Frontal and sagittal plane analyses of the stair climbing task in healthy adults aged over 40 years: what are the challenges compared to level walking?

Clinical Biomechanics 18 (2003) 950–959 www.elsevier.com/locate/clinbiomech

Frontal and sagittal plane analyses of the stair climbing task in healthy adults aged over 40 years: what are the challenges compared to level walking? S. Nadeau

a,b,*

, B.J. McFadyen

c,d

, F. Malouin

c,d

a

c

Centre de recherche Interdisciplinaire en R eadaptation (CRIR) du Montr eal m etropolitain, site Institut de r eadaptation de Montr eal, (Qc), Canada H3S 2J4 b Ecole de r eadaptation, Universit e de Montr eal, Canada H3C 3J7 Centre Interdisciplinaire de recherche en r eadaptation et int egration sociale (CIRRIS), Institut de r eadaptation en d eficience physique de Qu ebec, Canada G1M 2S8 d D epartement de r eadaptation, Universit e Laval, Canada G1K 7P4 Received 31 July 2002; accepted 21 July 2003

Abstract Objective. This study compared stair climbing and level walking in healthy adults aged over 40 years. Design. Eleven subjects performed at their comfortable speed. Background. The number of parameters studied during stair climbing has been limited, in particular in the frontal plane. Methods. Time–distance parameters and three-dimensional kinematic data were obtained using foot-switches and an Optotrak system. Ground reaction forces were collected with a force platform embedded in the second step of the staircase or in the ground for level walking. Relative angles were calculated using a Cardanic rotation matrix and the net moments and the powers at the ankle, knee and hip joints were estimated with an inverse dynamic approach. Results. A significant longer mean cycle duration and a shorter proportion of time in stance was obtained for stair climbing as compared to level walking. Profiles of the frontal plane joint angles, moments and powers indicated a different action of the hip abductors across tasks to control the pelvis in stance. Profiles of the sagittal plane confirmed the dominant role of the knee extensors during stair climbing but revealed also a knee-hip energy generation pattern that allows the avoidance of the intermediate step. Conclusions. Results suggest environment specific adaptations of the neuro-musculo-skeletal system that should be considered in the rehabilitation of stair climbing in patients. Relevance This study highlights the challenges of stair climbing compared to level walking in a within subject design. Key features of stair climbing that are important for the rehabilitation of step management are also reported. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Step management; Biomechanics; Locomotor tasks; Rehabilitation; Healthy adults

1. Introduction Stair climbing is a demanding locomotor task frequently performed in daily activities. In particular, for elderly persons and those with disabilities of the lower

* Corresponding author. Address: Faculty of medicine, School of Rehabilitation, University of Montreal, C.P. 6128, succursale Centreville, Montreal, Quebec, Canada H3C 3J7. E-mail address: [email protected] (S. Nadeau).

0268-0033/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0268-0033(03)00179-7

limbs, stair climbing, like stepping over an obstacle and rising from a chair, can be very challenging. Being able to negotiate stairs could dramatically improve the quality of life of a person with physical impairments, as well as it could facilitate the work load of the caregivers. Moreover, recovering safe locomotion is often a key factor that allows a patient to return home after a trauma or a disease attack (for review, see Startzell et al., 2000). To date, basic information on the biomechanical requirements of stair climbing as compared to level walking has been mostly restricted to young, fit

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adults or to elderly adults and has concentrated on sagittal plane patterns. More complete information is required for the rehabilitation of the general public. In the past two decades, researchers have begun to study stair walking using simulated staircase set-ups in gait laboratories. Assessment of the stair climbing kinematics in healthy subjects has revealed increased lower limb joint ranges of motion in the sagittal plane for stair climbing as compared to level walking (Andriacchi et al., 1980; Livingston et al., 1991; Laubenthal et al., 1972; Hoffman et al., 1977; Rowe et al., 2000; Riener et al., 2002). At the knee, some studies have reported maximum values ranging from 80° to 100° for typical step configurations (slopes from 30° to 35°), or approximately 12° to 20° more knee flexion than seen in level walking (Andriacchi et al., 1980; Livingston et al., 1991; Riener et al., 2002). Increases in the range of 15– 20° have also been reported in hip flexion during stair climbing (Andriacchi et al., 1980; Livingston et al., 1991). Kinetic and electromyography (EMG) analyses have further specified the muscle groups recruited in stair tasks and level walking. Previous analyses measuring the kinetics of the lower limbs have shown that greater knee moments were required in the stair climbing tasks than in level walking and that the largest increase in the sagittal moment in stair climbing occurs at the knee joint (Andriacchi et al., 1980; McFadyen and Winter, 1988). In ascent, the knee extensor muscles had a dominant role in the progression from one step to the next, assisted by the ankle plantar flexors and the hip extensors (McFadyen and Winter, 1988; Moffet et al., 1993; Townsend et al., 1978; Joseph and Watson, 1967). The result is a high increase of energy generation at the knee as compared to level walking where energy generation is provided mostly by the plantar flexors and the hip flexors and extensors (Winter, 1983; Winter, 1991). Comparison of the percentage of maximal activation level of some lower limb muscles also reveals significantly higher activation of the knee extensor muscles (vastus lateralis and medialis) and medial hamstring muscles during walking up stairs than level walking (Richards et al., 1989). Sagittal plane analyses have been useful in understanding the stair climbing task. However, it gives only a partial picture of information. As in level walking, the stair climbing task also needs to be analysed in other planes of movements. Only a few studies have provided data on lower limbs in the other planes of movement. Andriacchi et al. (1980) have reported the moments in the frontal and transverse planes in a group of ten young male adults (age range: 20–34 years). In general, moments in the transverse plane were small, under 15 N m at the hip, knee and ankle joints, when walking up from step 1 to step 3 on a staircase with a slope of 38°. The magnitude of the lower limb moments in the frontal plane was

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higher than in the transverse plane with the highest values observed in abduction at the hip (37 N m) and knee (33 N m) and in adduction at the ankle (43 N m). Kowalk et al. (1996) examined the magnitude of the knee abduction–adduction moments during stair climbing and descent in 10 normal subjects (age range: 22–40 years). Like Andriacchi et al. (1980), these authors found that the knee moment patterns were exclusively in abduction throughout stance (maximal value: 41.3 N m) during stair climbing. Kowalk et al. (1996) explained this finding by the location of the ground reaction vector that always passed medial to the knee joint centre during stair climbing. Kirkwood et al. (1999) investigated the hip moments during various exercises in a group of 30 healthy subjects (age range: 55–75 years). They found that among all the exercises assessed, only stair climbing generated peak moments higher than those obtained during level walking and only in medial rotation (transverse plane). The maximum hip abduction and extension moments were high (range: 0.77–1.0 N m/kg) but were similar in the two locomotion tasks (stair climbing and level walking), whereas the maximum hip adduction and flexion moments were significantly lower by 61% (0.18 vs. 0.04 N m/kg) and 59% (0.68 vs. 0.28 N m/kg), respectively in stair climbing than in level walking. Data at the knee and at the ankle joints were not presented in the study of Kirkwood et al. (1999). In a recent review on stair negotiation, Startzell et al. (2000) mentioned that much basic research remains to be conducted to determine the key determinants of difficulty and safety on stairs. Moreover, in order to provide rehabilitation professionals with a clearer picture of the specific requirements of the stair climbing task, research must be expanded to include a wider range of age groups that represent the general public, such as including middle-aged healthy persons, a more detailed and comprehensive analysis of frontal plane dynamics, and a better understanding of the different locomotor challenges between stair climbing and level walking. Therefore, our study is a broader attempt to address the question of how stair climbing challenges the lower limbs compared to level walking in healthy adults aged over 40 years. This will be done by providing a comparison of the kinetic and kinematic patterns of stair climbing and level walking in the frontal and sagittal planes of subjects performing at the speed they feel most comfortable (natural speed).

2. Methods 2.1. Subjects A sample of convenience of eleven healthy adults (5 females, 6 males) over 40 years old (median of 53.0

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years, range 41–70 years). Eight participants were middle-aged adults (age 45 and 64 years), one was slightly younger (41 years) and two were older (68 and 70 years). Their mean (SD) stature and body mass were of 168 (8.4) cm and 68.3 (12) kg, respectively. All were living independently at home and were engaged in recreational activity. They were free of any history of injury or disorder of the trunk or lower limbs that would affect their gait. The study was approved by the ethics committee of the Quebec Rehabilitation Institute and all subjects signed a consent form before participating. This study was carried out at the Centre for Interdisciplinary Research in Rehabilitation and Social Integration. 2.2. Staircase set-up The staircase included four steps extended at the top by a 2.44 m walkway to prevent deceleration during stair climbing (Fig. 1). The slope of the stairs was 33° with risers of 17 cm, tread depths of 26.0 cm and a width of 107 cm.Three AMTI force platforms (Advanced Medical Technologies Inc., Newton, MA, USA; Model OR65) were used to record the forces generated during ascent. One platform was imbedded in the floor just in front of the staircase, a second was under the first step and a third one was mounted on a solid frame that served as the second step of the stairs. Each platform was independent of the surrounding wooden pieces to ensure adequate recording of the forces generated on the stairs and was tested for accuracy for force and centre of pressure recordings. 2.3. Level and stair walking assessments In the stair climbing task, subjects stood on ground level in front of the stairs before ascending. All subjects were instructed to place only one foot on each step and to perform at the speed they feel most comfortable

Fig. 1. Staircase set-up used to assess stair climbing.

(natural speed), as if they were out taking a stroll. For each condition (level walking and stair climbing) subjects practised before collecting data until they felt that they performed each task naturally. For technical reasons and to avoid time delays related to the mounting of the staircase, the stair climbing task was always assessed before level walking. The level and stair gait assessments included simultaneous recordings of time–distance, kinematic and kinetic data. The time–distance parameters were recorded from three foot-switches located under each foot at the heel, metatarsal head and first toe. The kinematic data were obtained using the Optotrak system (Northern Digital Inc. Waterloo, Canada) sampled at 75 Hz. The three-dimensional co-ordinates of three non-collinear infrared markers, placed on the feet (lateral heel, dorsum, 5th metatarsal head), legs (lateral malleolus, mid shank, fibula head), thighs (greater trochanter, mid thigh, lateral femoral condyle), pelvis (left and right posterior superior iliac spines, left iliac crest) and trunk (T12 spinous process and right and left at the level of T8) were acquired during the two locomotion tasks. In addition, 12 specific anatomical areas were digitised on the feet (heel, posterior point, mid toe anterior point), the shanks (medial malleolus), thighs (medial femoral condyle), pelvis (right and left anterior superior iliac spines, right iliac crest) and on the trunk (left glenohumeral joint). Kinematic data were filtered with a 4th order Butterworth, zero lag filter, with a cut-off frequency of 6 Hz. Using the analysis package from Mishac Inc. (Mishac Kinetics, Waterloo, Canada), the relative angles were calculated using rotation matrices arranged in a Cardanic (x–y–z rotation) sequence such that the local x, y and z axes corresponded respectively to abduction–adduction, rotation and flexion–extention for the hip and knee joints, and eversion–inversion, rotation, and dorsiflexion, plantar flexion at the ankle joint. The ground reaction forces (kinetic data) were collected at 1000 Hz and later filtered with a 4th order Butterworth, zero lag filter, with a cut-off frequency of 50 Hz. An inverse dynamic approach (Bresler and Frankel, 1950) performed with the Kingait3 software (Mishac Kinetics) was used to estimate the net moments at the ankle, knee and hip joints. Then, the net muscle power at each joint was computed by multiplying the joint angular velocity by the local net muscle moment within each plane of movement. Data from the right lower limb are reported in the present study. The data collected when the subjects were on the second step were used for comparison with level walking data since steady state climbing was reached at that point. The stair gait cycle began at initial contact of the right foot on the second step and ended when the same foot contacted the fourth step. Five trials each of level walking and stair climbing were recorded for each

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participant. Ensemble averages were calculated for the angular displacements, moments and powers at each percent of the gait cycle (Figs. 2–4). Mean peak values of angles, moments and powers were obtained from data before ensemble averaging across subjects. Since the minimum and maximum values are not observed exactly at the same percentage from one subject to another, the peak values can differ slightly from those estimated from the ensemble averages. 2.4. Statistical analysis For both conditions the variables studied included the time–distance parameters (speed, cycle duration, cadence, stride length, percentages of stance, swing and double support), the kinematic and kinetic time-series profiles at the ankle, knee and hip and the peak values of the joint angles, moments and powers in the sagittal and frontal planes of movement. Descriptive statistics were calculated for the time–distance parameters and for selected peak values. Paired t-tests were performed to identify statistically significant differences between data from the two conditions. For each variable studied, before using the parametric test, assumptions relative to the normality of the distribution and the variance homogeneity were tested with Shapiro and Wilk’s W statistic and the Levine-test, respectively. A level of significance of P < 0:017 (P ¼ 0:05=3) adjusted for the number of joints (ankle, knee and hip) involved was chosen for all comparisons except for time–distance

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parameters (P ¼ 0:05). All data were analysed using the SPSS (version 10.0 for Windows) statistical analysis software.

3. Results 3.1. Time–distance parameters Compared to level walking, stair climbing was characterised by a lower cadence, a smaller proportion of stance phase, and a longer cycle duration (Table 1). As was expected, these differences during stair climbing were accompanied by a lower forward speed and a shorter stride length. 3.2. Joint angular displacements Overall, the main differences in the frontal plane movement profiles were observed at the hip joint (Fig. 2). The peak hip abduction was lower than 5° in both tasks and was reached at the same point (just after toeoff). However, the increase was progressive during stair climbing (from 10% of the gait cycle) which contrasted with the rapid abduction found during level walking. A hip adduction followed during stair climbing, whereas the hip remained in a neutral position during level walking. In both tasks, there was very small displacement at the knee in the frontal plane; the only significant difference was an adduction at the beginning of the

Fig. 2. Average (n ¼ 11) angular displacements at the ankle, knee and hip joint for the frontal and sagittal plane of movement during stair climbing (dashed lines) and during level walking (solid thick lines). Thin lines represent ± one standard deviation. The circles and squares indicate the end of the stance phase of the stair and level walking tasks, respectively. Positive values of the Y axis refer to abduction, dorsiflexion or flexion.

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Fig. 3. Average (n ¼ 11) joint moments at the ankle, knee and hip joint for the frontal and sagittal plane of movements in stair climbing (dashed lines) and during level walking (solid thick lines). Thin lines represent ± one standard deviation. The circles and squares indicate the end of the stance phase of the stair and level walking tasks, respectively. Positive values of the Y axis refer to abduction, plantar flexion and extension.

Fig. 4. Average (n ¼ 11) joint powers at the ankle, knee and hip joint for the frontal and sagittal plane of movements in stair climbing (dashed lines) and during level walking (solid thick lines). Thin lines represent ± one standard deviation. The circles and squares indicate the end of the stance phase of the stair and level walking tasks, respectively. The muscle power bursts in the frontal (F) and sagittal (S) plane of movements have been labelled according to Eng and Winter (1995) and McFadyen and Winter (1991). See text for details.

stance phase during stair climbing (Fig. 2). The movement profiles at the ankle had characteristics similar to that found at the hip. In both tasks, the peak ankle abduction at toe-off had similar magnitudes across tasks. Differences were seen, however, at the beginning of the single support phase: while the ankle reached an

adduction peak of 14.3° during stair climbing, it changed from abduction to adduction during level walking to reach a peak of 9° at 40% of gait cycle (Table 2). In the sagittal plane (Fig. 2), the greater differences between the kinematic profiles were observed at the knee and ankle rather than the hip. In early stance, the ankle

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Table 1 Mean (SD) time–distance variables for level walking and stair climbing

*

Variables

Level walking

Stair Climbing

P -value


Speed (m/s) Cadence (step/min) Cycle duration (ms) Stride length (m) Stance phase (%) Swing phase (%) Initial double support phase (%) Terminal double support phase (%) Total double support phase (%)

1.16 (0.10) 105.4 (8.2) 1145 (94) 1.32 (0.05) 63.0 (1.0) 37.0 (1.0) 13.2 (1.2) 12.9 (1.1) 26.1 (2.0)

0.46 (0.07) 93.6 (12.8) 1304 (183) 0.66 (0.09) 60.3 (1.1) 39.7 (1.1) 11.8 (0.9) 12.3 (1.3) 24.9 (2.1)

0.000 0.012 0.013 0.000 0.000 0.000 0.162 0.537 0.252

P -values lower than 0.05 revealed significant differences between level walking and stair climbing.

was already in dorsiflexion during stair climbing and attained a maximal value of about 30° around the transition to single support. During level walking, the ankle gradually went into dorsiflexion after the initial, small plantar flexion angle of 4.8° following foot contact. As in the frontal plane, sagittal plane ankle angles were similar for both locomotion tasks around toe-off, with peak plantar flexion angles of 9.4°. During the swing phase, the ankle dorsiflexion was greater during stair than level walking. Notably, in stair climbing, a large flexion at the knee was observed at the beginning of the stance phase and at the middle of the swing phase with peak values reaching 65° at foot contact and 93° at 81% of gait cycle. During level walking, these values were 1° at foot contact and 67° at 73% of gait cycle. Although hip profiles were similar between level and stair gait, a more flexed position at the hip was seen across the entire gait cycle in stair climbing. 3.3. Net joint moments In the frontal plane, the mean net joint moment profiles were very similar for the two tasks, particularly at the knee and hip joints (Fig. 3). However, the magnitude was different: the first peak abduction moment at the knee (see Table 2) was higher and the second peak abduction moment at the hip at 50% of the gait cycle (not shown in Table 2) was lower in stair climbing when compared to level walking. The adduction moment, observed at the transition from the stance phase to the swing phase at the knee and at the hip was very low (mean inferior to 0.1 N m/kg) and therefore are not reported in Table 2. The mean ankle net joint moment was low at the ankle and remained relatively neutral in stair climbing, but the overall pattern was variable for both locomotion tasks. In contrast to the frontal plane, more differences in muscle moments were seen in the sagittal plane at all three joints between the two tasks (Fig. 3). In stair climbing, an ankle plantar flexion moment was observed immediately following foot contact and during weight acceptance (loading response), as compared to a dorsiflexion moment in level walking. However, as during

level walking, the peak plantar flexion moment was observed at the end of the stance phase (49% of the gait cycle). The value was slightly lower, (1.17 N m/kg) on average, than during level walking (1.39 N m/kg). During early stance phase at the knee, the value of the net joint moment was very high in stair climbing. This value was more than twice the value observed in level walking at a similar point in the gait cycle (15%; Table 2). At the transition from stance to swing, contrary to level walking, a flexion moment was observed between 50% and 70% of the gait cycle. At the hip, in early stance, the extension moments were very similar for the two tasks. However, the extension moment lasted longer during the stair climbing task. In addition, the flexion moment in stair climbing never reached the high values observed during level walking at approximately 55% of the gait cycle. Moreover, it should be noted that a second burst of flexion moment appeared in the initial part of the swing phase (about 75% of the gait cycle) in stair climbing. 3.4. Net joint powers Comparison of the power curves clearly revealed that stair climbing was characterised by more positive power than level walking particularly at the hip joint in the frontal plane and at the ankle and knee joints in the sagittal plane of motion (Fig. 4). The muscle power bursts have been labelled according to Eng and Winter (1995). In the frontal plane, considerable power generation was produced by the abductor muscles at the hip in stance during stair climbing (H2-F), with the mean peak value at 21% of the gait cycle (Table 2). Despite similar moment patterns, the hip power curve in stair climbing differed dramatically from level walking. In contrast to stair climbing. both absorption (H1-F) and generation (H2-F and H3-F) of energy were seen at the hip in the frontal plane during level walking. Both the ankle and knee power patterns remained variable throughout the stance phase in the frontal plane. In the sagittal plane of motion, almost only positive powers (generation of energy) were employed during the

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Table 2 Mean peak values of angles, moments and powers and time of occurrence (% of gait cycle) at the hip, knee and ankle joints Variables Angle (degrees) Hip Abduction Adduction Flexion Extension Knee Abduction Adduction Flexion Extension Ankle Abduction Adduction Dorsiflexion Plantar flexion Moment (N m/kg) Hip Abduction Flexion Extension Knee Abduction Flexion Extension Ankleb Dorsiflexion Plantar flexion Power (W/kg) Hipb Abductors Flexors Extensors Kneeb Abductors Flexors Extensors Ankleb Dorsiflexors Plantar flexors

Level walking

P -valuea

Stair climbing

Mean

(SD)

%

Mean

(SD)

%

2.4 9.0 30.8 15.5

(5.2) (4.0) (5.6) (4.5)

73 46 89 52

4.1 9.7 60.1 )4.7

(4.3) (5.5) (5.6) (6.5)

68 10 91 52

0.050 0.511 0.000 0.000

5.8 4.6 67.0 1.1

(6.8) (4.1) (3.1) (5.0)

54 60 73 98

5.0 10.4 93.1 )10.0

(7.0) (7.1) (3.1) (2.7)

65 39 81 52

0.471 0.015 0.000 0.000

11.0 9.0 19.1 9.8

(5.0) (3.5) (3.0) (4.8)

64 40 49 50

12.2 14.3 29.8 9.4

(4.9) (3.6) (2.8) (6.0)

62 18 18 62

0.327 0.000 0.000 0.717

1.02 0.71 0.68

(0.23) (0.18) (0.13)

48 53 12.5

0.99 0.28 0.53

(0.15) (0.13) (0.17)

18 62 7

0.044 0.000 0.006

0.61 0.25 0.46

(0.13) (0.13) (0.15)

15 44 15

0.78 0.24 0.98

(0.16) (0.07) (0.18)

15 52 12

0.002 0.681 0.000

0.25 1.39

(0.04) (0.07)

6 49

1.17

(0.14)

48

0.001

0.62 (+) 1.17 (+) 0.61 (+)

(0.29) (0.27) (0.32)

54 61 10

0.71 (+) 0.58 (+) 1.01 (+)

(0.24) (0.14) (0.48)

21 71 7

0.441 0.000 0.019

0.38 ()) 0.87 ()) 0.60 ())

(0.22) (0.22) (0.28)

9 60 10

0.26 (+) 0.74 (+) 1.79 (+)

(0.30) (0.22) (0.50)

18 64 18

0.000 0.000 0.000

0.47 ()) 2.66 (+)

(0.09) (0.36)

4 56

0.40 ()) 2.53 (+)

(0.27) (0.52)

5 53

0.441 0.594

a P -values lower than 0.017 (adjusted probability values) revealed significant differences in maximal magnitude between level walking and stair climbing. b Peak values with magnitude less than 0.1 N m/kg for the moment and less than 0.1 W/kg for the power are not included.

stair climbing task. At the knee, both extensors and flexors contributed to the energy generation, as opposed to level walking where those muscles mainly absorbed energy in order to decelerate the segments. During the transition from stance to swing, a knee flexor generation pattern was observed during stair climbing (labelled K5 in obstructed walking McFadyen and Winter, 1991). This last burst of positive power completely differed from the knee extensor absorption pattern observed during level walking (K3). At the hip, the mean peak positive power (H1-S) by the extensor muscles at the beginning of the stance phase (around 20%) was similar to the one observed in level walking but lasted longer. Interestingly, for stair climbing, the power absorption

by the hip flexors (H2-S) was extremely reduced and the burst of energy generation during the swing phase (H3S) occurred later in the cycle. The magnitude of the H3S burst was almost two times lower in the stair task than during level walking (Table 2). This burst of energy generation by the hip flexors also appeared later than the one (K5) observed at the knee. As seen for level walking, a small quantity of energy was absorbed by the knee flexors at the end of the swing phase (at 94% of the gait cycle). At the ankle, the absorption by the ankle plantar flexors (A1) seen during level walking during the stance phase was partly eliminated in stair climbing whereas power generation reached a similar maximal value late in stance (Table 2).

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4. Discussion The objective of this paper was to provide a detailed description of the patterns, particularly within the frontal plane, of healthy persons aged over 40 years for stair climbing and to compare the general patterns with those of level walking. Results of the present study revealed interesting differences between level walking and stair climbing. Although other studies have provided such information, many of the present results have not been previously highlighted and are important to know when dealing with patient populations with decreased locomotor capacity. Regarding the time–distance parameters, significant differences were observed between the two locomotion tasks. Time–distance parameters in stair climbing were comparable to those of James and Parker (1989) obtained for a group of 10 elderly subjects (age range: 76– 83 years). They reported a mean (SD) cycle duration of 1.33 (0.15) s, a cadence of 92 (10.37) steps/min and a proportion of time in stance of 60% for a staircase with a slope of 38°. In the present study, when the time proportion spent in stance phase in both tasks was compared, we found that toe-off occurred significantly earlier by 3% in stair climbing than in level walking resulting in a longer swing phase proportion. These results are in agreement with those of Livingston et al. (1991). One explanation for this prolonged swing phase might come from the requirement to place the foot on the next step while avoiding hitting the intermediate step. However, there must be other factors as well because our results did not corroborate those of a recent study of Riener et al. (2002) that found the opposite timing. As in level walking, factors like speed of stair climbing probably need to be considered. Moreover, parameters, such as velocity, cadence and stride length are partly determined by the staircase characteristics (Livingston et al., 1991; Riener et al., 2002). Consequently, one should expect having more or less differences between the stair climbing parameters and those observed during level walking according to the stair design. In both level walking and stair climbing, the maximal magnitudes of movement in the frontal plane were small at all three joints; less than 10° at the hip and knee and less than 15° at the ankle in comparison to the maximum magnitudes observed at these joints in the sagittal plane. The profiles were different between tasks early in stance at the ankle and knee and throughout the gait cycle at the hip. In stair climbing, as in level walking, the hip abductor muscles control the lateral pelvic obliquity in order to allow the contralateral leg to swing properly. The magnitude of the abduction moment during stair climbing was similar to the values reported by Kirkwood et al. (1999) for a group of healthy subjects (age range: 55–75 years). Contrary to level walking, where

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the hip stayed in an adducted position in stance, stair climbing was characterised by a concentric action of the abductor muscles that raised the pelvis on the contralateral side. This lateral elevation of the pelvis most likely helps the swinging leg to avoid the intermediate step. In level walking, energy was firstly absorbed by the hip abductors and then followed by a burst of energy generation just before toe-off. Therefore, the same muscles are used but with different functions in each task. Considering the high level of net moment and power at the hip, future studies must provide additional data on healthy and pathological subjects in this plane of movement. Notably, these studies should provide information on how the stair climbing kinematics and kinetics are modified in subjects having different physical impairments. The stair climbing analysis in the frontal plane suggests that before re-training patients to manage steps one will need to assess if the hip abductors are able to sufficiently raise the pelvis and the contralateral limb. Even though there is no biomechanical study on this aspect, it is believed that patients with a weak abductor muscle group, for example those with a hip arthroplasty (Perron et al., 1998), could particularly have difficulty with this specific requirement of stair climbing. In addition, as stair climbing probably produces a higher stress on the lateral structures of the knee, indicated by higher abduction moments in the first part of the stance phase as compared to level walking, stair climbing training could be difficult or impossible in some knee instability disorders. The presently reported patterns and magnitudes of the knee joint moment during stair climbing in the frontal plane were similar to the findings reported by Kowalk et al. (1996). Finally, at the ankle, the challenge of stair climbing is not very different from that of level walking. Stair climbing induced major changes in the sagittal angular displacement patterns usually reported at the lower limbs during level walking. In comparison to level walking, more flexed attitudes of the lower limb were observed at the beginning of the stair climbing cycle (foot strike) and less extension at the hip was observed at toe-off. These observations reflected specific adaptations to the staircase environment. The knee and hip need to be flexed at foot contact to place the leg on the step. At the end of the stance phase of stair climbing, the hips do not need to extend as much as in level gait because the contralateral step length is reduced by the geometry of the staircase, as opposed to level walking where there is no such constraint. As supported by previous studies (Livingston et al., 1991; Riener et al., 2002), these differences in the range of motion between stair climbing and level walking may depend of the staircase configuration and subject characteristics. The angular displacements at the hip and knee were similar to those reported in studies using two-dimensional analyses (McFadyen and Winter, 1988; Moffet et al., 1993;

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Winter, 1991; Powers et al., 1997). Consequently, mild restrictions of range of motion in flexion at the knee such as those observed in subjects having knee osteoarthritis (Startzell et al., 2000; Collopy et al., 1977) can dramatically reduce the functional performance for climbing stairs. Some of these subjects might need to use a one step gait, placing both feet on each step, to successfully go up in the staircase. The results of this study also revealed that stair climbing required a large dorsiflexion magnitude and thus people with a foot drop problem or a restricted mobility in dorsiflexion at the ankle might need to compensate by using more knee and hip flexion to clear the intermediate step during the swing phase of the stair climbing, as well as greater hip abduction. Those unable to compensate adequately might present a higher risk to fall in ascending stair. Walking up the stairs is characterised by large moments and powers produced in the sagittal plane as previously shown (McFadyen and Winter, 1988; Joseph and Watson, 1967; James and Parker, 1989). A considerable portion of these moments and powers are required to support and propel the body against gravity and to generate movements that advance the body forward in the plane of progression like in level walking (Eng and Winter, 1995). In addition, stair climbing required raising the body while progressing to the next step; a main task that is very demanding for the lower limb muscles as shown by the large increase in the moments and powers in stair climbing in comparison to level walking. Mechanically, this essential task is performed by the extensor muscles of the lower limb (McFadyen and Winter, 1988). Particularly, the knee extension moment is doubled in comparison to level walking. This strong action of the knee extensors in stair climbing was also shown by EMG studies (Joseph and Watson, 1967; James and Parker, 1989) that revealed high and prolonged activities of the vastus medialis and rectus femoris during the first part of the stance phase (0–20%). Consequently, clinicians must be aware that, according to our data, stair climbing without arm rails might require values of extensor strength as high as 1.0 N m/kg to be executed normally. In the sagittal plane, the moment and power profiles of our group of subjects were almost identical to those previously reported (McFadyen and Winter, 1988; Kowalk et al., 1996; Duncan et al., 1997). The knee flexors in stair climbing have received little attention. Results of the present study revealed an important role of the knee flexors during stair climbing that has not been discussed related to the energy to avoid the intermediate step of the stairs. In addition to the hip adductor assistance for step avoidance mentioned above, the present work also confirms that stair climbing, like going over obstacles, requires a reorganisation of lower limb to a knee flexor strategy. From

the work of McFadyen and Winter (1991), it was clearly demonstrated that obstructed walking results in less absorption by the knee extensors followed by a novel burst of energy generation by the knee flexors at the transition from stance to swing (labelled K5 in obstructed walking McFadyen and Winter, 1991). This reorganisation of the gait patterns during obstructed walking also decreased or delayed energy generation by the hip flexor muscles during the swing phase of the trail limb (Niang and McFadyen, 2001). In stair climbing, the knee-hip coordination was similar to obstructed level walking. The burst of energy produced by the hip flexors (H3) was delayed in order to allow the knee flexors to generate sufficient energy to clear the intermediate step (K2) and the absorption of energy by the knee flexors was only seen late in swing (from 80% to 100% of the gait cycle). This may allow the leg to swing earlier than during level walking. Consequently, in comparison to level walking, walking up the stairs also required avoiding an obstacle, the intermediate step. In some patients having motor control problems or weaknesses at the lower limbs, an inadequate interplay between K3, K5 and H3 might result in an important difficulty to manage stair climbing. Facing this problem, rehabilitation professionals will have to provide exercises that will help the patients to compensate for, or to reestablish, this strategy of energy generation and absorption at the lower limbs. A detailed analysis of the stair climbing tasks in a group of chronic hemiparetic subjects, actually in process at our laboratory, will provide information on this specific aspect in this population. 4.1. Limits and future study This study presents two limits that are closely associated with the complexity of studying the biomechanics of functional activities. First, the tasks were not randomised across subjects. All participants began with the stair climbing task. This decision was taken in order to avoid time delays related to the mounting of the staircase when the subjects were in the laboratory. Since our subjects were healthy and active persons, we believed that their level walking patterns were not affected by factors such as fatigue. Moreover, to ensure that natural performance was assessed, subjects practised before collecting data until they felt that they performed each task naturally. Second, the small sample size limits the generalisation of the results. Only 11 healthy subjects participated in the study. This number can be judged, with reason, to be too small to generalise the results to the general population of adults over 40 years. This is often a limit of biomechanical studies involving three-dimensional kinematic and kinetic analyses at several joints in which, the amount of data computed is considerable and time-consuming. Consequently, we believed that future study is needed to

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confirm the different kinematic and kinetic patterns found in the present study between stair climbing and level walking.

5. Conclusion The stair climbing analyses revealed that substantial amounts of effort are required in the frontal plane mainly at the hip to control the pelvis. In comparison to level walking, walking up a set of stairs in healthy adults aged over 40 years required a reorganisation of the lower limb muscular strategies in order to respond to additional mechanical requirements such as rising the body to the next step and avoiding the intermediate step. Therefore there were major differences in patterns between level and stair walking concerning the knee flexors and extensors, hip abductors, and the magnitude of dorsiflexion at the ankle during the swing phase. These findings should particularly be considered in the rehabilitation of stair and level walking. Additional studies on stair climbing including comparisons of the joint kinematic and kinetic patterns across different speeds of stair walking and between young, middle aged and elderly adults will further allow a better understanding of stair walking patterns in healthy individuals.

Acknowledgements This research was carried out within the laboratory of Dr. McFadyen and received financial support from Natural Sciences and Engineering Research Council of Canada. Dr. Nadeau is supported by a the Canadian Institute of Health Research (CIHR).

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