Accepted Manuscript Postural and dynamic balance while walking in adults with an incomplete spinal cord injury Jean-François Lemay, Cyril Duclos, Sylvie Nadeau, Dany Gagnon, Émilie Desrosiers PII: DOI: Reference:
S1050-6411(14)00077-7 http://dx.doi.org/10.1016/j.jelekin.2014.04.013 JJEK 1706
To appear in:
Journal of Electromyography and Kinesiology
Received Date: Revised Date: Accepted Date:
28 October 2013 19 March 2014 17 April 2014
Please cite this article as: J-F. Lemay, C. Duclos, S. Nadeau, D. Gagnon, É. Desrosiers, Postural and dynamic balance while walking in adults with an incomplete spinal cord injury, Journal of Electromyography and Kinesiology (2014), doi: http://dx.doi.org/10.1016/j.jelekin.2014.04.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Journal of Electromyography and Kinesiology
2
Original Article
3
Postural and dynamic balance while walking in adults with an incomplete spinal
4
cord injury.
5 6
Jean-François Lemaya,b, Cyril Duclosa,b, Sylvie Nadeaua,b, Dany Gagnona,b, Émilie
7
Desrosiersb.
8 9
a
10 11 12
b
School of Rehabilitation (www.readap.umontreal.ca), Université de Montréal, Montreal, Canada Pathokinesiology Laboratory (www.pathokin.ca), Centre for Interdisciplinary Research in Rehabilitation of Greater Montreal — Institut de réadaptation Gingras-Lindsay de Montréal, Montreal, Canada
13 14 15 16 17 18 19 20 21
Jean-François Lemay is supported by a doctoral scholarship from the Fonds de la recherche du Québec-Santé (FRQS). Dany Gagnon holds a Junior 1 Award from the FRQS. Cyril Duclos, Sylvie Nadeau and Dany Gagnon are members of the QuebecOntario Spinal Cord Injury Research Team and the SensoriMotor Rehabilitation Research Team. The equipment and material required to complete this project at the Pathokinesiology Laboratory was financed in part by the Canada Foundation for Innovation. This project was funded in part by a research grant from the Craig H. Neilson Foundation.
22 23
Corresponding author:
24 25 26 27
Jean-François Lemay, Centre for Interdisciplinary Research in Rehabilitation of Greater Montreal – Institut de réadaptation Gingras-Lindsay-de-Montréal, Pathokinesiology Laboratory, 6300 avenue Darlington, Montreal, QC, Canada H3S 2J4. E-mail: [email protected]
28 29
Keywords:
30
Gait; biomechanics; balance; rehabilitation; spinal cord injury.
31
Abstract: 218 words
32
Manuscript: 4250 words (4400 max)
33
Tables and figures: 6
35
36
Introduction
37
Following a spinal cord injury, individuals commonly mention recovery of walking
38
function as one of the most important goals of rehabilitation (Ditunno et al., 2008). The
39
prognosis of independent walking among people with an incomplete spinal cord injury
40
(ISCI) (American Spinal Injury Association Impairment Scale (AIS), level D), being
41
greater than 80%, supports the likelihood of reaching this goal (Scivoletto et al., 2009).
42
In addition to impaired walking abilities, an ISCI disrupts the normal flow of sensory and
43
motor information involved in balance control (Barbeau et al., 1999). Standing balance is
44
indeed considered a major determinant of walking abilities (Barbeau et al., 2006,
45
Scivoletto et al., 2008) and is associated with walking speed and the use of ambulatory
46
assistive devices in this population (Lemay and Nadeau, 2009, Wirz et al., 2010).
47
Balance impairments may also partly explain the high incidence of falls in this population
48
(39-75% according to various studies (Amatachaya et al., 2011, Brotherton et al., 2007,
49
Phonthee et al., 2013)) and resulting secondary fall-related complications, such as
50
fractures and decreased social participation (Amatachaya et al., 2011, Brotherton et al.,
51
2007, Phonthee et al., 2013).
52
To date, few studies have quantified balance during gait in individuals with SCI. Day et
53
al. (2011) documented greater variability in step width and step length, margin of stability
54
as well as anteroposterior and mediolateral foot placement compared to able-bodied
55
individuals while walking on a treadmill (Day et al., 2011). Moreover, variability in step
56
length and foot placement was significantly and negatively associated with the Berg
57
Balance Scale, a clinical measure of balance (Berg et al., 1989). Leroux et al. (2006)
58
previously reported postural modification including accentuated trunk flexion and forward
59
pelvic tilt in individuals with an ISCI while walking on a treadmill as the grades of
60
inclination were increased, presumably to compensate for decreased balance or
61
secondary to the use of ambulatory assistive devices (Leroux et al., 2006). However, this
62
group of authors did not use specific measures of balance. Previous studies have also
63
shown that individuals with an ISCI have reduced balance during quasi-static stance
64
performed with eyes open or eyes closed as revealed by an increase in the center of
65
pressure (COP) mean velocity, sway area and root mean square distance compared to
66
able-bodied individuals (Lemay et al., 2013). Moreover, individuals with SCI display a
67
delayed onset of electromyographic activity in the ankle muscles following an external
68
perturbation (Thigpen et al., 2009), which may further impair the ability to control
69
balance.
70
Duclos et al. developed a model to assess balance difficulty during functional tasks such
71
as standing balance tasks (Duclos et al., 2012), walking on level surfaces (Duclos et al.,
72
2009) and sitting pivot transfers (Gagnon et al., 2012). Using the kinematic and ground
73
reaction force data collected in a laboratory environment, this model calculates the
74
stabilizing and destabilizing forces – the theoretical forces necessary to stabilize or
75
destabilize the person at each moment of the entire task (Duclos et al., 2009). This
76
model assesses both the postural and dynamic components of balance during the
77
performance of dynamic tasks when the base of support is moving and changing in
78
shape or in size (Duclos et al., 2012) and thus provides a comprehensive understanding
79
of balance difficulties. This model may therefore serve to optimize interventions in
80
rehabilitation aimed at improving walking abilities and reducing the risk of falls among
81
individuals with an ISCI.
82
We thus conducted a comprehensive laboratory-based protocol to calculate the
83
stabilizing/destabilizing forces to study balance during gait among individuals with an
84
ISCI and their able-bodied counterparts. The main purpose of this study was to compare
85
balance in these two groups during the single support phase of gait. This sub-phase of
86
the gait cycle was chosen because weight bearing on one foot while progressing forward
87
is a demanding and unstable subtask as previously shown in able-bodied and elderly
88
individuals (Duclos et al., 2009, Duclos et al., 2012). Our specific objectives were as
89
follows: (1) to compare the postural and dynamic components of balance in individuals
90
with an ISCI and able-bodied participants during the single support phase of gait, as well
91
as at specific moments during the single support phase; and (2) to identify
92
biomechanical variables that explain the values of these forces. The main hypothesis
93
was that compared to able-bodied individuals, both (postural and dynamic) components
94
of balance are impaired in individuals with an ISCI. Since the equations used to calculate
95
the stabilizing and destabilizing forces are based on recognized biomechanical factors
96
related to balance such as the COM speed (Pai and Patton, 1997) and the distance
97
between the COP and the limits of the base of support (Popovic et al., 2000), we
98
expected these to be important explanatory factor of the maximal stabilizing force and
99
the minimal destabilizing force respectively (Duclos et al., 2009, Duclos et al., 2012).
100
Methods
101
Participants
102
A convenience sample of 17 individuals with a traumatic ISCI (AIS D) were recruited
103
from both inpatients and outpatients of the SCI rehabilitation unit of the Institut de
104
réadaptation Gingras-Lindsay-de-Montréal (IRGLM) and compared to 17 age-matched
105
able-bodied controls. The inclusion criteria for the participants with an ISCI were: (1) the
106
ability to stand for five minutes without external support and (2) the ability to walk
107
independently for 15 m without ambulatory assistive devices. Participants were excluded
108
if they presented other neurological deficits in addition to the SCI or if they had walking
109
or balance difficulties prior to the SCI. None of the able-bodied participants reported
110
having physical impairments that interfered with walking or balance. Ethical approval
111
was obtained from the Research Ethics Committee of the Centre for Interdisciplinary
112
Research in Rehabilitation of Greater Montreal (CRIR). Each participant provided written
113
consent after having read and understood the information about the study. ***insert table 1 about here***
114 115
Clinical assessment
116
For the participants with an ISCI, demographic information regarding the date of injury
117
and AIS level were gathered from the participant’s chart. An experienced physical
118
therapist conducted the Lower Extremity Motor Score Assessment (LEMS) according to
119
American Spinal Cord Injury Association (ASIA) standards (Kirshblum et al., 2011). The
120
LEMS is an evaluation of lower extremity muscle strength of 5 major muscle groups,
121
representing the myotome from L2 to S1, using a 6 point (0 to 5) ordinal scale. Natural
122
walking speed was tested over a distance of 15 meters (Lam et al., 2008). Participants
123
were asked to perform three trials at their self-selected comfortable walking speed
124
without any walking assistive devices. Performance over the middle 10-m section was
125
timed and speed was calculated (10-m divided by the time in second). The three trials
126
were averaged and the mean walking speed was calculated.
127
Gait assessment
128
Kinematic and kinetic data for all participants were collected during a 3-hour assessment
129
session conducted at the Pathokinesiology Laboratory at the IRGLM. Thirty-six skin-
130
fixed infrared light emitting diodes (LEDs) were used to define the head, trunk, and
131
bilateral upper and lower extremity segments. In addition, a six-marker probe was used
132
to digitize the location of 13 bony landmarks to locate joint axes and to define the base
133
of support (BOS). A system linking four synchronized motion analysis camera bars
134
(Optotrak model 3020; Northern Digital Inc., Waterloo, Ontario, Canada) captured the
135
three-dimensional (3D) coordinates of the markers at a sampling frequency of 60 Hz.
136
Kinematic data were filtered with a fourth-order Butterworth zero-lag filter, with a cut-off
137
frequency of 6 Hz. Anthropometric measurements of the trunk, head, lower and upper
138
extremities were taken at the end of the laboratory session. These data were entered in
139
the biomechanical calculation using the 3D link model segment (Winter et al., 1990). In
140
terms of the kinetic data, three AMTI® force platforms, sequentially embedded in a 9-m
141
long walkway, recorded the ground reaction forces at a frequency of 600 Hz. The data
142
were filtered with a fourth-order Butterworth zero-lag filter, with a cut-off frequency of
143
10 Hz and down-sampled to 60 Hz to match the kinematic data.
144
Participants walked along the laboratory pathway at their self-selected comfortable
145
speed. After a period of familiarization, five trials for each foot were recorded. Trials were
146
analyzed for the lower extremity while stepping onto the force plate, between toe off and
147
the following heel strike of the contralateral lower extremity.
148
Data and statistical analyses
149
Analyses were performed on the single support phase of the weakest lower extremity
150
(right/left: 8/9 consecutive steps; based on the LEMS) for individuals with an ISCI to
151
better reveal balance difficulties. For the able-bodied participants, balance parameters of
152
eight right and nine left consecutive steps (i.e., single support phase only) were analyzed
153
to match the data of the individuals with an ISCI.
154
The single support phase of the gait cycle, from contralateral toe off to contralateral heel
155
strike, was normalized from 1 to 100 (100 data points). Data were then analyzed using
156
the stabilizing/destabilizing forces model. The stabilizing force (SF) is the theoretical
157
force necessary to stop both the COP and the COM at the limit of the potential base of
158
support (defined as the outside perimeter of the vertical projection of both feet on the
159
floor) at each moment of the task. It is calculated with the following equation: = −
160 161
∙ ∙ 2
where corresponds to the stabilizing force, to the participant’s mass, to to the horizontal distance between the position of the linear velocity of the COM and
162
the COP and the limit of the base of support in the direction of the COM linear velocity
163
(Duclos et al., 2009, Duclos et al., 2012). High stabilizing force values indicate higher
164
levels of difficulty in controlling the velocity of the centre of mass. The destabilizing force
165
(DF) is the force needed to move the COP to the limit of the base of support and is
166
calculated with the following equation: ∙ = ℎ
167 168 169
where corresponds to the destabilizing force, to the ground reaction force, to the
unitary vector normal to the contact surface, ℎ to the height of the COM from the to the horizontal distance between the position of the COP supporting surface and
170
and the limit of the base of support in the direction of the COM linear velocity (Duclos et
171
al., 2009, Duclos et al., 2012). Low destabilizing force values indicate a position of the
172
body that is destabilized more easily. Mean stabilizing force and destabilizing force
173
values over the single support phase, as well as the maximum stabilizing force value
174
and the minimum destabilizing force value were averaged for each participant and used
175
to characterize balance during the single support phase (Duclos et al., 2009). Mean
176
values characterize overall stability during the single support phase while extreme
177
values represent the maximal instability experienced during the task.
178
Descriptive statistics (mean and SD) were calculated for the participants’ characteristics
179
(e.g., age, height, mass, BMI, and time post lesion) and for the clinical assessments
180
(e.g., natural walking speed, unipodal stance phase, and LEMS). For both groups of
181
participants, the normality of the data distribution and the homogeneity of variance were
182
explored using the Shapiro-Wilk test and Levene’s test, respectively. The maximal
183
stabilizing force and the minimal destabilizing force of individuals with SCI and the COM
184
speed at maximal destabilizing force of able-bodied participants were abnormally
185
distributed. Logarithmic transformation was applied and successfully corrected the initial
186
departure from normality (Field, 2009). Between-group differences for all balance
187
difficulty parameters were explored using independent Student t-tests. To further
188
appreciate the importance of group differences of the various parameters, Cohen’s d
189
effect sizes were also computed (Cohen, 1988). The effect sizes were considered small
190
(
S1050-6411(14)00077-7 http://dx.doi.org/10.1016/j.jelekin.2014.04.013 JJEK 1706
To appear in:
Journal of Electromyography and Kinesiology
Received Date: Revised Date: Accepted Date:
28 October 2013 19 March 2014 17 April 2014
Please cite this article as: J-F. Lemay, C. Duclos, S. Nadeau, D. Gagnon, É. Desrosiers, Postural and dynamic balance while walking in adults with an incomplete spinal cord injury, Journal of Electromyography and Kinesiology (2014), doi: http://dx.doi.org/10.1016/j.jelekin.2014.04.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Journal of Electromyography and Kinesiology
2
Original Article
3
Postural and dynamic balance while walking in adults with an incomplete spinal
4
cord injury.
5 6
Jean-François Lemaya,b, Cyril Duclosa,b, Sylvie Nadeaua,b, Dany Gagnona,b, Émilie
7
Desrosiersb.
8 9
a
10 11 12
b
School of Rehabilitation (www.readap.umontreal.ca), Université de Montréal, Montreal, Canada Pathokinesiology Laboratory (www.pathokin.ca), Centre for Interdisciplinary Research in Rehabilitation of Greater Montreal — Institut de réadaptation Gingras-Lindsay de Montréal, Montreal, Canada
13 14 15 16 17 18 19 20 21
Jean-François Lemay is supported by a doctoral scholarship from the Fonds de la recherche du Québec-Santé (FRQS). Dany Gagnon holds a Junior 1 Award from the FRQS. Cyril Duclos, Sylvie Nadeau and Dany Gagnon are members of the QuebecOntario Spinal Cord Injury Research Team and the SensoriMotor Rehabilitation Research Team. The equipment and material required to complete this project at the Pathokinesiology Laboratory was financed in part by the Canada Foundation for Innovation. This project was funded in part by a research grant from the Craig H. Neilson Foundation.
22 23
Corresponding author:
24 25 26 27
Jean-François Lemay, Centre for Interdisciplinary Research in Rehabilitation of Greater Montreal – Institut de réadaptation Gingras-Lindsay-de-Montréal, Pathokinesiology Laboratory, 6300 avenue Darlington, Montreal, QC, Canada H3S 2J4. E-mail: [email protected]
28 29
Keywords:
30
Gait; biomechanics; balance; rehabilitation; spinal cord injury.
31
Abstract: 218 words
32
Manuscript: 4250 words (4400 max)
33
Tables and figures: 6
35
36
Introduction
37
Following a spinal cord injury, individuals commonly mention recovery of walking
38
function as one of the most important goals of rehabilitation (Ditunno et al., 2008). The
39
prognosis of independent walking among people with an incomplete spinal cord injury
40
(ISCI) (American Spinal Injury Association Impairment Scale (AIS), level D), being
41
greater than 80%, supports the likelihood of reaching this goal (Scivoletto et al., 2009).
42
In addition to impaired walking abilities, an ISCI disrupts the normal flow of sensory and
43
motor information involved in balance control (Barbeau et al., 1999). Standing balance is
44
indeed considered a major determinant of walking abilities (Barbeau et al., 2006,
45
Scivoletto et al., 2008) and is associated with walking speed and the use of ambulatory
46
assistive devices in this population (Lemay and Nadeau, 2009, Wirz et al., 2010).
47
Balance impairments may also partly explain the high incidence of falls in this population
48
(39-75% according to various studies (Amatachaya et al., 2011, Brotherton et al., 2007,
49
Phonthee et al., 2013)) and resulting secondary fall-related complications, such as
50
fractures and decreased social participation (Amatachaya et al., 2011, Brotherton et al.,
51
2007, Phonthee et al., 2013).
52
To date, few studies have quantified balance during gait in individuals with SCI. Day et
53
al. (2011) documented greater variability in step width and step length, margin of stability
54
as well as anteroposterior and mediolateral foot placement compared to able-bodied
55
individuals while walking on a treadmill (Day et al., 2011). Moreover, variability in step
56
length and foot placement was significantly and negatively associated with the Berg
57
Balance Scale, a clinical measure of balance (Berg et al., 1989). Leroux et al. (2006)
58
previously reported postural modification including accentuated trunk flexion and forward
59
pelvic tilt in individuals with an ISCI while walking on a treadmill as the grades of
60
inclination were increased, presumably to compensate for decreased balance or
61
secondary to the use of ambulatory assistive devices (Leroux et al., 2006). However, this
62
group of authors did not use specific measures of balance. Previous studies have also
63
shown that individuals with an ISCI have reduced balance during quasi-static stance
64
performed with eyes open or eyes closed as revealed by an increase in the center of
65
pressure (COP) mean velocity, sway area and root mean square distance compared to
66
able-bodied individuals (Lemay et al., 2013). Moreover, individuals with SCI display a
67
delayed onset of electromyographic activity in the ankle muscles following an external
68
perturbation (Thigpen et al., 2009), which may further impair the ability to control
69
balance.
70
Duclos et al. developed a model to assess balance difficulty during functional tasks such
71
as standing balance tasks (Duclos et al., 2012), walking on level surfaces (Duclos et al.,
72
2009) and sitting pivot transfers (Gagnon et al., 2012). Using the kinematic and ground
73
reaction force data collected in a laboratory environment, this model calculates the
74
stabilizing and destabilizing forces – the theoretical forces necessary to stabilize or
75
destabilize the person at each moment of the entire task (Duclos et al., 2009). This
76
model assesses both the postural and dynamic components of balance during the
77
performance of dynamic tasks when the base of support is moving and changing in
78
shape or in size (Duclos et al., 2012) and thus provides a comprehensive understanding
79
of balance difficulties. This model may therefore serve to optimize interventions in
80
rehabilitation aimed at improving walking abilities and reducing the risk of falls among
81
individuals with an ISCI.
82
We thus conducted a comprehensive laboratory-based protocol to calculate the
83
stabilizing/destabilizing forces to study balance during gait among individuals with an
84
ISCI and their able-bodied counterparts. The main purpose of this study was to compare
85
balance in these two groups during the single support phase of gait. This sub-phase of
86
the gait cycle was chosen because weight bearing on one foot while progressing forward
87
is a demanding and unstable subtask as previously shown in able-bodied and elderly
88
individuals (Duclos et al., 2009, Duclos et al., 2012). Our specific objectives were as
89
follows: (1) to compare the postural and dynamic components of balance in individuals
90
with an ISCI and able-bodied participants during the single support phase of gait, as well
91
as at specific moments during the single support phase; and (2) to identify
92
biomechanical variables that explain the values of these forces. The main hypothesis
93
was that compared to able-bodied individuals, both (postural and dynamic) components
94
of balance are impaired in individuals with an ISCI. Since the equations used to calculate
95
the stabilizing and destabilizing forces are based on recognized biomechanical factors
96
related to balance such as the COM speed (Pai and Patton, 1997) and the distance
97
between the COP and the limits of the base of support (Popovic et al., 2000), we
98
expected these to be important explanatory factor of the maximal stabilizing force and
99
the minimal destabilizing force respectively (Duclos et al., 2009, Duclos et al., 2012).
100
Methods
101
Participants
102
A convenience sample of 17 individuals with a traumatic ISCI (AIS D) were recruited
103
from both inpatients and outpatients of the SCI rehabilitation unit of the Institut de
104
réadaptation Gingras-Lindsay-de-Montréal (IRGLM) and compared to 17 age-matched
105
able-bodied controls. The inclusion criteria for the participants with an ISCI were: (1) the
106
ability to stand for five minutes without external support and (2) the ability to walk
107
independently for 15 m without ambulatory assistive devices. Participants were excluded
108
if they presented other neurological deficits in addition to the SCI or if they had walking
109
or balance difficulties prior to the SCI. None of the able-bodied participants reported
110
having physical impairments that interfered with walking or balance. Ethical approval
111
was obtained from the Research Ethics Committee of the Centre for Interdisciplinary
112
Research in Rehabilitation of Greater Montreal (CRIR). Each participant provided written
113
consent after having read and understood the information about the study. ***insert table 1 about here***
114 115
Clinical assessment
116
For the participants with an ISCI, demographic information regarding the date of injury
117
and AIS level were gathered from the participant’s chart. An experienced physical
118
therapist conducted the Lower Extremity Motor Score Assessment (LEMS) according to
119
American Spinal Cord Injury Association (ASIA) standards (Kirshblum et al., 2011). The
120
LEMS is an evaluation of lower extremity muscle strength of 5 major muscle groups,
121
representing the myotome from L2 to S1, using a 6 point (0 to 5) ordinal scale. Natural
122
walking speed was tested over a distance of 15 meters (Lam et al., 2008). Participants
123
were asked to perform three trials at their self-selected comfortable walking speed
124
without any walking assistive devices. Performance over the middle 10-m section was
125
timed and speed was calculated (10-m divided by the time in second). The three trials
126
were averaged and the mean walking speed was calculated.
127
Gait assessment
128
Kinematic and kinetic data for all participants were collected during a 3-hour assessment
129
session conducted at the Pathokinesiology Laboratory at the IRGLM. Thirty-six skin-
130
fixed infrared light emitting diodes (LEDs) were used to define the head, trunk, and
131
bilateral upper and lower extremity segments. In addition, a six-marker probe was used
132
to digitize the location of 13 bony landmarks to locate joint axes and to define the base
133
of support (BOS). A system linking four synchronized motion analysis camera bars
134
(Optotrak model 3020; Northern Digital Inc., Waterloo, Ontario, Canada) captured the
135
three-dimensional (3D) coordinates of the markers at a sampling frequency of 60 Hz.
136
Kinematic data were filtered with a fourth-order Butterworth zero-lag filter, with a cut-off
137
frequency of 6 Hz. Anthropometric measurements of the trunk, head, lower and upper
138
extremities were taken at the end of the laboratory session. These data were entered in
139
the biomechanical calculation using the 3D link model segment (Winter et al., 1990). In
140
terms of the kinetic data, three AMTI® force platforms, sequentially embedded in a 9-m
141
long walkway, recorded the ground reaction forces at a frequency of 600 Hz. The data
142
were filtered with a fourth-order Butterworth zero-lag filter, with a cut-off frequency of
143
10 Hz and down-sampled to 60 Hz to match the kinematic data.
144
Participants walked along the laboratory pathway at their self-selected comfortable
145
speed. After a period of familiarization, five trials for each foot were recorded. Trials were
146
analyzed for the lower extremity while stepping onto the force plate, between toe off and
147
the following heel strike of the contralateral lower extremity.
148
Data and statistical analyses
149
Analyses were performed on the single support phase of the weakest lower extremity
150
(right/left: 8/9 consecutive steps; based on the LEMS) for individuals with an ISCI to
151
better reveal balance difficulties. For the able-bodied participants, balance parameters of
152
eight right and nine left consecutive steps (i.e., single support phase only) were analyzed
153
to match the data of the individuals with an ISCI.
154
The single support phase of the gait cycle, from contralateral toe off to contralateral heel
155
strike, was normalized from 1 to 100 (100 data points). Data were then analyzed using
156
the stabilizing/destabilizing forces model. The stabilizing force (SF) is the theoretical
157
force necessary to stop both the COP and the COM at the limit of the potential base of
158
support (defined as the outside perimeter of the vertical projection of both feet on the
159
floor) at each moment of the task. It is calculated with the following equation: = −
160 161
∙ ∙ 2
where corresponds to the stabilizing force, to the participant’s mass, to to the horizontal distance between the position of the linear velocity of the COM and
162
the COP and the limit of the base of support in the direction of the COM linear velocity
163
(Duclos et al., 2009, Duclos et al., 2012). High stabilizing force values indicate higher
164
levels of difficulty in controlling the velocity of the centre of mass. The destabilizing force
165
(DF) is the force needed to move the COP to the limit of the base of support and is
166
calculated with the following equation: ∙ = ℎ
167 168 169
where corresponds to the destabilizing force, to the ground reaction force, to the
unitary vector normal to the contact surface, ℎ to the height of the COM from the to the horizontal distance between the position of the COP supporting surface and
170
and the limit of the base of support in the direction of the COM linear velocity (Duclos et
171
al., 2009, Duclos et al., 2012). Low destabilizing force values indicate a position of the
172
body that is destabilized more easily. Mean stabilizing force and destabilizing force
173
values over the single support phase, as well as the maximum stabilizing force value
174
and the minimum destabilizing force value were averaged for each participant and used
175
to characterize balance during the single support phase (Duclos et al., 2009). Mean
176
values characterize overall stability during the single support phase while extreme
177
values represent the maximal instability experienced during the task.
178
Descriptive statistics (mean and SD) were calculated for the participants’ characteristics
179
(e.g., age, height, mass, BMI, and time post lesion) and for the clinical assessments
180
(e.g., natural walking speed, unipodal stance phase, and LEMS). For both groups of
181
participants, the normality of the data distribution and the homogeneity of variance were
182
explored using the Shapiro-Wilk test and Levene’s test, respectively. The maximal
183
stabilizing force and the minimal destabilizing force of individuals with SCI and the COM
184
speed at maximal destabilizing force of able-bodied participants were abnormally
185
distributed. Logarithmic transformation was applied and successfully corrected the initial
186
departure from normality (Field, 2009). Between-group differences for all balance
187
difficulty parameters were explored using independent Student t-tests. To further
188
appreciate the importance of group differences of the various parameters, Cohen’s d
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effect sizes were also computed (Cohen, 1988). The effect sizes were considered small
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(
