Stilt walking: How do we learn those first steps?

Transcription

1 Ergonomics Vol. 52, No. 9, September 2009, Stilt walking: How do we learn those first steps? Sakineh B. Akram a * and James S. Frank b a Department of Kinesiology, University of Waterloo, 200 University Ave. West, Waterloo, Ontario, N2L 3G1, Canada; b Faculty of Graduate Studies, University of Windsor, 401 Sunset Ave., Windsor, Ontario, N9B 3P4, Canada This study examined how young healthy adults learn stilt walking. Ten healthy male university students attended two sessions of testing held on two consecutive days. In each session participants performed three blocks of 10 stiltwalking trials. Angular movements of head and trunk and the spatial and temporal gait parameters were recorded. When walking on stilts young adults improved their gait velocity through modifications of step parameters while maintaining trunk movements close to that observed during normal over-ground walking. Participants improved their performance by increasing their step frequency and step length and reducing the double support percentage of the gait cycle. Stilts are often used for drywall installation, painting over-the-head areas and raising workers above the ground without the burden of erecting scaffolding. This research examines the locomotor adaptation as young healthy adults learn the complex motor task of stilt walking; a task that is frequently used in the construction industry. Keywords: stilt walking; learning; skill acquisition; balance; adaptation 1. Introduction Stilt walking has a long history (Peacay 2006). Over the centuries people have been learning this skill for many different reasons. Stilts of different shapes and heights have been used by entertainers, mailmen, shepherds, fishermen, dancers, ice skaters, fruit pickers and warriors. Also, in many countries, stilts have long been popular toys for children. Wooden stilts have been, and still are, considered a gift that gives many hours of wholesome fun while helping build confidence and develop motor skills for children and adults. Today, although stilts are mainly used for entertainment, they still serve some practical purposes. Fruit farmers use aluminium stilts to prune and harvest their trees, claiming that wearing stilts facilitates accessibility allowing them to finish their job much faster (Armistead 2002). Stilts are used for washing large windows, repairing thatched roofs and installing or painting high ceilings. As an employable skill, stilts are most often used for drywall installation and painting over-thehead and hard-to-reach areas. Stilts are also used on construction sites to raise workers above the ground without the burden of erecting scaffolding or ladders (Whitaker 2006, Chiou et al. 2008). Despite the fact that stilt walking has been, and still is, of interest to many people, little is known about how humans learn this rather complex balance task. Stilts extend the lower part of the leg to raise the person above the walking surface; therefore, the person s body centre of mass is elevated. The mass of stilts modifies the mechanical properties of the lower limbs (Noble and Prentice 2006). Stilts not only increase the total mass of the lower limbs and the muscular work required for moving them, they also redistribute the inertia of the lower limbs more distally. The design of stilts does not allow the same range of movement that the ankle and foot are capable of when standing directly on the floor. Solid footplates prevent normal bending and twisting of the feet when walking. Movements of the ankles are also restricted. Furthermore, walking on stilts increases the possibility of loss of balance and falls (Schneider and Susi 1994). Even experienced drywall workers perceive the tasks performed on stilts as having greater fall potential than the same tasks performed using scaffolds and ladders (Pan et al. 2000). Previous research has shown that the perceived risk of injury in the event of a fall affects the postural control strategies employed by young healthy adults even during quiet stance (Brown and Frank 1997, Adkin et al. 2002). Fear of loss of balance and falling while walking on stilts may affect the postural adjustments adopted for proper balance control. The aforementioned restrictions and modifications make stilt walking different from normal over-ground walking; therefore, while walking on stilts individuals *Corresponding author. sbakram@ahsmail.uwaterloo.ca ISSN print/issn online Ó 2009 Taylor & Francis DOI: /

2 1120 S.B. Akram and J.S. Frank may need to adapt their locomotor pattern to maintain their balance. Adaptation is a critical component of locomotion. Several studies have shown adaptive changes in human locomotion to accommodate different locomotor tasks, such as walking on a split belt treadmill (Dietz et al. 1994, Reisman et al. 2005, Choi and Bastian 2007), a rotating treadmill (Earhart and Hong 2006), sloped or inclined surfaces (Leroux et al. 2002, Prentice et al. 2004) and compliant surfaces (Marigold and Patla 2005, MacLellan and Patla 2006). Adaptation to modifications of the lower limb properties such as added mass to the limbs (Noble and Prentice 2006) and disrupted muscular coordination of the lower limb (Gordon and Ferris 2007) has also been reported. Collectively, these studies demonstrate that human locomotor patterns are highly adaptable. The adaptive nature of the gait pattern ensures dynamic stability despite changes in the environment and/or properties of the lower limbs. The purpose of the present study is to examine locomotor adaptation as young healthy adults learn the complex motor task of stilt walking; a task that is frequently used in the construction industry (Whitaker 2006, Chiou et al. 2008). 2. Method 2.1. Participants Ten male university students, aged years, participated in this study. Volunteers had no known vestibular, neurological or musculoskeletal disorders as verified by self-report. Each participant performed a one-legged stance test to ensure normal function of the vestibular system. In this test, participants were asked to stand barefoot on one foot (their choice) with their eyes closed for at least 25 s. People with a vestibular deficit are not able to maintain their balance this long and will fall towards the affected side (Allum et al. 2001). In the present study all participants successfully passed this test, suggesting proper functioning of their vestibular system. Prior to testing, anthropometric measures including body mass, body height and foot length were recorded for each participant. Mean and standard deviation of participants body mass, body height and foot length were kg, cm and cm, respectively. All participants were naive to the task. They had no previous experience with stilt walking and were not allowed to observe other participants performance on stilts before they completed the experiment themselves. All participants were informed about the experimental procedure before signing a consent form. All procedures were approved by the Office of Research Ethics, University of Waterloo Procedure Participants were asked to attend two sessions of testing held on two consecutive days. The ISOTRAK 1 3SPACE electromagnetic tracking system (Polhemus, Colchester, VT, USA) was used to record the angular movements of head and trunk in pitch, roll and yaw planes. The 3SPACE transmitter was secured over the lower back at the level of L3 L4 using adhesive tape. A fabric belt was used to further secure the transmitter in place. Two receivers were used. One receiver was secured on the spinous process of T1 using adhesive tape. The other receiver was mounted on a plastic cap. The cap was adjusted on the participant s head so that the receiver always rested on the maximal occipital point. The 3SPACE system uses the position and orientation of the receivers relative to the transmitter to calculate the angular movements of head and trunk in real time. The accuracy of this system has previously been reported to be within 0.38 (McGill et al. 1994). Each session started with a calibration trial. Calibration trials were conducted while the participant stood quietly looking straight ahead. Position of the head and trunk in this posture was defined as zero. Calibration trials were repeated after every 10 trials of testing. Additional calibration trials were recorded anytime during the experiment when the experimenter suspected misalignment of the transmitter and/or receivers due to some unexpected movement of the participant. The ISOTRAK 1 signal was collected at 32 Hz. Each trial consisted of walking along a 7-m GaitRite 1 carpet. The GaitRite 1 carpet (CIR Systems Inc., Clifton, NJ, USA) is a pressure-sensitive carpet that allows recording of footfall position during walking trials. Gait parameters (velocity, step frequency (steps/min), step length, step width and the duration of the double and single support phases as percentage of the gait cycle) were determined using footfall position data. On the first session, the experiment started with participants performing 10 normal walking (NW) trials while wearing their shoes and without stilts. After completion of the NW trials, stilts were mounted while the participant was sitting on an elevated platform. Metal stilts (Sur-Stilts 1 ; The Forest Group Inc., Houston, TX, USA) were kg each and raised the participant 61cm above the floor (Figure 1). The footplates and floor plates of each stilt were 27cm long and 8.5 cm wide. Each stilt was fixed to the participant s leg by three sets of straps. One set of straps was attached to the two support bars running up the sides of the leg and fastened the upper part of the stilt to the shank below the knee. The other two

3 Ergonomics 1121 start point was approximately 1 m off the GaitRite 1 carpet. Each participant was tested for 30 stilt walking trials with 5-min seated rest after every 10 trials. In each trial, the participant was instructed to start walking from the start point and continue walking the length of the GaitRite 1 carpet (7 m). At the end of the trial, the participant was asked to keep walking until he was off the GaitRite 1 carpet and then stop. This approach was adopted to ensure that the data were not contaminated by acceleration and deceleration during the initiation and termination of the gait. Upon completion of each trial, the participant was allowed to use the rope for additional support while turning and returning to the start point. On the second day, participants performed only 30 stilt walking trials. The procedures were the same as the first session. No warm-up trials were allowed during either session. Figure 1. Metal stilts (Sur-Stilts 1 ; The Forest Group Inc., Houston, TX, USA) used in the current study. sets of straps were fastened at the back of the foot strapping the footplate of the stilt to the participant s foot. Stilt straps were tightened to prevent foot slippage from the stilts, while proper circulation and comfortable fit were considered. The lower bars connecting the footplate with the floor plate were spring loaded to facilitate stilt walking movements by allowing limited degree of plantar flexion and dorsi flexion. When attempting to stand, the participant grasped a rope suspended from a ceiling beam to assist balance. The participant was allowed to hold on to the rope until he was confident that he could stand without the rope support. Throughout the experiment the rope remained within the participant s hand reach in case he needed to grasp it to regain his balance. At the beginning of each session, participants were asked to put on a lightweight climbing harness. The safety harness was secured to a second rope suspended from the ceiling beam to protect against ground contact and injury in the event of a fall. Throughout the testing an assistant followed the participant closely to assist in the event of a fall. Holding the rope for support, the participant walked (about 2 3 m) to reach the start point. The 2.3. Data collection Spatial and temporal gait parameters, i.e. velocity, step frequency, step length, step width and double support time as percentage of gait cycle were extracted using the GaitRite 1 software. Step length was defined as the distance traversed between the heel strike of the right and left feet. Step width was defined as the perpendicular distance between the anterior/posterior axes of the two feet. The double support percentage of gait cycle was defined as the duration of the double support sub-phase of the gait cycle expressed as the percentage of the gait cycle duration. Angular movements of head and trunk in pitch, roll and yaw planes were recorded using the 3SPACE. All 3SPACE measurements started when the participant first stepped on to the GaitRite 1 carpet and continued until the participant traversed the length of the carpet. Measures used for analysis of trunk and head orientation included the mean angular position and standard deviation of angular displacement over the course of each trial Statistical analysis To compare the initial performance of the participants during stilt walking trials with NW, a two-way repeated measures ANOVA with condition (NW and stilt walking) and trial as factors was performed on the GaitRite 1 and 3SPACE data obtained during the NW trials and the first 10 trials of stilt walking during day 1. To determine how the postural strategies and walking patterns change with practice and learning, a two-way repeated measures ANOVA with trial and day as factors was performed on the GaitRite 1 and

4 1122 S.B. Akram and J.S. Frank 3SPACE data obtained during stilt walking trials on the first and second days of the experiment. Retention was examined by a one-way repeated measure ANOVA with trial as factor on the data obtained from the first trial of day 1, the last trial of day 1 and the first trial of day 2. If the participants performance on the first trial of day 2 was better than their performance on the first trial of day 1, and was similar to their performance on the last trial of day 1, it was concluded that the participant had been able to retain what they had learned during the first day. In conditions when a main effect of a factor on a dependent variable was revealed, Tukey s studentised range (HSD) test was performed to determine which means were significantly different from the others. For all tests, a significance value (p) of less than 0.05 was used to test statistical significance. 3. Results While walking on stilts, participants were able to maintain their balance without the use of external support from the very first trial. None of the participants fell during the experiment. Although the rope suspended from the ceiling beam remained within the participants hand reach throughout the experimental trials and the participants were instructed to grasp the rope anytime they felt they were losing their balance, only one participant used the rope to regain his balance and prevent a fall during the first trial. For this participant data obtained from this trial were discarded and the trial was repeated Modifications of the gait parameters with practice The two-way ANOVA revealed significant differences between participants performance during the NW trials and the first 10 trials of stilt walking on day 1. ANOVA revealed significant main effects of condition (F(1,9) ¼ 68.6, p ) and trial (F(9,81) ¼ 17, p ) on gait velocity. Condition*trial interaction effect was also significant (F(9,81) ¼ 11.24, p ). The source of this significant interaction was the greater changes in gait velocity over the course of trials during stilt walking than during NW trials (Figure 2a). ANOVA showed significant main effects of condition (F(1,9) ¼ 196.5, p ), and trial (F(9,81) ¼ 7.05, p ) on step frequency. Condition*trial interaction effect was also significant (F(9,81) ¼ 5.93, p ). The source of this significant interaction was the greater changes in step frequency over the course of trials during stilt walking than during NW trials (Figure 2b). Effects of condition (F(1,9) ¼ 18.59, p ¼ ), trial (F(9,81) ¼ 13.61, p ) and condition* Figure 2. Mean and standard deviation (error bars) of the gait velocity (a), step frequency (b), step length (c), step width (d) and double support percentage of the gait cycle (e) averaged across all participants for the 10 normal walking trials and the first 10 trials of stilt walking on day 1.

5 Ergonomics 1123 trial (F(9,81) ¼ 9.46, p ) on step length were significant. The significant condition*trial interaction effect was due to the greater changes in step length during the stilt walking trials than the NW trials (Figure 2c). ANOVA revealed only a significant effect of condition (F(1,9) ¼ , p ) on step width. Participants took significantly wider steps during stilt walking trials than during NW trials. The mean and standard deviation of step width during NW trials and the first 10 trials of stilt walking on the first day were and cm, respectively (Figure 2d). ANOVA showed significant main effects of condition (F(1,9) ¼ 21.93, p ¼ ) and trial (F(9,81) ¼ 9.50, p ) on the double support percentage of the gait cycle. Condition*trial interaction effect was also significant (F(9,81) ¼ 8.29, p ). The source of this significant interaction was the greater changes in the double support percentage of the gait cycle during the stilt walking trials than during the NW trials (Figure 2e). Modifications of the gait parameters during stilt walking over the course of the practice trials are reported next. Days 1 and 2 practice trials revealed changes in all gait parameters; gait velocity, step frequency, step length and step width increased, while double support time as a percentage of the gait cycle decreased. In general, gait velocity increased (165%) over the course of 2 days of practice (Figure 3a). The two-way ANOVA revealed significant main effects of day (F(1,9) ¼ 38.72, p ¼ ) and trial (F(29,261) ¼ 19.13, p ) on gait velocity. Day*trial interaction effect was also significant (F(29,261) ¼ 5.09, p ). The significant day* trial interaction effect was due to the greater increase in gait velocity during day 1 than day 2 (Figure 3a). Step frequency also increased over the course of practice trials (Figure 3b). ANOVA showed significant main effects of day (F(1,9) ¼ 18.65, p ¼ ) and trial (F(29,261) ¼ 3.91, p ). The day*trial interaction effect was also significant (F(29,261) ¼ 7.13, p ). The significant day*trial interaction effect was due to the greater increase in step frequency during day 1 than day 2 (Figure 3b). Step length increased with practice (Figure 3c). ANOVA revealed significant main effect of day (F(1,9) ¼ 21.66, p ¼ ) and trial (F(29,261) ¼ 13.90, p ) on step length. Tukey s analysis revealed that participants took significantly longer steps on the second day than on the first day (mean + SD and cm for day 2 and day 1, respectively). Furthermore, on both days the step length increased as the trials progressed. Averaged across both days, participants took shorter steps during the first five practice trials than during any other trial (Figure 3c). In general, changes in step width were small (less than 3 cm) (Figure 3d). The analysis of step width showed significant main effect of trial (F(29,261) ¼ 2.81, p ) only. Tukey s analysis revealed that, averaged across both days, step width was smallest in the first four trials and increased over the course of the trials (Figure 3d). Double support percentage of gait cycle decreased with practice, with most changes occurring during the first six trials on the first day (Figure 3e). The two-way ANOVA showed significant main effects of day (F(1,9) ¼ 27.75, p ¼ ) and trial (F(29,261) ¼ 9.68, p ) and day*trial (F(29,261) ¼ 5.02, p ) interaction effect. The source of this significant interaction effect was the greater decrease in the double support percentage of gait cycle in the initial trials of day 1 than day 2 (Figure 3e) Modifications of the trunk orientation with practice For one participant, a problem with the 3SPACE sensor positioned on the trunk went unnoticed during the data collection on the second day. Therefore, statistical analyses were performed on the trunk movement data obtained from nine participants. During the first 10 trials of stilt walking, the mean trunk orientation in all planes was within 28 of the trunk orientation recorded during the NW trials. Trunk orientation did not change with practice. The standard deviation values of the trunk movement showed no learning trend over the course of the practice trials. The average standard deviation values of the trunk pitch and roll movement while walking on stilts were comparable to NW (1.9 vs and 2.8 vs. 2.28, respectively). The average standard deviation value of the trunk movement in the yaw plane was slightly larger during stilt walking than NW (4.4 vs. 2.98) Modifications of the head orientation with practice During the first 10 trials of stilt walking, the mean head orientation in roll and yaw planes remained within 18 of head orientation recorded during NW trials. The mean and standard deviation of head position in the pitch plane during the NW trials and stilt walking trials were vs flexion, respectively, indicating significant flexion of the head during stilt walking. Over the course of the practice trials, the mean head orientation in roll and yaw planes remained unchanged. In the pitch plane, the head was always

6 1124 S.B. Akram and J.S. Frank Figure 3. Mean and standard deviation (error bars) of the gait velocity (a), step frequency (b), step length (c), step width (d) and double support percentage of the gait cycle (e) averaged across all participants for 30 practice trials performed on two consecutive days. flexed more than that observed in NW trials. The mean and standard deviation of head position in the pitch plane during the NW trials, stilt walking trials on day 1 and stilt walking trials on day 2 were , and flexion, respectively. The two-way ANOVA revealed significant main effect of trial (F(29,261) ¼ 1.63, p ¼ ) and day*trial interaction effect (F(29,261) ¼ 1.56, p ¼ ) on

7 Ergonomics 1125 the mean position of the head in pitch plane during the stilt walking trials. The source of this interaction was the greater changes in the mean position of the head in pitch plane over the course of the practice trials on the first day than on the second day (Figure 4). Changes in the standard deviation of angular displacement of the head in pitch, roll, and yaw planes at both days of practice were less than Retention The one-way ANOVA revealed significant effect of trial on gait velocity (F(2,18) ¼ 30.04, p ), step frequency (F(2,18) ¼ 18.27, p ), step length (F(2,18) ¼ 17.39, p ) and double support percentage of the gait cycle (F(2,18) ¼ 21.77, p ). Tukey s analysis revealed that while gait velocity, step frequency and step length during the first trial of day 2 were not different than the comparable values during the last trial of day 1, they were all significantly greater than the comparable values during the first trial of day 1. The mean and standard deviation values during the first trial of day 1, the last trial of day 1 and the first trial of day 2 were , , m/min for gait velocity, , , steps/min for step frequency and , , cm for step length. Tukey s analysis also revealed that while the double support percentage of the gait cycle during the first trial of day 2 was not different than the comparable value during the last trial of day 1, it was significantly smaller than the comparable value during the first trial of day 1. The mean and standard deviation of the double support percentage of the gait cycle during the first trial of day 1, the last trial of day 1 and the first trial of day 2 were , , Effect of trial on step width was not significant, indicating that averaged across all participants step width during the first trial of day 1, the last trial of day 1 and the first trial of day 2 was not different from each other. The mean and standard deviation of the step width during the first trial of day 1, the last trial of day 1 and the first trial of day 2 were , and cm. 4. Discussion The present study examined the locomotor adaptation as young healthy adults learned the complex motor task of stilt walking. Results indicated that while walking on stilts young healthy adults improve their gait velocity by adapting their gait parameters while showing negligible changes in trunk motion. Participants learned to walk faster without losing their balance by increasing the step frequency and step length and reducing the double support percentage of the gait cycle. Trunk motion during stilt walking was not significantly different from trunk motion during over-ground walking even during the early practice trials. The mean and standard deviation of angular displacement of trunk were very small even during the first 10 practice trials on the first day (mean + SD , , in pitch, roll and yaw planes, respectively) with minimum change over the course of practice trials in 2 days. The fact that none of the participants fell even during the very first trial and only one participant used the rope to regain his balance and prevent a fall during the first trial may indicate that the participants were able to use a previously learned motor control strategy to maintain their balance from their first attempts to walk on stilts. With practice, however, they were able to adapt their locomotor patterns to improve their performance. Human locomotion is energy efficient (Zarrugh et al. 1974). Previous studies have shown that the preferred pattern of locomotion in humans is the result of stability vs. energetic trade-off, i.e. humans walk with a certain speed and certain gait parameters, e.g. step width, step length and step frequency that allow Figure 4. Mean position of the head in pitch plane (8) averaged across all participants for the 30 practice trials performed on two consecutive days. Error bars represent the standard deviation of the mean position. Note: flexion is þve; extension is ve.

8 1126 S.B. Akram and J.S. Frank postural balance and stability while keeping the energy expenditure at a minimum (Zarrugh et al. 1974, Donelan et al. 2001, 2002, Kuo 2001). Deviation from these preferred gait parameters results in either loss of balance or greater energy demand. Zarrugh et al. (1974) showed that for walking velocities up to 145 m/min, for any given step length there is a certain step frequency that requires minimal energy to traverse a unit of distance. Vaida et al. (1981) examined performance of three healthy young men who had experience with stilt walking. Participants walking velocity, step length, step frequency and energy expenditure during stilt walking were recorded. Results showed that while on stilts participants walked faster than normal over-ground walking. The higher velocity was achieved by longer steps and slower step frequency. They also showed that increased step length and decreased step frequency reduces the energy requirements of stilt walking. Vaida and colleagues concluded that, while walking on stilts, walkers compensate for the increased energy requirement due to the load presented by the weight of the stilts by increasing their step length and decreasing their step frequency and therefore keeping their energy expenditure under control (Vaida et al. 1981). In the present study, parallel modifications of walking velocity, step frequency and step length were observed over the course of the practice trials. It is possible that as walking velocity increased, simultaneous changes of step frequency and step length allowed postural stability while keeping the energy cost to a minimum. Nevertheless, taking into account that the present study did not measure energy expenditure, further research is required to validate this assumption. While on stilts the participants adopted significantly wider steps. This may have been an adaptive strategy to facilitate lateral stabilisation. Lateral stabilisation during walking requires active control, Table 1. Gait parameters. Gait Parameter A* B{ Velocity 1.01 m/s 1.08 m/s Step frequency 78.3 steps/min 75.8 steps/min Step length 77.7 cm 85.5 cm Step width 18.7 cm 24 cm Double support % of gait cycle *The mean value of the gait parameters for the participants in the present study during the last trial of day 2. {The mean value of the gait parameters for 20 construction workers with more than 12 months of stilt walking experience as they walked while wearing stilts of a similar height in the study by Chiou et al. (2008). Note: The height of stilts was 61 cm in the present study and 60 cm in the study by Chiou et al. (2008). which is largely achieved by adjusting mediolateral foot placement (Donelan et al. 2004). However, walking with wide steps is metabolically expensive (Donelan et al. 2001). Furthermore, the mechanical work of step-to-step transition increases with square of step width (Donelan et al. 2001). The participants in the current study took significantly wider steps throughout the course of the practice trials (in comparison with the NW trials) to broaden their base of support and increase lateral stability. However, the greater metabolic cost of wider steps may have been compensated by the simultaneous changes of step length and step frequency. Although the head was kept flexed throughout the course of the practice trials, there was a significant decrease in the degree of head flexion over the course of the practice trials on day 1. It is speculated that while learning a novel and challenging walking task healthy young adults rely heavily on visual information especially earlier in practice. However, considering that eye movements were not monitored in the current study, further research is necessary to verify this assumption. Participants were able to retain the adapted locomotor pattern between testing sessions as evidenced by similar gait parameters on the last trial of day 1 and the first trial of day 2. It is possible that additional training would have resulted in further adaptation of the gait parameters. However, given the steady state nature of the gait parameters during the last 20 trials of day 2 (Figure 3), further modifications would likely be relatively small and/or require many more practice trials. In fact, the mean values of the gait parameters for the participants during the last trial of day 2 are comparable with the mean values of the gait parameters reported for a group of construction workers (n ¼ 20) with more than 12 months of stiltwalking experience as they walked on a straight path wearing stilts of a similar height (Chiou et al. 2008) (Table 1). 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