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Avoidance and Accommodation of Surface Height Changes by Healthy, Community-Dwelling, Young, and Elderly Men

^a CIRRIS, Québec Rehabilitation Institute and Department of Rehabilitation, Laval University, Québec, Canada
^b Départment of Kinesiology, University of Montréal, and Sherbrooke University Geriatric Institute, Canada

Bradford J. McFadyen, Centre for Interdisciplinary Research in Rehabilitation and Social Integration, Quebec Rehabilitation Institute, 525 Hamel, Québec, G1M 2S8 Canada E-mail: brad.mcfadyen{at}rea.ulaval.ca.

Decision Editor: John A. Faulkner, PhD

	Abstract

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The purpose of this study was to compare avoidance and accommodation strategies during gait between healthy, community-dwelling, young and elderly males. Ten young (28.4 ± 5.4 years) and ten elderly (69.5 ± 6.1 years) males were tested while walking on the level, avoiding a 11.75-cm-high obstacle, and accommodating a change in floor height of 11.75 cm. Three dimensional, kinematic, and kinetic analyses were performed bilaterally. Both age groups used the same general sagittal plane strategies in order to avoid and accommodate the obstructions, but the elderly group had significantly reduced lead toe clearances. These riskier toe clearances by the elderly group were found to be due to limited, frontal plane pelvic motion, shorter stride lengths, and subtle differences in the interplay of lower limb energetic patterns within the sagittal plane. These results are discussed with respect to diminishing physical capacity in the elderly populations.

ANTICIPATORY locomotor adjustments are crucial for safe walking. They can be broadly divided into avoidance strategies for stepping over objects and accommodation strategies for changing surfaces ⁽¹⁾. In young adults, obstacle avoidance always involves an active knee flexor strategy to increase both knee and hip flexion ^{(1)(2)(3)(4)(5)}, but stepping up to a new surface level with the leading limb mostly involves the exploitation of the level gait hip pull-off power ⁽¹⁾. Therefore, different gait strategies are used by young adults to lift the foot over versus onto a given surface height change (Fig. 1).

View larger version (17K): [in this window] [in a new window]   Figure 1. A graphical summary of the lower limb strategies found for unobstructed (A), obstacle avoidance (B), and height accommodation (C) in normal young adults at toe-off. Arrows represent the predominant muscle group at the hip and knee joints. Solid and dashed arrows represent power generation and absorption, respectively, by the muscle group. Also shown are the associated power burst names within the sagittal (D) and frontal (E) planes. K5S = knee flexor generation; K3S = knee extensor absorption; H3S = hip flexor generation; A2S = ankle push-off power; VHP = vertical power for raising the ipsilateral hip; H1F = hip abductor absorption power; A3F = ankle evertor absorptions; A1F = ankle invertor absorption; H3F = hip abductor generation.

Only one recent study has considered accommodation behavior in the elderly population, comparing young with elderly women ⁽¹⁶⁾. The results for stepping onto a 15-cm platform showed that the elderly women placed the trail limb at a greater distance from the platform as a percentage of stride length. The older group also showed lower heel clearances with the lead limb, but there were no differences for the trail limb. Finally, the lead heel following clearance was placed very close to the platform edge for the elderly groups.

In summary, although information is still very limited, elderly adults appear to be capable of exploiting intersegmental strategies for obstacle avoidance, but they show some differences from young adults. Surface height accommodation data are even more limited, and to our knowledge no information exists on the motor strategies used by the elderly population. This lack of information, along with conflicting spatial–temporal results between studies looking at obstacle avoidance, demands a more comprehensive study of anticipatory locomotor behavior differences between young and older adults. The purpose of the present work was to provide such details, adding new information related to frontal plane control and height accommodation behavior for elderly versus young male adults. It was expected that both young and elderly subjects would use similar avoidance and accommodation strategies, but that the role for particular lower limb joints would not be the same. Given that gait stability and muscle strength are known to decrease with age, it was hypothesized that elderly subjects would differ from younger subjects in their medial–lateral postural control and the relative contribution from hip abductors thought to aid in the hiking of the limb.

	Methods

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Ten young (age 28.4 ± 5.4 years; mean leg length 0.865 ± 0.052 m) and 10 elderly (age 69.5 ± 6.1 years; mean leg length 0.815 ± 0.037 m) adult males with no neurological or musculoskeletal problems provided signed consent to participate in the study. Young subjects were recruited from the community through the Quebec Rehabilitation Institute. Community-dwelling elderly subjects were chosen from a recent database at the Sherbrooke University Geriatric Institute. Elderly subjects were given additional physical exams by an occupational therapist (including foot vibration sensitivity and Ostwestry back-pain questionnaire) and a blood test by a nurse to rule out diabetic problems. Vision for all subjects was also confirmed to be normal, or corrected to normal, using the Snellen vision test. Ethical approval was received from both respective institutions.

All subjects were tested during three conditions of unobstructed walking, obstacle avoidance, and platform accommodation (Fig. 1) at natural walking speed. Both obstructions were 122 cm wide and 11.75 cm high. This height mimicked daily environmental constraints and was not too strenuous as to exaggerate differences between age groups, but it still ensured the use of anticipatory locomotor adjustments. The obstacle was 5 cm deep, and the platform was 366 cm long. Both age groups used the same obstructions and received the same instructions. Conditions were presented in blocks and counterbalanced across subjects, and six trials were recorded per subject.

The Optotrak system (Model 3020; Northern Digital, Inc., Waterloo, Ontario, Canada) was used to record the three-dimensional coordinates from three noncollinear infrared markers placed on the pelvis, trunk, and on each foot, leg, and thigh. The anterior end of the shoe was also digitized to calculate the displacement of the toe over time, and the heel was digitized to calculate stride length. Velocity was calculated by stride length divided by the time. Two force platforms (Advanced Mechanical Technology, Inc., Watertown, MA) recorded ground reaction forces applied by the leading and trailing limbs just before the obstructions. Link-segment analyses provided estimations of joint reaction forces and net muscle moments of force. Net muscle power was calculated as the product of the moment of force and relative joint angular velocity data. Vertical power at the hip joint was calculated as the product of the vertical reaction force and the vertical hip velocity.

Time series data were normalized to 100% of stride beginning with heel contact for each limb. Frontal plane knee data were not analyzed because of the very limited movement contribution at this joint. A repeated measures analysis of variance was used to test for main effects of age and obstruction condition for spatiotemporal data (maximum toe clearance, proximities of the foot to the obstruction, average gait velocity, stride length, and cadence) and specific power bursts associated with anticipatory locomotor adjustments (see Fig. 1). Significance was set to p < .05.

	Results

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The elderly group walked slower, F(1,17) = 16.76, p < .01, than the younger subjects (Table 1 ), which was due, in part, to a decreased stride length; F(1,17) = 9.19, p < .05. As well, the estimated cadence was observed to be lower for the elderly group (Table 1 ). Neither age group significantly changed their overall gait speed during obstacle clearance or platform accommodation, and no subject made contact with the obstacle or the platform face during testing.

Both age groups also presented the same trend across conditions and between limbs for toe clearance (Fig. 2). The obstacle was cleared by a higher margin than the platform [lead, F(1,17) = 119.8488, p < .01; trail, F(1,17) = 32.8153, p < .01], and the lead limb cleared both obstructions at a higher height than the trail limb [young, F(18,1) = 31.3265, p < .01; old, F(18,1) = 31.2395, p < .01]. Despite these same trends, clearance heights were lower for the elderly subjects as compared with the young adults for the lead limb [F(1,17) = 9.08, p < .01], but there were no differences for the trail limb. For foot proximity, there were no age or obstruction effects for the placement of the trail foot before the obstacle (Fig. 2). Elderly subjects, however, made contact closer [F(1,17) = 9.97, p < .01] to the obstacle or platform edge following clearance than younger subjects, and both groups placed the lead heel farther from the obstacle than from the platform edge following clearance [F(1,17) = 10.93, p < .01].

View larger version (22K): [in this window] [in a new window]   Figure 2. Mean toe clearances (A) across the obstacle (OBS) and platform (PLAT) conditions for the young () and elderly () subjects. Proximities of the trail toe before the obstruction and the lead heel at contact after clearance of the obstruction are also shown (B). Bars above the graph indicated significant differences.

View larger version (34K): [in this window] [in a new window]   Figure 3. Mean joint angular displacements for the lead (top) and trail (bottom) lower limbs within the sagittal (left) and frontal (right) planes. Data are presented across the young (thick curves) and elderly (thin curves) subjects for the unobstructed (solid line), obstacle (dashed curve), and platform (dotted curve) conditions. Positive slopes indicate hip and knee flexion and ankle dorsiflexion in the sagittal plane, and hip adduction and ankle inversion in the frontal plane. Main effects are shown for age () and for environment (). TO = toe-off; HC = heel contact.

View larger version (39K): [in this window] [in a new window]   Figure 4. Mean net muscle moments of force (left) and net muscle powers (right) in the sagittal plane for the lead (top) and trail (bottom) lower limb joints. Data are presented across the young (thick curves) and elderly (thin curves) subjects for the unobstructed (solid curve), obstacle (dashed curve), and platform (dotted curve) conditions. Main effects are shown for age () and for environment (). Positive power indicates generation and negative power indicates absorption. H1S = hip extensor generation; H3S = hip flexor generation; K1S = knee extensor absorption; K5S = knee flexor generation; K3S = knee extensor absorption; K4S = knee flexor absorption; A2S = ankle push-off power; TO = toe-off; HC = heel contact.

Within the frontal plane (Fig. 5), the elderly group showed less abductor absorption powers [H1F; lead, F(1,17) = 13.74, p < .01; trail, F(1,17) = 16.70, p < .01] and greater minimum abductor moments [lead, F(1,17) = 5.10, p < .05; trail, F(1,17) = 16.92, p < .001]. The elderly group also had a lower lead ankle eversion moment [F(1,17) = 5.81, p < .05], and related power absorption [A3F; F(1,17) = 14.34, p < .01] in late stance. During midstance, the young group produced greater trail ankle inversion moments [F(1,17) = 6.78, p < .05]. There were condition effects for the trail limb only for the first hip moment peak [F(2,34) = 12.24, p < .01] and minimum [F(2,34) = 16.12, p < .001] and their corresponding powers. In the latter part of stance, condition effects were observed for the muscle power absorption burst [F(2,34) = 3.61, p < .05]. Finally, the vertical power for raising the ipsilateral hip (VHP; see Table 3 ) was used similarly between age groups, but the elderly group tended to have lower values and appeared less variable. In general, both obstructions required more "hip hiking" than unobstructed gait [lead, F(2,34) = 60.1455, p < .01; trail, F(2,34) = 76.2364, p < .01].

View larger version (31K): [in this window] [in a new window]   Figure 5. Mean net muscle moments of force (right) and net muscle powers (left) in the frontal plane for the lead (top) and trail (bottom) lower limb joints. Data are presented across the young (thick curves) and elderly (thin curves) subjects for the unobstructed (solid curve), obstacle (dashed curve), and platform (dotted curve) conditions. Main effects are shown for age () and for environment (). Positive power indicates generation and negative power indicates absorption. H1F = hip abductor absorption; H2F = hip abductor generation; H3F = hip abductor generation; A1F = ankle inverter absorption; A2F = ankle evertor generation; A3F = ankle evertor absorption; TO = toe-off; HC = heel contact.

	Discussion

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Obstacle Avoidance
Chen and colleagues ⁽¹³⁾ showed a tendency (nonsignificant) for elderly subjects to have lower foot clearances than younger subjects. Despite this observation, the authors stated that the elderly subjects used a more cautious avoidance gait based on slower approach and crossing speeds, shorter stride lengths, and the tendency for placing the trail foot farther from the obstacle. The latter point in particular was explained as a risk reduction strategy that exploits toe lift occurring during the late swing phase. In the present study, the elderly subjects did not even tend toward placing their trail foot any further back from the obstruction as compared with the younger subjects, which could mean that the elderly group did not exploit the higher point of the foot trajectory during clearance as suggested by Chen and colleagues. This, therefore, could be part of the explanation for why Chen and colleagues only tended to see lower clearance in the elderly group, whereas the present work found it to be significantly different.

Patla and colleagues ⁽¹⁵⁾ suggested that elderly subjects used greater vertical hip energy (hiking) to increase toe clearances. In the present study, however, vertical movement of the hips (indicated by the frontal plane displacement of the pelvis) was much more constrained in the elderly men (Table 2 ), and the vertical energy at the hip actually tended to be lower, with significantly lower contralateral abductor power, in the elderly group. Such constrained pelvic movement by the older subjects is probably a main reason for the decreased toe clearances presently observed. Given that muscle strength decreases with age (cf. ⁽¹⁷⁾), this lack of exploitation of hip abduction in the elderly group appears more in line with what might be expected during this task. The reason for different results from other studies may be related to subject fitness levels.

Gait speed, per se, cannot be simply used to explain the toe clearance height differences observed between the age groups. Although slower walking speeds by the elderly group probably resulted in the decreased early stance to midstance moments, as well as lower ankle push-off moments and powers, their gait speed was by no means slow. Both age groups exploited the same intersegmental coupling effects and showed the same effects on toe velocities across conditions. Other, less obvious causes of decreased toe clearances in the elderly group are probably related to the combined nonsignificant differences in anticipatory locomotor adjustments between the age groups. The hip flexor power burst around toe-off (H3S) tended to be higher for the elderly subjects for unobstructed gait and for the trail limb during obstacle clearance. The knee extensor absorption burst at toe-off also always tended to be higher for the elderly group, and would affect the timing of the emergence of the knee flexor strategy (K5S) that tended to be smaller for the elderly group for the leading limb. These tendencies, although not individually statistically significant, will have an important combined effect on constraining toe clearances in addition to the significant frontal plane differences.

Height Accommodation
Only one other recent study ⁽¹⁶⁾ has looked at the transitional behavior in the elderly population for stepping up to a new height. Although the study compared young and elderly women, the same foot clearance behavior for the lead and trail limbs was shown as in the present study in terms of no differences between age groups for trail foot placement prior to climbing and a closer placement of the lead foot following clearance. However, the male subjects of the present study placed their foot approximately three times further away from the edge than the female subjects in the previous study ⁽¹⁶⁾. This particular point may simply be due to shorter stride lengths produced by women.

In terms of accommodation kinetic and energetic strategies, both age groups maintained the hip flexor strategy from unobstructed gait as has been previously discussed in the literature ⁽¹⁾. Trail limb accommodation strategies, which have not been previously presented, showed that a knee flexor strategy is also used. Such behavior would be necessary in order to avoid the edge of the platform. Therefore, mounting the platform with the trailing foot becomes a superimposed task of the accommodation hip strategy and the obstacle avoidance knee strategy similar to what has been already shown for the lead limb when a compound task is performed involving an obstacle placed before a surface level change ⁽¹⁾.

General Anticipatory Locomotor Adjustment Behavior
Because the elderly subjects of the present study were community dwelling, it is not surprising that they retained their ability to negotiate obstacles and platforms. However, even for these healthy independent walkers, age differences in locomotor strategies were seen. It is well documented that elderly people have decreased muscle strength as compared with that of young adults (cf. ⁽¹⁷⁾). Given that more energy is needed to lift the foot to a higher height, one can understand the lower toe clearances observed for both obstructed environments. However, the present study suggests that the decreased strength hypothesis may be most important for frontal plane patterns, particularly associated with hip abduction. Differences in stride lengths and the closer placement of the heel to the edge of the obstruction following clearance also appear to be important factors.

The generalization of the present results are limited by the number of subjects used and the use of independent, male subjects around the age of 70 years. Older or more sedentary subjects have to be tested separately. Although it is expected that community-dwelling elderly women will show similar patterns, as shown for spatiotemporal data ⁽¹⁶⁾, gender differences should also be further considered.

Overall, the present work showed that anticipatory locomotor adjustments are maintained by healthy, elderly males. However, they showed lower, and riskier, foot trajectories, which were found to be directly related to differences in frontal plane control at the hip, decreased stride lengths, and subtle differences in the interplay of lower limb energetic patterns in the sagittal plane. The present results are important for the assessment of the locomotor capacity of elderly adults affected by physical impairments.

	Acknowledgments

 
This work received partial financial support from the Natural Sciences and Engineering Research Council of Canada, as well as the Réseau Provincial en Adaptation-Réadaptation of the Fonds de la Recherche en Santé du Québec (FRSQ). F. Prince is an FRSQ reseach scholar.

We acknowledge the technical support of Mr. Guy St-Vincent, Mr. Michel Raîche, Mr. François Comeau, Mr. Jean-François Michaud, Mr. Yves Roy, and Ms. Caroline Gangnon.

Received March 26, 2001

Accepted December 13, 2001

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