Temporospatial and kinematic gait alterations during treadmill walking with body weight suspension

Gait and Posture 17 (2003) 235 /245 www.elsevier.com/locate/gaitpost

Temporospatial and kinematic gait alterations during treadmill walking with body weight suspension A. Joseph Threlkeld *, Lance D. Cooper, Brock P. Monger 1, Aric N. Craven 1, Howard G. Haupt 1 Biodynamics Laboratory, Department of Physical Therapy, Creighton University, Omaha, NE 68178, USA Accepted 28 June 2002

Abstract Our purpose was to analyze the effects of selected levels of body weight support (BWS) on lower extremity kinematics of normal subjects at a predetermined treadmill speed. Seventeen non-disabled volunteers walked on a treadmill at 1.25 m s
1. Temporospatial and kinematic data were collected while various support levels were applied (Minimal, 10, 30, 50 and 70% BWS). Compared to 10% BWS, significant temporospatial and kinematic changes were induced by 50 and 70% BWS. Fewer differences were induced by 30% BWS compared to 10% BWS. We concluded that gait patterns of unimpaired subjects are significantly changed by 50 and 70% BWS. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Gait; Motion analysis; Treadmill; Kinematics; Body weight suspension

1. Introduction Body weight suspension (BWS) uses a calibrated external lift mechanism in conjunction with a vest or sling to support a portion of a subject’s body weight. Variable BWS in conjunction with treadmill training has been reported as a means to facilitate functional ambulation in subjects with neurological impairments [1 /8] and to promote recovery from musculoskeletal injuries [9 /11]. The theoretical foundation of BWS as a therapeutic intervention is drawn in part from the treadmill gait training of cats with spinal cord transactions [12 /15]. These foundational theories imply that BWS with treadmill training provides peripheral sensory stimulation which promotes a neuroplastic response that in turn produces improvements in motor output. Implicit in these theories, peripheral sensation must be present and * Corresponding author. Tel./fax: /1-402-280-5676 E-mail address: [email protected] (A.J. Threlkeld). 1 Enrolled as a student in the Doctor of Physical Therapy program at Creighton University at the time this study was conducted.

appropriate in order to produce the desired motor result. Intervention protocols for subjects with neurological impairments have focused on imposing simulations of normal treadmill gait kinematics to provide an appropriate sensory stimulus for changing motor patterns [1 /8]. Researchers have reported the use of a spectrum of BWS levels ranging from as little as 10% to as much as 70% [1 /10,16,17]. Particularly with clinical interventions, the selection of a particular BWS level was based on a subjective judgment about which BWS level produced the most normal gait kinematics. A maximum BWS level has been selected based on the ability of a subject to attain a heel contact during gait or by the subject’s tolerance of harness discomfort [8,16]. The BWS levels were sometimes varied within and between treatment sessions making effects of any single BWS level on a subject’s kinematics difficult to detect or interpret. If correct limb kinematics during the training phase are necessary to provide appropriate sensory stimuli to produce a desirable motor outcome, then the effects of BWS on normal kinematics must be studied system-

0966-6362/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 6 6 - 6 3 6 2 ( 0 2 ) 0 0 1 0 5 - 4

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atically. We located only one study that reported the changes in gait kinematics of unimpaired subjects who were exposed to varying amounts of BWS. Finch et al. used sagittal plane videotape and footswitches to gather temporal and kinematic data from 10 subjects using 0% BWS at 1.36 m s
1 treadmill speed, 30% BWS at 0.97 m s
1, 50% BWS at 0.85 m s
1 and 70% BWS at 0.70 m s
1 [16]. The authors’ rationale for using a different treadmill speed for each of the BWS levels was based on the ‘comfortable walking speed’ determined by pilot testing. Finch et al. reported that increased BWS was associated with significant decreases in percentage of stance time and significant decreases in total percentage of double limb support (DLS) time. Their analysis of hip and knee sagittal kinematics showed that as BWS was increased from 0 to 70%, significant decreases occurred in the maximum hip and maximum knee flexion during the swing phase. No data were reported on ankle kinematics. Although Finch et al. [16] also reported the temporal and kinematic results from comparable treadmill speeds without BWS, it is impossible to separate the effects of variations in treadmill speed from the effects of varied BWS levels in their results. An understanding of the kinematic effects of BWS on normal gait is necessary to design or assess the effects of BWS rehabilitation protocols on subjects with gait dysfunction. The goal of this study was to investigate the changes induced by BWS during treadmill walking of subjects without gait pathology. These data can be used as a point for reference to help develop appropriate BWS guidelines for training and rehabilitation of patients. We hypothesized that the increased stability provided by BWS would reduce the subject’s stance time and double support time. Secondly, we hypothesized that the sagittal kinematics of the ankle, knee and foot would show progressive alterations in angular displacement as increasing levels of BWS were applied.

2. Methods The protocol for this study was reviewed and approved by the Creighton University Human Subjects Institutional Review Board. 2.1. Subjects Seventeen (eight male, nine female) volunteers were recruited from a sample of health science students at Creighton University. The mean age of the subjects was 24.3 years9/3.4 (S.D.) with an age range of 22 /37 years. Their mean height was 175.59/9.3 cm (range 157 /192 cm) and mean body mass was 73.29/14.9 kg (range 49.5 /98.4 kg). All subjects claimed previous treadmill walking experience and by self-report were free of

injuries, diseases or limitations that would have altered their treadmill walking abilities. The procedures of the study were explained thoroughly and all subjects read, understood and signed an approved consent form prior to engaging in the experimental protocol.

2.2. Experimental procedures The subjects walked continuously on a treadmill (Quinton Model Q-65, Quinton Instrument Company, Bothell, WA) at a constant rate of 1.25 m s
1 under five conditions of BWS: Minimal BWS, 10% BWS, 30% BWS, 50% BWS, and 70% BWS. The definition of % BWS was the percentage of the subject’s body weight supported by an external apparatus. Support was achieved using a commercial pneumatic BWS device (Vigor Neuro II, Vigor Equipment, Inc., Stevensville, MI) and harness assembly (Maine Anti-Gravity Systems, Portland, ME). The accuracy of the lifting force of the BWS device was assessed prior to beginning the experiment by comparing the lifting force indicated on the gauge of the BWS device to the tensile force indicated on a dynamometer (Chatillon CSD400, AMETEK Test and Calibration Instruments Division, Largo, FL) attached in series between the BWS lift cables and the base of the device. A comparison of five paired readings between 5 and 45 kg yielded a Pearson r value of 0.998. Gait data were collected from all subjects during treadmill ambulation using a reflective marker motion collection system (Motion Analysis HiRES with EVa v6.0 software, Motion Analysis Corporation, Santa Rosa, CA). Four 60 Hz video cameras with on-line digitizing were utilized with a set of 21 spherical reflective markers, 2.5 cm in diameter, for tracking the three-dimensional motion of the lower extremities [18]. The Motion Analysis system was calibrated prior to each data collection session according to the manufacturer’s standards. Subjects walked continuously on the treadmill. Sequential collection periods for each BWS condition provided 9 /12 stride cycles for analysis. Each subject walked for 1 /3 min with Minimal BWS to establish a consistent gait pattern then motion data were collected. The Minimal BWS condition was operationally defined as supplying enough lifting force to remove the slack from the harness supports and prevent downward slippage of the harness (:/3/6% of the subjects body weight). After Minimal BWS collection, the four other experimental BWS levels were sequentially applied. The order of BWS levels was systematically rotated between successive subjects to minimize the effect of presentation order on subject performance. The subjects walked for 1 min after each BWS change to allow adaptation then kinematic data were collected for that condition.

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237

alpha level of P 0/0.01 was set for statistical significance.

2.3. Data reduction The kinematic data were tracked then smoothed using a zero-lag Butterworth filter with a cut-off frequency of 6 Hz. Discontinuous kinematic segments were joined using the Motion Analysis EVa v6.0 algorithm. The kinematic data were exported to a commercial gait analysis package (OrthoTrak v4.2, Motion Analysis Corporation, Santa Rosa, CA) where joint centers and joint angles were calculated. Foot contact and toe-off events were identified by the OrthoTrak software using an algorithm based on foot marker position and acceleration. Using the foot contact data indexed to time, we calculated step length based on treadmill belt speed. OrthoTrak also converted the time base of the data to percent of cycle (% Cycle) and ensemble averaged within and across subjects. Temporal, spatial and kinematic grand means were extracted for each condition. The nomenclature for events and phases of the gait cycle was based on the Rancho Los Amigos system as described by Perry [19]. Selected temporospatial data and points from the sagittal ankle, knee, and hip kinematic data were extracted for each condition (Table 1). 2.4. Data analysis The data from the right and left sides of each dependent measure were averaged within subjects. The average values reflect the mean extremity performance independent of side and were utilized for all descriptive and comparative statistics. The data were compared using a one-way ANOVA for Repeated Measures to detect differences between the four BWS conditions. Post-hoc analyses for significant differences between the dependent measures were carried out with Fisher’s least significant difference test. An

3. Results 3.1. Comparison of Minimal BWS to 10% BWS There were no significant temporospatial or kinematic differences between the Minimal BWS condition and 10% BWS. Since the Minimal BWS provided a variable amount of lifting force (3 /6% of the subject’s mass), additional statistical comparisons are reported only between the fixed levels of BWS (10, 20, 50 and 70%). The Minimal BWS data are provided in the tables for reference. 3.2. Temporospatial comparisons The ensemble means and standard deviations for the overall temporospatial data at each BWS level are provided in Table 2. The P -values for statistical comparisons of the overall temporospatial data by level of BWS are listed in Table 3. Significant changes in cadence and step length were present when comparing 10 vs 50% BWS and 10 vs 70% BWS. The changes were small between the least and greatest BWS levels; cadence decreased 2.3 steps min
1 and step length increased 1.7 cm. The proportions of the gait cycle spent in stance, swing and initial DLS were altered in response to BWS and the threshold for significant changes was a change in BWS level of 40% (10 vs 50%; 10 vs 70%; 30 vs 70%). Additionally, the comparison of 30 vs 50% BWS was significantly different for total swing period and approached significance for total stance period and initial

Table 1 Lower extremity sagittal kinematic data extracted for analysis Ankle

Knee

Hip

Ankle Angle at IC: Average angular position of the ankle at the point of IC Ankle Max Angle during Early Stance: Average maximum angle of the ankle as it moved toward PF during the first 20% of the gait cycle Ankle Angle at PSw: Average angular position of the ankle at the beginning of PSw (at the beginning of terminal DLS) Ankle Angle at TO: Average angular position of the ankle at the point of toe-off Ankle Max PF Angle during Swing: Average maximum plantar flexion angle of the ankle during the swing phase

Knee Angle at IC: Average angular position of the knee at the point of foot strike Knee Max Flex Angle during Early Stance: Average maximum flexion angle of the knee during the first 20% of the gait cycle Knee Angle at PSw: Average angular position of the knee at the beginning of PSw (at the beginning of terminal DLS) Knee Angle at TO: Average angular position of the knee at the point of toe-off Knee Max Flex Angle during Swing: Average maximum flexion angle of the knee during the swing phase

Hip Angle at IC: Average angular position of the hip at the point of foot strike Hip Max Ext Angle during Stance: Average maximum extension angle of the hip during the stance phase Hip Angle at PSw: Average angular position of the hip at the beginning of PSw (at the beginning of terminal DLS) Hip Angle at TO: Average angular position of the hip at the point of toe-off Hip Max Flex Angle during Swing: Average maximum flexion angle of the knee during the swing phase

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238 Table 2 Overall temporospatial data Condition (%) Cadence (steps min
1)

Minimal 10 30 50 70

Step length (cm)

Total stance period (% of cycle)

Total swing period (% of cycle)

Initial double stance period (% of cycle)

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

111.1 110.0 108.7 108.0 107.7

5.3 5.7 5.2 4.6 6.5

67.9 68.3 69.3 69.8 70.0

3.7 3.6 3.5 3.4 4.2

64.1 64.1 63.5 62.5 60.1

1.3 1.3 1.3 1.7 3.1

35.9 36.0 36.5 37.5 40.2

1.3 1.3 1.3 1.7 2.8

14.1 14.1 13.4 12.5 9.8

1.4 1.4 1.3 1.8 2.8

DLS. The magnitude of change between the lowest and highest BWS level was approximate 4% cycle. 3.3. Joint angular comparisons The ensemble means and standard deviations for the sagittal angular data are provided in Tables 4 and 6 and Table 8, respectively. The P -values for comparisons of the sagittal angular data by BWS level are listed in Tables 5 and 7 and Table 9, respectively. Sagittal kinematic curves of the ankle hip and knee are provided in Fig. 1 through Fig. 4. Fig. 1 also includes normative kinematic bands taken from our laboratory data based on 14 unimpaired males (age 21 /47 years) walking on a level treadmill at 1.25 m s
1. The materials and methods used to collect the normative data were the same as used in the present study except no BWS vest or device was applied. Specific data from our normative database are included in the discussion as a comparative framework. 3.3.1. Ankle The sagittal plane ankle motion responded strongly to changes in BWS levels. As BWS increased, the ankle was positioned in progressively greater dorsiflexion at the time of initial contact (IC). After IC, subjects moved the ankle toward a more extended (plantarflexed) position as the limb was loaded during early stance. There were markedly significant differences for Ankle Max Angle during Early Swing between each of the BWS loading levels (P B/0.001). The mean ankle position did not pass

neutral and remained in dorsiflexion during early stance at 50 and 70% BWS. The lowest and highest levels of BWS produced a 58 difference in the maximum position of the ankle during the loading phase of stance with an increasingly dorsiflexed ankle position as BWS was increased. At the time of preswing (PSw), the subjects had a mean of 12.48 (9/3.6) of dorsiflexion at 10% BWS and became increasingly less dorsiflexed as BWS increased reaching a mean position of
/0.58 (9/6.0) of plantarflexion (PF) at 70% BWS. At the time of TO, subjects had a mean ankle angle of
/9.78 (9/4.5) of PF when 10% BWS was applied and a mean ankle position of
/19.98 (9/6.4) of PF at 70% BWS. The ankle positions at PSw and at TO were markedly different (P B/0.003) between each of the BWS conditions except Angle Angle at PSw between 10 and 30% BWS (P /0.178). With 10% BWS, the mean Ankle Max PF Angle during Swing was
/18.38 (9/4.6) of PF. At 70% BWS, subjects reached
/27.18 (9/5.6) of PF. The maximum ankle sagittal angular position during swing was statistically different between all four of the BWS conditions. There was a mean increase of
/8.88 of PF during the swing phase between the lowest and highest levels of BWS.

3.3.2. Knee The mean knee angle at the time of IC was slight hyperextension and was statistically the same regardless of BWS level. The subject’s mean Knee Max Angle

Table 3 P -values for overall tempororospatial data ( based on Fisher’s PLSD) Condition (%), interaction (%)

Cadence

Step length

Total stance period

Total swing period

Initial double stance period

10%, 10%, 10%, 30%, 30%, 50%,

0.0580 0.0032* 0.0007* 0.2621 0.1056 0.6117

0.0106 0.0003* B 0.0001* 0.2019 0.0565 0.5122

0.1074 B 0.0001* B 0.0001* 0.0111 B 0.0001* B 0.0001*

0.0905 B 0.0001* B 0.0001* 0.0074* B 0.0001* B 0.0001*

0.0679 B 0.0001* B 0.0001* 0.0133 B 0.0001* B 0.0001*

30% 50% 70% 50% 70% 70%

* Significant at P B 0.01.

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239

Table 4 Ankle kinematic data Condition (%) Ankle angle at IC (8)

Minimal 10 30 50 70 a

Ankle max angle during early stance (8)

Ankle angle at PSw Ankle angle at TO (8) (8)

Ankle max PF angle during swing (8)

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

3.7 4.0 4.6 5.4 7.3

2.5 2.8 2.8 2.0 2.5


2.1a
1.7a 0.0 1.6 3.7

2.8 2.8 2.9 2.6 2.9

12.6 12.4 11.0 7.2
0.5a

3.8 3.6 3.9 3.9 6.0


8.7a
9.7a
12.7a
16.1a
19.9a

4.7 4.5 5.7 5.8 6.4


17.9a
18.3a
21.9a
24.7a
27.1a

3.6 4.6 5.6 5.8 5.6

Negative values indicate ankle PF.

during early stance decreased as more BWS was applied: 16.58 with 10% BWS decreasing to 12.38 with 70% BWS. These differences in knee flexion during early stance were significant when support levels were changed at least 40% (10 vs 50% BWS, 10 vs 70% BWS; 30 vs 70% BWS). At the beginning of PSw, knee flexion increased as BWS increased. The mean Knee Angle at PSw was 4.38 (9/4.4) at 10% BWS and progressed to 13.88 (9/6.4) at 70% BWS. The Knee Angle at PSw was statistically different between every BWS level except 10 vs 30% BWS. At the time of TO, the knee displayed a small decrease in mean flexion as the BWS level was increased. The mean knee flexion was 35.38 (9/5.2) at 10% and decreased to 31.58 (9/6.6) at 70% BWS. There was a statistical difference between the Knee Angle at TO between 10% BWS vs 30, 50 and 70 BWS levels. The Knee Max Flex Angle during Swing showed a strong inverse relationship to the BWS level with 62.48 (9/3.3) at 10% BWS decreasing to 50.38 (9/3.9) at 70% BWS (Fig. 5). All of the statistical comparisons between BWS levels were markedly significant (P B/0.0001). 3.3.3. Hip When 10% BWS was applied, the subjects had a mean Hip Angle at IC of 22.38 (9/8.1). Hip flexion at time of IC diminished as greater BWS was applied until a mean of only 15.38 (9/8.3) of hip flexion was achieved at 70%

BWS. Statistical comparisons of Hip Angle at IC between all BWS levels was significant except 10 vs 30% BWS. A significant change in the mean Hip Max Ext Angle during Stance and the Hip Angle at PSw was produced only when 70% BWS was applied. The actual differences in mean hip extension at these two time points were small when comparing 10% BWS to 70% BWS: 3.28 at maximal extension during stance and 2.38 at PSw. By the time the gait cycle reached TO, there was no statistical difference in hip angle between BWS levels. The hip angle diverged again during the swing phase. Our subjects had a mean Hip Max Flex Angle during Swing of 26.08 (9/8.5) of flexion at 10% BWS decreasing to 17.98 (9/8.0) of flexion at 70% BWS. All of the BWS levels were statistically different from one another for Hip Max Flex Angle during Swing.

4. Discussion Our research hypotheses were supported. The application of BWS reduced the stance and DLS time while producing significant changes in the sagittal angular excursion at the hip, knee and ankle. These changes became more pronounced with increasing levels of BWS.

Table 5 P -values for ankle kinematic data (based on Fisher’s PLSD) Condition (%), interaction Ankle angle at (%) IC

Ankle max angle during early stance

Ankle angle at PSw

Ankle angle at TO

Ankle max PF angle during swing

10%, 10%, 10%, 30%, 30%, 50%,

B 0.0001* B 0.0001* B 0.0001* 0.0003* B 0.0001* B 0.0001*

0.1780 B 0.0001* B 0.0001* 0.0006* B 0.0001* B 0.0001*

0.0028* B 0.0001* B 0.0001* 0.0007* B 0.0001* 0.0002*

0.0001* B 0.0001* B 0.0001* 0.0025* B 0.0001* 0.0085*

30% 50% 70% 50% 70% 70%

* Significant at P B 0.01.

0.2650 0.0085* B 0.0001* 0.1167 B 0.0001* 0.0004*

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240 Table 6 Knee kinematic data Condition (%) Knee angle at IC (8)

Minimal 10 30 50 70 a

Knee max flex angle during early stance (8)

Knee angle at PSw Knee angle at TO (8) (8)

Knee max flex angle during swing (8)

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.


0.2a
1.3a
1.0a
2.4a
2.2a

5.1 5.1 5.1 4.8 5.0

16.6 16.5 15.1 13.6 12.3

4.6 4.0 4.3 3.9 4.7

3.8 4.3 5.0 7.7 13.8

4.5 4.4 4.6 5.3 6.4

34.9 35.3 33.1 32.7 31.5

5.6 5.2 6.0 5.8 6.6

63.0 62.4 58.4 54.8 50.3

3.7 3.3 3.7 3.2 3.9

Negative values indicate knee hyperextension.

4.1. Temporospatial gait variables Since we controlled walking velocity through a fixed treadmill speed, the temporospatial changes imply that as more external support was imposed, the subject progressively shifted more of the task of gait stability to the BWS device. The two highest levels of BWS (50 and 70%) produced a decrease in cadence, an increase in step length, a reduction in the stance phase and a decrease in DLS. Taken together, these changes indicate that subjects discarded the strategies associated with increasing gait stability [22,23]. Finch et al. also reported that increased levels of BWS caused a shortening of stance time and DLS time [16]. However, Finch et al. reported a mean total stance period of 59.9% Cycle at 0% BWS (treadmill speed of 1.36 m s
1) dropping to 51.7% Cycle at 70% BWS (treadmill speed of 0.7 m s
1), a change of 8.2% Cycle. Our subjects demonstrated a total stance period of 64.1% of cycle at the lowest levels of BWS (Minimal and 10%) dropping to 60.1% of cycle at 70% BWS (treadmill speed was constant at 1.25 m s
1), a change of only 4% Cycle. We consider the results of Finch et al. to reflect an interaction of BWS with treadmill speed. The interaction of treadmill walking speed with lower extremity kinematics has been documented by Nilsson and Thorstensson [25]. Although methodological details concerning the implementation of 0% BWS were not provided by Finch et al., other technical factors may have also contributed to the 4.2% Cycle difference between our stance time data and

that of Finch et al. such as the inherent stiffness of the BWS systems, the harness design and methods used to detect the beginning and end of stance. Data extracted from Finch et al. indicated initial DLS was 11.2% Cycle at 0% BWS (treadmill speed of 1.36 m s
1) dropping to 4.3% Cycle at 70% BWS (treadmill speed of 0.7 m s
1). Our data showed that initial DLS ranged from 14.1% Cycle at Minimal BWS dropping to 9.8% Cycle at 70% BWS (treadmill speed was constant at 1.25 m s
1). Considering the relatively small % Cycle normally spent in initial DLS, there is a striking difference between our results and those of Finch et al. and the differences appear too large to be attributable primarily to treadmill speed. The BWS system used in our experiment employed a pneumatic cylinder that provided a compliant lifting force that adjusted its lifting force to account for vertical movement of the subject’s center of mass. The BWS device used by Finch et al. utilized a motor and cotton webbing to supply a fixed amount of lift that would strongly resist the descent of the subject’s center of mass [16,24]. Dissimilar mechanical compliance of the BWS devices and vertical restraint of the center of mass may account for the difference in DLS time. 4.2. Ankle angular positions and temporal components During normal overground gait, the ankle is positioned in dorsiflexion at IC then rapidly moves into PF as weight is transferred to the limb [20,21]. Our

Table 7 P -values for knee kinematic data (based on from Fisher’s PLSD) Condition (%), interaction Knee angle at (%) IC

Knee max flex angle during early Knee angle at stance PSw

Knee angle at TO

Knee max flex angle during swing

10%, 10%, 10%, 30%, 30%, 50%,

0.0251 B 0.0001* B 0.0001* 0.0315 B 0.0001* 0.0445

0.0087* 0.0020* B 0.0001* 0.6109 0.0591 0.1633

B 0.0001* B 0.0001* B 0.0001* B 0.0001* B 0.0001* B 0.0001*

30% 50% 70% 50% 70% 70%

0.6614 0.0383 0.0972 0.013 0.0376 0.6672

* Significant at P B 0.01.

0.4220 0.0003* B 0.0001* 0.0040* B 0.0001* B 0.0001*

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241

Table 8 Hip kinematic data Condition (%) Hip angle at IC (8) Hip max ext angle during stance Hip angle at PSw (8) Hip angle at TO (8) Hip max flex angle during swing (8) (8)

Minimal 10 30 50 70 a

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

23.5 22.3 20.9 18.1 15.3

8.7 8.1 8.5 8.1 8.3


16.1a
16.2a
16.7a
15.9a
13.0a

8.1 7.4 8.1 7.7 9.0


14.5a
14.9a
15.6a
15.2a
12.6a

8.1 7.5 7.8 7.5 8.9


8.3a
8.0a
9.1a
8.7a
7.7a

8.6 8.3 8.7 8.1 8.9

26.8 26.0 23.8 21.0 17.9

8.8 8.5 8.4 7.6 8.0

Negative values indicate hip extension.

normative laboratory database for treadmill gait with no BWS indicates that the foot lands with the ankle in slight dorsiflexion (3.89/2.68) and moves to slight PF (
/ 3.59/2.48) during initial DLS (Fig. 1). The lowest levels of BWS (Minimal, 10 and 30%) allowed a mean Ankle Angle at FS that was similar to normal but induced a progressively more dorsiflexed Ankle Angle at FS with 50 and 70% BWS. With all levels of BWS, the ankle moved toward PF after IC although the mean movements had progressively less angular excursion during loading and did not move past neutral at the highest levels of BWS (50 and 70%). A much larger change was induced by BWS in the sagittal ankle position at PSw and at TO. Our normative laboratory database for treadmill gait indicates that mean ankle position is 12.58 (9/4.0) of dorsiflexion at PSw and moves to
/4.78 (9/5.3) of PF at TO (Fig. 1). Fig. 2 clearly shows that as BWS was increased, the subjects began to plantarflex their ankles progressively earlier in the gait cycle even beginning as early as midstance with 70% BWS. This signals an early end to the ankle rocker (foot flat) phase and an early beginning to the forefoot rocker (heel off) phase. In conjunction with this change, TO occurred progressively earlier in the gait cycle with increasing BWS resulting in a 4% Cycle shift of the stance period of the ankle kinematic curve to the left. The kinematic pattern of the ankle in late stance prior to terminal DLS is coupled to the energetics of gait such

that progressive dorsiflexion in late single limb stance is controlled by eccentric contraction of the posterior shank musculature and acts to slow the forward progression of the tibia, preventing collapse of the limb [20,22]. At the beginning of terminal DLS, the direction of ankle movement is reversed such that concentric contraction of the plantar flexor musculature assists in driving the tibia forward to prepare for the swing phase and assist with knee flexion [20,22]. In the presence of higher levels of BWS, the demand for damped dorsiflexion followed by energetic PF during the latter portion of stance may be reduced. During terminal DLS, the subject may be allowing the moving belt to pull the foot posteriorly while the ankle is quickly plantarflexed to maintain contact of the forefoot on the treadmill. This explanation is consistent with our temporospatial data, which indicate that the task of stance phase stability is shared with the BWS device and with the report of Griffin et al., which documents that anterior /posterior ground reaction forces are reduced as BWS is increased [26]. In normal overground gait, the maximum ankle PF angle occurs soon after swing phase begins [20,21]. Our normative laboratory treadmill data indicates that ankle PF during swing reaches a maximum of
/18.58 (9/6.3) which occurs at a 69.2 (9/0.7)% of the gait cycle (Fig. 1). The lowest levels of BWS (Minimal and 10% BWS) demonstrated essentially normal excursion of swing phase PF. The PF was progressively greater as lift was

Table 9 P -values for hip kinematic data (based on Fisher’s PLSD) Condition (%), interaction (%)

Hip angle at IC Hip max ext angle during stance

Hip angle at PSw

Hip angle at TO

Hip max flex angle during swing

10%, 10%, 10%, 30%, 30%, 50%,

0.0609 B 0.0001* B 0.0001* 0.0003* B 0.0001* 0.0002*

0.2005 0.5306 0.0003* 0.5097 B 0.0001* B 0.0001*

0.0937 0.2900 0.7053 0.5281 0.0414 0.1528

0.0065* B 0.0001* B 0.0001* 0.0003* B 0.0001* 0.0002*

30% 50% 70% 50% 70% 70%

* Significant at P B 0.01.

0.3796 0.6447 B 0.0001* 0.1824 B 0.0001* B 0.0001*

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Fig. 2. Sagittal plane kinematics of the ankle. Each line represents the ensemble average of all subjects for angular excursion of the ankle during the gait cycle for 10% (solid line), 30% (triangle), 50% (circle), and 70% (square) BWS conditions. Negative values for angular position represent ankle PF and positive values depict ankle dorsiflexion. The dashed vertical line shows the average point of toe-off under the 10% BWS condition for all subjects. The solid vertical line marks the average point of toe-off under the 70% BWS condition for all subjects. Two specific angular positions that were used in the analysis are indicated with labels on the graph: the maximum value for ankle PF during the early stance period (Max Angle during Early Stance) and the maximum angle of ankle PF during the swing period (Max PF Angle during Swing).

Fig. 1. Mean sagittal plane angular excursion for the hip (A), knee (B), and ankle (C) under the 10 and 70% BWS conditions. The solid line represents the 10% BWS condition and the line with squares illustrates the 70% BWS condition. The gray band in the background of the graph, represents 9/1 S.D. around the mean angular joint excursion of our laboratory reference database (Lab Norms) for normal, unsupported treadmill walking at 1.25 m s
1. The dashed vertical line shows the average point of toe-off under the 10% BWS condition. The solid vertical line indicates the average point of toe-off under the 70% BWS condition.

applied above 10% BWS. The maximum swing phase PF angle was reached within 4.1 /4.7% of cycle after TO regardless of BWS level. The greater PF angle produced a relative lengthening of the limb during early swing that in turn had to be overcome by compensatory proximal kinematic changes to permit foot clearance.

Fig. 3. Sagittal plane kinematics of the knee. Each line represents the ensemble average of all subjects for angular excursion of the knee during the gait cycle for 10% (solid line), 30% (triangle), 50% (circle), and 70% (square) BWS conditions. Negative values for angular position represent knee hyperextension and positive values depict knee flexion. The dashed vertical line shows the average point of toe-off under the 10% BWS condition for all subjects. The solid vertical line marks the average point of toe-off under the 70% BWS condition for all subjects. Two specific angular positions that were used in the analysis are indicated with labels on the graph: the maximum knee flexion at the onset of PSw (Angle of PSw) and the maximum angle of knee flexion during the swing period (Max Flex Angle during Swing).

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4.3. Knee angular positions and temporal components After IC, the changes in knee kinematics worked in conjunction with the changes in sagittal ankle motion through the stance phase. During the loading phase and early stance, the knee flexed less as BWS increased (Fig. 3). Simultaneously the ankle underwent progressively less PF (Fig. 2) thereby maintaining stance-limb length. Between 30 and 50% Cycle, the knee underwent a progressively earlier second wave of flexion as BWS increased. Over the same period, the ankle was rapidly plantarflexed to retain foot contact on the treadmill belt. The earlier knee flexion compensated for the relative limb lengthening caused by exaggerated ankle PF. Finch et al. reported a maximum knee flexion angle during swing of 72.48 at 0% BWS decreasing to 55.98 at 70% BWS. These angles are quite different from our subjects whose mean maximum flexion during swing was 62.48 at 10% BWS decreasing to 50.38 at 70% BWS. The shape and divergence of the sagittal kinematic knee curves through the gait cycle as published by Finch et al. are difficult to compare to ours due to their use of event normalization. Similar to our data, the sagittal knee kinematic curves published by Finch et al. show divergence between BWS levels in the period from mid-stance to maximum swing angle although no

Fig. 4. Sagittal plane kinematics of the hip. Each line represents the ensemble average of all subjects for angular excursion of the hip during the gait cycle for 10% (solid line), 30% (triangle), 50% (circle), and 70% (square) BWS conditions. Negative values for angular position represent hip extension and positive values depict hip flexion. The dashed vertical line shows the average point of toe-off under the 10% BWS condition for all subjects. The solid vertical line marks the average point of toe-off under the 70% BWS condition for all subjects. Two specific angular positions that were used in the analysis are indicated with labels on the graph: the maximum value for hip extension at the time of PSw (Angle at PSw) and the maximum angle of hip flexion during the swing period (Max Flex Angle during Swing).

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descriptive data or statistical comparisons were provided by Finch et al. 4.4. Hip angular positions and temporal components The kinematic graph of sagittal hip motion demonstrated a pattern of response to BWS that was much different than the knee and ankle (Fig. 4). Whereas the largest alterations of the ankle and knee kinematics in response to BWS occurred between 10 and 65% Cycle, the largest divergences of the sagittal hip kinematics centered around IC. The divergence began in mid- to late-stance (80 /100% Cycle) and returned to a nearnormal pattern following the loading response of the following stance phase (IC to 10% Cycle). Finch et al. did not include specific numeric data for the hip angle at IC but reported similar changes in maximum hip flexion during swing: mean hip maximum swing angle of 29.98 at 0% BWS decreasing to 20.28 at 70% BWS. Our subject’s maximum hip flexion during swing decreased from 268 (9/8.5) at 10% BWS to 17.98 (9/8.0) at 70% BWS. Intuitively, the reduction in hip flexion prior to IC associated with higher BWS levels would lead to a shorter step length but our data show that there was actually a slight increase in mean step length (1.7 cm) between the lowest and highest BWS levels. This seeming paradox could be explained by the strong compensatory plantar flexion of the ankle and by a change in the kinematics demands of treadmill walking with BWS. After IC, the stance foot moves posteriorly on the treadmill belt and passes behind the relatively stationary line of gravity by the time terminal DLS begins. Since a portion of the body mass was supported in the BWS device, the subject may allow the forefoot to remain on the treadmill longer than would be possible if the body mass was being propelled forward as in overground gait. Rather, the forefoot may be utilized to provide balance during terminal DLS, a function permitted by higher levels of BWS and reflected in the increase of ankle PF and reduced knee flexion at the time of TO. Although subjects may not have stepped as far forward because of less hip flexion, the increased posterior travel of the foot on the treadmill may have resulted in a small net increase in step length. The mean maximum hip extension angle from our normal treadmill gait database is 16.18 (9/6.6) and occurs in the midst of the terminal DLS period (55.59/ 1.1% Cycle) (Fig. 1). In symmetric gait, terminal DLS predictably begins at 50% Cycle. The hip motion in our normal database demonstrated continued extension between 50 and 55% Cycle while the transfer of weight to the contralateral limb was occurring. In contrast, at the highest level of BWS, the hip motion on the PSw side showed less total extension and a reversal of direction and extension toward flexion when the contralateral

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limb touched down at 50% Cycle. This supports our contention that at higher levels of BWS, the PSw limb had less need to supply an upward or forward thrust to propel the center of mass. Rather, the forefoot continued to ride posteriorly on the moving belt via rapid ankle PF coupled with a low knee flexion slope, providing additional stability.

5. Conclusions During treadmill walking at 1.25 m s
1, the temporospatial and kinematic patterns of the lower extremities of unimpaired subjects became significantly distorted at 50 and 70% BWS (compared to 10% BWS). Increasing the magnitude of BWS shifted the kinematic interplay between the lower extremity joints. If restoration of normal gait kinematics is a key rehabilitation or training goal, our results provide guidance in choosing BWS levels that minimize alterations in the normal lower extremity temporospatial and sagittal kinematic patterns. Support levels of 10 and 30% produced the least distortion of gait. If goals other than normal kinematics are paramount, our results can be utilized to distinguish the gait alterations induced by BWS from those due to pathology or intervention. Additionally, specific motions may be targeted by selection of higher BWS levels. For example, 50 and 70% BWS produced large increases in ankle PF during PSw and early swing. Sensory input from ankle PF at this time has been shown to be a powerful stimulus for the stepping response and may assist in early phases of gait training for persons with neurologic impairment [1 / 8]. The interaction of BWS and treadmill speed on lower extremity kinematics has yet to be clearly described. In addition, the mechanical characteristics of BWS devices appear to have an effect on gait. Given the widespread utilization of BWS in clinical situations, additional research is warranted.

Acknowledgements Supported in part by Creighton Health Futures Foundation Grant #200256-726000.

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