influence of moving visual environment on sit-to-stand kinematics in children and adults ...

Perceptual and Motor Skills, 2009, 109, 121-132. © Perceptual and Motor Skills 2009

DOI 10.2466/PMS.109.1.121-132

INFLUENCE OF MOVING VISUAL ENVIRONMENT ON SIT-TO-STAND KINEMATICS IN CHILDREN AND ADULTS1

JILL C. SLABODA, JOSEPH E. BARTON

Department of Physical Therapy

EMILY A. KESHNER

Department of Electrical and Computer Engineering

Department of Physical TherapyTemple University

Summary.—The effect of visual field motion on the sit-to-stand kinematics of adults and children was investigated. Children (8 to12 years of age) and adults (21 to 49 years of age) were seated in a virtual environment that rotated in the pitch and roll directions. Participants stood up either (1) concurrent with onset of visual motion or (2) after an immersion period in the moving visual environment, and (3) without visual input. Angular velocities of the head with respect to the trunk, and trunk with respect to the environment, were calculated as was head and trunk center of mass. Both adults and children reduced head and trunk angular velocity after immersion in the moving visual environment. Unlike adults, children demon-strated significant differences in displacement of the head center of mass during the immersion and concurrent trials when compared to trials without visual input. Re-sults suggest a time-dependent effect of vision on sit-to-stand kinematics in adults, whereas children are influenced by the immediate presence or absence of vision.

Sit-to-stand kinematics are often modified when task constraints such as seat height, foot position, base of support, or the presence or absence of arm rests are altered (Burdett, Habasevich, Pisciotta, & Simon, 1985; Stevens, Bojen-Moller, & Soames, 1989; Khemlani, Carr, & Crosbie, 1999; Kawagoe, Tajima, & Chosa, 2000; Scholz, Reisman, & Schoner, 2001; Jan-essen, Bussman, & Stam, 2002; Hennington, Johnson, Penrose, Barr, Mc-Mulkin, & Vander Linden, 2004). For example, children were observed to increase trunk flexion and ankle dorsiflexon when performing sit-to-stand with a weighted backpack (Seven, Akalan, & Yucesoy, 2007). Adults increased trunk angular velocity in flexion during sit-to-stand when the height of the chair was decreased (Schenkman, Riley, & Pieper, 1996). Thus, as the mechanical constraints of the task vary, adults and children clearly modify their response kinematics to perform the action of rising from a seated to a standing position.

Although it might be expected that mechanical constraints will alter task kinematics, it is not as obvious that changing the sensory constraints will modify a task that is regularly engaged in, such as rising from a chair. One such sensory constraint often encountered in daily activities is the pa-1Address correspondence to Jill Slaboda, Ph.D., Temple University, 1800 North Broad Street, 40 Pearson Hall, Philadelphia, PA 19122 or e-mail ([email protected]).

J. C. SLABODA, ET AL.122

rameters of the optic flow field. The action of rising to stance from a seated position can take place in the quiet environment of a home, or in the very visually active environment of a shopping mall. The goal of this investiga-tion was to assess how the presence or absence of visual motion modified sit-to-stand kinematics in children and adults, and to explore whether ex-posure to visual motion had a time-dependent effect on the kinematics of the sit-to-stand task.

Immersion in an optic flow field influences the speed and direction of locomotion in adults, suggesting that adults adapt their kinematics to the moving visual environment (Prokop, Schubert, & Berger, 1997; Keshner & Kenyon, 2000). Age effects on an optic flow field have also been demon-strated in postural responses (Foster & Sveistrup, 1996; Sparto, Redfern, Jasko, Casselbrant, Mandel, & Furman, 2006; Casselbrant, Mandel, Sparto, Redfern, & Furman, 2007; Bair, Kiemel, Jeka, & Clark, 2007; Viel, Vaugo-yeau, & Assaiante, 2009), and suggest that children process vision differ-ently than adults. For example, Baumberger, Isableu, and Flückiger (2004) found that quiet sway responses occurred in the direction of an approach-ing linear optic field projected on the floor within 1.7 to 3.4 sec. in young children 7 to 9 years old, whereas adults demonstrated sway responses opposite to the direction of optic flow within 1.4 sec. Greffou, Bertone, Hanssens, and Faubert (2008) found that children younger than 16 years had much larger sway responses than adults 20 to 25 years old or children older than 16 years when immersed in a sinusoidal moving virtual envi-ronment for 68 sec. Even though these studies suggest that visual motion influences locomotion and balance, currently little research has been con-ducted in either adults or children that focused on identifying whether visual inputs cause modifications in a functionally oriented, yet mechani-cally constrained task such as sit-to-stand.

This study examined how a moving visual field influenced sit-to-stand kinematics and whether these effects differ between children and adults. Two visual time delays were used: participants were instructed to stand (1) concurrently with the onset of visual scene motion or (2) after a 10-sec. period of immersion in the moving visual environment. These two visual time delays were chosen to assess whether experiencing optic flow while seated (during motor planning) or once the motion has started (motor execution) would have a differential effect on the sit-to-stand ki-nematics. The guiding hypothesis was that immersion in the disorienting visual scene during motor planning would result in modifications of the sit-to-stand kinematics in both children and adults. In addition, it was hy-pothesized that when the visual scene was moving at the onset of motor execution, sit-to-stand kinematics in adults and children would not differ significantly from trials without visual input.

VISUAL ENVIRONMENT AND SIT-TO-STAND 123

MethodTen adults (ages 21 to 49 years) and eight children (ages 8 to 12 years)

participated and gave informed consent as approved by the Temple Uni-versity Institutional Review Board. None of the participants had a his-tory of central or peripheral neurological disorders, problems related to movement of the spinal column, or any orthopedic problems related to the knee, hip, or ankle. All participants had a minimum of 20/40 corrected vision.

Participants were seated on an adjustable stool (Tama, Seto, Japan) with the ankles, knees, and hips at 90° of flexion. The feet were positioned with toes forward and parallel to one another, and the arms were at their sides when told to stand. Seated posture was confirmed by experiment-er’s visual inspection. The stool was a hard surface (seat diameter was 12 in.), and had no armrests or any support for the participant’s back (i.e., no support from the stool or a wall). Participants practiced the sit-to-stand motion before the start of the experiment to become comfortable with the seated posture and to practice not using their arms during the motion.

Each participant performed the sit-to-stand motion in a three-wall virtual environment that consisted of three 2.4 m × 1.7 m screens (Stewart, Torrance, California), located in front and to either side of the participant, which are wide enough to encompass the peripheral visual field. Two Panasonic PT-D5600U DLP-based projectors, located behind each screen, projected a full-color workstation field (1024 × 768 stereo) at 60 Hz onto the screens. Different polarized filters placed in front of the projector pro-vided left-eye and right-eye views of the image on each screen and special passive stereo glasses worn by the participant delivered the correct view to each eye. Three dual processor computers with NVIDIA Quadro graph-ics cards created the imagery projected in the virtual environment and were synchronized via the CAVELib application (VRCO, Virginia Beach, Virginia) to display a single contiguous image of the virtual world across all three screens. A three-dimensional, stereo visual scene with columns and a distant horizon (see Fig. 1 in Keshner & Kenyon, 2000) was dis-played during the experiment.

Participants were instructed to stand at their normal pace without us-ing their arms when given a verbal command to stand either concurrently with the onset of visual scene motion or after a 10-sec. period of immer-sion in the moving visual environment. During the task, the visual scene rotated either in the pitch (up or down) or roll (counterclockwise or clock-wise) direction. The visual scene rotated at a constant velocity that was cal-ibrated to match each participant’s initial sit-to-stand velocity performed without visual input (i.e., in a dark room). This standing velocity was cal-culated as the displacement of the head from a seated to a standing posi-


tion in the vertical direction divided by the time required to stand from a seated position when the participant was in the dark room. Standing was the point when the head displacement reached a maximum height. The two visual time delay conditions (concurrent and immersion), two visual scenes (pitch and roll), and two directions per visual scene (up and down or left and right) resulted in a total of eight trials. In addition, three trials were performed without visual input. The order of the trials was random-ized and all of the participants had the same randomized order.

Motion of the head, shoulders, neck, lower back, and hips was tracked at 120 Hz with a Motion Analysis (Santa Rosa, California) infrared Hawk system. From the marker data, angles of the trunk with respect to the lab-oratory environment (trunk-in-space), the head angle with respect to the trunk coordinates (head-re-trunk), and the displacement of the center of mass of the head and trunk in the x, y, and z directions were calculated. The center of mass and angle trajectories were separated into three phas-es to describe successive portions of the sit-to-stand motion. These phases were: sitting to liftoff of the body from the seat, liftoff to ascent of the body vertically (upright), and upright to standing. These phases are similar to the sit-to-stand intervals described by Riley, Mann, and Hodge (1990) and Riley, Schenkman, Mann, and Hodge (1991). The sitting-liftoff phase was identified as the time when the flexion angle of the trunk moved more than two standard deviations from the seated position to the time when the forward velocity of the trunk center of mass reached a maximum. The liftoff-upright phase was the time after liftoff until the trunk center of mass velocity in the z direction reached a maximum. Finally, the upright-stand phase was the time after upright to when trunk center of mass ve-locity was at a minimum. All of the angular trajectories were plotted with the phases identified and these phases were checked by an experimenter to assure the algorithm correctly identified the phases.

For each sit-to-stand phase, velocity of the angles and center of mass were calculated, and a root mean square measure was used for compari-son. In addition, root mean square of center of mass and angle displace-ments at each phase were also calculated and used for comparison be-tween the visual conditions.

Head and trunk center of mass displacement were normalized to con-trol for differences in height between adults and children. The normalized center of mass displacement was calculated by subtracting the beginning center of mass position from the center of mass trajectories and then di-viding the trajectories by the standing center of mass position which re-sulted in a 0 to 1 scale of the center of mass trajectory. Velocity of the nor-malized center of mass was calculated and a root mean square measure of normalized center of mass and of the normalized center of mass velocity was used for comparisons between adults and children.


The effect of immersion on sit-to-stand motion was assessed by com-paring root mean square velocities, root mean square center of mass dis-placement, and root mean square angle displacement between visual conditions (no visual input, immersion, and concurrent) using repeated measures analysis of variance (ANOVA). If significant differences were found, post hoc paired t tests with a Bonferroni correction (p < .02) were performed to assess which sit-to-stand phases were significantly differ-ent between the visual conditions. In addition, the sit-to-stand parameters were compared between adults and children for each visual condition us-ing repeated-measures ANOVA, to assess if the sit-to-stand kinematics of children were different than the sit-to-stand kinematics of adults. If sig-nificant, post hoc independent t tests with a Bonferroni correction (p < .005) were performed to assess which sit-to-stand phases were significantly dif-ferent between adults and children.

ResultsSit-to-stand Responses of Adults to Visual Motion

Adult trajectories of the trunk-in-space angular velocity for each visu-al condition are shown in Fig. 1. Head-re-trunk and trunk-in-space angu-lar velocities were significantly reduced over all sit-to-stand phases when adults were immersed in the pitch or roll visual scene compared to con-

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Fig. 1. Trunk-in-space angular velocity of an adult (left) and of a child (right) when no visual input was presented (solid), visual scene rotated in roll concurrently with the onset of sit-to-stand motion (dashed), or after the adult or child was immersed in the roll rotation of the visual scene (dots).

current motion of the scene or without visual input (Fig. 2). For both pitch and roll rotations of the visual scene, head and trunk root mean square center of mass velocities were significantly decreased over all sit-to-stand phases when immersed compared to concurrent motion or without visu-


al input. No significant difference was found in root mean square angular or center of mass velocity when trials with concurrent motion of the scene were compared to trials without visual input. Root mean square head and trunk center of mass displacement and root mean square head and trunk angular displacement were not significantly different between any of the visual conditions. Sit-to-stand Responses of Children to Visual Motion

Trajectories of the trunk-in-space for each visual condition of the chil-dren are shown in Fig. 1. Trunk-in-space and head-re-trunk angular veloc-ities were reduced during all sit-to-stand phases when children or adults were immersed in the pitching and rolling visual scenes compared to tri-als with concurrent motion of the visual scene or without visual input (Fig. 3). Head and trunk center of mass velocities were also significantly

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Sit-to-stand PhasesFig. 2. Adult average root mean squared angular velocity of the trunk (top) and the

head with respect to the trunk (bottom) when no visual input was presented (black), when the scene moved concurrently with the onset of sit-to-stand motion (checkered), and when immersed in the moving visual scene (vertical lines). The error bars are the standard devia-tions. The asterisks indicate that angular velocity when the adults were immersed in visual environment was significantly slower than when no visual input was presented or visual field was presented with the onset of motion.


reduced over all sit-to-stand phases when children were immersed in ei-ther the roll or pitch visual scene compared to standing up concurrently with visual scene motion or without visual input.

Unlike adults, however, root mean square measures of the head cen-ter of mass displacement were significantly larger in the trials without vi-sual input than the trials with concurrent motion of the roll visual scene for all sit-to-stand phases (Fig. 4). When immersed in the roll or pitch vi-sual scene, root mean square of the head center of mass displacement was reduced for the first and last sit-to-stand phases compared to head center of mass displacement when no visual input was provided. No significant difference was found for head center of mass displacement between im-mersion and concurrent motion of the scene in either roll or pitch. Root mean square of the trunk center of mass and angular root mean square

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A Roll Scene Pitch Scene

B Children’s Head-re-trunk Angular Velocity

Sit-to-stand PhasesFig. 3. Children’s root mean squared average angular velocity of the trunk (top) and

the head with respect to the trunk (bottom) when no visual input was presented (black), when the scene moved concurrently with the onset of sit-to-stand motion (checkered), and when immersed in the moving visual scene (vertical lines). The error bars are the standard deviations. The asterisks indicate that angular velocity when the children were immersed in visual environment was significantly slower than when no visual input was presented or visual field was presented with the onset of motion.


displacement were not significantly different between any of the visual conditions. Differential Sit-to-stand Responses of Adults and Children

No significant differences were found between adults and children for root mean square head or trunk center of mass displacement or veloc-ity of the center of mass for any of the trials. Head and trunk root mean square angles and trunk-in-space angular velocities were not significant-ly different between adults and children. However, head-re-trunk angu-lar velocity of children was significantly faster for the sitting-liftoff phase than adults for all but the trial without visual input.

DiscussionThe influence of visual field motion on sit-to-stand kinematics when

presented at two time delays was studied in children and adults to assess whether there is a time-dependent influence on the kinematic responses and whether children and adults react differently to the moving visual environment. Differential kinematic responses were observed following immersive and concurrent visual inputs, which indicates that vision does have a time-dependent influence on sit-to-stand kinematics. Both chil-dren and adults when seated in a moving visual environment compensat-ed for the constantly changing environment by reducing their head and trunk angular velocities as well as head and trunk center of mass velocities

Children Head Center of Mass

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Fig. 4. Left graph is the head center of mass trajectory of a child during sit-to-stand motion when no visual input was presented (solid), when the visual scene rotated in roll concurrently with the onset of sit-to-stand motion (dashed), or after the child was immersed in the roll rotation of the visual scene (dots). The right graph is the average root mean square head center of mass with standard deviation error bars when no visual input was presented (dark), when the scene moved concurrently in roll rotation with motion (checkered), and during immersion in roll visual scene (vertical bars).


from the initiation of sit-to-stand motion to the standing position. Visual field motion did not affect sit-to-stand kinematics when the environment moved at the onset of motion, and head and trunk angular and center of mass velocities were the same in trials with concurrent visual field motion or no visual input in both age groups. These results provide important in-sight into how adults and children use visual cues when standing from a seated position during everyday life. It was expected that adults and chil-dren would stand slower when watching a train pass by them than when standing up in the dark or standing up at the same time as an object, such as a bird flying, moves within their field of view.

Differences between the two groups did emerge, however, in re-sponse to directionality of the visual field. It was somewhat unexpected to find that adults performed the task similarly regardless of the direction of optic flow and were only influenced by the timing of optic flow. Previ-ous studies have revealed that when adults are walking within an optic flow field, walking velocity as well as trunk and head displacement were modulated by both speed and direction of optic flow (Keshner & Ken-yon, 2000; Mulavara, Richards, Ruttley, Marshburn, Nomura, & Bloom-berg, 2005; Schneider, Jahn, Dieterich, Brandt, & Strupp, 2008). Based on these studies, it was expected that roll rotation of the visual scene would be more disorienting because this plane is perpendicular to the primary plane of sit-to-stand motion. Equivalent responses to different directions of optic flow suggest that sit-to-stand responses are more controlled than gait which may be related to complexity of sit-to-stand motion. During sit-to-stand motion, the center of mass is moved from a seated to a stand-ing position through the coordinated motion of the upper and lower body segments (Hirschfeld, Thorsteinsdottir, & Olsson, 1999). If adults altered displacement in response to the optic flow, sit-to-stand mechanics would be changed, which could potentially cause instability if the center of mass moved away from the lower limbs. In reducing velocity, adults displace their bodies the same distance in the sagittal plane but at a slower pace. This strategy may be used by adults to control the location of the center of mass when adults perceive that the visual environment is moving sepa-rately from their own motion and they have no stationary visual cues to help orient them in space (Isableu, Ohlmann, Cremieux, & Amblard, 2003; Streepey, Kenyon, & Keshner, 2007; Horlings, Carpenter, Kung, Honegger, Wiederhold, & Allum, 2009).

In contrast to adults, sit-to-stand kinematics in children were influ-enced by both the timing and the rotation plane of optic flow. During both immersion and concurrent roll rotation of the visual scene, children reduced the degrees of freedom of the head and exhibited the strategy of locking the head motion to the trunk that has been found in younger children during walking (Assaiante & Amblard, 1993, 1995). This shift in


strategy may suggest that the children believed the task was more diffi-cult when the visual scene rotated in roll. Previous research has revealed that as a reaching task became more difficult, older children shifted their motion patterns and demonstrated motion patterns that were more simi-lar to those of younger children than to those of adults (Streepey & An-gulo-Kinzler, 2002). Future studies may focus on identifying the influence of rotation plane and timing of optic flow on the sit-to-stand kinematics in younger children compared to older children to assess how kinematics change with development and perceived task difficulty.

The modified response to concurrent inputs of the roll scene was only observed in motion of the head center of mass in the children and not in the angular displacement of the head with respect to the trunk. This dis-crepancy could be due to how these values were calculated. Head center of mass was calculated from the magnitude of the three-dimensional vec-tor, whereas the head-re-trunk angular displacement represented only the angle in the sagittal plane. It is possible that the presence of head motions in planes other than the primary plane of motion was modulated by the presence or absence of vision. Another discrepancy in the results is that the comparison between adults and children did not show any signifi-cant differences for head center of mass displacement during the concur-rent motion of the visual scene in the roll rotation. The lack of significance may be due to the method of normalization, since this analysis could have smoothed any differences between the groups for head center of mass dis-placement.This study explored head and trunk motion to define response kinematics of children and adults instead of the more commonly inves-tigated kinematics of the lower limbs during sit-to-stand tasks (Mathi-yakom, McNitt-Gray, Requejo, & Costa, 2005; Tully, Fotoohabadi, & Ga-lea, 2005). Upper body segments were chosen because both the head and trunk were shown to be influenced by optic flow in postural control stud-ies (Keshner & Kenyon, 2000) and the trunk is the principal axis of mo-tion over the entire task. The initiation of sit-to-stand motion begins with the flexion of the trunk while the participants are still seated, and trunk flexion provides the initial momentum to move the body from a seated to a standing position. The lower limbs are most responsible for providing support once the participant begins to lift off from the seat. Thus, analyz-ing kinematics at the trunk makes it possible to examine the entire sit-to-stand period. If only the lower limbs were examined, differences in ki-nematics during the sitting-to-liftoff phase when immersed compared to trials concurrent or without visual input would probably not be found be-cause the lower limbs are stationary during this portion of the task.

In conclusion, it was found that a moving visual field influenced sit-to-stand kinematics of adults and children differently. In adults, optic flow was a time dependent influence on response kinematics causing adults to


adapt to visual field motion only when it was presented during the plan-ning of their response. Children, however, were influenced by both the direction and timing of the optic flow, and they were responsive to visu-al field motion even if it appeared when they were already in the process of executing the sit-to-stand movement. Considering that both children and adults encounter continuous optic flow in daily life, such as on busy streets, identifying responses to optic flow in healthy adults and children may help in developing training programs for the treatment of individu-als with balance or motor development problems.

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Accepted June 11, 2009.

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