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Locomotor adaptations for changes in the slope of the walking surface
Gait and Posture 20 (2004) 255–265
Locomotor adaptations for changes in the slope of the walking surface Stephen D. Prentice∗ , Erika N. Hasler, Jennifer J. Groves, James S. Frank Gait and Posture Laboratory, Department of Kinesiology, University of Waterloo, 200 University Ave. West, Waterloo, Ont., Canada N2L 3G1 Accepted 27 September 2003
Abstract The goal of this study was to examine the transition of walking from a level surface onto different inclined surfaces. Kinematic data of limb and trunk segments were recorded from individuals as they approached and stepped onto four different ramped surfaces (slopes = 3◦ , 6◦ , 9◦ , 12◦ ). This transition introduced significant adaptations to the swing limb trajectory that were evident in even the lowest ramp condition and appear to be scaled to the ramp inclination although the nature of this scaling seemed to change between the 6◦ and 9◦ conditions. An increased forward pitch of the trunk orientation during all ramp conditions was initiated early on during the preceding stance phase on level ground. The swing limb modification essentially consisted of a two-stage response. The initial response of the limb trajectory changes was not specific to the degree of inclination but later changes were dependent on the ramp condition. The initial response is to ensure a safe toe clearance as the foot approaches the edge of the ramp and then later modifications provide the appropriate positioning of the limb to prepare for an elevated foot contact. Early changes were actively produced through an increased pull-off by the hip flexors and an elevation of the swing limb by the active muscle control of the stance limb. Ankle dorsiflexion also appears to have a supporting role increasing toe clearance. Absorption at the hip and knee during later swing contribute to control and position the limb in preparation for foot contact. These strategies were similar to those adopted for step changes in the level of the walking surface where there are similar demands of the quickly moving the limb forward and upward, however, the positioning of the limb for new angled landing surface requires further adaptations. © 2003 Elsevier B.V. All rights reserved. Keywords: Ramp; Kinetics; Kinematics; Human; Locomotion; Gait modification
1. Introduction The transition from a level to an inclined surface presents a number of challenges to locomotor control system. Swing limb trajectories must be modified to ensure safe toe clearance and foot placement as the elevation and orientation of the support surface changes. Similarly, the geometric configuration of body segments and postural support must be altered as the direction at which the ground reaction forces act upon the foot changes with the support surface orientation. Although we encounter inclined walking surfaces regularly and walking on uneven surfaces has been recognized as activity that is highly associated with falls in the elderly [1], very little research [2–9] has been directed towards the examination of how human walking patterns are modified in these circumstances, particularly the transition between level and sloped surfaces. It has also been indicated that slope ∗ Corresponding author. Tel.: +519-888-4567x6830; fax: +519-746-6776. E-mail address: [email protected] (S.D. Prentice).
0966-6362/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2003.09.006
walking can present substantial complications for many individuals coping with muscle or joint pathologies [6]. In fact, the use of ramp or slope walking has recently proven advantageous to assessing specific demands of pathological gait in orthopedic and neurological conditions. [7–9]. The present study will examine how limb trajectory and body posture are modified when walking onto ramps of different inclinations. The challenges of ramp walking are dependent on the physical characteristics of the ramp. Legislated standards and recommendations for the construction of walkways cite maximum slopes ranging anywhere from 3◦ to 6◦ and ramps upto 9◦ are permitted depending on use and accessibility needs [10,11]. However, many natural and existing urban environments can often exceed such recommendations. A significant decrease in walking speed has been found when walking either up or down a steep ramp of greater than 9◦ , while little difference in walking speed has been noted between level ground and lower grade ramps [2]. As the slope increased, walking down the ramp was characterized by a decrease in step length while ascending the ramp was characterized by a decrease in cadence. Changes in muscle
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activity have also been linked to the slope of the walking surface. Tokuhiro et al. [4] noted that muscle activation patterns during walking changed with ramps having a slope greater than 6◦ . While this study only examined the phasic timing and duration of electromyographic (EMG) bursts it does indicate the roles of these muscles do change dramatically with the change in slope of the walking surface and can be related to specific changes in the postural and movement demands. It appears that the typical level walking pattern may require little modification for small grade ramps; however, steeper grades seem to require changes in the motor patterns to provide the additional joint stability, propulsion and limb guidance to accommodate the changing support surface. Treadmill walking at different inclinations has also provided insight into the postural adaptations that are needed to walk on a slope. Leroux et al. [8,9] have documented how forward trunk inclination and stance limb flexion increases with increased treadmill inclination. While detailed kinetic and kinematic analyses of downhill walking have been performed [5,6], little attention has been given to ascending ramps. Further, those examinations of walking on a slope have dealt primarily with steady state conditions and have not examined the transition from level walking to slope walking. Similar transitions, including stepping onto a platform or stairs, have been examined more thoroughly using robust biomechanical analyses [12–14]. The current task had individuals walking towards a ramp and making the transition from a level to a sloped walking surface which can vary from a 3◦ to a 12◦ slope. It was hypothesized that walking onto low grade slopes will involve very similar kinematic and kinetic patterns to that of level walking and that more significant adaptations will occur at steeper slopes. We anticipated that the approach to the ramp will not be different from level walking and that majority of ramp specific changes coming just before the first contact with the ramp. Analysis of the kinetic actions will provide insight into how the motor patterns are adapted to shape the limb and trunk for their new landing postures.
2. Methods To investigate these challenges, participants were instructed to walk on a walkway which included a ramp upto an elevated, level walking surface. An approach similar to that used in investigations of other gait modifications [12,14–18] was adopted to evaluate the kinematic and kinetic adaptations adopted when approaching ramps of different inclinations (3◦ , 6◦ , 9◦ , or 12◦ ). This protocol was reviewed and received ethical clearance from the University of Waterloo, Office of Research Ethics. Six participants (three male, three female; right limb dominant; mean height = 1.74 m; mean wt = 67.5 kg; mean age = 26 years) were recruited from the student population at the University of Waterloo. Prior to testing, infrared emitting diodes were placed the following anatom-
ical landmarks of the individual’s right lower limb: the greater trochanter, lateral femoral condyle, lateral malleolus, posterior calcaneous, fifth metatarsal, and great toe. Diodes were also placed on the right shoulder and head (×2) as well as the left great toe. This permitted collection of kinematic data for a five-segment model of the right limb, trunk, and head using the OPTOTRAK 3D motion measurement system (Northern Digital Inc., Waterloo, Canada). Two Optotrack sensors each having three lenses were used to capture the kinematic volume. Experiments were performed on a walkway in which the middle portion consisted of an adjustable 3 m long ramp upto a higher level walking surface. Participants were instructed to approach the ramp and continue walking upto the higher level walkway. The location of the ramp and the subject’s starting position was set to ensure that the right limb was always the first to step onto the ramp and that the edge of the ramp would be located mid-step. The walking trials were separated into five blocks of 10 trials. The first block had participants’ walk on a level walkway while the remaining blocks included trials at one of four inclinations (3◦ , 6◦ , 9◦ , or 12◦ ). The specific ramp conditions were used to follow the conditions performed by previous ramp studies [2,4]. Marker coordinate data were filtered using a fourth-order zero lag low-pass Butterworth filter (fc = 6 Hz). Segment and joint kinematics were calculated for the stride onto the ramp and joint kinetics (joint moments and mechanical power) were calculated for the swing phase of this same stride using standard inverse dynamics and anthropometric data as described by Winter [19,20]. Sagittal joint angles (hip, knee, and ankle) were calculated as the relative angles between adjacent segments as described by Winter [19,20]. The hip angle was calculated between the thigh and the trunk as defined by the greater trochanter and the shoulder marker (aka: head, arms, and trunk segment or HAT). The sagittal orientation (i.e., pitch) of the trunk segment was measured with respect to the horizontal axis of the global references system of the laboratory. Kinematic and kinetic profiles were ensemble averaged for each participant across the 10 trials of each condition and a population ensemble average was formed for each ramp condition from the mean profiles of all individuals. Prior to averaging, individual profiles were normalized with respect to time such that the time from right heel contact (RHC) to the subsequent RHC represented 0%–100% of the gait cycle. Information relating limb and trunk posture was also determined by reporting discrete kinematic values at key events in the stride onto the ramp. These events included right toe-off (RTO), the point when the leading (right) foot passed the edge of the ramp and finally when landing on the ramp at RHC. These kinematic values at each of the key events were compared across ramp conditions using separate one way repeated measures analysis of variance. A least squared means post hoc measure was employed to identify differences between specific ramp conditions. Further kinetic analysis was performed by integrating the joint power profiles to determine the net work performed at each
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joint during two periods of the swing phase. Early swing work was calculated from toe-off to the point when the toe first passed the edge of the ramp and late swing work was calculated across the remaining duration of swing. 3. Results Participants were observed to modify their walking patterns to accommodate the step onto the new walking surface. The approach to the ramp was virtually identical to level walking with the majority of the changes in body posture and limb trajectory being initiated during the swing phase as the limb approaches and lands onto the sloped surface. These changes included modifications to the trunk orientation and the lower limb trajectory. 3.1. Limb and trunk kinematics The posture of the leading limb during the stance phase during the approach to ramps of different inclinations did not differ from that during level walking. There were no sig-
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nificant differences in orientation of the limb or the trunk at the heel contact preceding the step onto the ramp and the only significant finding at toe-off was the trunk orientation which was modified for the ramp conditions (F(4, 20) = 9.58, P = 0.0002). Here, the participants adopted a posture that increased slightly the amount of forward pitch of the trunk (see Fig. 1) from 87.9◦ observed during level walking to 85.5◦ at the highest ramp inclination. The greatest changes in limb kinematics were found during the swing phase immediately prior to landing on the new surface. The swing limb motions demonstrated an increased elevation of the toe trajectory that was observed during all ramp inclinations. These changes are illustrated in the spatial trajectory of the toe marker in the sagittal plane (Fig. 2) during level walking as well as steps onto the ramp. The increases in the height of the toe began directly following toe-off and continued until heel contact on the ramp surface. Fig. 3 illustrates the associated changes in trunk and joint angle time histories observed for the five ramp conditions. An increased flexion at all three joints was observed beginning approximately at toe-off and continuing until foot contact.
Fig. 1. Histograms illustrating the significant changes in sagittal trunk and joint angles measured at toe-off, over the edge of the obstacle and at the first heel contact on the ramp. Trunk angle represents the segment orientation with respect to the horizontal (ground). The hip, knee, and ankle angle are relative joint angles with positive representing flexion (or dorisflexion) and negative indicating extension (or plantarflexion). Mean and inter-subject standard deviation values calculated across all six individuals are presented for all five ramp conditions (0◦ , 3◦ , 6◦ , 9◦ , 12◦ ). Results of a least square means post hoc analysis are indicated by the brackets located above the histograms. The arrow indicates a category that was significantly different (P < 0.01) from the connected bracketed categories.
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Fig. 2. Spatial trajectories for the hip and toe are plotted for all five ramp conditions (0◦ : thick solid line, 3◦ : thin solid line, 6◦ : dashed line, 9◦ : dotted line, 12◦ : open circles). Each trajectory is an ensemble average of the mean results from all six participants plotted from heel contact to heel contact. The vertical axis represents the vertical height (mm) of each marker and the horizontal axis displays the horizontal position (mm) of each marker with respect to the origin of the walkway coordinate system. The location of the ramp is indicated by the vertical line.
Initial changes of increased flexion in each of the joint angles did not differ among the different non-zero ramp inclinations; however, as the limb approached mid-swing the hip, knee, and ankle angles each showed further increases in joint flexion that were scaled to the slope of the ramp. At the point when the leading foot first crosses the edge of the ramp, there was a significant increase in toe clearance (F(4, 20) = 21.54, P < 0.0001) from 5.0 cm during level walking to 10.2 cm at the steepest ramp condition (see Fig. 4). This increase in toe clearance was a product of a significant increase in flexion at all three joints (hip: F(4, 20) = 31.01, P < 0.0001; knee: F(4, 20) = 12.74, P < 0.0001; ankle: F(4, 20) = 4.54, P = 0.009) and hip elevation (F(4, 20) = 19.72, P < 0.0001) when compared to level walking. The hip, knee, and ankle angles increased from 17.7◦ , 34.7◦ , and −1.5◦ during level walking to 28.0◦ , 46.8◦ , and 4.2◦ , respectively, for the 12◦ ramp condition (see Fig. 1) while the hip elevation increased relatively slightly from 89.9 cm during level walking to 91.4 cm for the 12◦ condition (see Fig. 4). Accompanying this added flexion and hip elevation was a significant increase in: forward pitching of the trunk (F(4, 20) = 20.32, P < 0.0001) and toe velocity, both horizontal (F(4, 20) = 9.65, P < 0.002) and vertical (F(4, 20) = 17.99, P < 0.0001) components (see Fig. 4). The arrows and the connected brackets in Figs. 1 and 4 indicate the least squared means post hoc analysis of significant main effects. It can be seen that all measures apart from the ankle angle revealed a difference between level walking and all ramp conditions, however, many of these measures further indicated differences across the range of ramp slopes. In most of these cases, significant differences were noted between lower and higher ramp inclinations. For example in the 3◦ ramp condition, measurements of toe velocity, joint and trunk flexion all were significantly different
from values for higher slopes. Similar findings were found for the 6◦ ramp for hip angle, vertical toe velocity, and toe clearance. Interestingly, the ankle and trunk angle values were not significantly changed between level walking and the 3◦ ramp. Similar changes in the limb and trunk kinematics were observed as the participants contact the ramp with the leading limb. Trunk angle (F(4, 20) = 11.81, P < 0.0001) once again indicated a more forward pitch during the ramp conditions while amount of limb flexion (hip: F(4, 20) = 16.71, P < 0.0001; knee: F(4, 20) = 9.27, P = 0.0002; ankle: F(4, 20) = 6.1, P < 0.0022) and hip elevation (F(4, 20) = 14.99, P < 0.0001) all increased as they had earlier in the swing phase. The hip, knee, and ankle angles at heel contact increased from 16.3◦ , −3.6◦ , and −1.8◦ during level walking to 34.4◦ , 15.5◦ , and 5.5◦ , respectively, on the 12◦ ramp (see Fig. 1). Similarly, the vertical position of the above level ground (hip elevation in Fig. 4) increased from 86.1 cm during level walking to 89.7 cm during the steepest ramp contact. Once again, many of the measures indicated evidence of scaling across ramp conditions where values on lower slopes were found to be different from the higher conditions (see Figs. 1 and 4). Specifically, differences in hip elevation as well as the amount of hip and knee flexion were found to differ between 3◦ ramp and the higher ramp conditions. Again the ankle angle did not appear to be modified until the higher ramp inclinations were performed. The trunk angle was found to be less sensitive to ramp angle despite the further increase in forward pitch from earlier in the gait cycle. At this point of the swing phase the heel was used to indicate the endpoint trajectory where a significant increase (F(4, 20) = 7.59, P = 0.0007) in the horizontal displacement of the heel ranged from 57.7 cm in level walking to 66.4 cm onto the 12◦ ramp (see Fig. 4). It was also notable
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Fig. 3. Trunk and joint angle time histories are plotted for all five ramp conditions (0◦ : thick solid line, 3◦ : thin solid line, 6◦ : dashed line, 9◦ : dotted line, 12◦ : open circles). Trunk angle represents the segment orientation with respect to the horizontal (ground). The hip, knee, and ankle angle are relative joint angles with positive representing flexion (or dorisflexion) and negative indicating extension (or plantarflexion). Each trajectory is an ensemble average of the mean results from all six participants plotted from heel contact to heel contact. The location of the ramp is indicated by the vertical line.
that no increases were evident in the vertical and horizontal contact velocities of the heel (see Fig. 4). 3.2. Limb kinetics The underlying kinetics responsible for the movements observed were evaluated using net joint moments of force, mechanical powers, and work (Figs. 5A, B and 6, respectively) at ankle, knee, and hip joints. As the primary changes to limb motion appeared during the swing phase, kinetic analysis was limited to this phase. Other than a slightly increased dorsiflexor moment observed in early swing, the ankle moment did not change considerably with the introduction of the ramp (Fig. 5A). This increased dorsiflexor moment was, however, associated with a small but significant (F(4, 20) = 5.6, P = 0.0034) increase in energy applied at the ankle (see Figs. 5B and 6). The knee joint, during the ramp conditions, exhibited a
slight increase in the extensor moment after toe-off followed later by an increased flexor moment during later stages of swing (Fig. 5A). Apart from the longer duration of this increased flexor moment for the higher ramp inclinations, little differences were found between the ramp conditions. Interestingly, substantial differences in the ramp conditions were noted in the power profiles for the knee joint (Fig. 5B). Following toe-off there was a significant increase (F(4, 20) = 9.11, P = 0.0002) in the amount of absorption (K3 in Fig. 5B, see Winter [19]) by the knee extensors during the ramp conditions especially for the higher slopes (Fig. 6). Work done in early swing ranged from −0.014 J/kg during level walking to −0.026 J/kg for the steepest ramp. The biggest differences amongst ramp conditions came during late swing where the magnitude of the absorption burst (K4 in Fig. 5B, see Winter [19]) was inversely related to ramp inclinations, higher absorption were noted with lower ramp angles and the timing of this
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Fig. 4. Histograms illustrating the significant changes in discrete hip elevation and foot (heel/toe) positions and velocities measured over the edge of the obstacle and at the first heel contact on the ramp. Hip elevation, toe and heel kinematics represent the spatial kinematics of these respective markers with respect to the global reference frame of the laboratory. Mean and inter-subject standard deviation values calculated across all six individuals are presented for all five ramp conditions (0◦ , 3◦ , 6◦ , 9◦ , 12◦ ). Results of a least square means post hoc analysis are indicated by the brackets located above the histograms. The arrow indicates a category that was significantly different (P < 0.01) from the connected bracketed categories. Note the results for heel contact velocities are displayed for comparison even though they were not found to be significantly different across ramp conditions.
absorption in the different ramp conditions was similar but occurred much earlier than during level walking. The work done at the knee, however, was not significantly different between any of the walking conditions. Some of the largest changes in limb kinetics occurred at the hip (Fig. 5A and B). Once again during early swing, the differences were largely found with the introduction of the ramp rather than specific to any one ramp condition. Larger hip flexor moments were observed during this phase for all ramp conditions which in turn resulted in significantly increased (F(4, 20) = 13.05, P < 0.0001) hip pull-off powers resulting in work values that ranged from 0.024 J/kg in level walking to 0.062 J/Kg during the 12◦ ramp (H3 in Fig. 5B, see Winter [19]; Fig. 6). The changes during later swing were more specific to each ramp condition. All ramp conditions resulted in increased hip extensor moments of similar magnitudes that peaked earlier than level walking although there
was differences in the phasing between the two lower and the two higher ramp conditions (Fig. 5A). The timing of the peak hip moments for the 9◦ and 12◦ conditions were closer to that of level walking while the peaks during 3◦ and 6◦ ramps occurred earlier. These increased extensor moment resulted in the introduction of a absorptive power burst not observed during level walking and despite the similarity in the size of the extensor moments the timing of these moments led to changes in both the timing and amplitude of the absorption (Figs. 5B and 6). The absorption of energy by the hip extensors was found to be significantly greater (F(4, 20) = 5.28, P = 0.0045) for the higher ramp conditions when compared to the lower ramp conditions. In fact, level walking and the lowest ramp resulted in a net energy generation. In addition to the rotational power applied at each joint, an additional measure representing the translational power applied vertically at the hip joint was evaluated (Fig. 7).
S.D. Prentice et al. / Gait and Posture 20 (2004) 255–265 Fig. 5. Joint moments (A) and powers (B) time histories are plotted for all five ramp conditions (0◦ : thick solid line, 3◦ :thin solid line, 6◦ : dashed line, 9◦ : dotted line, 12◦ : open circles). Positive joint moments represent extensor moments and positive power represents energy generation. Each trajectory is an ensemble average of the mean results from all six participants plotted from toe-off to heel contact. The location of the ramp is indicated by the vertical line.
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Fig. 6. Histograms illustrating the significant changes in work performed at the hip, knee, and ankle during early and late swing. Mean and standard deviation values calculated across all six individuals are presented for all five ramp conditions (0◦ , 3◦ , 6◦ , 9◦ , 12◦ ). Results of a least square means post hoc analysis are indicated by the brackets located above the histograms. The arrow indicates a category that was significantly different (P < 0.01) from the connected bracketed categories.
Fig. 7. Vertical translational power applied at the hip is plotted for all five ramp conditions (0◦ : thick solid line, 3◦ : thin solid line, 6◦ : dashed line, 9◦ : dotted line, 12◦ : open circles). Each trajectory is an ensemble average of the results from all six participants plotted from toe-off to heel contact. The location of the ramp is indicated by the vertical line.
This power measure represents the influence of the stance limb in elevating the swinging limb. During early swing, the stance limb assists the elevation of the limb by the generation of energy applied at the proximal end of the swing limb and during late swing the stance limb absorbed energy by opposing the lowering of the limb to allow a for a more gentler weight acceptance. Very large changes in this translational power were evident for all ramp conditions with an increased generation of energy during early swing followed by a decreased energy absorption during late swing. While the initial increase in energy generation was non-specific, the ultimate amount of energy generation during early swing in-
creased with ramp inclination and the following absorption during late swing actually decreased with ramp inclination.
4. Discussion Stepping onto surfaces of different inclinations required significant yet subtle modifications in both limb and trunk motion. It further appeared that individuals scaled these actions to the slope of the new walking surface. Little modification was observed to occur during the stance phase
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immediately before the step onto the ramp and thus the primary alterations in the walking movement occurred during the swing phase. This is in agreement with many other reports of gait modifications that suggest that limb trajectory changes can be effectively made within the stride immediately before the change is required [21,22]. Essentially, the response involved a very generalized modification in the early swing phase action to increase the amount of limb clearance that was not related to the specific ramp condition. This then was followed by a more specific control of later swing to adjust the limb posture for the specific conditions of the new walking surface. Similar two-stage modulations have been identified in avoiding unexpected obstacles and stepping over fragile obstacles [23,24]. In these studies, it was suggested that this staging is likely implemented for safety reasons where a general response is followed by a more condition-specific correction. It would appear that a similar process is evident in stepping onto different ramps where we witnessed two required elements of this task. First identifying that a change in the walking surface exists and taking evasive action to elevate the trajectory of the swing limb to avoid tripping. The second task of forming the correct positioning of the limb for landing did not appear to require any major modifications in walking actions until late swing. 4.1. Changes in trunk orientation Steady state walking on an inclined surface (treadmill) has been characterized by a forward inclination of the trunk that is scaled to the angle of inclination [8,9]. This adaptation is in response to the increased postural and propulsion demands associated with the change in orientation of the support surface. The small but significant increase in the forward inclination of the trunk throughout both the stance and swing phases of all ramp conditions would indicate that individuals were able to anticipate these added demands. More slope specific modifications of trunk orientation were observed during the swing phase indicating the individuals ability to further anticipate the magnitude of the perturbations introduced by higher slopes. It should be noted that trunk inclination is a component of the hip flexion measure used in this study, however, its importance in relation to the postural demands of this task required separate examination. The use of the hip angle was maintained as the conventional measure for relating the contribution of the thigh position to changes in the toe trajectory. While independence of measurement was compromised, the changes observed in hip angle were much greater than the very small changes in trunk orientation and can be largely attributed to changes in thigh orientation. 4.2. Changes during early swing The early changes seen during the swing limb motion during ramp condition included an increase in the amount of
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toe clearance over the edge of the ramp which has it similarities to obstructed walking tasks and perhaps more closely to step changes in the height of the walking surface. Indeed the magnitude approaching a 10 cm clearance is quite similar to what has been found with obstacles and platforms of varying height [12,14,25]. The response of increased limb flexion and trunk elevation are quite comparable to the primary motions adopted by individuals when traversing obstacles or steps. Patla and Rietdyk [25] have further noted that when stepping over obstructions, the velocity of the toe is reduced to minimize the effects of any consequential impact with the barrier. It is interesting that velocity of the toe crossing the edge of the ramp increased greatly from level walking. While a slowing of foot movement over an obstacle has its advantages in minimizing the consequences of contact, the movement of the foot over the edge of the ramp is also influenced by the necessity to safely move the foot higher to the new level of the walking surface. With such a large margin of clearance this increased velocity of the foot is unlikely to pose a threat but if the edge of the ramp is not well identified by the walker then this fast movement may be more problematic. It was somewhat surprising that even the smallest ramp inclination was sufficient to require significant adaptations to the swing limb trajectory. This can be explained in part by fact that the ramp walking surface was readily identifiable through its contrast to the surrounding floor. Nonetheless, it was anticipated that the transition to lower ramp conditions would perhaps be more of reactive process where individuals simply would maintain more of a normal walking motion with subtle adjustments in limb positioning. The large adjustment of the limb motion to clear the edge of the ramp suggests that crossing onto even the smallest ramps was a very proactive avoidance process. During obstructed walking, it has been well documented that a reduction in the amount of absorption by knee extensors which can also lead to an increased energy generation by knee flexors is a primary strategy used to increase the elevation of the foot [12,14,16,18]. The mechanism used to elevate the foot during early swing onto the ramp was quite similar to that used stepping onto a step or platform. Elevation of the foot during ramp conditions was created by an increased absorption at the knee accompanied by a very large increase in the amount of pull-off power generated by the hip flexors. Similar to stepping onto a step, the manner in which the foot was elevated can be attributed to the necessity of ensuring that the foot moves quickly forward and upward given the new height of the landing location. McFadyen and Carnahan [12], McFadyen and Prince [14], and McFadyen et al. [18] have noted that stepping onto a step, platform, or even over obstacles placed at a further distance with respect to the point where the swing foot leaves the ground will lead to an increase in the hip pull-off burst to allow the limb to reach over the more distant and or elevated landing surfaces. The increased absorption at the knee during ramp conditions would indicate that there was a greater requirement to
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reverse the backward motion of the leg segment (i.e., knee flexion) following toe-off and this accompanied by an increase in the hip flexion would lead to the quick forward and upward movement of the foot. Thus, the resulting toe trajectory while similar to an obstacle clearance task, exhibits a different kinetic strategy that more closely resembles an enhanced level walking strategy or that similar to stepping onto a platform or a set of stairs [12,14]. A somewhat similar strategy, however, has been observed in amputees when traversing an obstacles of different heights [17]. 4.3. Changes during late swing The limb positioning during late stance was found to be quite specific to the particular ramp conditions. While much of this would be expected given the increased height of the walking surface as the foot comes in contact with the ramp, individuals were able to judge and anticipate the geometry of the different sloped surfaces. Evidence for this comes as the amount of joint flexion and elevation of the hip and limb were scaled to the different slopes both prior to and at heel contact. Further support comes from the finding that heel contact velocities did not differ from level walking. This implies that individuals were providing a gentle contact onto the surface and not simply reacting to the foot impacting the ramp. If individuals were not planning appropriately for the ramp then we would have expected to see significant elevations in the contact velocities. However, one limitation is that the contact velocities are with reference to the level walking surface and may not best reflect the velocities that are normal and tangential to the walking surface. A more thorough analysis of contact orientations and velocities would shed more insight for the magnitude of impact and the tangential/slip velocities. Nonetheless, individuals were able to anticipate the body segment orientations for the new walking surface. The control of the positioning of the limb for contact and eventual support of the body on the inclined surface was accomplished primarily through absorptive actions at the hip and knee. Here, the knee flexor muscles served to brake the extension of the knee in a fashion quite similar as that during level walking. The main difference was an earlier phasing of this braking burst and while higher ramp conditions actually exhibited a decreased peak absorption the amount of negative work done was not different. The actions at the hip were more sensitive to the different ramp conditions. The hip extensors during this period were responsible for absorption to slow the rate of hip flexion during the higher ramp conditions while very similar joint moments served to generate energy to promote hip extension at lower ramp angles and level walking. Interestingly, the hip extensor moments observed for all ramp conditions were of similar peak magnitude and exhibited only subtle differences in timing yet the power profiles and work values were quite different. It is not clear if these differences in the action at hip were the result of the subtle timing changes in the application of
hip joint moments or a more coordinated effort amongst the joints of the swing limb with or without the actions of the supporting limb (i.e., via trunk/hip elevation). It was intriguing to note that an increase in absorption at the hip was accompanied by a decrease in the absorption at the knee joint in the two higher ramp conditions while the power generation/absorption for the lower grade ramps were more similar in magnitude to level walking. This seems to imply that stepping onto higher grade ramps requires a different strategy to achieve the correct placement and positioning of the limb for support and propulsion on steeper surfaces. These findings are consistent with those observed during steady state walking on a ramp [4] where muscle activity records which indicate the onset of hamstring activity during swing occurs earlier as the grade of the ramp is increased and similarly the onset of the rectus femoris becomes delayed. These data would support our finding of earlier onsets of the knee flexor and hip extensor moments and indicate that an increased dominance of hamstring action over quadriceps actions at both the knee and hip would explain these results. The reverse would be true for the observed changes in early swing where quadriceps actions would increase the hip pull-off and knee absorption. While muscular activity was not directly monitored in this study, the changes in limb kinetics would indicate that locomotor control system would be required to change both the timing and amplitude of muscle activity in the lower limb to achieve this particular gait modification. Overall the findings of this study provide additional support for the flexibility the control of walking patterns within complex physical environments. The growing need for community accessibility has increased the prevalence of inclined walking surfaces. It is comforting to know that we can so readily modify our level walking to these new surfaces. In this study, the ramp surface was easily identifiable with respect to the surrounding walking surface and thus may have influenced the magnitude of the clearance individuals require as they cross onto the ramp. Further investigation is needed to determine if the proactive strategies adopted in this studies would hold for ramps that are less distinctive visually. This would also be an important factor for those parties designing ramps for community mobility. Additionally, the substantial change in the kinetic actions evident between 6◦ and 9◦ ramp conditions needs to be examined further and would also have implications for the design of community walking surfaces.
5. Conclusion A two-stage anticipatory modification was observed in this experiment as the physical characteristics of the walking surface were changed. Changing the slope of the walking surface altered both the level and orientation of the contact surface. The ability of individuals to scale their limb trajectory in advance to relatively small changes in ramp
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inclination indicated that we are able to not only detect these changes but the locomotor control system appears to be able to manage the mechanics of the limb very effectively with very subtle changes in the timing and magnitude of muscular action. The control system could have employed a more reactive approach where level walking movements or a simple modification of these movements that was not ramp specific were executed and then the sensory information detecting the new surface could have been used to modify subsequent actions. The ramp specific modification further indicate the central nervous systems awareness of limb mechanics and its ability to harness these properties to provide safe and functional locomotor actions.
Acknowledgements This research has been supported by the Department of Kinesiology, the University of Waterloo and Natural Science and Engineering Research Council of Canada. I would also like to acknowledge T. Tomko, M.J. Reid, and J.L. Zettle for their assistance in this experiment.
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[8] Leroux A, Fung J, Barbeau H. Adaptation of the walking pattern to uphill walking in normal and spinal-cord injured subjects. Exp Brain Res 1999;126:359–68. [9] Leroux A, Fung J, Barbeau H. Postural adaptation to walking on inclined surfaces I: Normal Strategies. Gait Posture 2002;15:64–74. [10] US Department of Justice, ADA standards for accessible design. Nondiscrimination on the basis of disability by public accommodations and in commercial facilities. 1994; 28 CFR Part 36:518–521. [11] Ontario Building Code Act 1992; 3.4.6.6 & 9.8.6.2. [12] McFadyen BJ, Carnahan H. Anticipatory locomotor adjustments for accommodating versus avoiding level changes in humans. Exp Brain Res 1997;114:500–6. [13] Gélat T, Brenièr Y. Adaptation of the gait initiation process for stepping onto a new level using a single step. Exp Brain Res 2000;133: 538–46. [14] McFadyen BJ, Prince F. Avoidance and accommodation of surface height changes by healthy, community-dwelling, young and elderly men. J Gerentol Bio Sci 2002;57A:B166–74. [15] McFadyen BJ, Winter DA. Anticipatory locomotor adjustments during obstructed human walking. Neurosci Res Commun 1991;9:37– 44. [16] Patla AE, Prentice SD. The role of active forces and intersegmental dynamics in the control of limb trajectory over obstacles during locomotion in humans. Exp Brain Res 1995;106:499–504. [17] Hill SW, Patla AE, Ishac MG, Adkin AL, Supan TJ, Barth DG. Altered kinetic strategy for the control of swing limb elevation over obstacles in unilateral below-knee amputee gait. J Biomech 1999;32:545–9. [18] McFadyen BJ, Magnan GA, Boucher JP. Anticipatory locomotor adjustments for avoiding visible, fixed obstacles of varying proximity. Hum Mov Sci 1993;12:259–72. [19] Winter DA, Biomechanics and motor control of human gait: normal, elderly and pathological. Waterloo: Waterloo Biomechanics; 1991. [20] Winter DA, Biomechanics and motor control of human movement. New York: Wiley-Interscience; 1990. [21] Patla AE, Prentice SD, Robinson C, Neufeld J. Visual control of locomotion: strategies for changing direction and for going over obstacles. J Exp Psychol Hum Percept Perform 1991;17:603–34. [22] Patla AE. Understanding the roles of vision in the control of human locomotion. Gait Posture 1997;5:54–69. [23] Patla AE, Beuter A, Prentice S. A two-stage correction of limb trajectory to avoid obstacles during stepping. Neurosci Res Commun 1991;8:153–9. [24] Patla AE, Rietdyk S, Martin C, Prentice S. Locomotor patterns of the leading and trailing limbs as solid and fragile obstacles are stepped over: Some insights into the role of vision during locomotion. J Motor Behav 1996;28:35–47. [25] Patla AE, Rietdyk S. Visual control of limb trajectory over obstacles during locomotion: effect of obstacle height and width. Gait Posture 1993;1:45–60.
Locomotor adaptations for changes in the slope of the walking surface Stephen D. Prentice∗ , Erika N. Hasler, Jennifer J. Groves, James S. Frank Gait and Posture Laboratory, Department of Kinesiology, University of Waterloo, 200 University Ave. West, Waterloo, Ont., Canada N2L 3G1 Accepted 27 September 2003
Abstract The goal of this study was to examine the transition of walking from a level surface onto different inclined surfaces. Kinematic data of limb and trunk segments were recorded from individuals as they approached and stepped onto four different ramped surfaces (slopes = 3◦ , 6◦ , 9◦ , 12◦ ). This transition introduced significant adaptations to the swing limb trajectory that were evident in even the lowest ramp condition and appear to be scaled to the ramp inclination although the nature of this scaling seemed to change between the 6◦ and 9◦ conditions. An increased forward pitch of the trunk orientation during all ramp conditions was initiated early on during the preceding stance phase on level ground. The swing limb modification essentially consisted of a two-stage response. The initial response of the limb trajectory changes was not specific to the degree of inclination but later changes were dependent on the ramp condition. The initial response is to ensure a safe toe clearance as the foot approaches the edge of the ramp and then later modifications provide the appropriate positioning of the limb to prepare for an elevated foot contact. Early changes were actively produced through an increased pull-off by the hip flexors and an elevation of the swing limb by the active muscle control of the stance limb. Ankle dorsiflexion also appears to have a supporting role increasing toe clearance. Absorption at the hip and knee during later swing contribute to control and position the limb in preparation for foot contact. These strategies were similar to those adopted for step changes in the level of the walking surface where there are similar demands of the quickly moving the limb forward and upward, however, the positioning of the limb for new angled landing surface requires further adaptations. © 2003 Elsevier B.V. All rights reserved. Keywords: Ramp; Kinetics; Kinematics; Human; Locomotion; Gait modification
1. Introduction The transition from a level to an inclined surface presents a number of challenges to locomotor control system. Swing limb trajectories must be modified to ensure safe toe clearance and foot placement as the elevation and orientation of the support surface changes. Similarly, the geometric configuration of body segments and postural support must be altered as the direction at which the ground reaction forces act upon the foot changes with the support surface orientation. Although we encounter inclined walking surfaces regularly and walking on uneven surfaces has been recognized as activity that is highly associated with falls in the elderly [1], very little research [2–9] has been directed towards the examination of how human walking patterns are modified in these circumstances, particularly the transition between level and sloped surfaces. It has also been indicated that slope ∗ Corresponding author. Tel.: +519-888-4567x6830; fax: +519-746-6776. E-mail address: [email protected] (S.D. Prentice).
0966-6362/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2003.09.006
walking can present substantial complications for many individuals coping with muscle or joint pathologies [6]. In fact, the use of ramp or slope walking has recently proven advantageous to assessing specific demands of pathological gait in orthopedic and neurological conditions. [7–9]. The present study will examine how limb trajectory and body posture are modified when walking onto ramps of different inclinations. The challenges of ramp walking are dependent on the physical characteristics of the ramp. Legislated standards and recommendations for the construction of walkways cite maximum slopes ranging anywhere from 3◦ to 6◦ and ramps upto 9◦ are permitted depending on use and accessibility needs [10,11]. However, many natural and existing urban environments can often exceed such recommendations. A significant decrease in walking speed has been found when walking either up or down a steep ramp of greater than 9◦ , while little difference in walking speed has been noted between level ground and lower grade ramps [2]. As the slope increased, walking down the ramp was characterized by a decrease in step length while ascending the ramp was characterized by a decrease in cadence. Changes in muscle
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activity have also been linked to the slope of the walking surface. Tokuhiro et al. [4] noted that muscle activation patterns during walking changed with ramps having a slope greater than 6◦ . While this study only examined the phasic timing and duration of electromyographic (EMG) bursts it does indicate the roles of these muscles do change dramatically with the change in slope of the walking surface and can be related to specific changes in the postural and movement demands. It appears that the typical level walking pattern may require little modification for small grade ramps; however, steeper grades seem to require changes in the motor patterns to provide the additional joint stability, propulsion and limb guidance to accommodate the changing support surface. Treadmill walking at different inclinations has also provided insight into the postural adaptations that are needed to walk on a slope. Leroux et al. [8,9] have documented how forward trunk inclination and stance limb flexion increases with increased treadmill inclination. While detailed kinetic and kinematic analyses of downhill walking have been performed [5,6], little attention has been given to ascending ramps. Further, those examinations of walking on a slope have dealt primarily with steady state conditions and have not examined the transition from level walking to slope walking. Similar transitions, including stepping onto a platform or stairs, have been examined more thoroughly using robust biomechanical analyses [12–14]. The current task had individuals walking towards a ramp and making the transition from a level to a sloped walking surface which can vary from a 3◦ to a 12◦ slope. It was hypothesized that walking onto low grade slopes will involve very similar kinematic and kinetic patterns to that of level walking and that more significant adaptations will occur at steeper slopes. We anticipated that the approach to the ramp will not be different from level walking and that majority of ramp specific changes coming just before the first contact with the ramp. Analysis of the kinetic actions will provide insight into how the motor patterns are adapted to shape the limb and trunk for their new landing postures.
2. Methods To investigate these challenges, participants were instructed to walk on a walkway which included a ramp upto an elevated, level walking surface. An approach similar to that used in investigations of other gait modifications [12,14–18] was adopted to evaluate the kinematic and kinetic adaptations adopted when approaching ramps of different inclinations (3◦ , 6◦ , 9◦ , or 12◦ ). This protocol was reviewed and received ethical clearance from the University of Waterloo, Office of Research Ethics. Six participants (three male, three female; right limb dominant; mean height = 1.74 m; mean wt = 67.5 kg; mean age = 26 years) were recruited from the student population at the University of Waterloo. Prior to testing, infrared emitting diodes were placed the following anatom-
ical landmarks of the individual’s right lower limb: the greater trochanter, lateral femoral condyle, lateral malleolus, posterior calcaneous, fifth metatarsal, and great toe. Diodes were also placed on the right shoulder and head (×2) as well as the left great toe. This permitted collection of kinematic data for a five-segment model of the right limb, trunk, and head using the OPTOTRAK 3D motion measurement system (Northern Digital Inc., Waterloo, Canada). Two Optotrack sensors each having three lenses were used to capture the kinematic volume. Experiments were performed on a walkway in which the middle portion consisted of an adjustable 3 m long ramp upto a higher level walking surface. Participants were instructed to approach the ramp and continue walking upto the higher level walkway. The location of the ramp and the subject’s starting position was set to ensure that the right limb was always the first to step onto the ramp and that the edge of the ramp would be located mid-step. The walking trials were separated into five blocks of 10 trials. The first block had participants’ walk on a level walkway while the remaining blocks included trials at one of four inclinations (3◦ , 6◦ , 9◦ , or 12◦ ). The specific ramp conditions were used to follow the conditions performed by previous ramp studies [2,4]. Marker coordinate data were filtered using a fourth-order zero lag low-pass Butterworth filter (fc = 6 Hz). Segment and joint kinematics were calculated for the stride onto the ramp and joint kinetics (joint moments and mechanical power) were calculated for the swing phase of this same stride using standard inverse dynamics and anthropometric data as described by Winter [19,20]. Sagittal joint angles (hip, knee, and ankle) were calculated as the relative angles between adjacent segments as described by Winter [19,20]. The hip angle was calculated between the thigh and the trunk as defined by the greater trochanter and the shoulder marker (aka: head, arms, and trunk segment or HAT). The sagittal orientation (i.e., pitch) of the trunk segment was measured with respect to the horizontal axis of the global references system of the laboratory. Kinematic and kinetic profiles were ensemble averaged for each participant across the 10 trials of each condition and a population ensemble average was formed for each ramp condition from the mean profiles of all individuals. Prior to averaging, individual profiles were normalized with respect to time such that the time from right heel contact (RHC) to the subsequent RHC represented 0%–100% of the gait cycle. Information relating limb and trunk posture was also determined by reporting discrete kinematic values at key events in the stride onto the ramp. These events included right toe-off (RTO), the point when the leading (right) foot passed the edge of the ramp and finally when landing on the ramp at RHC. These kinematic values at each of the key events were compared across ramp conditions using separate one way repeated measures analysis of variance. A least squared means post hoc measure was employed to identify differences between specific ramp conditions. Further kinetic analysis was performed by integrating the joint power profiles to determine the net work performed at each
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joint during two periods of the swing phase. Early swing work was calculated from toe-off to the point when the toe first passed the edge of the ramp and late swing work was calculated across the remaining duration of swing. 3. Results Participants were observed to modify their walking patterns to accommodate the step onto the new walking surface. The approach to the ramp was virtually identical to level walking with the majority of the changes in body posture and limb trajectory being initiated during the swing phase as the limb approaches and lands onto the sloped surface. These changes included modifications to the trunk orientation and the lower limb trajectory. 3.1. Limb and trunk kinematics The posture of the leading limb during the stance phase during the approach to ramps of different inclinations did not differ from that during level walking. There were no sig-
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nificant differences in orientation of the limb or the trunk at the heel contact preceding the step onto the ramp and the only significant finding at toe-off was the trunk orientation which was modified for the ramp conditions (F(4, 20) = 9.58, P = 0.0002). Here, the participants adopted a posture that increased slightly the amount of forward pitch of the trunk (see Fig. 1) from 87.9◦ observed during level walking to 85.5◦ at the highest ramp inclination. The greatest changes in limb kinematics were found during the swing phase immediately prior to landing on the new surface. The swing limb motions demonstrated an increased elevation of the toe trajectory that was observed during all ramp inclinations. These changes are illustrated in the spatial trajectory of the toe marker in the sagittal plane (Fig. 2) during level walking as well as steps onto the ramp. The increases in the height of the toe began directly following toe-off and continued until heel contact on the ramp surface. Fig. 3 illustrates the associated changes in trunk and joint angle time histories observed for the five ramp conditions. An increased flexion at all three joints was observed beginning approximately at toe-off and continuing until foot contact.
Fig. 1. Histograms illustrating the significant changes in sagittal trunk and joint angles measured at toe-off, over the edge of the obstacle and at the first heel contact on the ramp. Trunk angle represents the segment orientation with respect to the horizontal (ground). The hip, knee, and ankle angle are relative joint angles with positive representing flexion (or dorisflexion) and negative indicating extension (or plantarflexion). Mean and inter-subject standard deviation values calculated across all six individuals are presented for all five ramp conditions (0◦ , 3◦ , 6◦ , 9◦ , 12◦ ). Results of a least square means post hoc analysis are indicated by the brackets located above the histograms. The arrow indicates a category that was significantly different (P < 0.01) from the connected bracketed categories.
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Fig. 2. Spatial trajectories for the hip and toe are plotted for all five ramp conditions (0◦ : thick solid line, 3◦ : thin solid line, 6◦ : dashed line, 9◦ : dotted line, 12◦ : open circles). Each trajectory is an ensemble average of the mean results from all six participants plotted from heel contact to heel contact. The vertical axis represents the vertical height (mm) of each marker and the horizontal axis displays the horizontal position (mm) of each marker with respect to the origin of the walkway coordinate system. The location of the ramp is indicated by the vertical line.
Initial changes of increased flexion in each of the joint angles did not differ among the different non-zero ramp inclinations; however, as the limb approached mid-swing the hip, knee, and ankle angles each showed further increases in joint flexion that were scaled to the slope of the ramp. At the point when the leading foot first crosses the edge of the ramp, there was a significant increase in toe clearance (F(4, 20) = 21.54, P < 0.0001) from 5.0 cm during level walking to 10.2 cm at the steepest ramp condition (see Fig. 4). This increase in toe clearance was a product of a significant increase in flexion at all three joints (hip: F(4, 20) = 31.01, P < 0.0001; knee: F(4, 20) = 12.74, P < 0.0001; ankle: F(4, 20) = 4.54, P = 0.009) and hip elevation (F(4, 20) = 19.72, P < 0.0001) when compared to level walking. The hip, knee, and ankle angles increased from 17.7◦ , 34.7◦ , and −1.5◦ during level walking to 28.0◦ , 46.8◦ , and 4.2◦ , respectively, for the 12◦ ramp condition (see Fig. 1) while the hip elevation increased relatively slightly from 89.9 cm during level walking to 91.4 cm for the 12◦ condition (see Fig. 4). Accompanying this added flexion and hip elevation was a significant increase in: forward pitching of the trunk (F(4, 20) = 20.32, P < 0.0001) and toe velocity, both horizontal (F(4, 20) = 9.65, P < 0.002) and vertical (F(4, 20) = 17.99, P < 0.0001) components (see Fig. 4). The arrows and the connected brackets in Figs. 1 and 4 indicate the least squared means post hoc analysis of significant main effects. It can be seen that all measures apart from the ankle angle revealed a difference between level walking and all ramp conditions, however, many of these measures further indicated differences across the range of ramp slopes. In most of these cases, significant differences were noted between lower and higher ramp inclinations. For example in the 3◦ ramp condition, measurements of toe velocity, joint and trunk flexion all were significantly different
from values for higher slopes. Similar findings were found for the 6◦ ramp for hip angle, vertical toe velocity, and toe clearance. Interestingly, the ankle and trunk angle values were not significantly changed between level walking and the 3◦ ramp. Similar changes in the limb and trunk kinematics were observed as the participants contact the ramp with the leading limb. Trunk angle (F(4, 20) = 11.81, P < 0.0001) once again indicated a more forward pitch during the ramp conditions while amount of limb flexion (hip: F(4, 20) = 16.71, P < 0.0001; knee: F(4, 20) = 9.27, P = 0.0002; ankle: F(4, 20) = 6.1, P < 0.0022) and hip elevation (F(4, 20) = 14.99, P < 0.0001) all increased as they had earlier in the swing phase. The hip, knee, and ankle angles at heel contact increased from 16.3◦ , −3.6◦ , and −1.8◦ during level walking to 34.4◦ , 15.5◦ , and 5.5◦ , respectively, on the 12◦ ramp (see Fig. 1). Similarly, the vertical position of the above level ground (hip elevation in Fig. 4) increased from 86.1 cm during level walking to 89.7 cm during the steepest ramp contact. Once again, many of the measures indicated evidence of scaling across ramp conditions where values on lower slopes were found to be different from the higher conditions (see Figs. 1 and 4). Specifically, differences in hip elevation as well as the amount of hip and knee flexion were found to differ between 3◦ ramp and the higher ramp conditions. Again the ankle angle did not appear to be modified until the higher ramp inclinations were performed. The trunk angle was found to be less sensitive to ramp angle despite the further increase in forward pitch from earlier in the gait cycle. At this point of the swing phase the heel was used to indicate the endpoint trajectory where a significant increase (F(4, 20) = 7.59, P = 0.0007) in the horizontal displacement of the heel ranged from 57.7 cm in level walking to 66.4 cm onto the 12◦ ramp (see Fig. 4). It was also notable
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Fig. 3. Trunk and joint angle time histories are plotted for all five ramp conditions (0◦ : thick solid line, 3◦ : thin solid line, 6◦ : dashed line, 9◦ : dotted line, 12◦ : open circles). Trunk angle represents the segment orientation with respect to the horizontal (ground). The hip, knee, and ankle angle are relative joint angles with positive representing flexion (or dorisflexion) and negative indicating extension (or plantarflexion). Each trajectory is an ensemble average of the mean results from all six participants plotted from heel contact to heel contact. The location of the ramp is indicated by the vertical line.
that no increases were evident in the vertical and horizontal contact velocities of the heel (see Fig. 4). 3.2. Limb kinetics The underlying kinetics responsible for the movements observed were evaluated using net joint moments of force, mechanical powers, and work (Figs. 5A, B and 6, respectively) at ankle, knee, and hip joints. As the primary changes to limb motion appeared during the swing phase, kinetic analysis was limited to this phase. Other than a slightly increased dorsiflexor moment observed in early swing, the ankle moment did not change considerably with the introduction of the ramp (Fig. 5A). This increased dorsiflexor moment was, however, associated with a small but significant (F(4, 20) = 5.6, P = 0.0034) increase in energy applied at the ankle (see Figs. 5B and 6). The knee joint, during the ramp conditions, exhibited a
slight increase in the extensor moment after toe-off followed later by an increased flexor moment during later stages of swing (Fig. 5A). Apart from the longer duration of this increased flexor moment for the higher ramp inclinations, little differences were found between the ramp conditions. Interestingly, substantial differences in the ramp conditions were noted in the power profiles for the knee joint (Fig. 5B). Following toe-off there was a significant increase (F(4, 20) = 9.11, P = 0.0002) in the amount of absorption (K3 in Fig. 5B, see Winter [19]) by the knee extensors during the ramp conditions especially for the higher slopes (Fig. 6). Work done in early swing ranged from −0.014 J/kg during level walking to −0.026 J/kg for the steepest ramp. The biggest differences amongst ramp conditions came during late swing where the magnitude of the absorption burst (K4 in Fig. 5B, see Winter [19]) was inversely related to ramp inclinations, higher absorption were noted with lower ramp angles and the timing of this
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Fig. 4. Histograms illustrating the significant changes in discrete hip elevation and foot (heel/toe) positions and velocities measured over the edge of the obstacle and at the first heel contact on the ramp. Hip elevation, toe and heel kinematics represent the spatial kinematics of these respective markers with respect to the global reference frame of the laboratory. Mean and inter-subject standard deviation values calculated across all six individuals are presented for all five ramp conditions (0◦ , 3◦ , 6◦ , 9◦ , 12◦ ). Results of a least square means post hoc analysis are indicated by the brackets located above the histograms. The arrow indicates a category that was significantly different (P < 0.01) from the connected bracketed categories. Note the results for heel contact velocities are displayed for comparison even though they were not found to be significantly different across ramp conditions.
absorption in the different ramp conditions was similar but occurred much earlier than during level walking. The work done at the knee, however, was not significantly different between any of the walking conditions. Some of the largest changes in limb kinetics occurred at the hip (Fig. 5A and B). Once again during early swing, the differences were largely found with the introduction of the ramp rather than specific to any one ramp condition. Larger hip flexor moments were observed during this phase for all ramp conditions which in turn resulted in significantly increased (F(4, 20) = 13.05, P < 0.0001) hip pull-off powers resulting in work values that ranged from 0.024 J/kg in level walking to 0.062 J/Kg during the 12◦ ramp (H3 in Fig. 5B, see Winter [19]; Fig. 6). The changes during later swing were more specific to each ramp condition. All ramp conditions resulted in increased hip extensor moments of similar magnitudes that peaked earlier than level walking although there
was differences in the phasing between the two lower and the two higher ramp conditions (Fig. 5A). The timing of the peak hip moments for the 9◦ and 12◦ conditions were closer to that of level walking while the peaks during 3◦ and 6◦ ramps occurred earlier. These increased extensor moment resulted in the introduction of a absorptive power burst not observed during level walking and despite the similarity in the size of the extensor moments the timing of these moments led to changes in both the timing and amplitude of the absorption (Figs. 5B and 6). The absorption of energy by the hip extensors was found to be significantly greater (F(4, 20) = 5.28, P = 0.0045) for the higher ramp conditions when compared to the lower ramp conditions. In fact, level walking and the lowest ramp resulted in a net energy generation. In addition to the rotational power applied at each joint, an additional measure representing the translational power applied vertically at the hip joint was evaluated (Fig. 7).
S.D. Prentice et al. / Gait and Posture 20 (2004) 255–265 Fig. 5. Joint moments (A) and powers (B) time histories are plotted for all five ramp conditions (0◦ : thick solid line, 3◦ :thin solid line, 6◦ : dashed line, 9◦ : dotted line, 12◦ : open circles). Positive joint moments represent extensor moments and positive power represents energy generation. Each trajectory is an ensemble average of the mean results from all six participants plotted from toe-off to heel contact. The location of the ramp is indicated by the vertical line.
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Fig. 6. Histograms illustrating the significant changes in work performed at the hip, knee, and ankle during early and late swing. Mean and standard deviation values calculated across all six individuals are presented for all five ramp conditions (0◦ , 3◦ , 6◦ , 9◦ , 12◦ ). Results of a least square means post hoc analysis are indicated by the brackets located above the histograms. The arrow indicates a category that was significantly different (P < 0.01) from the connected bracketed categories.
Fig. 7. Vertical translational power applied at the hip is plotted for all five ramp conditions (0◦ : thick solid line, 3◦ : thin solid line, 6◦ : dashed line, 9◦ : dotted line, 12◦ : open circles). Each trajectory is an ensemble average of the results from all six participants plotted from toe-off to heel contact. The location of the ramp is indicated by the vertical line.
This power measure represents the influence of the stance limb in elevating the swinging limb. During early swing, the stance limb assists the elevation of the limb by the generation of energy applied at the proximal end of the swing limb and during late swing the stance limb absorbed energy by opposing the lowering of the limb to allow a for a more gentler weight acceptance. Very large changes in this translational power were evident for all ramp conditions with an increased generation of energy during early swing followed by a decreased energy absorption during late swing. While the initial increase in energy generation was non-specific, the ultimate amount of energy generation during early swing in-
creased with ramp inclination and the following absorption during late swing actually decreased with ramp inclination.
4. Discussion Stepping onto surfaces of different inclinations required significant yet subtle modifications in both limb and trunk motion. It further appeared that individuals scaled these actions to the slope of the new walking surface. Little modification was observed to occur during the stance phase
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immediately before the step onto the ramp and thus the primary alterations in the walking movement occurred during the swing phase. This is in agreement with many other reports of gait modifications that suggest that limb trajectory changes can be effectively made within the stride immediately before the change is required [21,22]. Essentially, the response involved a very generalized modification in the early swing phase action to increase the amount of limb clearance that was not related to the specific ramp condition. This then was followed by a more specific control of later swing to adjust the limb posture for the specific conditions of the new walking surface. Similar two-stage modulations have been identified in avoiding unexpected obstacles and stepping over fragile obstacles [23,24]. In these studies, it was suggested that this staging is likely implemented for safety reasons where a general response is followed by a more condition-specific correction. It would appear that a similar process is evident in stepping onto different ramps where we witnessed two required elements of this task. First identifying that a change in the walking surface exists and taking evasive action to elevate the trajectory of the swing limb to avoid tripping. The second task of forming the correct positioning of the limb for landing did not appear to require any major modifications in walking actions until late swing. 4.1. Changes in trunk orientation Steady state walking on an inclined surface (treadmill) has been characterized by a forward inclination of the trunk that is scaled to the angle of inclination [8,9]. This adaptation is in response to the increased postural and propulsion demands associated with the change in orientation of the support surface. The small but significant increase in the forward inclination of the trunk throughout both the stance and swing phases of all ramp conditions would indicate that individuals were able to anticipate these added demands. More slope specific modifications of trunk orientation were observed during the swing phase indicating the individuals ability to further anticipate the magnitude of the perturbations introduced by higher slopes. It should be noted that trunk inclination is a component of the hip flexion measure used in this study, however, its importance in relation to the postural demands of this task required separate examination. The use of the hip angle was maintained as the conventional measure for relating the contribution of the thigh position to changes in the toe trajectory. While independence of measurement was compromised, the changes observed in hip angle were much greater than the very small changes in trunk orientation and can be largely attributed to changes in thigh orientation. 4.2. Changes during early swing The early changes seen during the swing limb motion during ramp condition included an increase in the amount of
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toe clearance over the edge of the ramp which has it similarities to obstructed walking tasks and perhaps more closely to step changes in the height of the walking surface. Indeed the magnitude approaching a 10 cm clearance is quite similar to what has been found with obstacles and platforms of varying height [12,14,25]. The response of increased limb flexion and trunk elevation are quite comparable to the primary motions adopted by individuals when traversing obstacles or steps. Patla and Rietdyk [25] have further noted that when stepping over obstructions, the velocity of the toe is reduced to minimize the effects of any consequential impact with the barrier. It is interesting that velocity of the toe crossing the edge of the ramp increased greatly from level walking. While a slowing of foot movement over an obstacle has its advantages in minimizing the consequences of contact, the movement of the foot over the edge of the ramp is also influenced by the necessity to safely move the foot higher to the new level of the walking surface. With such a large margin of clearance this increased velocity of the foot is unlikely to pose a threat but if the edge of the ramp is not well identified by the walker then this fast movement may be more problematic. It was somewhat surprising that even the smallest ramp inclination was sufficient to require significant adaptations to the swing limb trajectory. This can be explained in part by fact that the ramp walking surface was readily identifiable through its contrast to the surrounding floor. Nonetheless, it was anticipated that the transition to lower ramp conditions would perhaps be more of reactive process where individuals simply would maintain more of a normal walking motion with subtle adjustments in limb positioning. The large adjustment of the limb motion to clear the edge of the ramp suggests that crossing onto even the smallest ramps was a very proactive avoidance process. During obstructed walking, it has been well documented that a reduction in the amount of absorption by knee extensors which can also lead to an increased energy generation by knee flexors is a primary strategy used to increase the elevation of the foot [12,14,16,18]. The mechanism used to elevate the foot during early swing onto the ramp was quite similar to that used stepping onto a step or platform. Elevation of the foot during ramp conditions was created by an increased absorption at the knee accompanied by a very large increase in the amount of pull-off power generated by the hip flexors. Similar to stepping onto a step, the manner in which the foot was elevated can be attributed to the necessity of ensuring that the foot moves quickly forward and upward given the new height of the landing location. McFadyen and Carnahan [12], McFadyen and Prince [14], and McFadyen et al. [18] have noted that stepping onto a step, platform, or even over obstacles placed at a further distance with respect to the point where the swing foot leaves the ground will lead to an increase in the hip pull-off burst to allow the limb to reach over the more distant and or elevated landing surfaces. The increased absorption at the knee during ramp conditions would indicate that there was a greater requirement to
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reverse the backward motion of the leg segment (i.e., knee flexion) following toe-off and this accompanied by an increase in the hip flexion would lead to the quick forward and upward movement of the foot. Thus, the resulting toe trajectory while similar to an obstacle clearance task, exhibits a different kinetic strategy that more closely resembles an enhanced level walking strategy or that similar to stepping onto a platform or a set of stairs [12,14]. A somewhat similar strategy, however, has been observed in amputees when traversing an obstacles of different heights [17]. 4.3. Changes during late swing The limb positioning during late stance was found to be quite specific to the particular ramp conditions. While much of this would be expected given the increased height of the walking surface as the foot comes in contact with the ramp, individuals were able to judge and anticipate the geometry of the different sloped surfaces. Evidence for this comes as the amount of joint flexion and elevation of the hip and limb were scaled to the different slopes both prior to and at heel contact. Further support comes from the finding that heel contact velocities did not differ from level walking. This implies that individuals were providing a gentle contact onto the surface and not simply reacting to the foot impacting the ramp. If individuals were not planning appropriately for the ramp then we would have expected to see significant elevations in the contact velocities. However, one limitation is that the contact velocities are with reference to the level walking surface and may not best reflect the velocities that are normal and tangential to the walking surface. A more thorough analysis of contact orientations and velocities would shed more insight for the magnitude of impact and the tangential/slip velocities. Nonetheless, individuals were able to anticipate the body segment orientations for the new walking surface. The control of the positioning of the limb for contact and eventual support of the body on the inclined surface was accomplished primarily through absorptive actions at the hip and knee. Here, the knee flexor muscles served to brake the extension of the knee in a fashion quite similar as that during level walking. The main difference was an earlier phasing of this braking burst and while higher ramp conditions actually exhibited a decreased peak absorption the amount of negative work done was not different. The actions at the hip were more sensitive to the different ramp conditions. The hip extensors during this period were responsible for absorption to slow the rate of hip flexion during the higher ramp conditions while very similar joint moments served to generate energy to promote hip extension at lower ramp angles and level walking. Interestingly, the hip extensor moments observed for all ramp conditions were of similar peak magnitude and exhibited only subtle differences in timing yet the power profiles and work values were quite different. It is not clear if these differences in the action at hip were the result of the subtle timing changes in the application of
hip joint moments or a more coordinated effort amongst the joints of the swing limb with or without the actions of the supporting limb (i.e., via trunk/hip elevation). It was intriguing to note that an increase in absorption at the hip was accompanied by a decrease in the absorption at the knee joint in the two higher ramp conditions while the power generation/absorption for the lower grade ramps were more similar in magnitude to level walking. This seems to imply that stepping onto higher grade ramps requires a different strategy to achieve the correct placement and positioning of the limb for support and propulsion on steeper surfaces. These findings are consistent with those observed during steady state walking on a ramp [4] where muscle activity records which indicate the onset of hamstring activity during swing occurs earlier as the grade of the ramp is increased and similarly the onset of the rectus femoris becomes delayed. These data would support our finding of earlier onsets of the knee flexor and hip extensor moments and indicate that an increased dominance of hamstring action over quadriceps actions at both the knee and hip would explain these results. The reverse would be true for the observed changes in early swing where quadriceps actions would increase the hip pull-off and knee absorption. While muscular activity was not directly monitored in this study, the changes in limb kinetics would indicate that locomotor control system would be required to change both the timing and amplitude of muscle activity in the lower limb to achieve this particular gait modification. Overall the findings of this study provide additional support for the flexibility the control of walking patterns within complex physical environments. The growing need for community accessibility has increased the prevalence of inclined walking surfaces. It is comforting to know that we can so readily modify our level walking to these new surfaces. In this study, the ramp surface was easily identifiable with respect to the surrounding walking surface and thus may have influenced the magnitude of the clearance individuals require as they cross onto the ramp. Further investigation is needed to determine if the proactive strategies adopted in this studies would hold for ramps that are less distinctive visually. This would also be an important factor for those parties designing ramps for community mobility. Additionally, the substantial change in the kinetic actions evident between 6◦ and 9◦ ramp conditions needs to be examined further and would also have implications for the design of community walking surfaces.
5. Conclusion A two-stage anticipatory modification was observed in this experiment as the physical characteristics of the walking surface were changed. Changing the slope of the walking surface altered both the level and orientation of the contact surface. The ability of individuals to scale their limb trajectory in advance to relatively small changes in ramp
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inclination indicated that we are able to not only detect these changes but the locomotor control system appears to be able to manage the mechanics of the limb very effectively with very subtle changes in the timing and magnitude of muscular action. The control system could have employed a more reactive approach where level walking movements or a simple modification of these movements that was not ramp specific were executed and then the sensory information detecting the new surface could have been used to modify subsequent actions. The ramp specific modification further indicate the central nervous systems awareness of limb mechanics and its ability to harness these properties to provide safe and functional locomotor actions.
Acknowledgements This research has been supported by the Department of Kinesiology, the University of Waterloo and Natural Science and Engineering Research Council of Canada. I would also like to acknowledge T. Tomko, M.J. Reid, and J.L. Zettle for their assistance in this experiment.
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