take larger than necessary steps over obstacles. The findings are interpreted as a refinement of the essential sensorimotor elements involved in obstacle avoidance before the age of 8 and the more mature integration of these elements after the age of 8. PsycINFO
classification:
Keywords: Age differences;
2330 Human locomotion;
Motor performance
1. Introduction
Obstacle avoidance - maneuvering over and around obstacles in the environment to avoid collision - is an integral part of skilled locomotion and provides * Corresponding 746-6776.
author. E-mail: [email protected];
0167-9457/97/$17.00 Copyright PII SO167-9457(96)00064-4
Tel.:
+ 1 519 885-1211;
0 1997 Elsevier Science B.V. All rights reserved.
Fax: + 1 519
508
K.M. Pryde et al./ Human Movement Science I6 (1997) 507-516
us with a high degree of flexibility for navigating various terrains. Although obstacle avoidance has been studied in a variety of experimental contexts, including manual skill (Bruner, 1970) and cognitive mapping of the environment (Pick and Lockman, 1981>, little is known about its development throughout childhood for locomotor tasks. The present study was designed to investigate age-related changes that occur in the ability to locomote or maneuver through a cluttered environment with control and accuracy. The two sensory modalities most important for locomotion and obstacle avoidance are vision and kinesthesis (Patla et al., 1995). Vision is essential for providing exteroceptive information about the layout of the environment obstacle location and characteristics such as height and width. Vision also provides exproprioceptive information about limb position and orientation as it goes over obstacles. In addition to the two types of information described above, vision also serves a role in postural stability as information from peripheral flow is used to maintain balance. Schmuckler and Gibson (1989) showed that maintaining postural stability and steering through the environment are two discrete functions guided by different aspects of optical flow. They also showed that very ‘inexperienced’ walkers (e.g. 5 months or less walking experience) relative to ‘experienced’ walkers (e.g. between 12 and 24 months walking experience) have difficulty differentiating the information relevant for balance from that for steering. The resultant conflict in visual information causes increases in postural instability when steering through the environment as opposed to when walking along a straight path without having to steer. Although Schmuckler and Gibson (1989) did not find this differentiating process to be complete in their ‘experienced’ walkers, they concluded that it is only with increasing age and walking experience that children are able to distinguish between these two types of visual information and to eventually integrate the two functions and use them with a greater degree of automaticity. Consider next the contributions of the kinesthetic system. Kinesthesis is critical for providing information about the position and movement of the limbs through space. Information determined from the relative joint angles of the leading and trailing limbs provide knowledge about the location of the limb to ensure obstacle clearance. In order to steer oneself through the environment, both visual and kinesthetic information must be integrated and coupled with the motor system to perceive and react to various directional changes. Additional demands are placed on intersensory and sensorimotor integration abilities once objects are added to a locomotor task. In addition to steering oneself along the path the mover is now required to make decisions based on spatial and temporal
K.M. Pryde et al./Human
Movement Science 16 (1997) 507-516
509
information pertaining to the size and location of objects. Timing when to initiate movements to avoid an obstacle based on location information must be coordinated with information about the size of obstacles (i.e. height and width). These properties indicate how high to lift one’s foot to step over on-ground obstacles or how low to bend to successfully avoid overhead obstacles. Patla et al. (1995) have studied the effect of the aging process on obstacle avoidance by examining avoidance strategies in adults and toddlers. Based on the toddlers’ pattern and rate of failure in obstacle avoidance, Patla et al. (1995) point out that interesting signposts exist for the development of a stable avoidance strategy but successful obstacle avoidance must await maturation of the sensory systems and the motor mechanism as well as the coupling between them. Patla et al. (1995) have used these findings to develop a jigsaw puzzle metaphor which summarizes the salient features of obstacle avoidance. As shown in Fig. 1, these features include the visual, kinesthetic and effector system properties which begin as individual pieces during the fast few years of life. During the course of development these pieces or elements become refined until they can be brought together into an integrated whole. It is only when all the pieces of the puzzle or elements of the system can be fully integrated that adult-like strategies can be used for successful obstacle avoidance. This puzzle metaphor suggests that young children with less maneuvering experience and less developed sensory and sensorimotor integration systems may have difficulty modulating their actions to obstacles in the environment. Thus, young children may be more likely to lift their feet too high or bend excessively to avoid obstacles in comparison to older more experienced children who should leave minimal clearance between themselves and objects to produce the most cost efficient movements. The following experiment examined age-related changes that occur in chil-
wGRW_%
DEVELOPMENT eis
\““”
Fig. 1. Jigsaw puzzle metaphor (adapted from Patla et al., 1995).
System
]
510
KM. Plyde et al. /Human Movement Science I6 (1997) 507-516
dren’s ability to guide themselves under conditions of increased pathway complexity. Given the findings of Schmuckler and Gibson (1989) and Patla et al. (1995) we predicted that the increased abilities with age in postural stability and sensorimotor integration along with greater experience in locomotor tasks would enable older children to steer themselves and avoid obstacles with a greater degree of control and automaticity. More specifically, age differences in performance would be relatively small in a simple straight path requiring basic walking abilities and would increase in more complex paths requiring steering and obstacle avoidance. Furthermore, we predicted that younger children with less avoidance experience and less integrated sensorimotor systems would demonstrate overcompensated steps over and/or under obstacles to produce larger safety margins for obstacle clearance.
2. Method 2. I. Subjects The subjects consisted of twenty four female children aged 5 to 16 years with no known physical or mental impairments. The children were volunteers from the Carousel Dance Centre at the University of Waterloo whose parents gave written consent to their child’s involvement in the study. The study was approved and carried out according to the ethical guidelines laid down by the Office of Human Research at the University of Waterloo. The subjects were placed in one of three groups based on their stage of motor development as defined by Gallahue (1982). Group 1 (n = 9) was 5-7 years of age (fundamental motor abilities stage); Group 2 (n = 11) was 8-10 years of age (general movement abilities stage); and Group 3 (n = 4) was 11-16 years of age (specific movement abilities stage) (Gallahue, 1982). 2.2. Apparatus
and experimental
conditions
All children participated in three conditions. The first condition (Straight) involved a straight pathway without obstacles measuring 55 feet in length and 3.5 feet in width. The second condition (Winding) involved a winding U-shaped pathway without obstacles. The pathway measured 55 feet in length and varied in width from 2 feet to 3.5 feet. The third condition (Obstacle) involved the same winding pathway used in the Winding condition with the inclusion of 33 obstacles placed at irregular intervals along the path. The obstacles were made
K.M. Pryde et al. /Human Movement Science 16 (1997) 507-516
511
of soft foam and varied in size, shape, and colour. The various shapes and sizes of the obstacles included: 7 flat rectangular (8”l X 4” w X l”h), 5 rectangular (24”l X 5”~ X S’h), 10 cylindrical (eight 8”diam. X 20”h and two 12” diam. X 12”h used as hanging obstacles), 9 tube-like (4”diam. X 36”1), and 2 block-like (2O”l X 8”~ X 15”h). The on-ground obstacles were arranged in horizontal, vertical, and oblique positions such that the participants were required to maneuver around some obstacles and to step over others. None of the participants were required to step over an obstacle greater than five inches in height. The hanging obstacles hung 55 inches from the floor and were positioned over the centre of the path. A colour video camera was used to record all trials in this condition and was set up in a position such that the performance of all participants was within viewing range. 2.3. Procedure Subjects were tested individually and participated in each of the three conditions in a random order. For each condition, subjects began on a starting line and were instructed to walk at a normal pace along the pathway until they crossed the finish line. Each subject’s walking time from start to finish was timed to the nearest one hundredth of a second. Three trials were collected for each condition. Additional instructions were given in the Winding and Obstacle conditions. In the Winding condition subjects were instructed to ensure that they stayed within the lines of the winding pathway. Any step taken outside the pathway line was recorded as an error. In the Obstacle condition subjects were not only instructed to walk within the lines of the winding pathway but also to ensure they avoided bumping into, knocking over, or stepping on any obstacles. An error in this condition was defined as a step outside of the pathway line and/or any obstacle hits. Because the initial analysis revealed a low number of errors in both the Winding and Obstacle conditions, the different types of errors were not differentiated in this analysis. All errors were recorded during the initial analysis and were later reanalyzed for accuracy from the videotape by two trained observers. A qualitative measure, overcompensating for obstacle clearance, was also recorded in this condition. An overcompensation was defined as any step over an obstacle with toe clearance greater than the length of the foot and any crouch under an obstacle greater than the length of the head from the top of the head to the bottom of the chin. All overcompensations were determined from the videotape.
512
KM. Pryde et al. /Human Mouemenr Science 16 (1997) 507-516
3. Results
Inter-rater reliability was assessed for the error and overcompensation measures. A random sample of children was chosen from each group and analyzed by two trained observers. The number of errors and the number of overcompensations observed and recorded by each examiner were then correlated to determine the reliability of inter-rater judgements. The inter-rater correlation of errors was 0.88, p < 0.02. The inter-rater correlation of overcompensations was 0.98, p < 0.001. The dependent variables, movement time, errors, and overcompensations were calculated for each trial. The mean values for the three trials in each condition were examined in a 3 (group) X 3 (condition) analysis of variance. Significance was evaluated at the 0.05 level and post hoc tests were performed using LSD t-tests. 3.1. hkwement time Analyses of movement time revealed slower overall movement times for Groups A (5-7 years) and B (8-10 years) than for Group C (1 l-16 years), F(2,21) = 4.08, p < 0.05 and increases in movement time for increasing complexity of pathways, F(2,42) = 116.31, p < 0.01. An interaction effect between group and condition (see Fig. 2) was also revealed, F(4,42) = 2.93, p < 0.05. Analyses of the simple main effects of group in each condition revealed an age effect only in the obstacle condition. Post hoc analyses revealed that the
35
qsttaight n wtrld!ng n obstacle
30 --
A (5-7 years)
E (8-10 years)
C (11.16years)
Age Gmup
Fig. 2. Interaction
between group and condition for movement time
KM. Pryde et al./ Human Movement Science 16 (1997) 507-516 Table 1 Effect of group on errors and overcompensations
for obstacle condition
Agegroup
Errors
Overcompensations
A (5-7 years) B (8-10 years) C (11-16 years)
1.OO a (0.55) 0.33 b (0.52) 0.33 b (0.27)
5.12 a (2.42) 3.33 a.b (2.07) 2.17 b (2.98)
Means with different superscripts are significantly The standard deviations arc given in parentheses.
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different at p < 0.05.
youngest groups (A and B), demonstrated than did the oldest group CC>.
significantly
longer movement
times
3.2. Errors ’ Error analyses for the obstacle condition (see Table 1) revealed that Group A (5-7 years) made significantly more errors than Groups B (8- 10 years) and C (11-16 years), F(2,21) = 5.38, p < 0.05. An insufficient number of errors was made on the Winding condition to enable a statistical analysis. 3.3. Over-compensations (see fi. 1) Analyses of overcompensations for the obstacle pathway (see Table 1) revealed that Group A (5-7 years) made significantly more overcompensations than Group C (11-16 years), F(2,21) = 19.77, p < 0.01.
4. Discussion Our findings indicate that there are age-related changes in locomotor ability, although, as we predicted, the effect of these changes is dependent on the nature of the task. Age does have its greatest effect in the more complex pathways. As predicted the differences between the age groups for movement time on the Straight path were small and insignificant. As shown in Fig. 2, the increase in movement time from the Straight path to the Winding path is also approximately the same for all age groups suggesting that the addition of a steering component
’ Analyses of error and overcompensation measures revealed some differences in within-group Further analyses of these differences in variability using the Fm, statistic revealed no significant F,,,,,,(3,10) = 2.07, p > 0.01. errors: Fmro.pP(3,10)= 4.13, p > 0.01; overcompensations:
variability. differences,
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does not involve sufficient motor control and information processing demands to affect performance. In other words, adding steering requirements did not challenge the locomotor control system of even the youngest children beyond that afforded by the basic walking task. This unexpected finding combined with the fact that no errors were made in these conditions suggests that 5-7-year-olds are able to distinguish the different types of visual information necessary for steering as shown by Schmuckler and Gibson (1989). Recall that Schmuckler and Gibson (1989) found 2-3-year-old children incapable of differentiating the visual information relevant for balance from that for steering which led to decreases in stability. Possibly this differentiating process becomes complete between three and five years of age and therefore the effects of steering would be seen in children of this age group. The addition of avoidance requirements to a steering task does, however, place sufficient demands on motor control and information processing as it has a much larger effect on the movement time of the two younger groups. Although it was not the purpose of the present study to de-confound steering and avoidance abilities, the nature of the age by pathway interaction suggests that adding obstacles alone to the basic walking task (Straight condition) without adding the demands for steering might have the same effect on performance. It is interesting and somewhat surprising that the two younger groups did not demonstrate greater differences in movement time. Analyses of performance in the present study for the Obstacle condition indicate that there were considerable costs paid by the 5-7-year-olds for moving at the same rate as the 8-lo-year-olds in that they made significantly more errors. This apparent trade-off in accuracy suggests that the youngest children should have adopted a slower time to move through the path. In addition to making more errors in the Obstacle path, the 5--7-year-olds also exhibited more overcompensations when stepping over and bending under obstacles, as we predicted. Of particular interest, is the observation that the same movement times caused an increase in errors and overcompensations for the 5-7-year-old children only. One explanation for the greater number of errors and overcompensations in the youngest group comes from the work of Shumway-Cooke and Woollacott ( 1985) which examined the development of postural adjustments in the presence of conflicting sensory information in children aged 15 months to 10 years. Their youngest children, 15-3 1 months, demonstrated exaggerated postural reactions which overcompensated for the externally generated body sway. This finding was consistent with the results of Schmuckler and Gibson’s (1989) study. Of particular interest were the postural patterns of the 4-6-year-old children which were highly variable. Shumway-Cooke and Woollacott (1985) interpreted the
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postural patterns of this age group as evidence for a shift away from visual domination of balance control and a greater reliance on kinesthetic information for the maintenance of stability. Finally they found that adult-like postural adjustments were not achieved until 7-10 years of age. If 4-6-year-old children are experiencing difficulties in postural stability due to changes in sensorimotor control, then their fast movement times would create greater perturbations to balance control resulting in more errors. Furthermore, because of the tendency of these children to make more errors they may also adopt a secondary strategy of clearing obstacles with a larger than average safety margin to ensure obstacle clearance in their haste. This compensatory strategy is reflected in the greater number of overcompensations exhibited by the 5-7-year-olds. This explanation is in keeping with the jigsaw puzzle metaphor presented by Patla et al. (1995) suggesting that it is the unrefined elements and immature integration of the sensorimotor system of young children that results in their compensatory strategies. The performance of the 8-lo-year-old group provides evidence for the refinement and more fully developed integration of the sensorimotor elements necessary for adult-like obstacle avoidance as they exhibit fewer errors and overcompensations while moving in the same time. A second explanation that would account for the performance of the 5-7year-olds comes from our hypothesis based on the puzzle metaphor (Patla et al., 1995). This explanation suggests that the greater number of overcompensations exhibited by the youngest children is a direct result of the immature sensorimotor system as opposed to a compensatory strategy. Because the elements of the system necessary for adult-like obstacle avoidance, that is the visual, kinesthetic and effector systems, are in the process of refinement and change as shown by Shumway-Cooke and Woollacott (1985), children before the age of eight years may have difficulty modulating their actions to the size of the obstacles encountered in their path. This theory could be confirmed by requiring young children to visually scale how high they would need to lift their foot to clear various obstacle heights without actually stepping over the obstacles. If in fact children in this age range do have difficulty modulating their actions to visual information about objects in their environment, then they should have significant difficulty performing this task relative to older children. A difficulty in using visual information to modulate their actions combined with a fast movement time would explain an increase in errors since faster movement would afford less time to process the visual information necessary for producing efficient movement. Although the explanations presented cannot be verified on the basis of the methodology of the present study, one way to investigate these ideas is to
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control the rate at which children are able to complete the obstacle task. If one or both of these explanations accounts for the increased errors and overcompensations in 5-7-year-olds, then requiring children in this age group to move at a slower rate should improve their performance. In sum, our findings support our predictions about age-related changes in obstacle avoidance and also raise a number of interesting questions regarding the development of this fundamental skill. While mature strategies for steering are achieved at an early age, adult-like strategies for obstacle avoidance do not begin until 8-10 years of age. In keeping with the jigsaw puzzle analogy, these findings suggest that the refinement of the essential properties involved in obstacle avoidance occur before the age of seven years and that these pieces or properties become more firmly integrated between the ages of eight and ten years. Before eight years of age, children show significant difficulty modifying their walking speed and their actions to successfully and efficiently avoid obstacles. It is not until after eight years of age that children begin to show signs of using more adult-like strategies for successful obstacle avoidance. Whether or not the performance of the youngest children can be attributed to postural instability and compensatory strategies or to difficulty modulating their actions based on visual information about the environment must be examined in future research.
References Bruner, J.S., 1970. ‘The growth and structure of skill’. In: K. Connolly (Ed.), Mechanisms of motor skill development. New York Academic Press. Gallahue, D.L., 1982. Developmental movement experiences for children. New York: Wiley. Patla, A.E., S.D. Prentice and L.T. Gobbi, 1995. ‘Visual control of obstacle avoidance during locomotion: Strategies in young children, young and older adults’. In: A.M. Femandez and N. Teasdale (Eds.), Changes in sensorimotor behavior in aging. Pick, H.L. and J.J. Lockman, 1981. ‘From frames of reference to spatial representations’. In: L.S. Liben, A.H. Patterson, and N. Newcombe (Eds.), Spatial representation and behavior across the lifespan. New York: Academic Press. Schmuckler, M.A. and E.J. Gibson, 1989. The effect of imposed optical flow on guided locomotion in young walkers. British Journal of Developmental Psychology, 193-206. Shumway-Cooke, A. and M.J. Woollacott, 1985. The growth of stability: Postural control from a developmental perspective. Journal of Motor Behavior 17, 131-147.