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Swing phase mechanics of healthy young and elderly men
Human Movement Science 20 (2001) 427±446
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Swing phase mechanics of healthy young and elderly men Peter M. Mills *, Rod S. Barrett Biomechanics-Dynamics Group, School of Physiotherapy and Exercise Science, Grith University, Gold Coast, Qld, Australia
Abstract This study examined the eect of ageing on the swing phase mechanics of young and elderly gait. Sagittal plane marker trajectories and force plate data were collected while 10 young (24:9 0:9 years) and eight elderly (68:9 0:4 years) subjects walked at their preferred walking speeds. Comparison between young and elderly gait was made for a range of spatial± temporal, kinematic and kinetic variables with emphasis given to identifying possible dierences at toe-o, minimum metatarsal±phalangeal joint clearance and heel contact. In order to control for the confounding eect of gait velocity on the dependent variables, a multivariate analysis of covariance was used to identify dierences between the young and elderly subjects due to age. In contrast to studies that have reported lower preferred walking speeds in the elderly compared to the young [J.O. Judge, R.B. Davis III, S. Ounpuu, Step length reductions in advanced age: the role of ankle and hip kinetics, Journal of Gerontology: Medical Sciences 51 (1996) M303±312; D.C. Kerrigan, M.K. Todd, U. Della Croce, L.A. Lipsitz, J.J. Collins, Biomechanical gait alterations independent of speed in the healthy elderly: evidence for speci®c limiting impairments, Archives of Physical and Medical Rehabilitation 79 (1998) 317± 322], no dierences in walking speed nor in the spatial±temporal variables that determine walking speed were detected. The elderly were however, found to have a greater hip extension moment at the time of minimum metatarsal±phalangeal joint clearance, and a signi®cantly higher anterior±posterior velocity heel contact velocity that was linked to a signi®cantly higher shank and foot angular velocity at heel contact. Since many gait variables are highly correlated with walking speed [C. Kirtley, M.W. Whittle, R.J. Jeerson, In¯uence of walking speed on
*
Corresponding author. Tel.: +61-7-5552-8357; fax: +61-7-5552-8674. E-mail address: [email protected] (P.M. Mills).
0167-9457/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 9 4 5 7 ( 0 1 ) 0 0 0 6 1 - 6
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gait parameters, Journal of Biomechanical Engineering 7 (1985) 282±288; D.A. Winter, Biomechanical motor patterns in normal walking, Journal of Motor Behaviour 15 (1983) 302± 330], dierences between young and elderly gait found in the present study may therefore be attributed to ageing, rather than a secondary eect of dierences in gait velocity. Ó 2001 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2540; 2330 Keywords: Biomechanics; Gait; Ageing; Kinematics; Kinetics
1. Introduction The gait cycle is comprised of two distinct phases, the support phase and the swing phase, which require dierent motor strategies (Winter, 1983). During the support phase a net extensor moment generated by the hip, knee and ankle joints is required to prevent the collapse of the stance limb (Winter, 1983). The principal swing phase task is the progression of the foot of the swing limb from the previous to the next support position, providing the basis for the forward progression of the body (Winter, 1991). The motion of the swing leg is achieved primarily through the precise coordination of a seven segment kinematic chain consisting of the thigh, shank and foot segments of both the swing and support limbs, and the pelvis (Winter, 1992). Many researchers have likened the motion of the swing limb to that of a compound pendulum (Maillardet, 1977; Mena, Mansour, & Simon, 1981; Mochon & McMahon, 1980), however a more appropriate analogy is that of a force driven damped oscillator because muscular activity is required throughout the swing phase (Cavanagh & Gregor, 1975; Holt, Hamill, & Andres, 1990; Piazza & Delp, 1996; Whittlesey, van Emmerik, & Hamill, 2000). An ankle dorsi¯exion moment caused by the concentric contraction of the pre-tibial muscles occurs following toe-o (TO) to ensure clearance of the ground by the toe (Mena et al., 1981; Winter, 1991). During late swing, activation of the hamstrings group causes a ¯exion moment at the knee, and an extension moment at the hip, both of which contribute to the reduction of the anterior±posterior (A±P) velocity of the foot prior to heel contact (HC) (Cavanagh & Gregor, 1975; Winter, 1991). An understanding of the eect of ageing on swing phase mechanics is important, because the two critical points in the gait cycle from a falls perspective are minimum toe clearance and HC, which occur during the swing phase and the swing to stance transition, respectively (Winter, 1991). Falls
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during walking are the primary cause of accidental injury in the elderly population (Berg, Alessio, Mills, & Tong, 1997; Tinetti, Speechley, & Ginter, 1988) and are predominantly the result of a slip or trip (Berg et al., 1997; Tinetti et al., 1988). It is therefore important to identify age-related dierences in swing phase mechanics that may predispose the elderly to slipping or tripping. Trips occur when an external force interrupts the progress of the swing leg (Karst, Hageman, Jones, & Bunner, 1999; Winter, 1992), and the potential for a slip is greatest during mid-swing, when the toe reaches a minimum height of around 10 mm at which time the A±P velocity of the foot is greater than 4 m/s (Winter, 1991). The majority of slips occur in the moments following HC (Gronquist, Roine, Jarvinen, & Korhonen, 1989) and a high anterior±posterior heel contact velocity (A±P HCV) is thought to increase the risk of slipping (Karst et al., 1999; Myung & Smith, 1997; Winter, 1991). Toe clearance and A±P HCV are sensitive to the angular displacements and velocities of the segments of the lower extremity. Toe clearance is especially sensitive to the angles at the hip, knee and ankle joint of the swing leg, while A±P HCV is largely determined by the angular velocities of the thigh and shank of the swing leg (Winter, 1992). A±P HCV has been reported to be greater in elderly (1:15 0:04 m/s) than young (0:87 0:05 m/s) men (Winter, 1991). The biomechanical factors responsible for the greater A±P HCV of the elderly are yet to be established, however data have been presented identifying trends towards a lower knee ¯exion velocity at heel contact, a lower peak late swing knee ¯exion moment, and a lower late swing peak knee ¯exion power K4 in elderly compared with young subjects (Winter, 1991). It is well established that the velocity of gait is correlated with spatiotemporal (Grieve & Gear, 1966; Kirtley, Whittle, & Jeerson, 1985; Rosenrot, Wall, & Charteris, 1980), kinematic (Oberg, Karsznia, & Oberg, 1994; Winter, 1989) and kinetic gait variables (Cavanagh & Gregor, 1975; Chen, Kuo, & Andriacchi, 1997). Studies that have compared the preferred gait velocity of young and elderly subjects have mainly reported a lower gait velocity for the elderly than the young subjects. This age-related dierence in gait velocity has not been taken into account by many of the studies that have reported dierences in the biomechanical gait patterns of the young and elderly (Gabell & Nayak, 1984; Imms & Edholm, 1981; Winter, Patla, Frank, & Walt, 1990), and therefore, it is dicult to ascertain whether the dierences reported by these studies are due to the eect of ageing or gait velocity.
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To date there have been no detailed comparisons of the swing phase biomechanics of young and elderly subjects walking at their preferred gait velocities. Thus, the purpose of this study was to compare the swing phase kinematics and kinetics of healthy young and elderly men walking at their preferred walking velocities, while statistically controlling for gait velocity. An emphasis was placed on the assessment of kinematic and kinetic variables at the time of TO, HC and minimum metatarsal±phalangeal joint clearance MTPMIN .
2. Methods 2.1. Subjects and experimental procedure 10 young (age 24:9 0:9 years) and eight elderly (age 68:9 0:4 years) male volunteers participated in the study. The inclusion criterion for the subjects were: to be currently living in the community, to have normal or corrected to normal vision, to have no known musculo-skeletal or neurological abnormalities, not require mechanical aids or devices for walking, and to be aged between 20 and 30 years, for the young group, and between 65 and 75 years, for the elderly group. The experiment was carried out in accordance with the NHMRC guidelines for ethical conduct in research involving human subjects (National Health & Medical Research Council, 1999). Subjects were required to walk along a 12 m walkway at their preferred walking speed. Each subject performed a minimum of two practice trials prior to ®ve trials for which video and force plate data were collected. Subjects wore their own soft-soled shoes to facilitate natural gait. 2.2. Spatial±temporal variables Gait velocity was calculated from the time taken to pass between two light gates positioned 5 m apart at shoulder height. Stride length and stride duration were determined from consecutive ipsilateral HC events. Single support, double support and swing durations (seconds) and periods (% of gait cycle) were calculated from the ipsilateral HC events and the intermediate TO event. The initial HC event was determined from the peak negative A±P acceleration of the heel marker, and TO was determined from the peak positive A±P acceleration of the MTP marker (Kirtley, 1998). The second HC event occurred
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on the force platform and was de®ned as the time that the vertical component of the ground reaction force exceeded a threshold of 15 N. 2.3. Kinematics Seven re¯ective spherical markers (20 mm diameter), were placed on the right-hand side of each subject over the following anatomical landmarks: acromion of the scapula, greater trochanter, lateral femoral epicondyle, head of ®bula, lateral malleolus, and on the subjects' footwear overlying the lateral posterior aspect of the calcaneus and the ®fth metatarsal±phalangeal joint (MTP). Sagittal plane marker trajectories were recorded at 50 Hz using a Panasonic WV-CP610/B video camera and digitised using the Peak Motus Motion Measurement System (Version 4.0). Raw coordinate data were low-pass ®ltered at 6 Hz using a fourth order zero lag digital Butterworth ®lter (Winter, Sidwall, & Hobson, 1974) and interpolated from 50 to 1000 Hz to correspond with the force plate data. The ®ltered marker coordinates were used to de®ne the absolute angles of the trunk, thigh, shank, foot and a virtual ankle±heel segment from which the relative ankle, knee, and hip joint angles were determined. Segmental angles were de®ned counter-clockwise relative to the right-hand horizontal. Relative joint angles were de®ned as included angles between adjacent segments that were positive in ¯exion/dorsi¯exion (Winter, 1991). For the hip and knee, full extension was de®ned as zero degrees, and for the ankle, a 90° angle between the shank and foot segments was de®ned as zero degrees. Segment and joint angular velocities, and the vertical and A±P velocities of the hip, heel and toe markers were calculated using ®nite dierence equations. The height of the MTP marker at MTPMIN was calculated by subtracting the height of the MTP marker during mid-stance from the minimum height of the MTP marker during mid swing. A±P HCV was de®ned as the A±P velocity of the heel marker at HC. All kinematic variables were temporally normalised to the duration of the swing phase (i.e. 0% at TO and 100% at HC). A link segment model of the swing limb was constructed (Eq. (1)) to identify the kinematic factor(s) that may be responsible for any dierences in A±P HCV identi®ed between the young and elderly subjects in the present study. The lengths of the thigh and shank segments, and the distance from the lateral malleolus to the lateral aspect of the calcaneus (ankle±heel) were determined from the ®ltered marker coordinates.
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A±P HCV mhipx rthigh xthigh sin hthigh rshank xshank sin hshank rankle±heel xankle±heel sin hankle±heel
1
where mhipx is the anterior±posterior velocity of the hip marker at HC, rthigh ; rshank , and rankle±heel are the lengths of the thigh, shank and virtual ankle± heel segments, respectively, xthigh ; xshank , and xankle±heel are the angular velocities of the thigh, shank and virtual ankle±heel segments respectively, at HC, and, hthigh ; hshank , and hankle±heel are the absolute angles of the thigh, shank and virtual ankle-heel segments respectively, at HC. 2.4. Kinetics A Kistler 900 600 mm (Type 9287A) multicomponent force platform and a Kistler (Type 9865C) 8-channel charge ampli®er were used to sample the vertical ground reaction force which was used to identify HC. Force plate data was sampled at 1000 Hz. Newtonian equations of motion were used to calculate the net hip, knee and ankle joint moments throughout the swing phase (Elftman, 1939), with extension/plantar ¯exion moments de®ned as positive. Centres of mass and moments of inertia of the trunk, thigh, shank and foot required for the estimation of joint moments were calculated from the anthropometric data of Winter (1990). Joint power was also calculated and both joint moments and powers were normalised with respect to body mass and temporally normalised to the duration of the swing phase. 2.5. Within group variability Within age-group coecients of variation (COV) were calculated for each of the kinematic and kinetic variables and averaged over the entire swing phase. 2.6. Statistical analysis All dependent variables are reported as the mean the standard error of the mean (SEM) and COV's are expressed as percentages. Dierences in the dependent variables between the young and elderly were assessed using a multivariate analysis of variance (MANOVA). Additionally, in order to control for the eect of gait velocity, a multivariate analysis of covariance
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(MANCOVA) with gait velocity as a covariate was used to assess dierences due to the eect of age. Signi®cance values are presented for the MANOVA and the MANCOVA for those variables that achieved signi®cance. An a level of 0.05 was used for all statistical comparisons.
3. Results 3.1. Anthropometric variables Height, body-mass and the anthropometric variables required for the linksegment model (Eq. (1)) for the young and elderly subjects are presented in Table 1. 3.2. Spatial±temporal variables Gait velocity, stride length, stride duration, and single support, double support and swing durations and periods for the young and elderly subjects are presented in Table 2. The swing duration of the elderly subjects was signi®cantly less than that of the young subjects (0:43 0:01 vs. 0:46 0:01 s, p 0:046, MANOVA) however, when corrected for gait velocity, the difference in swing duration was not signi®cant (p 0:060, MANCOVA). 3.3. Foot trajectory kinematics The ensemble averaged A±P and vertical displacements and velocities of the heel and MTP joint throughout the swing phase for the young and elderly subjects are displayed in Fig. 1. MTPMIN for the young and elderly subjects was not signi®cantly dierent (20 3 vs. 21 2 mm) nor was the relative Table 1 Subject characteristics and anthropometrical variables Variable
Young Mean (SEM)
Elderly Mean (SEM)
Height (cm) Mass (kg) Thigh length (cm) Shank length (cm) Ankle-heel distance (cm)
176.8 74.2 40.1 38.5 8.2
172.9 82.2 38.0 36.9 8.6
(1.4) (2.3) (0.7) (0.7) (0.2)
(1.6) (3.7) (0.8) (0.8) (0.2)
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Table 2 Spatial±temporal variables of the young and elderly subjects Variable
Young Mean (SEM)
Elderly Mean (SEM)
Gait velocity (m/s) Stride length (m) Stride duration (s) Support duration (s) Swing duration (s) Double support duration (s) Support period (% stride) Swing period (% stride) Double support (% stride)
1.41 (0.04) 1.70 (0.03) 1.18 (0.02) 0.72 (0.02) 0.46 (0.01) 0.27 (0.02) 61.3 (0.7) 38.7 (0.7) 22.5 (1.3)
1.44 (0.03) 1.68 (0.03) 1.13 (0.03) 0.70 (0.02) 0.43 (0.01) 0.28 (0.01) 62.3 (0.6) 37.7 (0.6) 24.6 (1.2)
*
Signi®cantly dierent from young (MANOVA), p < 0:05.
timing of MTPMIN (61:2 1:2 vs. 62:8 1:2% of swing). The elderly subjects had a signi®cantly greater A±P HCV than the young subjects, independent of gait velocity (1:32 0:05 vs. 1:05 0:05 m/s, p 0:002, MANOVA; p 0:0004, MANCOVA). 3.4. Segmental kinematics Ensemble averaged trunk, thigh, shank and foot angles and angular velocities for the young and elderly subjects throughout the swing phase are presented in Fig. 2. At HC the angular velocity of the shank was signi®cantly less for the elderly than for the young subjects, independent of gait velocity ( 0:70 0:09 vs. 1:35 0:16 rad/s, p 0:004, MANOVA; p 0:005, MANCOVA), as was the angular velocity of the foot ( 2:14 0:10 vs. 2:61 0:15 rad/s, p 0:026, MANOVA; p 0:005, MANCOVA). 3.5. Joint kinematics Ensemble averaged hip, knee and ankle joint angles and angular velocities for the young and elderly subjects throughout the swing phase are presented in Fig. 3. There were no signi®cant dierences between the young and elderly subjects for any of the joint angles or angular velocities, or the A±P hip velocity at TO, MTPMIN or HC. A summary of the joint kinematic variables for the young and elderly subjects is presented in Table 3.
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Fig. 1. A±P and vertical displacements of the MTP (a) and (b) and the heel (c) and (d) throughout the swing phase. Solid lines represent the means of the young subjects and dotted lines represent the means of the elderly subjects. COV's of each of the variables for the young and elderly groups are presented in Table 5.
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Fig. 2. Trunk, thigh, shank and foot segmental angles (a) and angular velocities (b) for the young subjects (solid lines) and elderly subjects (dotted lines). Angles are de®ned counter-clockwise relative to the righthand horizontal. COV's of each of the variables for the young and elderly groups are presented in Table 5.
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Fig. 3. Hip, knee and ankle joint angles (a) and angular velocities (b) throughout the swing phase. Solid lines represent the means of the young subjects and dotted lines represent the means of the elderly subjects. Joint angles are included angles that are positive in ¯exion/dorsi¯exion and negative in extension/plantar ¯exion. For the hip and knee, full extension is de®ned as zero degrees, and for the ankle, a 90° angle between the shank and foot was de®ned as zero degrees. COV's of each of the variables for the young and elderly groups are presented in Table 5.
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Table 3 Kinematic variables at TO, MTPMIN , and HC for the young and elderly subjects Event TO
vhipx (m/s) htrunk (deg) hthigh (deg) hshank (deg) hfoot (deg) hankle heel (deg) xtrunk (rad/s) xthigh (rad/s) xshank (rad/s) xfoot (rad/s) hhip (deg) hknee (deg) hankle (deg) xhip (rad/s) xknee (rad/s) xankle (rad/s)
MTPMIN
HC
Young Mean (SEM)
Elderly Mean (SEM)
Young Mean (SEM)
Elderly Mean (SEM)
Young Mean (SEM)
Elderly Mean (SEM)
1.81 (0.06) 84.8 (0.7) 79.0 (1.3) 41.4 (0.8) 121.3 (1.5) 2.5 (2.3) 0.44 (0.02) 2.31 (0.11) )3.20 (0.09) )6.33 (0.34) )5.8 (1.9) 37.7 (1.6) )10.0 (2.8) )1.87 (0.12) )5.50 (0.15) 3.13 (0.32)
1.87 (0.04) 85.2 (1.0) 80.8 (1.3) 41.9 (0.8) 120.3 (1.6) 4.3 (2.1) 0.41 (0.03) 2.43 (0.08) )3.42 (0.10) )6.13 (0.20) )4.4 (1.9) 38.9 (1.2) )11.6 (1.0) )2.01 (0.08) )5.85 (0.13) 2.71 (0.23)
1.55 (0.06) 90.9 (0.8) 117.0 (1.1) 70.4 (1.3) 165.8 (1.3) 47.8 (1.8) 0.13 (0.03) 0.58 (0.15) 6.50 (0.08) 6.00 (0.29) 26.1 (1.6) 46.6 (1.9) 5.4 (2.4) )0.45 (0.16) 5.92 (0.22) 0.51 (0.27)
1.63 (0.06) 90.3 (0.7) 119.5 (1.1) 73.5 (1.7) 166.6 (0.6) 50.6 (0.8) 0.15 (0.04) 0.54 (0.16) 6.76 (0.22) 6.00 (0.24) 29.2 (0.9) 46.0 (1.8) 3.2 (1.7) )0.39 (0.16) 6.23 (0.33) 0.77 (0.24)
1.89 (0.07) 92.3 (0.9) 114.2 (1.4) 111.6 (1.1) 196.2 (1.3) 80.0 (1.0) 0.24 (0.04) )0.17 (0.12) )1.35 (0.16) )2.61 (0.15) 22.0 (1.8) 2.6 (2.3) )5.4 (1.4) 0.41 (0.14) )1.17 (0.23) 1.26 (0.11)
1.86 (0.04) 91.4 (0.7) 116.9 (0.9) 113.7 (1.0) 195.3 (1.2) 79.3 (1.1) 0.16 (0.03) )0.12 (0.12) )0.70 (0.09); )2.14 (0.10); 25.4 (0.7) 3.1 (1.5) )8.4 (1.0) 0.28 (0.13) )0.58 (0.17) 1.44 (0.13)
Segmental angles are de®ned counter-clockwise relative to the right-hand horizontal. Joint angles are included angles that are positive in ¯exion/ dorsi¯exion and negative in extension/plantar ¯exion. For the hip and knee, full extension is de®ned as zero degrees, and for the ankle a 90° angle between the shank and foot was de®ned as zero degrees. * Signi®cantly dierent from young (MANOVA), p < 0:05. ** Signi®cantly dierent from young (MANCOVA), p < 0:05.
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Variable
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Fig. 4. Hip, knee and ankle joint moments (a) and powers (b) throughout the swing phase. Solid lines represent the means of the young subjects and dotted lines represent the means of the elderly subjects. Positive moments are extension/plantar ¯exion moments and negative moments are ¯exion/dorsi¯exion moments. Positive power represents joint power generation and negative power represents joint power absorption. COV's of each of the variables for the young and elderly groups are presented in Table 5.
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3.6. Kinetics Ensemble averaged joint moments and powers for the hip, knee and ankle joints throughout the swing phase are presented in Fig. 4. The only kinetic dierence identi®ed was a greater hip extension moment at MTPMIN for the elderly compared with the young subjects, independent of gait velocity (0:14 0:2 vs. 0:09 0:01 Nm/kg, p 0:007 (MANOVA), p 0:010 (MANCOVA)). A summary of the kinetic variables for the young and elderly Table 4 Hip, knee and ankle joint moments (M) and powers (P ) at TO, MTPMIN and HC Variable
Event TO
MTPMIN
HC
Young Elderly Young Elderly Young Elderly Mean (SEM) Mean (SEM) Mean (SEM) Mean (SEM) Mean (SEM) Mean (SEM) Mhip (Nm/kg) Mknee (Nm/kg) Mankle (Nm/kg) Phip (W/kg) Pknee (W/kg) Pankle (W/kg)
)0.37 0.06 )0.04 0.69 )0.35 )0.12
(0.03) (0.01) (0.00) (0.07) (0.06) (0.02)
)0.33 0.08 )0.05 0.67 )0.44 )0.12
(0.02) (0.01) (0.01) (0.06) (0.05) (0.01)
0.09 )0.08 )0.01 )0.03 )0.48 )0.01
(0.01) (0.01) (0.00) (0.02) (0.04) (0.00)
0.14 )0.09 )0.01 )0.05 )0.56 )0.01
(0.02); 0.21 (0.00) )0.21 (0.00) 0.03 (0.02) 0.08 (0.05) 0.24 (0.00) 0.03
(0.03) (0.02) (0.00) (0.02) (0.05) (0.00)
0.30 )0.23 0.03 0.09 0.11 0.04
(0.04) (0.02) (0.00) (0.04) (0.04) (0.01)
Positive moments represent net joint extension moments/plantar ¯exion and negative moments represent net joint ¯exion/dorsi¯exion moments, positive powers represents net power generation and negative powers represent net power absorption at the joint. * Signi®cantly dierent from young (MANOVA), p < 0:05. ** Signi®cantly dierent from young (MANCOVA), p < 0:05.
Table 5 Maximum and minimum moments (M) and powers (P ) at the hip, knee and ankle during the swing phase Variable
Mhip (Nm/kg) Mknee (Nm/kg) Mankle (Nm/kg) Phip (W/kg) Pknee (W/kg) Pankle (W/kg)
Maximum
Minimum
Young Mean (SEM)
Elderly Mean (SEM)
Young Mean (SEM)
Elderly Mean (SEM)
0.36 0.11 0.04 0.83 0.24 )0.13
0.40 0.13 0.04 0.85 0.15 )0.10
)0.37 )0.30 )0.06 )0.05 )1.43 )0.18
)0.33 )0.30 )0.05 )0.08 )1.47 )0.12
(0.03) (0.01) (0.00) (0.09) (0.04) (0.04)
(0.02) (0.01) (0.00) (0.08) (0.02) (0.01)
(0.03) (0.01) (0.01) (0.02) (0.08) (0.06)
(0.02) (0.02) (0.00) (0.01) (0.15) (0.01)
Positive moments represent net extension/plantar ¯exion joint moments and negative moments represent net ¯exion/dorsi¯exion joint moments, positive powers represents net power generation and negative powers represent net power absorption at the joint.
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Table 6 Coecients of variation (COV) for the kinematic and kinetic variables of the young and elderly subjects Variable
A±P MTP displacement (m) Vertical MTP displacement (m) A±P heel displacement (m) Vertical heel displacement (m) A±P MTP velocity (m/s) Vertical MTP velocity (m/s) A±P heel velocity (m/s) Vertical heel velocity (m/s) htrunk (deg) hthigh (deg) hshank (deg) hfoot (deg) xtrunk (rad/s) xthigh (rad/s) xshank (rad/s) xfoot (rad/s) hhip (deg) hknee (deg) hankle (deg) xhip (rad/s) xknee (rad/s) xankle (rad/s) Mhip (Nm/kg) Mknee (Nm/kg) Mankle (Nm/kg) Phip (W/kg) Pknee (W/kg) Pankle (W/kg)
COV (%) Young
Elderly
8 20 7 17 9 32 8 23 3 4 5 4 37 18 12 16 31 13 94 22 15 39 32 25 28 45 32 42
6 16 6 9 8 26 7 20 2 3 3 3 43 16 11 16 15 9 58 22 12 38 35 30 18 46 34 45
subjects at the times of TO, MTPMIN and HC is presented in Table 4, and the maximum and minimum joint moments and powers of the young and elderly subjects are presented in Table 5. 3.7. Within age-group variability Within age-group COV's for each of the kinematic and kinetic variables throughout the swing phase are presented in Table 6. The COV's within the young group were greater than those within the elderly group for 22 out of 29 of the variables assessed.
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4. Discussion The present study examined the kinematics and kinetics of the swing phase in healthy young and elderly men walking at their preferred speed. The preferred walking speed of elderly men has previously been reported as being lower than young men (Judge, Davis III, & Ounpuu, 1996; Kerrigan, Todd, Della Croce, Lipsitz, & Collins, 1998; Murray, Kory, & Clarkson, 1969), however in the present study no age-related dierence in gait velocity was identi®ed. The preferred gait velocity of the young subjects was consistent with previous ®ndings, however the gait velocity of the elderly subjects study was considerably higher than in the majority of previous studies (Kerrigan et al., 1998; Murray et al., 1969; Winter et al., 1990). The relatively high gait velocity of the elderly subjects in the present study is likely to be re¯ective of the selection criterion, which aimed to exclude individuals with any conditions that may have aected their gait. Dierences in swing phase mechanics between the young and elderly subjects in the present study may therefore be interpreted as being indicative of the eect of ageing per se and not of conditions that have an increased prevalence in the elderly population such as osteoarthritis (Hughes & Dunlop, 1995). It is well established that the velocity of walking is correlated with spatiotemporal (Grieve & Gear, 1966; Kirtley et al., 1985; Rosenrot et al., 1980), kinematic (Oberg et al., 1994; Winter, 1989) and kinetic gait variables (Cavanagh & Gregor, 1975; Chen et al., 1997). Because dierences in the gait velocities of the young and elderly have been identi®ed in the majority of studies that have investigated the eect of ageing on gait, it is dicult to ascertain whether the dierences in their gait patterns are a primary consequence of age, or re¯ective of dierences in preferred walking speed (DeVita & Hortobagyi, 2000). In order to control for the eect of gait velocity on biomechanical gait patterns, some investigators have either set a criterion walking speed for both young and elderly subjects (DeVita & Hortobagyi, 2000), or compared dierences between fast elderly gait and young preferred speed gait (Kerrigan et al., 2000). A potential problem with these approaches is that the resulting gait patterns may be dierent from the subjects' natural gait patterns. In the present study no dierence was identi®ed between the preferred gait velocities of the young and elderly subjects, and additionally, the eect of ageing on the dependent variables was assessed independently of gait velocity using MANCOVA. Therefore, the mechanical dierences presented here are a primary eect of ageing and not simply a secondary eect of dierences in gait velocity.
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Swing duration was the only spatial±temporal variable that was signi®cantly dierent between the young and elderly, with the swing duration of the elderly being less than that of the young. However, the dierence between the swing duration of the young and elderly was not found to be signi®cant when swing duration was normalised for stride duration (i.e. swing period), or when the eect of gait velocity was taken into account. Therefore, the shorter swing period of the elderly compared with the young reported by others (e.g. Winter et al., 1990) is likely to be due to dierences in gait velocity or stride duration, and not to ageing per se. No dierences in joint kinematics were identi®ed between the young and elderly subjects at TO, MTPMIN , or HC. The hip, knee, and ankle joint angles of the young and elderly men at MTPMIN are similar to those reported by Winter (1992) for young subjects. The ®ndings of previous studies regarding the eect of ageing on joint kinematics during walking are inconsistent. For example, Judge et al. (1996) reported greater hip ¯exion at HC in the elderly than the young, Kerrigan et al. (1998) reported no dierences in peak hip ¯exion during late swing, but a decrease in peak hip extension in the elderly while both Elble, Thomas, Higgins, and Colliver (1991) and Winter (1991) reported a greater range of motion at the hip for the elderly than the young. According to DeVita and Hortobagyi (2000) and Kerrigan et al. (1998), many of the dierences in joint kinematics reported between the young and elderly are related to dierences in the gait velocities and/or stride lengths of the groups, which may explain why no dierences were detected in the present study. Dierences in the health status and activity levels of elderly subjects in dierent studies may also explain the range of results reported. The importance of de®ning what is meant by ``elderly'' in gait studies involving elderly subjects should therefore not be underestimated. In agreement with previous ®ndings (Winter, 1991), A±P HCV was greater for the elderly than for the young men. Additionally, it was shown that the greater A±P HCV of the elderly was independent of gait velocity. The majority of slips and slip-related falls while walking are initiated in the 50 milliseconds following heel contact (Gronquist et al., 1989; Strandberg & Lanshammar, 1981) and a high A±P HCV has been proposed as a risk factor for slip induced falls (Karst et al., 1999; Winter, 1991). Therefore, the greater A±P HCV of the elderly may provide an explanation as to why slips are the primary cause of falls in elderly men (Berg et al., 1997). A proximal to distal examination of the variables on the right-hand side of the link segment model (Eq. (1)) identi®ed that the lower shank angular velocity of the elderly was responsible for their greater A±P HCV (Tables 1
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and 3). Thus, while the dierences between the young and elderly in hip and knee angular velocities at HC were not statistically signi®cant in isolation, their combined eect was a signi®cantly lower rotational velocity of the shank and foot segments, resulting in a greater A±P HCV for the elderly compared with the young subjects. Winter (1991) identi®ed a trend towards a lower K4 power burst in the elderly than the young, which was proposed as the cause of the elderly's greater A±P HCV. In the present study, no dierences in peak K4 were identi®ed which may have been due to the lack of dierences in the gait velocities of the young and elderly subjects. The magnitude of the peak knee ¯exor moment during the swing phase is however positively correlated with gait velocity (Cavanagh & Gregor, 1975) and thus, the lower gait velocity of the elderly (1.29 m/s) compared with the young subjects (1.43 m/s) in Winter's study may have been the primary cause of their lower peak K4 power burst (c.f. Winter, 1991). The smaller dierence in gait velocity between the young and elderly in the present study (1.41 vs. 1.44 m/s, respectively) is therefore a probable explanation why an age-related trend in peak K4 was not identi®ed in the present study. A kinetic explanation for the dierences in A±P HCV between the young and elderly was not identi®ed in the present study however, at heel contact the knee ¯exors were generating 118% more power in the young than the elderly subjects. There was a tendency towards lower inter-subject variability of kinematic variables within the elderly group, which is in agreement with Winter (1991). In contrast with the ®ndings of Winter (1991) who reported a decrease in inter-subject kinetic variability in the elderly compared with the young, within group variability of joint moments and powers did not appear to display any age-related pattern in the present study. The kinetic variability of both the young and elderly subjects in the present study was also lower than that reported by Winter (1991) for the entire gait cycle, which is consistent with the ®nding that kinetic variability is greater during the stance than the swing phase (Winter, 1989). Therefore, a possible explanation for the similar kinetic variability of the young and elderly groups in the present study is that the majority of the age-related decrease in inter-subject kinetic variability may occur during the stance phase rather than the swing phase. In this paper, a comprehensive description of the mechanics of the swing phase of gait has been presented for young and elderly men. Many of the agerelated kinematic and kinetic dierences reported by previous studies were not found in the present study. The lack of biomechanical dierences between the young and elderly is likely to be related to the lack of signi®cant dier-
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ences in the preferred gait velocities, stride lengths, and stride durations of the young and elderly subjects in this study. The main ®nding of the study was a signi®cantly greater A±P HCV for the elderly than the young, independent of gait velocity. At the kinematic level, the greater A±P HCV of the elderly than the young men was attributed to their reduced shank velocity at heel contact. The greater A±P HCV of the elderly men compared with the young men may help to explain why slips are the predominant cause of falls in elderly men. References Berg, W. P., Alessio, H. M., Mills, E. M., & Tong, C. (1997). Circumstances and consequences of falls in independent community-dwelling older adults. Age and Ageing, 26, 261±268. Cavanagh, P., & Gregor, R. (1975). Knee joint torque during the swing phase of normal treadmill walking. Journal of Biomechanics, 8, 337±344. Chen, I. H., Kuo, K. N., & Andriacchi, T. P. (1997). The in¯uence of walking speed on mechanical joint power during gait. Gait and Posture, 6, 171±176. DeVita, P., & Hortobagyi, T. (2000). Age causes a redistribution of joint torques and powers during gait. Journal of Applied Physiology, 88, 1804±1811. Elble, R. J., Thomas, S. S., Higgins, C., & Colliver, J. (1991). Stride-dependent changes in gait of older people. Journal of Neurology, 238, 1±5. Elftman, H. (1939). Forces and energy changes in the leg during walking. American Journal of Physiology, 125, 339±356. Gabell, A., & Nayak, U. S. (1984). The eect of age on variability of gait. Journal of Gerontology, 39, 662± 666. Grieve, D. W., & Gear, R. J. (1966). The relationship between length of stride, step frequency, time of swing and speed of walking for children and adults. Ergonomics, 5, 379±399. Gronquist, R., Roine, J., Jarvinen, E., & Korhonen, E. (1989). An apparatus and a method for determining the slip resistance of shoes and ¯oors by simulation of human foot motions. Ergonomics, 32, 979±995. Holt, K. G., Hamill, J., & Andres, R. O. (1990). The force-driven harmonic oscillator as a model for human locomotion. Human Movement Science, 9, 55±68. Hughes, S. L., & Dunlop, D. (1995). The prevalence and impact of arthritis in older persons. Arthritis Care & Research, 8, 257±264. Imms, F. J., & Edholm, O. G. (1981). Studies of gait and mobility in the elderly. Age and Ageing, 10, 147± 156. Judge, J. O., Davis III, R. B., & Ounpuu, S. (1996). Step length reductions in advanced age: The role of ankle and hip kinetics. Journal of Gerontology: Medical Sciences, 51, M303±312. Karst, G. M., Hageman, P. A., Jones, T. F., & Bunner, S. H. (1999). Reliability of foot trajectory measures within and between testing sessions. Journal of Gerontology: Medical Sciences, 54A, M343±M347. Kerrigan, D. C., Lee, L. W., Nieto, T. J., Markman, J. D., Collins, J. J., & Riley, P. O. (2000). Kinetic alterations independent of walking speed in elderly fallers. Archives of Physical and Medical Rehabilitation, 81, 730±735. Kerrigan, D. C., Todd, M. K., Della Croce, U., Lipsitz, L. A., & Collins, J. J. (1998). Biomechanical gait alterations independent of speed in the healthy elderly: Evidence for speci®c limiting impairments. Archives of Physical and Medical Rehabilitation, 79, 317±322.
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Kirtley, C. (1998). Determining initial contact and toe-o from kinematic data, [On-line]. Clinical Gait Analysis. Available: http://guardian.curtin/.edu/cga/faq/toe-o.html [1999, June 20] (March 8, 2001). Kirtley, C., Whittle, M. W., & Jeerson, R. J. (1985). In¯uence of walking speed on gait parameters. Journal of Biomechanical Engineering, 7, 282±288. Maillardet, F. (1977). The swing phase of locomotion. International Mechanical Engineering, 6, 67±75. Mena, D., Mansour, J. M., & Simon, S. R. (1981). Analysis and synthesis of human swing leg motion during gait and its clinical applications. Journal of Biomechanics, 14, 823±832. Mochon, S., & McMahon, T. A. (1980). Ballistic walking. Journal of Biomechanics, 13, 49±57. Murray, M., Kory, R., & Clarkson, B. (1969). Walking patterns in healthy old men. Journal of Gerontology, 24, 169±178. Myung, R., & Smith, J. L. (1997). The eect of load carrying and ¯oor contaminants on slip and fall parameters. Ergonomics, 40, 235±246. National Health and Medical Research Council. (1999). National Statement on Ethical Conduct in Research Involving Humans, [On-line]. Available: http://www.nhmrc.health.gov.au/publicat/humans/ contents.htm [2001, 10/2/99]. Oberg, T., Karsznia, A., & Oberg, K. (1994). Joint angle parameters in gait: Reference data for normal subjects, 10±79 years of age. Journal of Rehabilitation Research and Development, 31, 199±213. Piazza, S. J., & Delp, S. L. (1996). The in¯uence of muscles on knee ¯exion during the swing phase of gait. Journal of Biomechanics, 29, 723±733. Rosenrot, P., Wall, J. C., & Charteris, J. (1980). The relationship between velocity, stride time, support time and swing time during normal walking. Journal of Human Movement Studies, 6, 323±335. Strandberg, L., & Lanshammar, H. (1981). The dynamics of slipping accidents. Journal of Occupational Accidents, 3, 153±162. Tinetti, M. E., Speechley, M., & Ginter, S. F. (1988). Risk factors for falls among elderly persons living in the community. The New England Journal of Medicine, 319, 1701±1707. Whittlesey, S. N., van Emmerik, R. E., & Hamill, J. (2000). The swing phase of human walking is not a passive movement. Motor Control, 4, 273±292. Winter, D. A. (1983). Biomechanical motor patterns in normal walking. Journal of Motor Behaviour, 15, 302±330. Winter, D. A. (1989). Coordination of motor tasks during human gait. In S. A. Wallace (Ed.), Advances in psychology: perspectives on the coordination of movement (Vol. 60). Amsterdam: Elsevier. Winter, D. A. (1990). Biomechanics and motor control of human movement (2nd ed.). New York: Wiley. Winter, D. A. (1991). The biomechanics and motor control of human gait: normal, elderly and pathological (2nd ed.). Waterloo, ON: University of Waterloo Press. Winter, D. A. (1992). Foot trajectory in human gait: A precise and multifactorial motor control task. Physical Therapy, 72, 45±53. Winter, D. A., Patla, A. E., Frank, J. S., & Walt, S. E. (1990). Biomechanical walking pattern changes in the ®t and healthy elderly. Physical Therapy, 70, 340±347. Winter, D. A., Sidwall, H. G., & Hobson, D. A. (1974). Measurement and reduction of noise in kinematics of locomotion. Journal of Biomechanics, 7, 157±159.
www.elsevier.com/locate/humov
Swing phase mechanics of healthy young and elderly men Peter M. Mills *, Rod S. Barrett Biomechanics-Dynamics Group, School of Physiotherapy and Exercise Science, Grith University, Gold Coast, Qld, Australia
Abstract This study examined the eect of ageing on the swing phase mechanics of young and elderly gait. Sagittal plane marker trajectories and force plate data were collected while 10 young (24:9 0:9 years) and eight elderly (68:9 0:4 years) subjects walked at their preferred walking speeds. Comparison between young and elderly gait was made for a range of spatial± temporal, kinematic and kinetic variables with emphasis given to identifying possible dierences at toe-o, minimum metatarsal±phalangeal joint clearance and heel contact. In order to control for the confounding eect of gait velocity on the dependent variables, a multivariate analysis of covariance was used to identify dierences between the young and elderly subjects due to age. In contrast to studies that have reported lower preferred walking speeds in the elderly compared to the young [J.O. Judge, R.B. Davis III, S. Ounpuu, Step length reductions in advanced age: the role of ankle and hip kinetics, Journal of Gerontology: Medical Sciences 51 (1996) M303±312; D.C. Kerrigan, M.K. Todd, U. Della Croce, L.A. Lipsitz, J.J. Collins, Biomechanical gait alterations independent of speed in the healthy elderly: evidence for speci®c limiting impairments, Archives of Physical and Medical Rehabilitation 79 (1998) 317± 322], no dierences in walking speed nor in the spatial±temporal variables that determine walking speed were detected. The elderly were however, found to have a greater hip extension moment at the time of minimum metatarsal±phalangeal joint clearance, and a signi®cantly higher anterior±posterior velocity heel contact velocity that was linked to a signi®cantly higher shank and foot angular velocity at heel contact. Since many gait variables are highly correlated with walking speed [C. Kirtley, M.W. Whittle, R.J. Jeerson, In¯uence of walking speed on
*
Corresponding author. Tel.: +61-7-5552-8357; fax: +61-7-5552-8674. E-mail address: [email protected] (P.M. Mills).
0167-9457/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 9 4 5 7 ( 0 1 ) 0 0 0 6 1 - 6
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gait parameters, Journal of Biomechanical Engineering 7 (1985) 282±288; D.A. Winter, Biomechanical motor patterns in normal walking, Journal of Motor Behaviour 15 (1983) 302± 330], dierences between young and elderly gait found in the present study may therefore be attributed to ageing, rather than a secondary eect of dierences in gait velocity. Ó 2001 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2540; 2330 Keywords: Biomechanics; Gait; Ageing; Kinematics; Kinetics
1. Introduction The gait cycle is comprised of two distinct phases, the support phase and the swing phase, which require dierent motor strategies (Winter, 1983). During the support phase a net extensor moment generated by the hip, knee and ankle joints is required to prevent the collapse of the stance limb (Winter, 1983). The principal swing phase task is the progression of the foot of the swing limb from the previous to the next support position, providing the basis for the forward progression of the body (Winter, 1991). The motion of the swing leg is achieved primarily through the precise coordination of a seven segment kinematic chain consisting of the thigh, shank and foot segments of both the swing and support limbs, and the pelvis (Winter, 1992). Many researchers have likened the motion of the swing limb to that of a compound pendulum (Maillardet, 1977; Mena, Mansour, & Simon, 1981; Mochon & McMahon, 1980), however a more appropriate analogy is that of a force driven damped oscillator because muscular activity is required throughout the swing phase (Cavanagh & Gregor, 1975; Holt, Hamill, & Andres, 1990; Piazza & Delp, 1996; Whittlesey, van Emmerik, & Hamill, 2000). An ankle dorsi¯exion moment caused by the concentric contraction of the pre-tibial muscles occurs following toe-o (TO) to ensure clearance of the ground by the toe (Mena et al., 1981; Winter, 1991). During late swing, activation of the hamstrings group causes a ¯exion moment at the knee, and an extension moment at the hip, both of which contribute to the reduction of the anterior±posterior (A±P) velocity of the foot prior to heel contact (HC) (Cavanagh & Gregor, 1975; Winter, 1991). An understanding of the eect of ageing on swing phase mechanics is important, because the two critical points in the gait cycle from a falls perspective are minimum toe clearance and HC, which occur during the swing phase and the swing to stance transition, respectively (Winter, 1991). Falls
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during walking are the primary cause of accidental injury in the elderly population (Berg, Alessio, Mills, & Tong, 1997; Tinetti, Speechley, & Ginter, 1988) and are predominantly the result of a slip or trip (Berg et al., 1997; Tinetti et al., 1988). It is therefore important to identify age-related dierences in swing phase mechanics that may predispose the elderly to slipping or tripping. Trips occur when an external force interrupts the progress of the swing leg (Karst, Hageman, Jones, & Bunner, 1999; Winter, 1992), and the potential for a slip is greatest during mid-swing, when the toe reaches a minimum height of around 10 mm at which time the A±P velocity of the foot is greater than 4 m/s (Winter, 1991). The majority of slips occur in the moments following HC (Gronquist, Roine, Jarvinen, & Korhonen, 1989) and a high anterior±posterior heel contact velocity (A±P HCV) is thought to increase the risk of slipping (Karst et al., 1999; Myung & Smith, 1997; Winter, 1991). Toe clearance and A±P HCV are sensitive to the angular displacements and velocities of the segments of the lower extremity. Toe clearance is especially sensitive to the angles at the hip, knee and ankle joint of the swing leg, while A±P HCV is largely determined by the angular velocities of the thigh and shank of the swing leg (Winter, 1992). A±P HCV has been reported to be greater in elderly (1:15 0:04 m/s) than young (0:87 0:05 m/s) men (Winter, 1991). The biomechanical factors responsible for the greater A±P HCV of the elderly are yet to be established, however data have been presented identifying trends towards a lower knee ¯exion velocity at heel contact, a lower peak late swing knee ¯exion moment, and a lower late swing peak knee ¯exion power K4 in elderly compared with young subjects (Winter, 1991). It is well established that the velocity of gait is correlated with spatiotemporal (Grieve & Gear, 1966; Kirtley, Whittle, & Jeerson, 1985; Rosenrot, Wall, & Charteris, 1980), kinematic (Oberg, Karsznia, & Oberg, 1994; Winter, 1989) and kinetic gait variables (Cavanagh & Gregor, 1975; Chen, Kuo, & Andriacchi, 1997). Studies that have compared the preferred gait velocity of young and elderly subjects have mainly reported a lower gait velocity for the elderly than the young subjects. This age-related dierence in gait velocity has not been taken into account by many of the studies that have reported dierences in the biomechanical gait patterns of the young and elderly (Gabell & Nayak, 1984; Imms & Edholm, 1981; Winter, Patla, Frank, & Walt, 1990), and therefore, it is dicult to ascertain whether the dierences reported by these studies are due to the eect of ageing or gait velocity.
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To date there have been no detailed comparisons of the swing phase biomechanics of young and elderly subjects walking at their preferred gait velocities. Thus, the purpose of this study was to compare the swing phase kinematics and kinetics of healthy young and elderly men walking at their preferred walking velocities, while statistically controlling for gait velocity. An emphasis was placed on the assessment of kinematic and kinetic variables at the time of TO, HC and minimum metatarsal±phalangeal joint clearance MTPMIN .
2. Methods 2.1. Subjects and experimental procedure 10 young (age 24:9 0:9 years) and eight elderly (age 68:9 0:4 years) male volunteers participated in the study. The inclusion criterion for the subjects were: to be currently living in the community, to have normal or corrected to normal vision, to have no known musculo-skeletal or neurological abnormalities, not require mechanical aids or devices for walking, and to be aged between 20 and 30 years, for the young group, and between 65 and 75 years, for the elderly group. The experiment was carried out in accordance with the NHMRC guidelines for ethical conduct in research involving human subjects (National Health & Medical Research Council, 1999). Subjects were required to walk along a 12 m walkway at their preferred walking speed. Each subject performed a minimum of two practice trials prior to ®ve trials for which video and force plate data were collected. Subjects wore their own soft-soled shoes to facilitate natural gait. 2.2. Spatial±temporal variables Gait velocity was calculated from the time taken to pass between two light gates positioned 5 m apart at shoulder height. Stride length and stride duration were determined from consecutive ipsilateral HC events. Single support, double support and swing durations (seconds) and periods (% of gait cycle) were calculated from the ipsilateral HC events and the intermediate TO event. The initial HC event was determined from the peak negative A±P acceleration of the heel marker, and TO was determined from the peak positive A±P acceleration of the MTP marker (Kirtley, 1998). The second HC event occurred
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on the force platform and was de®ned as the time that the vertical component of the ground reaction force exceeded a threshold of 15 N. 2.3. Kinematics Seven re¯ective spherical markers (20 mm diameter), were placed on the right-hand side of each subject over the following anatomical landmarks: acromion of the scapula, greater trochanter, lateral femoral epicondyle, head of ®bula, lateral malleolus, and on the subjects' footwear overlying the lateral posterior aspect of the calcaneus and the ®fth metatarsal±phalangeal joint (MTP). Sagittal plane marker trajectories were recorded at 50 Hz using a Panasonic WV-CP610/B video camera and digitised using the Peak Motus Motion Measurement System (Version 4.0). Raw coordinate data were low-pass ®ltered at 6 Hz using a fourth order zero lag digital Butterworth ®lter (Winter, Sidwall, & Hobson, 1974) and interpolated from 50 to 1000 Hz to correspond with the force plate data. The ®ltered marker coordinates were used to de®ne the absolute angles of the trunk, thigh, shank, foot and a virtual ankle±heel segment from which the relative ankle, knee, and hip joint angles were determined. Segmental angles were de®ned counter-clockwise relative to the right-hand horizontal. Relative joint angles were de®ned as included angles between adjacent segments that were positive in ¯exion/dorsi¯exion (Winter, 1991). For the hip and knee, full extension was de®ned as zero degrees, and for the ankle, a 90° angle between the shank and foot segments was de®ned as zero degrees. Segment and joint angular velocities, and the vertical and A±P velocities of the hip, heel and toe markers were calculated using ®nite dierence equations. The height of the MTP marker at MTPMIN was calculated by subtracting the height of the MTP marker during mid-stance from the minimum height of the MTP marker during mid swing. A±P HCV was de®ned as the A±P velocity of the heel marker at HC. All kinematic variables were temporally normalised to the duration of the swing phase (i.e. 0% at TO and 100% at HC). A link segment model of the swing limb was constructed (Eq. (1)) to identify the kinematic factor(s) that may be responsible for any dierences in A±P HCV identi®ed between the young and elderly subjects in the present study. The lengths of the thigh and shank segments, and the distance from the lateral malleolus to the lateral aspect of the calcaneus (ankle±heel) were determined from the ®ltered marker coordinates.
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A±P HCV mhipx rthigh xthigh sin hthigh rshank xshank sin hshank rankle±heel xankle±heel sin hankle±heel
1
where mhipx is the anterior±posterior velocity of the hip marker at HC, rthigh ; rshank , and rankle±heel are the lengths of the thigh, shank and virtual ankle± heel segments, respectively, xthigh ; xshank , and xankle±heel are the angular velocities of the thigh, shank and virtual ankle±heel segments respectively, at HC, and, hthigh ; hshank , and hankle±heel are the absolute angles of the thigh, shank and virtual ankle-heel segments respectively, at HC. 2.4. Kinetics A Kistler 900 600 mm (Type 9287A) multicomponent force platform and a Kistler (Type 9865C) 8-channel charge ampli®er were used to sample the vertical ground reaction force which was used to identify HC. Force plate data was sampled at 1000 Hz. Newtonian equations of motion were used to calculate the net hip, knee and ankle joint moments throughout the swing phase (Elftman, 1939), with extension/plantar ¯exion moments de®ned as positive. Centres of mass and moments of inertia of the trunk, thigh, shank and foot required for the estimation of joint moments were calculated from the anthropometric data of Winter (1990). Joint power was also calculated and both joint moments and powers were normalised with respect to body mass and temporally normalised to the duration of the swing phase. 2.5. Within group variability Within age-group coecients of variation (COV) were calculated for each of the kinematic and kinetic variables and averaged over the entire swing phase. 2.6. Statistical analysis All dependent variables are reported as the mean the standard error of the mean (SEM) and COV's are expressed as percentages. Dierences in the dependent variables between the young and elderly were assessed using a multivariate analysis of variance (MANOVA). Additionally, in order to control for the eect of gait velocity, a multivariate analysis of covariance
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(MANCOVA) with gait velocity as a covariate was used to assess dierences due to the eect of age. Signi®cance values are presented for the MANOVA and the MANCOVA for those variables that achieved signi®cance. An a level of 0.05 was used for all statistical comparisons.
3. Results 3.1. Anthropometric variables Height, body-mass and the anthropometric variables required for the linksegment model (Eq. (1)) for the young and elderly subjects are presented in Table 1. 3.2. Spatial±temporal variables Gait velocity, stride length, stride duration, and single support, double support and swing durations and periods for the young and elderly subjects are presented in Table 2. The swing duration of the elderly subjects was signi®cantly less than that of the young subjects (0:43 0:01 vs. 0:46 0:01 s, p 0:046, MANOVA) however, when corrected for gait velocity, the difference in swing duration was not signi®cant (p 0:060, MANCOVA). 3.3. Foot trajectory kinematics The ensemble averaged A±P and vertical displacements and velocities of the heel and MTP joint throughout the swing phase for the young and elderly subjects are displayed in Fig. 1. MTPMIN for the young and elderly subjects was not signi®cantly dierent (20 3 vs. 21 2 mm) nor was the relative Table 1 Subject characteristics and anthropometrical variables Variable
Young Mean (SEM)
Elderly Mean (SEM)
Height (cm) Mass (kg) Thigh length (cm) Shank length (cm) Ankle-heel distance (cm)
176.8 74.2 40.1 38.5 8.2
172.9 82.2 38.0 36.9 8.6
(1.4) (2.3) (0.7) (0.7) (0.2)
(1.6) (3.7) (0.8) (0.8) (0.2)
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Table 2 Spatial±temporal variables of the young and elderly subjects Variable
Young Mean (SEM)
Elderly Mean (SEM)
Gait velocity (m/s) Stride length (m) Stride duration (s) Support duration (s) Swing duration (s) Double support duration (s) Support period (% stride) Swing period (% stride) Double support (% stride)
1.41 (0.04) 1.70 (0.03) 1.18 (0.02) 0.72 (0.02) 0.46 (0.01) 0.27 (0.02) 61.3 (0.7) 38.7 (0.7) 22.5 (1.3)
1.44 (0.03) 1.68 (0.03) 1.13 (0.03) 0.70 (0.02) 0.43 (0.01) 0.28 (0.01) 62.3 (0.6) 37.7 (0.6) 24.6 (1.2)
*
Signi®cantly dierent from young (MANOVA), p < 0:05.
timing of MTPMIN (61:2 1:2 vs. 62:8 1:2% of swing). The elderly subjects had a signi®cantly greater A±P HCV than the young subjects, independent of gait velocity (1:32 0:05 vs. 1:05 0:05 m/s, p 0:002, MANOVA; p 0:0004, MANCOVA). 3.4. Segmental kinematics Ensemble averaged trunk, thigh, shank and foot angles and angular velocities for the young and elderly subjects throughout the swing phase are presented in Fig. 2. At HC the angular velocity of the shank was signi®cantly less for the elderly than for the young subjects, independent of gait velocity ( 0:70 0:09 vs. 1:35 0:16 rad/s, p 0:004, MANOVA; p 0:005, MANCOVA), as was the angular velocity of the foot ( 2:14 0:10 vs. 2:61 0:15 rad/s, p 0:026, MANOVA; p 0:005, MANCOVA). 3.5. Joint kinematics Ensemble averaged hip, knee and ankle joint angles and angular velocities for the young and elderly subjects throughout the swing phase are presented in Fig. 3. There were no signi®cant dierences between the young and elderly subjects for any of the joint angles or angular velocities, or the A±P hip velocity at TO, MTPMIN or HC. A summary of the joint kinematic variables for the young and elderly subjects is presented in Table 3.
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Fig. 1. A±P and vertical displacements of the MTP (a) and (b) and the heel (c) and (d) throughout the swing phase. Solid lines represent the means of the young subjects and dotted lines represent the means of the elderly subjects. COV's of each of the variables for the young and elderly groups are presented in Table 5.
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Fig. 2. Trunk, thigh, shank and foot segmental angles (a) and angular velocities (b) for the young subjects (solid lines) and elderly subjects (dotted lines). Angles are de®ned counter-clockwise relative to the righthand horizontal. COV's of each of the variables for the young and elderly groups are presented in Table 5.
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Fig. 3. Hip, knee and ankle joint angles (a) and angular velocities (b) throughout the swing phase. Solid lines represent the means of the young subjects and dotted lines represent the means of the elderly subjects. Joint angles are included angles that are positive in ¯exion/dorsi¯exion and negative in extension/plantar ¯exion. For the hip and knee, full extension is de®ned as zero degrees, and for the ankle, a 90° angle between the shank and foot was de®ned as zero degrees. COV's of each of the variables for the young and elderly groups are presented in Table 5.
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Table 3 Kinematic variables at TO, MTPMIN , and HC for the young and elderly subjects Event TO
vhipx (m/s) htrunk (deg) hthigh (deg) hshank (deg) hfoot (deg) hankle heel (deg) xtrunk (rad/s) xthigh (rad/s) xshank (rad/s) xfoot (rad/s) hhip (deg) hknee (deg) hankle (deg) xhip (rad/s) xknee (rad/s) xankle (rad/s)
MTPMIN
HC
Young Mean (SEM)
Elderly Mean (SEM)
Young Mean (SEM)
Elderly Mean (SEM)
Young Mean (SEM)
Elderly Mean (SEM)
1.81 (0.06) 84.8 (0.7) 79.0 (1.3) 41.4 (0.8) 121.3 (1.5) 2.5 (2.3) 0.44 (0.02) 2.31 (0.11) )3.20 (0.09) )6.33 (0.34) )5.8 (1.9) 37.7 (1.6) )10.0 (2.8) )1.87 (0.12) )5.50 (0.15) 3.13 (0.32)
1.87 (0.04) 85.2 (1.0) 80.8 (1.3) 41.9 (0.8) 120.3 (1.6) 4.3 (2.1) 0.41 (0.03) 2.43 (0.08) )3.42 (0.10) )6.13 (0.20) )4.4 (1.9) 38.9 (1.2) )11.6 (1.0) )2.01 (0.08) )5.85 (0.13) 2.71 (0.23)
1.55 (0.06) 90.9 (0.8) 117.0 (1.1) 70.4 (1.3) 165.8 (1.3) 47.8 (1.8) 0.13 (0.03) 0.58 (0.15) 6.50 (0.08) 6.00 (0.29) 26.1 (1.6) 46.6 (1.9) 5.4 (2.4) )0.45 (0.16) 5.92 (0.22) 0.51 (0.27)
1.63 (0.06) 90.3 (0.7) 119.5 (1.1) 73.5 (1.7) 166.6 (0.6) 50.6 (0.8) 0.15 (0.04) 0.54 (0.16) 6.76 (0.22) 6.00 (0.24) 29.2 (0.9) 46.0 (1.8) 3.2 (1.7) )0.39 (0.16) 6.23 (0.33) 0.77 (0.24)
1.89 (0.07) 92.3 (0.9) 114.2 (1.4) 111.6 (1.1) 196.2 (1.3) 80.0 (1.0) 0.24 (0.04) )0.17 (0.12) )1.35 (0.16) )2.61 (0.15) 22.0 (1.8) 2.6 (2.3) )5.4 (1.4) 0.41 (0.14) )1.17 (0.23) 1.26 (0.11)
1.86 (0.04) 91.4 (0.7) 116.9 (0.9) 113.7 (1.0) 195.3 (1.2) 79.3 (1.1) 0.16 (0.03) )0.12 (0.12) )0.70 (0.09); )2.14 (0.10); 25.4 (0.7) 3.1 (1.5) )8.4 (1.0) 0.28 (0.13) )0.58 (0.17) 1.44 (0.13)
Segmental angles are de®ned counter-clockwise relative to the right-hand horizontal. Joint angles are included angles that are positive in ¯exion/ dorsi¯exion and negative in extension/plantar ¯exion. For the hip and knee, full extension is de®ned as zero degrees, and for the ankle a 90° angle between the shank and foot was de®ned as zero degrees. * Signi®cantly dierent from young (MANOVA), p < 0:05. ** Signi®cantly dierent from young (MANCOVA), p < 0:05.
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Variable
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Fig. 4. Hip, knee and ankle joint moments (a) and powers (b) throughout the swing phase. Solid lines represent the means of the young subjects and dotted lines represent the means of the elderly subjects. Positive moments are extension/plantar ¯exion moments and negative moments are ¯exion/dorsi¯exion moments. Positive power represents joint power generation and negative power represents joint power absorption. COV's of each of the variables for the young and elderly groups are presented in Table 5.
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3.6. Kinetics Ensemble averaged joint moments and powers for the hip, knee and ankle joints throughout the swing phase are presented in Fig. 4. The only kinetic dierence identi®ed was a greater hip extension moment at MTPMIN for the elderly compared with the young subjects, independent of gait velocity (0:14 0:2 vs. 0:09 0:01 Nm/kg, p 0:007 (MANOVA), p 0:010 (MANCOVA)). A summary of the kinetic variables for the young and elderly Table 4 Hip, knee and ankle joint moments (M) and powers (P ) at TO, MTPMIN and HC Variable
Event TO
MTPMIN
HC
Young Elderly Young Elderly Young Elderly Mean (SEM) Mean (SEM) Mean (SEM) Mean (SEM) Mean (SEM) Mean (SEM) Mhip (Nm/kg) Mknee (Nm/kg) Mankle (Nm/kg) Phip (W/kg) Pknee (W/kg) Pankle (W/kg)
)0.37 0.06 )0.04 0.69 )0.35 )0.12
(0.03) (0.01) (0.00) (0.07) (0.06) (0.02)
)0.33 0.08 )0.05 0.67 )0.44 )0.12
(0.02) (0.01) (0.01) (0.06) (0.05) (0.01)
0.09 )0.08 )0.01 )0.03 )0.48 )0.01
(0.01) (0.01) (0.00) (0.02) (0.04) (0.00)
0.14 )0.09 )0.01 )0.05 )0.56 )0.01
(0.02); 0.21 (0.00) )0.21 (0.00) 0.03 (0.02) 0.08 (0.05) 0.24 (0.00) 0.03
(0.03) (0.02) (0.00) (0.02) (0.05) (0.00)
0.30 )0.23 0.03 0.09 0.11 0.04
(0.04) (0.02) (0.00) (0.04) (0.04) (0.01)
Positive moments represent net joint extension moments/plantar ¯exion and negative moments represent net joint ¯exion/dorsi¯exion moments, positive powers represents net power generation and negative powers represent net power absorption at the joint. * Signi®cantly dierent from young (MANOVA), p < 0:05. ** Signi®cantly dierent from young (MANCOVA), p < 0:05.
Table 5 Maximum and minimum moments (M) and powers (P ) at the hip, knee and ankle during the swing phase Variable
Mhip (Nm/kg) Mknee (Nm/kg) Mankle (Nm/kg) Phip (W/kg) Pknee (W/kg) Pankle (W/kg)
Maximum
Minimum
Young Mean (SEM)
Elderly Mean (SEM)
Young Mean (SEM)
Elderly Mean (SEM)
0.36 0.11 0.04 0.83 0.24 )0.13
0.40 0.13 0.04 0.85 0.15 )0.10
)0.37 )0.30 )0.06 )0.05 )1.43 )0.18
)0.33 )0.30 )0.05 )0.08 )1.47 )0.12
(0.03) (0.01) (0.00) (0.09) (0.04) (0.04)
(0.02) (0.01) (0.00) (0.08) (0.02) (0.01)
(0.03) (0.01) (0.01) (0.02) (0.08) (0.06)
(0.02) (0.02) (0.00) (0.01) (0.15) (0.01)
Positive moments represent net extension/plantar ¯exion joint moments and negative moments represent net ¯exion/dorsi¯exion joint moments, positive powers represents net power generation and negative powers represent net power absorption at the joint.
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Table 6 Coecients of variation (COV) for the kinematic and kinetic variables of the young and elderly subjects Variable
A±P MTP displacement (m) Vertical MTP displacement (m) A±P heel displacement (m) Vertical heel displacement (m) A±P MTP velocity (m/s) Vertical MTP velocity (m/s) A±P heel velocity (m/s) Vertical heel velocity (m/s) htrunk (deg) hthigh (deg) hshank (deg) hfoot (deg) xtrunk (rad/s) xthigh (rad/s) xshank (rad/s) xfoot (rad/s) hhip (deg) hknee (deg) hankle (deg) xhip (rad/s) xknee (rad/s) xankle (rad/s) Mhip (Nm/kg) Mknee (Nm/kg) Mankle (Nm/kg) Phip (W/kg) Pknee (W/kg) Pankle (W/kg)
COV (%) Young
Elderly
8 20 7 17 9 32 8 23 3 4 5 4 37 18 12 16 31 13 94 22 15 39 32 25 28 45 32 42
6 16 6 9 8 26 7 20 2 3 3 3 43 16 11 16 15 9 58 22 12 38 35 30 18 46 34 45
subjects at the times of TO, MTPMIN and HC is presented in Table 4, and the maximum and minimum joint moments and powers of the young and elderly subjects are presented in Table 5. 3.7. Within age-group variability Within age-group COV's for each of the kinematic and kinetic variables throughout the swing phase are presented in Table 6. The COV's within the young group were greater than those within the elderly group for 22 out of 29 of the variables assessed.
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4. Discussion The present study examined the kinematics and kinetics of the swing phase in healthy young and elderly men walking at their preferred speed. The preferred walking speed of elderly men has previously been reported as being lower than young men (Judge, Davis III, & Ounpuu, 1996; Kerrigan, Todd, Della Croce, Lipsitz, & Collins, 1998; Murray, Kory, & Clarkson, 1969), however in the present study no age-related dierence in gait velocity was identi®ed. The preferred gait velocity of the young subjects was consistent with previous ®ndings, however the gait velocity of the elderly subjects study was considerably higher than in the majority of previous studies (Kerrigan et al., 1998; Murray et al., 1969; Winter et al., 1990). The relatively high gait velocity of the elderly subjects in the present study is likely to be re¯ective of the selection criterion, which aimed to exclude individuals with any conditions that may have aected their gait. Dierences in swing phase mechanics between the young and elderly subjects in the present study may therefore be interpreted as being indicative of the eect of ageing per se and not of conditions that have an increased prevalence in the elderly population such as osteoarthritis (Hughes & Dunlop, 1995). It is well established that the velocity of walking is correlated with spatiotemporal (Grieve & Gear, 1966; Kirtley et al., 1985; Rosenrot et al., 1980), kinematic (Oberg et al., 1994; Winter, 1989) and kinetic gait variables (Cavanagh & Gregor, 1975; Chen et al., 1997). Because dierences in the gait velocities of the young and elderly have been identi®ed in the majority of studies that have investigated the eect of ageing on gait, it is dicult to ascertain whether the dierences in their gait patterns are a primary consequence of age, or re¯ective of dierences in preferred walking speed (DeVita & Hortobagyi, 2000). In order to control for the eect of gait velocity on biomechanical gait patterns, some investigators have either set a criterion walking speed for both young and elderly subjects (DeVita & Hortobagyi, 2000), or compared dierences between fast elderly gait and young preferred speed gait (Kerrigan et al., 2000). A potential problem with these approaches is that the resulting gait patterns may be dierent from the subjects' natural gait patterns. In the present study no dierence was identi®ed between the preferred gait velocities of the young and elderly subjects, and additionally, the eect of ageing on the dependent variables was assessed independently of gait velocity using MANCOVA. Therefore, the mechanical dierences presented here are a primary eect of ageing and not simply a secondary eect of dierences in gait velocity.
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Swing duration was the only spatial±temporal variable that was signi®cantly dierent between the young and elderly, with the swing duration of the elderly being less than that of the young. However, the dierence between the swing duration of the young and elderly was not found to be signi®cant when swing duration was normalised for stride duration (i.e. swing period), or when the eect of gait velocity was taken into account. Therefore, the shorter swing period of the elderly compared with the young reported by others (e.g. Winter et al., 1990) is likely to be due to dierences in gait velocity or stride duration, and not to ageing per se. No dierences in joint kinematics were identi®ed between the young and elderly subjects at TO, MTPMIN , or HC. The hip, knee, and ankle joint angles of the young and elderly men at MTPMIN are similar to those reported by Winter (1992) for young subjects. The ®ndings of previous studies regarding the eect of ageing on joint kinematics during walking are inconsistent. For example, Judge et al. (1996) reported greater hip ¯exion at HC in the elderly than the young, Kerrigan et al. (1998) reported no dierences in peak hip ¯exion during late swing, but a decrease in peak hip extension in the elderly while both Elble, Thomas, Higgins, and Colliver (1991) and Winter (1991) reported a greater range of motion at the hip for the elderly than the young. According to DeVita and Hortobagyi (2000) and Kerrigan et al. (1998), many of the dierences in joint kinematics reported between the young and elderly are related to dierences in the gait velocities and/or stride lengths of the groups, which may explain why no dierences were detected in the present study. Dierences in the health status and activity levels of elderly subjects in dierent studies may also explain the range of results reported. The importance of de®ning what is meant by ``elderly'' in gait studies involving elderly subjects should therefore not be underestimated. In agreement with previous ®ndings (Winter, 1991), A±P HCV was greater for the elderly than for the young men. Additionally, it was shown that the greater A±P HCV of the elderly was independent of gait velocity. The majority of slips and slip-related falls while walking are initiated in the 50 milliseconds following heel contact (Gronquist et al., 1989; Strandberg & Lanshammar, 1981) and a high A±P HCV has been proposed as a risk factor for slip induced falls (Karst et al., 1999; Winter, 1991). Therefore, the greater A±P HCV of the elderly may provide an explanation as to why slips are the primary cause of falls in elderly men (Berg et al., 1997). A proximal to distal examination of the variables on the right-hand side of the link segment model (Eq. (1)) identi®ed that the lower shank angular velocity of the elderly was responsible for their greater A±P HCV (Tables 1
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and 3). Thus, while the dierences between the young and elderly in hip and knee angular velocities at HC were not statistically signi®cant in isolation, their combined eect was a signi®cantly lower rotational velocity of the shank and foot segments, resulting in a greater A±P HCV for the elderly compared with the young subjects. Winter (1991) identi®ed a trend towards a lower K4 power burst in the elderly than the young, which was proposed as the cause of the elderly's greater A±P HCV. In the present study, no dierences in peak K4 were identi®ed which may have been due to the lack of dierences in the gait velocities of the young and elderly subjects. The magnitude of the peak knee ¯exor moment during the swing phase is however positively correlated with gait velocity (Cavanagh & Gregor, 1975) and thus, the lower gait velocity of the elderly (1.29 m/s) compared with the young subjects (1.43 m/s) in Winter's study may have been the primary cause of their lower peak K4 power burst (c.f. Winter, 1991). The smaller dierence in gait velocity between the young and elderly in the present study (1.41 vs. 1.44 m/s, respectively) is therefore a probable explanation why an age-related trend in peak K4 was not identi®ed in the present study. A kinetic explanation for the dierences in A±P HCV between the young and elderly was not identi®ed in the present study however, at heel contact the knee ¯exors were generating 118% more power in the young than the elderly subjects. There was a tendency towards lower inter-subject variability of kinematic variables within the elderly group, which is in agreement with Winter (1991). In contrast with the ®ndings of Winter (1991) who reported a decrease in inter-subject kinetic variability in the elderly compared with the young, within group variability of joint moments and powers did not appear to display any age-related pattern in the present study. The kinetic variability of both the young and elderly subjects in the present study was also lower than that reported by Winter (1991) for the entire gait cycle, which is consistent with the ®nding that kinetic variability is greater during the stance than the swing phase (Winter, 1989). Therefore, a possible explanation for the similar kinetic variability of the young and elderly groups in the present study is that the majority of the age-related decrease in inter-subject kinetic variability may occur during the stance phase rather than the swing phase. In this paper, a comprehensive description of the mechanics of the swing phase of gait has been presented for young and elderly men. Many of the agerelated kinematic and kinetic dierences reported by previous studies were not found in the present study. The lack of biomechanical dierences between the young and elderly is likely to be related to the lack of signi®cant dier-
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ences in the preferred gait velocities, stride lengths, and stride durations of the young and elderly subjects in this study. The main ®nding of the study was a signi®cantly greater A±P HCV for the elderly than the young, independent of gait velocity. At the kinematic level, the greater A±P HCV of the elderly than the young men was attributed to their reduced shank velocity at heel contact. The greater A±P HCV of the elderly men compared with the young men may help to explain why slips are the predominant cause of falls in elderly men. References Berg, W. P., Alessio, H. M., Mills, E. M., & Tong, C. (1997). Circumstances and consequences of falls in independent community-dwelling older adults. Age and Ageing, 26, 261±268. Cavanagh, P., & Gregor, R. (1975). Knee joint torque during the swing phase of normal treadmill walking. Journal of Biomechanics, 8, 337±344. Chen, I. H., Kuo, K. N., & Andriacchi, T. P. (1997). The in¯uence of walking speed on mechanical joint power during gait. Gait and Posture, 6, 171±176. DeVita, P., & Hortobagyi, T. (2000). Age causes a redistribution of joint torques and powers during gait. Journal of Applied Physiology, 88, 1804±1811. Elble, R. J., Thomas, S. S., Higgins, C., & Colliver, J. (1991). Stride-dependent changes in gait of older people. Journal of Neurology, 238, 1±5. Elftman, H. (1939). Forces and energy changes in the leg during walking. American Journal of Physiology, 125, 339±356. Gabell, A., & Nayak, U. S. (1984). The eect of age on variability of gait. Journal of Gerontology, 39, 662± 666. Grieve, D. W., & Gear, R. J. (1966). The relationship between length of stride, step frequency, time of swing and speed of walking for children and adults. Ergonomics, 5, 379±399. Gronquist, R., Roine, J., Jarvinen, E., & Korhonen, E. (1989). An apparatus and a method for determining the slip resistance of shoes and ¯oors by simulation of human foot motions. Ergonomics, 32, 979±995. Holt, K. G., Hamill, J., & Andres, R. O. (1990). The force-driven harmonic oscillator as a model for human locomotion. Human Movement Science, 9, 55±68. Hughes, S. L., & Dunlop, D. (1995). The prevalence and impact of arthritis in older persons. Arthritis Care & Research, 8, 257±264. Imms, F. J., & Edholm, O. G. (1981). Studies of gait and mobility in the elderly. Age and Ageing, 10, 147± 156. Judge, J. O., Davis III, R. B., & Ounpuu, S. (1996). Step length reductions in advanced age: The role of ankle and hip kinetics. Journal of Gerontology: Medical Sciences, 51, M303±312. Karst, G. M., Hageman, P. A., Jones, T. F., & Bunner, S. H. (1999). Reliability of foot trajectory measures within and between testing sessions. Journal of Gerontology: Medical Sciences, 54A, M343±M347. Kerrigan, D. C., Lee, L. W., Nieto, T. J., Markman, J. D., Collins, J. J., & Riley, P. O. (2000). Kinetic alterations independent of walking speed in elderly fallers. Archives of Physical and Medical Rehabilitation, 81, 730±735. Kerrigan, D. C., Todd, M. K., Della Croce, U., Lipsitz, L. A., & Collins, J. J. (1998). Biomechanical gait alterations independent of speed in the healthy elderly: Evidence for speci®c limiting impairments. Archives of Physical and Medical Rehabilitation, 79, 317±322.
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