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Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke
JCLB-03937; No of Pages 7 Clinical Biomechanics xxx (2015) xxx–xxx
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Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
art in science 2012 award finalist
Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke Karen J. Nolan a,b,⁎, Mathew Yarossi a,c, Patrick Mclaughlin d a
Human Performance and Engineering Research, Kessler Foundation, West Orange, NJ, USA Department of Physical Medicine and Rehabilitation, Rutgers - New Jersey Medical School, Newark, NJ, USA Graduate School of Biomedical Sciences, Rutgers - New Jersey Medical School, Newark, NJ, USA d College of Health and Biomedicine/ISEAL, Victoria University, Melbourne, Australia b c
a r t i c l e
i n f o
Article history: Received 14 June 2013 Accepted 16 March 2015 Keywords: Stroke Stroke patients Foot drop Hemiplegic gait Orthoses Gait Walking speed Functional electrical stimulation Rehabilitation
a b s t r a c t Background: Center of pressure measured during gait can provide information about underlying control mechanisms and the efficacy of a foot drop stimulator. This investigation evaluated changes in center of pressure displacement in individuals with stroke with and without a foot drop stimulator. Methods: Individuals with stroke-related foot drop (n = 11) using a foot drop stimulator and healthy controls (n = 11). Walking speed and bilateral center of pressure variables: 1) net displacement; 2) position and maximum displacement; and 3) mean velocity during walking. Findings: On the affected limb with the foot drop stimulator as compared to the affected limb without the foot drop stimulator: 1) increased anterior/posterior maximum center of pressure excursion 8% during stance; 2) center of pressure at initial contact was 6% more posterior; 3) medial/lateral mean, maximum and minimum center of pressure position during stance all significantly decreased; 4) anterior/posterior net displacement increased during stance and single support; and 5) anterior/posterior velocity of the center of pressure increased during stance. Interpretation: Individuals with stroke using a foot drop stimulator contacted the ground more posterior at initial contact and utilized more of the anterior/posterior plantar surface of the foot on the affected limb during stance. With the foot drop stimulator there was a shift in center of pressure toward the medial side possibly indicating an improvement in equinovarus gait where there is a tendency to load the lateral foot throughout stance. For individuals with stroke a foot drop stimulator can improve displacement of the center of pressure which indicates improved forward progression and stability during walking. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Hemiplegia of the lower limb is one of the most serious and common disabling impairments resulting from stroke (American Heart Association, 2014). Hemiplegic gait is characterized by poor single limb stance and difficulty controlling forward progression (Perry, 1969). Foot drop secondary to stroke, results from weakness or lack of voluntary control in the ankle and toe dorsiflexor muscles. During walking this results in ineffective ankle dorsiflexion during swing and failure to achieve heel strike at initial contact (Burridge and Mclellan, 2000; Stein et al., 2010); these disturbances in healthy walking patterns contribute to decreased speed, a disruption in weight acceptance and weight transfer, and an inefficient and unstable gait (Burridge and Mclellan, 2000; Nolan and Yarossi, 2011a,b).
⁎ Corresponding author at: Kessler Foundation, 1199 Pleasant Valley Way, West Orange, NJ, USA. E-mail address: [email protected] (K.J. Nolan).
The standard of care for treating foot drop in chronic stroke has been the application of an ankle foot orthosis (AFO) to assist with ambulation. The AFO traditionally places the ankle in a neutral fixed position (~90°, dorsiflexed) and passively compensates for foot drop throughout the gait cycle (Stein et al., 2010). Existing literature evaluating gait biomechanics indicates that applying an AFO in individuals with stroke can improve gait speed at the expense of ankle range of motion and power generation during push-off (Fatone and Hansen, 2007; Perry and Burnfield, 2010). Although the AFO can mitigate some of the walking difficulty, as a rehabilitation intervention it is not targeted to provide or preserve dynamic function. An alternative rehabilitation approach is to apply functional electrical stimulation (FES) to the common peroneal nerve to help provide active movement during ambulation (Bethoux et al., 2014, 2015; Everaert et al., 2013; Sabut et al., 2010; Stein et al., 2010). The stimulation paradigm for FES is to elicit task-specific movement patterns that result in dynamic functional activity (Daly and Ruff, 2007). Commercially available foot drop stimulators (FDS) can be specifically programmed to provide active dorsiflexion at the correct timing
http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016 0268-0033/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
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K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
and phase of gait through surface stimulation. The FDS is an alternative to traditional AFOs. FDS technology provides electrically induced muscle activation during the swing phase of gait and at initial contact. Ankle– foot movements are actively produced using the FDS in contrast to the rigid compensatory assistance provided through an AFO (Burridge et al., 2007; Pilkar et al., 2014). Depending on the placement of the electrodes, ankle dorsiflexion can be combined with eversion. Added eversion can provide ankle stability during foot contact and weight acceptance (Stein et al., 2010). FDS technology provides dynamic movements to the ankle–foot complex. Previous research evaluating the immediate orthotic effect has shown only small changes in walking speed, improved dorsiflexion angle, and improved temporal–spatial characteristics (Knutson and Chae, 2010; Kottink et al., 2007; Sabut et al., 2010; Taylor et al., 1999a,b). These results demonstrate the efficacy for FDS utilization in poststroke rehabilitation but they fail to precisely indicate how FDS technology can improve gait mechanisms by helping to restore or maintain function (Everaert et al., 2013; Stein et al., 2010). Measurements of the center of pressure (CoP) have been used previously to characterize hemiplegic gait and orthotic interventions (Fatone et al., 2009; Mizelle et al., 2006). The CoP represents the cumulative neuromuscular response that controls center of mass (CoM) movement to help maintain forward progression and balance (Chisholm et al., 2011). Changes in anterior/posterior (AP) CoP during stance can provide precise information on the control of forward progression. Modifications to the medial/lateral (ML) CoP may indicate changes in the control process that regulate lateral stability or the ability to transfer weight between legs during gait (Chisholm et al., 2011). Previous research evaluating CoP in individuals with stroke and AFO intervention have described a smoother anterior progression of the CoP, elimination of posterior directed movement of the CoP during weight acceptance, and a larger CoP displacement (Fatone and Hansen, 2007; Mizelle et al., 2006; Mueller et al., 1992). CoP is a robust and comprehensive measure that can demonstrate the efficacy of FDS technology for individuals with foot drop and provides precise quantifiable information on performance and function during gait (Mizelle et al., 2006). There is limited research evaluating the effect of FDS on CoP during walking gait in individuals with hemiplegia. Precise changes in the CoP during gait can provide information about underlying control mechanisms of the neuromuscular system and have been previously used to characterize hemiplegic gait (Mizelle et al., 2006). The purpose of this investigation was to evaluate changes in center of pressure displacement during walking in individuals with stroke, with and without a foot drop stimulator (FDS) and in healthy controls. 2. Methods
placed near the head of the fibula, directly over the motor nerve and proximal musculature. FES is applied to the peroneal nerve during the gait cycle with programmable timing, intensity and duration controlled by a tilt sensor and accelerometer. The foot drop stimulator (FDS) provides electrically induced muscle activation during the swing phase of gait and at initial contact. The selected technology does not rely on a foot switch, telemetry or external wires in order to initiate dorsiflexion and does not restrict the user to a particular plantar surface area to initiate dorsiflexion. Each participant with stroke used their own WalkAide® device that they normally used for daily ambulation for all walking tests. Each device had been previously custom programmed (stimulus intensity and timing of muscle activation) by a licensed clinician. 2.3. Procedures Individuals in the stroke group completed four 5-meter walks (2 with FDS and 2 without FDS) at a self-selected speed on level ground. The healthy control (HC) group performed a 2-minute walk as part of a larger research study at a self-selected pace and data from the first 18 s of the walking test were used for analysis. Participants wore neutral walking shoes with average heel heights for all walking tests and no comparisons were made to a barefoot condition. Members of the study team provided supervision and non-contact guarding during all walking tests for safety. All procedures performed in this investigation were approved by the Human Subjects Review Board and informed consent was obtained prior to study participation. Wireless plantar pressure data were collected bilaterally at 100 Hz using the pedar®-X Expert System (Novel GmbH, Munich, Germany) during walking tests. The insole sensor technology allows for bilateral analysis of multiple steps. Using the pedar®-X, force is calculated by multiplying the recorded pressure by the sensor area resulting in a force “normal” to each sensor in the matrix (Kernozek et al., 1996). Using pressure data from each of the 99 sensors, the centroid of the pressure distribution is provided in terms of x and y insole coordinates for each foot independently. The origin (0, 0) was defined at the point most medial and posterior with reference to the insole, regardless of foot orientation and line of progression (Chisholm et al., 2011). Increased x-coordinate indicated a movement toward the lateral border of the insole and increased y-coordinate indicated a movement toward the anterior border. 2.4. Data analysis and outcome measures Demographic information including age, gender, and time since stroke were collected and verified with medical records. Data from all assessments are presented as mean (standard deviation).
2.1. Participants Individuals with hemiplegia and foot drop secondary to stroke (N3 months) and healthy controls were recruited for participation. Individuals with stroke were recruited from a larger multi-site clinical trial. All participants with stroke were currently using a commercially available foot drop stimulator (FDS) (WalkAide®, Innovative Neurotronics, Inc., Austin, TX, USA) for assistance with gait deviations. Individuals with stroke were able to walk independently for 10 m without FDS. 2.2. Foot drop stimulator (FDS) The WalkAide® is a battery operated, single-channel, asymmetrical biphasic stimulator with programmable pulse width and frequency that is utilized during walking as a functional electrical stimulation (FES) orthotic device (Melo et al., 2015). This small device (87.9 g, 8.2 cm(H) × 6.1 cm(W) × 2.1 cm(T)) attaches to a molded cuff located just below the knee (Fig. 1). Two surface electrodes are specifically
Fig. 1. Foot Drop Stimulator (WalkAide®, Innovative Neurotronics, Inc., Austin, TX, USA).
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
Plantar pressure data was exported using the Novel pedar-X Recorder software (Novel Electronics, Inc., Munich, Germany). Per convention, the first two steps were omitted from analysis for all walking tests. All data was imported into Matlab (The Mathworks, Inc., Natick, MA, USA) for custom analysis. Time series force data was used to determine gait cycle events and subdivide the stance phase into 3 sub phases: IDS (initial double support); SS (single support); and TDS (terminal double support), as previously described in Nolan and Yarossi (Nolan and Yarossi, 2011a). Time series CoP data were used to calculate bilateral position, displacement and velocity variables for each phase of the gait cycle as well as the stance phase. Primary outcome variables included CoP: 1) net displacement (anterior/posterior (AP), % insole length); 2) position and maximum displacement (AP, % insole length; and medial/lateral (ML), % insole width); and 3) mean velocity (AP, cm/s). Self-selected walking speed (m/s) was also measured to assess changes in functional ambulation, Table 1. 2.5. Statistical analysis Data were analyzed using descriptive statistics in Matlab (The Mathworks, Inc., Natick, MA, USA). Independent two sample t-tests were performed to determine if there was a significant difference in CoP outcome variables between the stroke group and the healthy control group (P ≤ 0.05). Comparisons were made during stance, IDS, SS, and TDS, between the affected limb and unaffected limb of the stroke group (with and without FDS) to the right limb of the healthy control group. Paired sample t-tests were used to determine if there were significant differences in the stroke group between conditions, with and without FDS (P ≤ 0.05) for CoP outcome variables: 1) net displacement; 2) velocity; 3) position at initial contact and toe off; 4) maximum displacement and 5) posterior displacement during stance. Comparisons were made bilaterally during stance, IDS, SS and TDS for the affected and unaffected limbs to examine the difference in CoP with and without FDS. 3. Results Data from 11 participants with hemiplegia and foot drop 112.8 (76.8) months poststroke (10 males and 1 female) and 11 healthy controls (3 males and 8 females) were available for analysis. In the stroke group, 7 participants were affected on the right side and 4 participants were affected on the left. All participants with stroke used their own FDS custom programmed by a licensed clinician on their affected limb during ambulation in the FDS condition. The FDS was positioned below the knee, secured with a latch, and properly aligned using anatomical landmarks (tibial tuberosity) and visual indicators (locator on cuff). A detailed description of participant characteristics can be found in Table 2. Individuals in the healthy control group had a significantly faster walking speed 1.27 (0.19) m/s than individuals with stroke regardless of condition, with (P ≤ 0.0001) and without (P ≤ 0.0001) FDS. There were no significant differences in walking speed (m/s) for individuals with stroke, with FDS 0.60 (0.25) m/s or without FDS 0.62 (0.28) m/s. Table 1 CoP outcome variables. Name
Description
AP net displacement AP max displacement ML max displacement Posterior displacement
CoP position (cm % insole length) at the end of phase–CoP position at the beginning of phase (Stance, IDS, SS, TDS). Maximum CoP position (cm % insole length)–minimum CoP position during phase. Maximum CoP position (cm % insole width)–minimum CoP position during phase. CoP position (cm % insole length) at initial contact–CoP position at minimum position during IDS.
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Table 2 Participant characteristics; mean (sd). Group
Age (yrs.)
Height (m)
Weight (kg)
Time using FDS (months)
Stroke (n = 11) Healthy controls (n = 11)
60.5 (8.3) 54.2 (15.7)
1.78 (0.11) 1.67 (0.10)
85.3 (11.9) 68.5 (13.2)
4.6 (4.7) –
There were no significant interlimb differences for the healthy control group for any CoP outcome variables. As a result the right limb CoP variables were used for all comparisons. 3.1. Anterior/posterior CoP net displacement and velocity AP net displacement of the CoP was significantly longer during stance and SS, and significantly shorter during TDS in the healthy control group as compared to the affected limb of the stroke group regardless of condition, with (stance P = 0.02; SS P ≤ 0.0001, TDS P ≤ 0.0001) and without (stance P = 0.0004; SS P ≤ 0.0001, TDS P ≤ .0001) FDS. The AP net displacement of the CoP was significantly longer during SS and significantly shorter during IDS and TDS in healthy controls as compared to the unaffected limb of the stroke group regardless of condition, with (SS P = 0.0001, IDS P = 0.0003; TDS P = 0.026) and without (SS P = 0.0003, IDS P = 0.0008, TDS P = 0.019) FDS. The AP net displacement of the CoP during the stance phase in healthy controls was not significantly different than the unaffected limb of the stroke group regardless of condition, with (P = 0.121) and without (P = 0.087) FDS. AP net displacement of the CoP was significantly longer during stance (16.8%, P = 0.006), IDS (10.9%, P = 0.047), and SS (20.4%, P = 0.0001), and significantly shorter during TDS (14.3%, P = 0.005) on the unaffected limb compared to the affected limb of the stroke group without FDS. AP net displacement of the CoP remained significantly longer during SS (16.3%, P = 0.0002) and shorter during TDS (15.4%, P = 0.001) on the unaffected limb compared to the affected limb of the stroke group when the FDS was added to the affected limb. AP net displacement of the CoP was not significantly different between the affected and unaffected limb of the stroke group during stance (P = 0.064) and IDS (P = 0.069) with the addition of the FDS to the affected limb. AP net displacement of the CoP was significantly longer during stance (8%, P = 0.039) and SS (2.2% P = 0.019) on the affected limb with the FDS as compared to without the FDS. Anterior/posterior CoP net displacement variables for individuals with stroke (with and without FDS) and healthy subjects can be found in Table 3. AP velocity of the CoP was significantly faster during stance and SS and significantly slower during TDS in the healthy control group, as compared to the affected limb of the stroke group regardless of condition: with (stance P = 0.005; SS P ≤ .0001, TDS P ≤ 0.0001) and without (stance P = 0.001; SS P ≤ 0.0001, TDS P = 0.001) FDS. AP velocity of the CoP was significantly faster during stance and SS and significantly slower during TDS in the healthy control group, as compared to the unaffected limb of the stroke group regardless of condition: with (stance P = 0.001; SS P = 0.0001, TDS P = 0.006) and without (stance P = 0.001; SS P = 0.0002, TDS P = 0.004) FDS. AP velocity of the CoP was significantly faster during SS and significantly slower during TDS on the unaffected limb, as compared to the affected limb of the stroke group: with (SS P = 0.003, TDS P = 0.027) and without (SS P = 0.0004, TDS P = 0.030) FDS. AP velocity of the CoP on the affected limb increased with the FDS as compared to without the FDS, but this trend was not significant. 3.2. CoP position and maximum displacement CoP position at initial contact was significantly more medial and posterior on the affected limb of the stroke group with the FDS (ML P =
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
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K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
Table 3 Anterior/posterior CoP net displacement and velocity.
Stance IDS SS TDS
Displacement Velocity Displacement Velocity Displacement Velocity Displacement Velocity
Stroke group — affected limb
Stroke group — unaffected limb
Healthy control
With FDS
Without FDS
With FDS
Without FDS
Right
46.4 (18.1)a,e 15.5 (7.0)e 6.1 (13.4) 16.5 (20.5) 15.8 (14.9)a,c,e 10.5 (9.8)c,e 23.8 (15.0) 23.4 (14.3)c,e
38.4 (18.1)a,b,d 13.3 (7.0)d,f 2.9 (15.8)b 1.2 (31.6) 13.6 (4.4)a,b,d 9.1 (10.0)b,d,f 21.5 (14.6) 23.2 (15.8)b,d,f
56.4 (6.4) 14.8 (5.1)g 15.8 (5.9)g 18.3 (8.1) 32.1 (14.7)c,g 15.2 (8.7)c,g 8.4 (9.7) 9.3 (9.2)c,g
55.2 (8.0)b 14.5 (5.4) 13.8 (4.8)b,f 17.7 (7.2) 34.0 (14.5)b,f 15.9 (8.7)b 7.2 (7.5) 8.9 (7.8)b
57.0 (5.8)d,e 26.4 (9.2)d,e,f,g 4.8 (3.7)f,g 18.9 (15.2) 53.9 (6.1)d,e,f,g 36.0 (12.3)d,e,f,g −1.4 (6.6) −0.8 (6.3)d,e,f,g
All AP variables presented as % insole length (displacement) or % insole length per second (velocity). a P ≤ 0.05 with vs. without FDS for the affected limb of the stroke group. b P ≤ 0.05 affected vs. unaffected limb, without FDS in the stroke group. c P ≤ 0.05 affected vs. unaffected limb, with FDS in the stroke group. d P ≤ 0.05 stroke group affected limb without FDS vs. healthy control. e P ≤ 0.05 stroke group affected limb with FDS vs. healthy control. f P ≤ 0.05 stroke group unaffected limb without FDS vs. healthy control. g P ≤ 0.05 stroke group unaffected limb with FDS vs. healthy control.
.005, AP P = 0.043). AP and ML changes at toe-off were not significant with the addition of the FDS, and in both conditions the CoP position at toe off on the affected limb resembled the unaffected limb and the healthy controls. ML mean (P = 0.025), maximum (P = 0.014) and minimum (P = 0.027) CoP position on the affected limb during stance were all significantly more medial with the FDS in the stroke group. AP minimum (P = 0.009) CoP on the affected limb during stance was significantly more posterior with the FDS; but the AP mean (P = 0.153) and maximum (P = 0.114) CoP on the affected limb with FDS were not significant. Maximum ML displacement on the affected limb with FDS was not significantly different (P = 0.346) but maximum AP displacement on the affected limb was significantly longer (P =0.001) with FDS. The only significant differences found on the unaffected limb were ML maximum CoP position was significantly more medial (P = 0.017) and maximum ML displacement was shorter (P = 0.014) when the FDS was added to the affected limb. CoP position and maximum displacement variables for individuals with stroke on the affected and unaffected limb (with and without FDS) and healthy controls can be found in Table 4. 4. Discussion CoP provides information specific to the lower extremity and the neuromuscular fluctuations that are part of motor control during
walking (Mizelle et al., 2006). This study represents a novel application of bilateral CoP data for individuals with stroke currently utilizing FDS for assistance with gait deviations. The primary endpoint in this investigation was to investigate changes in CoP displacement during walking in individuals with stroke with and without a foot drop stimulator (FDS). Multiple comparisons were provided between the stroke group (affected and unaffected) and individuals in the healthy controls as a reference for the directionality of the results, Fig. 2. Research utilizing measurements of CoP as a primary outcome measure typically quantify the entire stance phase and do not comprehensively describe CoP changes throughout weight acceptance (initial double support, IDS), single support (SS) and weight transfer (terminal double support, TDS). These subphases have been shown to be essential for maintaining walking speed in hemiplegic gait (Nolan and Yarossi, 2011a). The current investigation evaluated the entire stance phase and broke down the stance phase into IDS, SS, and TDS in order to understand how CoP changes during gait. Gait speed did not increase with the addition of the FDS to the affected limb in the stroke group. The average gait speed attained was ~ 0.6 (0.2) m/s in the stroke group, regardless of condition, and 1.27 (0.2) m/s in the healthy control group. Healthy self-selected walking speed is 1.48 m/s for men and 1.23 m/s for women (Blessey et al., 1976). According to Perry et al. individuals with a gait speed between 0.4 and
Table 4 CoP position and maximum displacement variables.
Initial contact Toe off Mean (during stance) Max (during stance) Min (during stance) Max displacement Posterior displacement
ML AP ML AP ML AP ML AP ML AP ML AP
Stroke group — affected limb
Stroke group — unaffected limb
Healthy controls
With FDS
Without FDS
With FDS
Without FDS
Right Limb
63.4 (6.8)a 26.7 (33.8)a 45.3 (4.6) 73.1 (9.3) 58.1 (7.7)a 45.1 (8.5) 67.3 (5.8)a 74.9 (8.3) 42.2 (14.0)a 20.8 (9.9)a 25.0 (12.0) 54.1 (11.9)a −5.9 (8.8)
69.6 (7.2)a 33.8 (18.9)a 52.5 (17.0) 72.1 (8.1) 61.4 (7.6)a 47.3 (8.9) 71.6 (6.3)a 73.0 (7.5) 49.2 (23.2)a 26.9 (10.4)a 22.3 (13.3) 46.0 (10.7)a −6.8 (10.1)
62.9 (4.3) 17.4 (3.9) 41.5 (8.1) 73.8 (5.4) 56.1 (5.1) 46.3 (2.9) 67.1 (3.6)b 76.5 (3.0) 38.9 (8.2) 16.2 (2.7) 28.2 (8.5)b 60.3 (4.5) −1.3 (2.0)
63.0 (4.7) 17.9 (4.5) 41.6 (6.6) 73.2 (5.7) 57.0 (4.8) 46.0 (2.7) 68.9 (4.2)b 76.0 (3.6) 38.1 (6.7) 16.4 (3.1) 30.7 (8.0)b 59.6 (5.4) −1.6 (2.3)
66.2 (1.3) 13.4 (1.1) 45.1 (3.3) 74.4 (1.0) 58.3 (3.4) 47.0 (1.9) 68.9 (2.0) 76.0 (0.5) 43.3 (3.0) 12.1 (0.4) 25.6 (2.5) 63.8 (0.6) −1.3 (0.9)
ML variables presented as % insole width; AP variables presented as % insole length. a P ≤ 0.05 with vs. without FDS for the affected limb of the stroke group. b P ≤ 0.05 with vs. without FDS for the unaffected limb of the stroke group.
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
AP Displacement (% Insole Length)
K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
Affected without FDS Affected with FDS Unaffected without FDS Unaffected with FDS Healthy Control Group (Right)
AP COP Velocity (cm/sec)
% Stance
% Stance Fig. 2. Anterior/posterior mean position and velocity for during stance for individuals with stroke (with and without FDS) and healthy controls.
0.8 m/s would have limited community ambulation equivalent to moderate gait impairments (Perry et al., 1995). Individuals with stroke using a FDS contacted the ground 6% more posterior at initial contact and utilized 8% more of the AP plantar surface of the foot on the affected limb during stance. This change in CoP in the stroke group with the FDS more closely resembled the healthy control
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group but individuals with stroke still utilized 10.6% less of the plantar surface of the foot throughout stance than the healthy control group. Previous research in hemiplegic gait indicated similar improvements with traditional AFO intervention in hemiplegic gait (Fatone and Hansen, 2007). Initial contact was 6% more medial on the affected limb of the stroke group with the FDS. In addition, the mean, maximum and minimum position of the CoP, were also all shifted more medially on the affected limb of the stroke group with FDS. Overall CoP ML maximum displacement did not change indicating that there was the same amount of ML variability during stance in the affected limb of the stroke group with FDS but this variability shifted medially. This medial shift may represent an overall improvement in gait, specifically a reduction in equinovarus gait where there is a tendency to load the lateral foot throughout stance. Although the medial shift is small, previous research has indicated that improved ML CoP can improve the control process that regulates stability (Chisholm et al., 2011). Changes in AP posterior displacement during IDS with the addition of the FDS were variable for individuals with stroke. In four individuals with stroke the FDS removed the existence of posterior movement of the CoP, therefore improving forward progression. In the remaining participants the amount of posterior displacement was unchanged or worsened by the FDS. A representative subject with stroke (with and without FDS) and healthy controls can be seen in Fig. 3. Posterior movement (retropulsion) of the CoP during IDS is indicative of inefficient weight transfer on the affected limb (Nolan and Yarossi, 2011b). Further research is needed to explore how an FDS can be utilized to effectively remove retropulsion during stance and what effect that has on gait quality. AP velocity of the CoP on the affected limb increased during stance in the stroke group with the use of the FDS, although this trend did not reach significance. The majority of that increase occurred during IDS. In individuals with stroke, AP velocity of the CoP is impaired on the affected limb during IDS. When the FDS was added, AP velocity of the CoP increased during IDS on the affected limb and resembled the AP velocity during IDS of a healthy adult. This finding is consistent with previous research indicating that a FDS added to the paretic limb during gait could improve weight acceptance (Nardone et al., 2009; Nolan and Yarossi, 2011b) and therefore provide a smoother forward progression of the CoP. Regardless of these improvements on the affected limb
Fig. 3. CoP displacement, one representative individual: A) stroke without FDS; B) stroke with FDS; and C) healthy control.
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
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with FDS for individuals with stroke, the AP velocity of the CoP was still 10.9 cm/s faster throughout stance in the healthy controls. This is primarily due to decreased AP velocity of the CoP during stance and more specifically during SS in the stroke group on the affected and unaffected limb regardless of condition. One of the most interesting findings in this investigation was that the amount of AP progression characteristic during each phase of gait was distinctly different for individuals with stroke compared to healthy controls. In healthy controls the majority of the AP net displacement of the CoP occurs during SS (57%) with relatively little AP displacement of the CoP during double support. For individuals with stroke SS remains the dominant phase of AP progression on the unaffected limb (32.1% with FDS and 34.0% without FDS) but AP displacement during IDS and TDS is larger than found in healthy controls. Progression by phase is markedly different on the affected limb, the largest AP net displacement of the CoP occurs during TDS (23.8% with FDS and 21.5% without FDS). On the affected limb of the stroke group AP displacement of the CoP is delayed until late in stance and this effect remains regardless of FDS intervention. Stroke subjects displayed the majority of anterior progression of the CoP during TDS with an associated increased velocity of the CoP in the anterior direction during this phase. In contrast the healthy controls displayed very little AP movement of the CoP during TDS. For individuals with stroke, this may indicate a reluctance to transfer weight anterior (onto the affected limb) during single support, hindering forward progression. As a result, individuals with stroke wait until TDS to shift weight forward. When the FDS is added to the paretic limb this effect remains but the FDS allows it to happen faster (increased AP velocity). Although the increase in AP velocity during TDS for individuals with stroke on the affected limb is small, it is indicative of positive changes in gait mechanisms that contribute to overall quality of gait. 5. Conclusions Objectively quantifying the CoP in individuals with foot drop is a key factor for understanding forward progression and changes in stability after FDS intervention in individuals with stroke. Analysis of the CoP progression throughout the subphases of stance provided a comprehensive method to evaluate the changes in gait mechanics that effect gait quality. For individuals with stroke, on the affected limb, FDS can improve AP and ML displacement, as well as position and velocity of the CoP to improve forward progression and stability. Measuring outcomes that can provide quantitative information about the potential effectiveness of an FDS orthotic device will provide meaningful clinical evidence for implementation into rehabilitation. Disclaimers None. Grant support Supported by Innovative Neurotronics, Inc. and Kessler Foundation. PMcL was partly supported by the Australian Government funded Collaborative Research Networks Program at Victoria University. Declaration of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper. Appendix 1
AFO AP
ankle foot orthosis anterior/posterior
CoM CoP FDS FES HC IDS ML SS TDS
center of mass center of pressure foot drop stimulator functional electrical stimulator healthy control initial double support medial/lateral single support terminal double support
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K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx Pilkar, R., Yarossi, M., Nolan, K.J., 2014. EMG of the tibialis anterior demonstrates a training effect after utilization of a foot drop stimulator. NeuroRehabilitation 35, 299–305. Sabut, S.K., Lenka, P.K., Kumar, R., Mahadevappa, M., 2010. Effect of functional electrical stimulation on the effort and walking speed, surface electromyography activity, and metabolic responses in stroke subjects. J. Electromyogr. Kinesiol. 20, 1170–1177. Stein, R.B., Everaert, D.G., Thompson, A.K., Chong, S.L., Whittaker, M., Robertson, J., Kuether, G., 2010. Long-term therapeutic and orthotic effects of a foot drop stimulator on walking performance in progressive and nonprogressive neurological disorders. Neurorehabil. Neural Repair 24, 152–167.
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Taylor, P.N., Burridge, J.H., Dunkerley, A.L., Lamb, A., Wood, D.E., Norton, J.A., Swain, I.D., 1999a. Patients' perceptions of the Odstock Dropped Foot Stimulator (ODFS). Clin. Rehabil. 13, 439–446. Taylor, P.N., Burridge, J.H., Dunkerley, A.L., Wood, D.E., Norton, J.A., Singleton, C., Swain, I.D., 1999b. Clinical use of the Odstock dropped foot stimulator: its effect on the speed and effort of walking. Arch. Phys. Med. Rehabil. 80, 1577–1583.
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
Contents lists available at ScienceDirect
Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
art in science 2012 award finalist
Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke Karen J. Nolan a,b,⁎, Mathew Yarossi a,c, Patrick Mclaughlin d a
Human Performance and Engineering Research, Kessler Foundation, West Orange, NJ, USA Department of Physical Medicine and Rehabilitation, Rutgers - New Jersey Medical School, Newark, NJ, USA Graduate School of Biomedical Sciences, Rutgers - New Jersey Medical School, Newark, NJ, USA d College of Health and Biomedicine/ISEAL, Victoria University, Melbourne, Australia b c
a r t i c l e
i n f o
Article history: Received 14 June 2013 Accepted 16 March 2015 Keywords: Stroke Stroke patients Foot drop Hemiplegic gait Orthoses Gait Walking speed Functional electrical stimulation Rehabilitation
a b s t r a c t Background: Center of pressure measured during gait can provide information about underlying control mechanisms and the efficacy of a foot drop stimulator. This investigation evaluated changes in center of pressure displacement in individuals with stroke with and without a foot drop stimulator. Methods: Individuals with stroke-related foot drop (n = 11) using a foot drop stimulator and healthy controls (n = 11). Walking speed and bilateral center of pressure variables: 1) net displacement; 2) position and maximum displacement; and 3) mean velocity during walking. Findings: On the affected limb with the foot drop stimulator as compared to the affected limb without the foot drop stimulator: 1) increased anterior/posterior maximum center of pressure excursion 8% during stance; 2) center of pressure at initial contact was 6% more posterior; 3) medial/lateral mean, maximum and minimum center of pressure position during stance all significantly decreased; 4) anterior/posterior net displacement increased during stance and single support; and 5) anterior/posterior velocity of the center of pressure increased during stance. Interpretation: Individuals with stroke using a foot drop stimulator contacted the ground more posterior at initial contact and utilized more of the anterior/posterior plantar surface of the foot on the affected limb during stance. With the foot drop stimulator there was a shift in center of pressure toward the medial side possibly indicating an improvement in equinovarus gait where there is a tendency to load the lateral foot throughout stance. For individuals with stroke a foot drop stimulator can improve displacement of the center of pressure which indicates improved forward progression and stability during walking. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Hemiplegia of the lower limb is one of the most serious and common disabling impairments resulting from stroke (American Heart Association, 2014). Hemiplegic gait is characterized by poor single limb stance and difficulty controlling forward progression (Perry, 1969). Foot drop secondary to stroke, results from weakness or lack of voluntary control in the ankle and toe dorsiflexor muscles. During walking this results in ineffective ankle dorsiflexion during swing and failure to achieve heel strike at initial contact (Burridge and Mclellan, 2000; Stein et al., 2010); these disturbances in healthy walking patterns contribute to decreased speed, a disruption in weight acceptance and weight transfer, and an inefficient and unstable gait (Burridge and Mclellan, 2000; Nolan and Yarossi, 2011a,b).
⁎ Corresponding author at: Kessler Foundation, 1199 Pleasant Valley Way, West Orange, NJ, USA. E-mail address: [email protected] (K.J. Nolan).
The standard of care for treating foot drop in chronic stroke has been the application of an ankle foot orthosis (AFO) to assist with ambulation. The AFO traditionally places the ankle in a neutral fixed position (~90°, dorsiflexed) and passively compensates for foot drop throughout the gait cycle (Stein et al., 2010). Existing literature evaluating gait biomechanics indicates that applying an AFO in individuals with stroke can improve gait speed at the expense of ankle range of motion and power generation during push-off (Fatone and Hansen, 2007; Perry and Burnfield, 2010). Although the AFO can mitigate some of the walking difficulty, as a rehabilitation intervention it is not targeted to provide or preserve dynamic function. An alternative rehabilitation approach is to apply functional electrical stimulation (FES) to the common peroneal nerve to help provide active movement during ambulation (Bethoux et al., 2014, 2015; Everaert et al., 2013; Sabut et al., 2010; Stein et al., 2010). The stimulation paradigm for FES is to elicit task-specific movement patterns that result in dynamic functional activity (Daly and Ruff, 2007). Commercially available foot drop stimulators (FDS) can be specifically programmed to provide active dorsiflexion at the correct timing
http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016 0268-0033/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
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K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
and phase of gait through surface stimulation. The FDS is an alternative to traditional AFOs. FDS technology provides electrically induced muscle activation during the swing phase of gait and at initial contact. Ankle– foot movements are actively produced using the FDS in contrast to the rigid compensatory assistance provided through an AFO (Burridge et al., 2007; Pilkar et al., 2014). Depending on the placement of the electrodes, ankle dorsiflexion can be combined with eversion. Added eversion can provide ankle stability during foot contact and weight acceptance (Stein et al., 2010). FDS technology provides dynamic movements to the ankle–foot complex. Previous research evaluating the immediate orthotic effect has shown only small changes in walking speed, improved dorsiflexion angle, and improved temporal–spatial characteristics (Knutson and Chae, 2010; Kottink et al., 2007; Sabut et al., 2010; Taylor et al., 1999a,b). These results demonstrate the efficacy for FDS utilization in poststroke rehabilitation but they fail to precisely indicate how FDS technology can improve gait mechanisms by helping to restore or maintain function (Everaert et al., 2013; Stein et al., 2010). Measurements of the center of pressure (CoP) have been used previously to characterize hemiplegic gait and orthotic interventions (Fatone et al., 2009; Mizelle et al., 2006). The CoP represents the cumulative neuromuscular response that controls center of mass (CoM) movement to help maintain forward progression and balance (Chisholm et al., 2011). Changes in anterior/posterior (AP) CoP during stance can provide precise information on the control of forward progression. Modifications to the medial/lateral (ML) CoP may indicate changes in the control process that regulate lateral stability or the ability to transfer weight between legs during gait (Chisholm et al., 2011). Previous research evaluating CoP in individuals with stroke and AFO intervention have described a smoother anterior progression of the CoP, elimination of posterior directed movement of the CoP during weight acceptance, and a larger CoP displacement (Fatone and Hansen, 2007; Mizelle et al., 2006; Mueller et al., 1992). CoP is a robust and comprehensive measure that can demonstrate the efficacy of FDS technology for individuals with foot drop and provides precise quantifiable information on performance and function during gait (Mizelle et al., 2006). There is limited research evaluating the effect of FDS on CoP during walking gait in individuals with hemiplegia. Precise changes in the CoP during gait can provide information about underlying control mechanisms of the neuromuscular system and have been previously used to characterize hemiplegic gait (Mizelle et al., 2006). The purpose of this investigation was to evaluate changes in center of pressure displacement during walking in individuals with stroke, with and without a foot drop stimulator (FDS) and in healthy controls. 2. Methods
placed near the head of the fibula, directly over the motor nerve and proximal musculature. FES is applied to the peroneal nerve during the gait cycle with programmable timing, intensity and duration controlled by a tilt sensor and accelerometer. The foot drop stimulator (FDS) provides electrically induced muscle activation during the swing phase of gait and at initial contact. The selected technology does not rely on a foot switch, telemetry or external wires in order to initiate dorsiflexion and does not restrict the user to a particular plantar surface area to initiate dorsiflexion. Each participant with stroke used their own WalkAide® device that they normally used for daily ambulation for all walking tests. Each device had been previously custom programmed (stimulus intensity and timing of muscle activation) by a licensed clinician. 2.3. Procedures Individuals in the stroke group completed four 5-meter walks (2 with FDS and 2 without FDS) at a self-selected speed on level ground. The healthy control (HC) group performed a 2-minute walk as part of a larger research study at a self-selected pace and data from the first 18 s of the walking test were used for analysis. Participants wore neutral walking shoes with average heel heights for all walking tests and no comparisons were made to a barefoot condition. Members of the study team provided supervision and non-contact guarding during all walking tests for safety. All procedures performed in this investigation were approved by the Human Subjects Review Board and informed consent was obtained prior to study participation. Wireless plantar pressure data were collected bilaterally at 100 Hz using the pedar®-X Expert System (Novel GmbH, Munich, Germany) during walking tests. The insole sensor technology allows for bilateral analysis of multiple steps. Using the pedar®-X, force is calculated by multiplying the recorded pressure by the sensor area resulting in a force “normal” to each sensor in the matrix (Kernozek et al., 1996). Using pressure data from each of the 99 sensors, the centroid of the pressure distribution is provided in terms of x and y insole coordinates for each foot independently. The origin (0, 0) was defined at the point most medial and posterior with reference to the insole, regardless of foot orientation and line of progression (Chisholm et al., 2011). Increased x-coordinate indicated a movement toward the lateral border of the insole and increased y-coordinate indicated a movement toward the anterior border. 2.4. Data analysis and outcome measures Demographic information including age, gender, and time since stroke were collected and verified with medical records. Data from all assessments are presented as mean (standard deviation).
2.1. Participants Individuals with hemiplegia and foot drop secondary to stroke (N3 months) and healthy controls were recruited for participation. Individuals with stroke were recruited from a larger multi-site clinical trial. All participants with stroke were currently using a commercially available foot drop stimulator (FDS) (WalkAide®, Innovative Neurotronics, Inc., Austin, TX, USA) for assistance with gait deviations. Individuals with stroke were able to walk independently for 10 m without FDS. 2.2. Foot drop stimulator (FDS) The WalkAide® is a battery operated, single-channel, asymmetrical biphasic stimulator with programmable pulse width and frequency that is utilized during walking as a functional electrical stimulation (FES) orthotic device (Melo et al., 2015). This small device (87.9 g, 8.2 cm(H) × 6.1 cm(W) × 2.1 cm(T)) attaches to a molded cuff located just below the knee (Fig. 1). Two surface electrodes are specifically
Fig. 1. Foot Drop Stimulator (WalkAide®, Innovative Neurotronics, Inc., Austin, TX, USA).
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
Plantar pressure data was exported using the Novel pedar-X Recorder software (Novel Electronics, Inc., Munich, Germany). Per convention, the first two steps were omitted from analysis for all walking tests. All data was imported into Matlab (The Mathworks, Inc., Natick, MA, USA) for custom analysis. Time series force data was used to determine gait cycle events and subdivide the stance phase into 3 sub phases: IDS (initial double support); SS (single support); and TDS (terminal double support), as previously described in Nolan and Yarossi (Nolan and Yarossi, 2011a). Time series CoP data were used to calculate bilateral position, displacement and velocity variables for each phase of the gait cycle as well as the stance phase. Primary outcome variables included CoP: 1) net displacement (anterior/posterior (AP), % insole length); 2) position and maximum displacement (AP, % insole length; and medial/lateral (ML), % insole width); and 3) mean velocity (AP, cm/s). Self-selected walking speed (m/s) was also measured to assess changes in functional ambulation, Table 1. 2.5. Statistical analysis Data were analyzed using descriptive statistics in Matlab (The Mathworks, Inc., Natick, MA, USA). Independent two sample t-tests were performed to determine if there was a significant difference in CoP outcome variables between the stroke group and the healthy control group (P ≤ 0.05). Comparisons were made during stance, IDS, SS, and TDS, between the affected limb and unaffected limb of the stroke group (with and without FDS) to the right limb of the healthy control group. Paired sample t-tests were used to determine if there were significant differences in the stroke group between conditions, with and without FDS (P ≤ 0.05) for CoP outcome variables: 1) net displacement; 2) velocity; 3) position at initial contact and toe off; 4) maximum displacement and 5) posterior displacement during stance. Comparisons were made bilaterally during stance, IDS, SS and TDS for the affected and unaffected limbs to examine the difference in CoP with and without FDS. 3. Results Data from 11 participants with hemiplegia and foot drop 112.8 (76.8) months poststroke (10 males and 1 female) and 11 healthy controls (3 males and 8 females) were available for analysis. In the stroke group, 7 participants were affected on the right side and 4 participants were affected on the left. All participants with stroke used their own FDS custom programmed by a licensed clinician on their affected limb during ambulation in the FDS condition. The FDS was positioned below the knee, secured with a latch, and properly aligned using anatomical landmarks (tibial tuberosity) and visual indicators (locator on cuff). A detailed description of participant characteristics can be found in Table 2. Individuals in the healthy control group had a significantly faster walking speed 1.27 (0.19) m/s than individuals with stroke regardless of condition, with (P ≤ 0.0001) and without (P ≤ 0.0001) FDS. There were no significant differences in walking speed (m/s) for individuals with stroke, with FDS 0.60 (0.25) m/s or without FDS 0.62 (0.28) m/s. Table 1 CoP outcome variables. Name
Description
AP net displacement AP max displacement ML max displacement Posterior displacement
CoP position (cm % insole length) at the end of phase–CoP position at the beginning of phase (Stance, IDS, SS, TDS). Maximum CoP position (cm % insole length)–minimum CoP position during phase. Maximum CoP position (cm % insole width)–minimum CoP position during phase. CoP position (cm % insole length) at initial contact–CoP position at minimum position during IDS.
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Table 2 Participant characteristics; mean (sd). Group
Age (yrs.)
Height (m)
Weight (kg)
Time using FDS (months)
Stroke (n = 11) Healthy controls (n = 11)
60.5 (8.3) 54.2 (15.7)
1.78 (0.11) 1.67 (0.10)
85.3 (11.9) 68.5 (13.2)
4.6 (4.7) –
There were no significant interlimb differences for the healthy control group for any CoP outcome variables. As a result the right limb CoP variables were used for all comparisons. 3.1. Anterior/posterior CoP net displacement and velocity AP net displacement of the CoP was significantly longer during stance and SS, and significantly shorter during TDS in the healthy control group as compared to the affected limb of the stroke group regardless of condition, with (stance P = 0.02; SS P ≤ 0.0001, TDS P ≤ 0.0001) and without (stance P = 0.0004; SS P ≤ 0.0001, TDS P ≤ .0001) FDS. The AP net displacement of the CoP was significantly longer during SS and significantly shorter during IDS and TDS in healthy controls as compared to the unaffected limb of the stroke group regardless of condition, with (SS P = 0.0001, IDS P = 0.0003; TDS P = 0.026) and without (SS P = 0.0003, IDS P = 0.0008, TDS P = 0.019) FDS. The AP net displacement of the CoP during the stance phase in healthy controls was not significantly different than the unaffected limb of the stroke group regardless of condition, with (P = 0.121) and without (P = 0.087) FDS. AP net displacement of the CoP was significantly longer during stance (16.8%, P = 0.006), IDS (10.9%, P = 0.047), and SS (20.4%, P = 0.0001), and significantly shorter during TDS (14.3%, P = 0.005) on the unaffected limb compared to the affected limb of the stroke group without FDS. AP net displacement of the CoP remained significantly longer during SS (16.3%, P = 0.0002) and shorter during TDS (15.4%, P = 0.001) on the unaffected limb compared to the affected limb of the stroke group when the FDS was added to the affected limb. AP net displacement of the CoP was not significantly different between the affected and unaffected limb of the stroke group during stance (P = 0.064) and IDS (P = 0.069) with the addition of the FDS to the affected limb. AP net displacement of the CoP was significantly longer during stance (8%, P = 0.039) and SS (2.2% P = 0.019) on the affected limb with the FDS as compared to without the FDS. Anterior/posterior CoP net displacement variables for individuals with stroke (with and without FDS) and healthy subjects can be found in Table 3. AP velocity of the CoP was significantly faster during stance and SS and significantly slower during TDS in the healthy control group, as compared to the affected limb of the stroke group regardless of condition: with (stance P = 0.005; SS P ≤ .0001, TDS P ≤ 0.0001) and without (stance P = 0.001; SS P ≤ 0.0001, TDS P = 0.001) FDS. AP velocity of the CoP was significantly faster during stance and SS and significantly slower during TDS in the healthy control group, as compared to the unaffected limb of the stroke group regardless of condition: with (stance P = 0.001; SS P = 0.0001, TDS P = 0.006) and without (stance P = 0.001; SS P = 0.0002, TDS P = 0.004) FDS. AP velocity of the CoP was significantly faster during SS and significantly slower during TDS on the unaffected limb, as compared to the affected limb of the stroke group: with (SS P = 0.003, TDS P = 0.027) and without (SS P = 0.0004, TDS P = 0.030) FDS. AP velocity of the CoP on the affected limb increased with the FDS as compared to without the FDS, but this trend was not significant. 3.2. CoP position and maximum displacement CoP position at initial contact was significantly more medial and posterior on the affected limb of the stroke group with the FDS (ML P =
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
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K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
Table 3 Anterior/posterior CoP net displacement and velocity.
Stance IDS SS TDS
Displacement Velocity Displacement Velocity Displacement Velocity Displacement Velocity
Stroke group — affected limb
Stroke group — unaffected limb
Healthy control
With FDS
Without FDS
With FDS
Without FDS
Right
46.4 (18.1)a,e 15.5 (7.0)e 6.1 (13.4) 16.5 (20.5) 15.8 (14.9)a,c,e 10.5 (9.8)c,e 23.8 (15.0) 23.4 (14.3)c,e
38.4 (18.1)a,b,d 13.3 (7.0)d,f 2.9 (15.8)b 1.2 (31.6) 13.6 (4.4)a,b,d 9.1 (10.0)b,d,f 21.5 (14.6) 23.2 (15.8)b,d,f
56.4 (6.4) 14.8 (5.1)g 15.8 (5.9)g 18.3 (8.1) 32.1 (14.7)c,g 15.2 (8.7)c,g 8.4 (9.7) 9.3 (9.2)c,g
55.2 (8.0)b 14.5 (5.4) 13.8 (4.8)b,f 17.7 (7.2) 34.0 (14.5)b,f 15.9 (8.7)b 7.2 (7.5) 8.9 (7.8)b
57.0 (5.8)d,e 26.4 (9.2)d,e,f,g 4.8 (3.7)f,g 18.9 (15.2) 53.9 (6.1)d,e,f,g 36.0 (12.3)d,e,f,g −1.4 (6.6) −0.8 (6.3)d,e,f,g
All AP variables presented as % insole length (displacement) or % insole length per second (velocity). a P ≤ 0.05 with vs. without FDS for the affected limb of the stroke group. b P ≤ 0.05 affected vs. unaffected limb, without FDS in the stroke group. c P ≤ 0.05 affected vs. unaffected limb, with FDS in the stroke group. d P ≤ 0.05 stroke group affected limb without FDS vs. healthy control. e P ≤ 0.05 stroke group affected limb with FDS vs. healthy control. f P ≤ 0.05 stroke group unaffected limb without FDS vs. healthy control. g P ≤ 0.05 stroke group unaffected limb with FDS vs. healthy control.
.005, AP P = 0.043). AP and ML changes at toe-off were not significant with the addition of the FDS, and in both conditions the CoP position at toe off on the affected limb resembled the unaffected limb and the healthy controls. ML mean (P = 0.025), maximum (P = 0.014) and minimum (P = 0.027) CoP position on the affected limb during stance were all significantly more medial with the FDS in the stroke group. AP minimum (P = 0.009) CoP on the affected limb during stance was significantly more posterior with the FDS; but the AP mean (P = 0.153) and maximum (P = 0.114) CoP on the affected limb with FDS were not significant. Maximum ML displacement on the affected limb with FDS was not significantly different (P = 0.346) but maximum AP displacement on the affected limb was significantly longer (P =0.001) with FDS. The only significant differences found on the unaffected limb were ML maximum CoP position was significantly more medial (P = 0.017) and maximum ML displacement was shorter (P = 0.014) when the FDS was added to the affected limb. CoP position and maximum displacement variables for individuals with stroke on the affected and unaffected limb (with and without FDS) and healthy controls can be found in Table 4. 4. Discussion CoP provides information specific to the lower extremity and the neuromuscular fluctuations that are part of motor control during
walking (Mizelle et al., 2006). This study represents a novel application of bilateral CoP data for individuals with stroke currently utilizing FDS for assistance with gait deviations. The primary endpoint in this investigation was to investigate changes in CoP displacement during walking in individuals with stroke with and without a foot drop stimulator (FDS). Multiple comparisons were provided between the stroke group (affected and unaffected) and individuals in the healthy controls as a reference for the directionality of the results, Fig. 2. Research utilizing measurements of CoP as a primary outcome measure typically quantify the entire stance phase and do not comprehensively describe CoP changes throughout weight acceptance (initial double support, IDS), single support (SS) and weight transfer (terminal double support, TDS). These subphases have been shown to be essential for maintaining walking speed in hemiplegic gait (Nolan and Yarossi, 2011a). The current investigation evaluated the entire stance phase and broke down the stance phase into IDS, SS, and TDS in order to understand how CoP changes during gait. Gait speed did not increase with the addition of the FDS to the affected limb in the stroke group. The average gait speed attained was ~ 0.6 (0.2) m/s in the stroke group, regardless of condition, and 1.27 (0.2) m/s in the healthy control group. Healthy self-selected walking speed is 1.48 m/s for men and 1.23 m/s for women (Blessey et al., 1976). According to Perry et al. individuals with a gait speed between 0.4 and
Table 4 CoP position and maximum displacement variables.
Initial contact Toe off Mean (during stance) Max (during stance) Min (during stance) Max displacement Posterior displacement
ML AP ML AP ML AP ML AP ML AP ML AP
Stroke group — affected limb
Stroke group — unaffected limb
Healthy controls
With FDS
Without FDS
With FDS
Without FDS
Right Limb
63.4 (6.8)a 26.7 (33.8)a 45.3 (4.6) 73.1 (9.3) 58.1 (7.7)a 45.1 (8.5) 67.3 (5.8)a 74.9 (8.3) 42.2 (14.0)a 20.8 (9.9)a 25.0 (12.0) 54.1 (11.9)a −5.9 (8.8)
69.6 (7.2)a 33.8 (18.9)a 52.5 (17.0) 72.1 (8.1) 61.4 (7.6)a 47.3 (8.9) 71.6 (6.3)a 73.0 (7.5) 49.2 (23.2)a 26.9 (10.4)a 22.3 (13.3) 46.0 (10.7)a −6.8 (10.1)
62.9 (4.3) 17.4 (3.9) 41.5 (8.1) 73.8 (5.4) 56.1 (5.1) 46.3 (2.9) 67.1 (3.6)b 76.5 (3.0) 38.9 (8.2) 16.2 (2.7) 28.2 (8.5)b 60.3 (4.5) −1.3 (2.0)
63.0 (4.7) 17.9 (4.5) 41.6 (6.6) 73.2 (5.7) 57.0 (4.8) 46.0 (2.7) 68.9 (4.2)b 76.0 (3.6) 38.1 (6.7) 16.4 (3.1) 30.7 (8.0)b 59.6 (5.4) −1.6 (2.3)
66.2 (1.3) 13.4 (1.1) 45.1 (3.3) 74.4 (1.0) 58.3 (3.4) 47.0 (1.9) 68.9 (2.0) 76.0 (0.5) 43.3 (3.0) 12.1 (0.4) 25.6 (2.5) 63.8 (0.6) −1.3 (0.9)
ML variables presented as % insole width; AP variables presented as % insole length. a P ≤ 0.05 with vs. without FDS for the affected limb of the stroke group. b P ≤ 0.05 with vs. without FDS for the unaffected limb of the stroke group.
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
AP Displacement (% Insole Length)
K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
Affected without FDS Affected with FDS Unaffected without FDS Unaffected with FDS Healthy Control Group (Right)
AP COP Velocity (cm/sec)
% Stance
% Stance Fig. 2. Anterior/posterior mean position and velocity for during stance for individuals with stroke (with and without FDS) and healthy controls.
0.8 m/s would have limited community ambulation equivalent to moderate gait impairments (Perry et al., 1995). Individuals with stroke using a FDS contacted the ground 6% more posterior at initial contact and utilized 8% more of the AP plantar surface of the foot on the affected limb during stance. This change in CoP in the stroke group with the FDS more closely resembled the healthy control
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group but individuals with stroke still utilized 10.6% less of the plantar surface of the foot throughout stance than the healthy control group. Previous research in hemiplegic gait indicated similar improvements with traditional AFO intervention in hemiplegic gait (Fatone and Hansen, 2007). Initial contact was 6% more medial on the affected limb of the stroke group with the FDS. In addition, the mean, maximum and minimum position of the CoP, were also all shifted more medially on the affected limb of the stroke group with FDS. Overall CoP ML maximum displacement did not change indicating that there was the same amount of ML variability during stance in the affected limb of the stroke group with FDS but this variability shifted medially. This medial shift may represent an overall improvement in gait, specifically a reduction in equinovarus gait where there is a tendency to load the lateral foot throughout stance. Although the medial shift is small, previous research has indicated that improved ML CoP can improve the control process that regulates stability (Chisholm et al., 2011). Changes in AP posterior displacement during IDS with the addition of the FDS were variable for individuals with stroke. In four individuals with stroke the FDS removed the existence of posterior movement of the CoP, therefore improving forward progression. In the remaining participants the amount of posterior displacement was unchanged or worsened by the FDS. A representative subject with stroke (with and without FDS) and healthy controls can be seen in Fig. 3. Posterior movement (retropulsion) of the CoP during IDS is indicative of inefficient weight transfer on the affected limb (Nolan and Yarossi, 2011b). Further research is needed to explore how an FDS can be utilized to effectively remove retropulsion during stance and what effect that has on gait quality. AP velocity of the CoP on the affected limb increased during stance in the stroke group with the use of the FDS, although this trend did not reach significance. The majority of that increase occurred during IDS. In individuals with stroke, AP velocity of the CoP is impaired on the affected limb during IDS. When the FDS was added, AP velocity of the CoP increased during IDS on the affected limb and resembled the AP velocity during IDS of a healthy adult. This finding is consistent with previous research indicating that a FDS added to the paretic limb during gait could improve weight acceptance (Nardone et al., 2009; Nolan and Yarossi, 2011b) and therefore provide a smoother forward progression of the CoP. Regardless of these improvements on the affected limb
Fig. 3. CoP displacement, one representative individual: A) stroke without FDS; B) stroke with FDS; and C) healthy control.
Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016
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K.J. Nolan et al. / Clinical Biomechanics xxx (2015) xxx–xxx
with FDS for individuals with stroke, the AP velocity of the CoP was still 10.9 cm/s faster throughout stance in the healthy controls. This is primarily due to decreased AP velocity of the CoP during stance and more specifically during SS in the stroke group on the affected and unaffected limb regardless of condition. One of the most interesting findings in this investigation was that the amount of AP progression characteristic during each phase of gait was distinctly different for individuals with stroke compared to healthy controls. In healthy controls the majority of the AP net displacement of the CoP occurs during SS (57%) with relatively little AP displacement of the CoP during double support. For individuals with stroke SS remains the dominant phase of AP progression on the unaffected limb (32.1% with FDS and 34.0% without FDS) but AP displacement during IDS and TDS is larger than found in healthy controls. Progression by phase is markedly different on the affected limb, the largest AP net displacement of the CoP occurs during TDS (23.8% with FDS and 21.5% without FDS). On the affected limb of the stroke group AP displacement of the CoP is delayed until late in stance and this effect remains regardless of FDS intervention. Stroke subjects displayed the majority of anterior progression of the CoP during TDS with an associated increased velocity of the CoP in the anterior direction during this phase. In contrast the healthy controls displayed very little AP movement of the CoP during TDS. For individuals with stroke, this may indicate a reluctance to transfer weight anterior (onto the affected limb) during single support, hindering forward progression. As a result, individuals with stroke wait until TDS to shift weight forward. When the FDS is added to the paretic limb this effect remains but the FDS allows it to happen faster (increased AP velocity). Although the increase in AP velocity during TDS for individuals with stroke on the affected limb is small, it is indicative of positive changes in gait mechanisms that contribute to overall quality of gait. 5. Conclusions Objectively quantifying the CoP in individuals with foot drop is a key factor for understanding forward progression and changes in stability after FDS intervention in individuals with stroke. Analysis of the CoP progression throughout the subphases of stance provided a comprehensive method to evaluate the changes in gait mechanics that effect gait quality. For individuals with stroke, on the affected limb, FDS can improve AP and ML displacement, as well as position and velocity of the CoP to improve forward progression and stability. Measuring outcomes that can provide quantitative information about the potential effectiveness of an FDS orthotic device will provide meaningful clinical evidence for implementation into rehabilitation. Disclaimers None. Grant support Supported by Innovative Neurotronics, Inc. and Kessler Foundation. PMcL was partly supported by the Australian Government funded Collaborative Research Networks Program at Victoria University. Declaration of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper. Appendix 1
AFO AP
ankle foot orthosis anterior/posterior
CoM CoP FDS FES HC IDS ML SS TDS
center of mass center of pressure foot drop stimulator functional electrical stimulator healthy control initial double support medial/lateral single support terminal double support
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Please cite this article as: Nolan, K.J., et al., Changes in center of pressure displacement with the use of a foot drop stimulator in individuals with stroke, Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.03.016