Model and measurement studies on stages of prosthetic gait.   

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Chapter 1

Gait initiation
Controlling propulsive forces in gait initiation in transfemoral amputees.
Helco G. van Keeken, Aline H. Vrieling, At L. Hof, Jan P K. Halbertsma, Tanneke Schoppen, Klaas Postema, Bert Otten,
In J Biomech Eng, volume 130, 2008. [bib] [pdf] [doi]  
 
NOTICE: this is the author's version of a work that was accepted for publication in J Biomech Eng. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication.

Gait initiation with the prosthetic (l) and sound (r) limb leading.
Notice the temporal and spatial differences.

Controlling propulsive forces; gait initiation in transfemoral amputees.

During prosthetic gait initiation transfemoral amputees control the spatial and temporal parameters which modulate the propulsive forces, the positions of the center of pressure and the center of mass. Whether their sound limb or prosthetic limb is leading, the transfemoral amputees reach the same end velocity. We wondered how the center of mass velocity build up is influenced by the differences in propulsive components in the limbs and how the trajectory of the center of pressure differs from the center of pressure trajectory in healthy subjects. Seven transfemoral subjects and eight able-bodied subjects were tested on a force plate and on an eight meter long walkway. On the force plate, they initiated gait two times with their sound limb leading and two times with their prosthetic limb leading. Force data were used to calculate the center of mass velocity curves in horizontal and vertical directions. Gait initiated on the walkway was used to determine the limb preference. We hypothesized that because of the differences in propulsive components, the motions of the center of pressure and the center of mass have to be different, as ankle muscles are used to help generate horizontal ground reaction force components. Also, due to the absence of active ankle function in the prosthetic limb, the vertical center of mass velocity during gait initiation may be different when leading with the prosthetic limb compared to when leading with the sound limb. The data showed that whether the transfemoral subjects initiated gait with their prosthetic limb or with their sound limb, their horizontal end velocity was equal. The subjects compensated the loss of propulsive force under the prosthesis with the sound limb, both when the prosthetic limb was leading and when the sound limb was leading. In the vertical center of mass velocity a tendency for differences between the two conditions was found. When initiating gait with the sound limb, the downward vertical center of mass velocity at the end of the gait initiation was higher compared to when leading with the prosthetic limb. Our subjects used a gait initiation strategy that depended mainly on the active ankle function of the sound limb; therefore they changed the relative durations of the gait initiation anticipatory postural adjustment phase and the step execution phase. Both limbs were controlled in one single system of gait propulsion.
The shape of the center of pressure trajectories, the applied forces and the center of mass velocity curves are described in this chapter.
 
ground reaction forces, prosthetic gait initiation, motor strategy, center of pressure, center of mass, velocity

Introduction

Gait initiation is a task that challenges the balance control system by forcing an individual from a state of stable balance to an unstable posture during walking 1; 2; 3; 4. Gait initiation demands a complex integration of neural mechanisms, muscle activity and biomechanical forces 5; 6; 7.
In persons with an amputation of the lower extremity, gait initiation may cause difficulties, because of the disability to use an active ankle strategy 8 and the reduced sensory input system in the prosthetic limb. The lack of ankle strategy, which normally contributes to the posterior displacement of the center of pressure (CoP) at gait initiation and thereby creating a forward momentum 4; 9 on the center of mass (CoM) (figure 1), has to be compensated for with other strategies. The lack of propulsive force during the end of the stance phase of the trailing prosthetic limb due to the absence of the calf muscles also influences the amputees’ performance 8; 10; 11; 12.
Gait initiation can be divided in an anticipatory postural adjustment (Apa) phase and a step execution (Exe) phase. During the Apa phase postural adjustments are made. These adjustments are invariably proportional to the focal gait initiation movement in the Exe phase in which the leading limb is moved forward. The postural adjustments are an integral part of the planning of the movement. The adjustments consist of muscle activation which take in account the coming change in posture and assist the movement by creating a horizontal distance between the CoP and the CoM 13; 14; 15; 16.
After an amputation, a reorganization of motor strategies in gait initiation takes place. To regulate the speed of progression during gait initiation, the amputees control the spatial and temporal parameters of the propulsive forces. When gait is initiated with the sound limb, amputees control the intensity of the propulsive forces during the Apa phase and the Exe phase. In contrast, when gait is initiated with the prosthetic limb, the modulation of the horizontal CoM (CoMy) velocity results mainly from the propulsive forces generated during the Exe phase 17.
In a study of Michel and Chong (2004), the CoMy end velocity (VmHor) at heel contact of the leading limb, was not significantly different in the prosthetic limb compared to the sound limb. This finding may imply that the subjects treat their different limbs as a functional unit, resulting in the same VmHor when initiating gait.
Another study of Michel and Do (2002) showed that the absence of ankle and knee muscles did not affect the CoMy velocity within amputee subjects. The average VmHor of amputees is lower than the average VmHor of healthy subjects. Michel and Chong stated that the absence of ankle and knee muscles did not affect the CoMy velocity. However, a study by Kerrigan et al. (2000) implied that the vertical CoM (CoMz) velocity is influenced by the absence of the ankle musculature. In the study by Kerrigan et al. the relevance of heel rise in the reduction of CoMz vertical displacement was shown. Heel rise during the push off phase in the gait cycle results in an elongation of the CoP and CoM distance and therefore prevents the CoMz moving in a downwards direction during gait initiation. A study by Nolan and Kerrigan (2003) showed that toe-standing gait initiation influences the anterior - posterior CoP (CoPy) trajectory. Less posterior CoPy translation was seen when initiating gait in toe-standing position compared to heel-toe standing gait initiation. To compensate for this decrease CoPy translation to posterior, the subjects used a different muscle activation pattern, resulting in a delayed forward translation of the CoPy and therefore creating a greater forward momentum. Kim et al. (2003) showed in a laboratory setting in which a cadaveric leg was mounted on a foot and ankle joint simulator, that the trajectory of the CoPy was influenced by the angle of the tibia and the foot, and the extrinsic ankle muscles. When applying an amount of 5 kg of muscle loading on the calf muscle with the tibia in upright position, a CoPy displacement in anterior direction of maximal 0.008 meter was reached. Kim et al. indicated that the study may be interpreted as simulation of a bipedal stance with a small amount of postural sway.
A prosthetic limb lacks the active adjustment of the ankle dorsiflexion and plantarflexion angle; most of the artificial ankles are passive systems. During gait initiation the CoPy moves forward as a result of an increasing torque on the ankle caused by the forward rotation of the tibia. The CoPy under the passive prosthetic ankle can not be moved to posterior during the gait initiation and therefore does not contribute to increasing forward momentum during the gait initiation in amputees. The range of this CoPy motion increases with increasing stiffness of the prosthetic ankle.
During gait initiation a prosthesis user has to deal with both an active sound ankle and a passive prosthetic ankle. It is not known how prosthesis users have incorporated the CoPy motion and the disability to actively control the trajectory in their gait initiation process under the prosthetic limb and the sound limb.
We hypothesized that because of the absence of the active ankle function in the prosthetic limb a relative smaller range of motion of the CoPy is shown within the prosthesis users during gait initiation when standing on the prosthetic limb compared to standing on the sound limb with an active ankle function. This trajectory is not actively influenced and therefore the CoPy moves in a trajectory which is related to the CoMy and tibia orientation. Still, posterior positioning of the CoPy at the beginning of gait initiation, as seen in healthy subjects, is necessary for gait initiation. Therefore we hypothesized that this posterior positioning of the CoPy is only possible when the sound limb is still in contact with the ground. Furthermore, due to the absence of active ankle function in the prosthetic limb, the CoMz velocity during gait initiation and the CoMz end velocity (VmVert) may be different when leading with the prosthetic limb compared to leading with the sound limb.
Finally, based on the findings of Michel and Chong, we expected that although there are differences in the two leading limb conditions, our subjects should be able to initiate gait with the prosthetic limb and the sound limb. They use a strategy in which their two limbs operate as a functional unit, resulting in the same CoM end velocity when initiating gait in the two conditions. However, because of the differences in the two conditions, we expect that our subjects prefer to use the prosthetic limb as leading limb during voluntarily gait initiation. In addition, observations during rehabilitation therapy show that transfemoral (TF) amputee subjects initiate gait with the prosthetic limb more often. During quiet stance at ease the TF subjects tend to stand mainly on their sound limb. This posture makes initiating gait with the prosthetic limb more likely, because the body weight is already shifted towards the stance limb.
The aim of this study is to identify modifications in the CoP and CoM movement control strategies in TF amputees in the anticipatory postural adjustment (Apa) phase and the step execution (Exe) phase during gait initiation. These data can be of interest to improve prosthetic knees and feet, to improve rehabilitation programs and to understand how the central nervous system adjusts to impairments caused by absence of active muscular control of joints.


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Figure 1: Posterior displacement of the center of pressure (CoP) creating a forward momentum.


Methods

Subjects

For this study, amputee subjects were recruited by a prosthetics workshop with clients in the three northern provinces of the Netherlands. Inclusion criteria for the amputee group were: a TF amputation because of trauma or oncology for at least one year, daily use of a prosthesis and the ability to walk with the prosthesis more than 50 m without walking aids. Amputees were excluded if they had any medical conditions affecting their mobility or balance, like neurological, orthopedic or rheumatic disorders, otitis media, cognitive problems, severely impaired vision, reduced sensibility of the sound limb, or the use of antipsychotic drugs, antidepressants or tranquilizers. Furthermore, amputee subjects with pain or wounds of the stump, and fitting problems of the prosthesis were excluded. A matched control group of able bodied (AB) subjects was also selected. They were recruited via advertisements at the local blood bank, hospital, television and radio station. AB subjects were included when they did not suffer from lack fitness during common daily activities. Exclusion criteria for AB subjects were orthopedic or neurological disorders, otitis media, reduced sensibility in the lower limbs and the use of antipsychotic drugs, antidepressants or tranquilizers.
The study group consisted of seven TF amputees and eight AB subjects. The medical ethics committee of the University Medical Center Groningen approved the study protocol. All subjects signed an informed consent before testing. The characteristics of the subjects are shown in table 1. The groups showed no statistically significant differences in gender, age, height and weight (in amputees with prosthesis). The TF group contained five right-sided and two left-sided amputees. The TF amputees used different types of prosthetic knees, all supplied with a free movable knee: Teh Lin (3), C-leg (1), Total knee (1), Otto Bock 3R60 (1) and Proteval (1). The following prosthetic feet were used by the subjects: C-walk (2), dynamic SACH (2) and Endolite (3). The subjects walked with their own shoes.


Table 1: Patient characteristics of the TF subjects and the AB subjects. Mean values and standard deviations of age, weight, height, time since amputation, and side of amputation. Gender and side of amputation are provided in absolute numbers. No statistically significant differences were found (p 0.05).
Group TF (n = 7) AB (n = 8)



Gender (male / female) 61 71
Age (years) 44.0 14.1 46.4 9.7
Weight (kg) 81.4 12.4 85.0 10.1
Height (m) 1.82 0.06 1.84 0.07
Time since amputation (months)210.7 158.1-
Side amputation (right / left) 5 / 2 -




Apparatus

The study was performed at the Motion Analysis Laboratory of the Center for Rehabilitation of the University Medical Center Groningen. A Bertec force plate of 0.40 x 0.60 m was used to collect the ground reaction forces (GRF) in three directions and the position of the CoP in two directions. The forces were sampled at 100 Hz. The gait initiation was recorded with two video cameras: one scanning the coronal plane, the other the sagittal plane. The video sampling frequency was 25 Hz. Recording and analysis of the force measurements and video registration was done with a custom-developed Gait Analysis System (GAS) based on LabView software. An eight meter long walkway was used to assess the leading limb preference.

Procedure

Force plate trials were used to obtain data on the GRF, moments, and the CoP. The data were obtained in vertical (GRFz; CoPz), anteroposterior (GRFy; CoPy) and mediolateral (GRFx; CoPx) direction. The CoM (CoMz; CoMy; CoMx) end velocity (VmVert; VmHor) and duration of the Apa phase and the Exe phase were calculated from the GRF data (figure 2; figure 3).
The start of the Apa phase was defined as the moment in time when GRFy was larger than one percent of the body weight in Newton. The end of the Apa phase and start of the Exe phase was defined as the moment in time the CoPx reached the highest velocity when transitioning from the leading limb towards the trailing limb side just before foot off. The end of the Exe phase was defined as the moment in time that GRFz was at its maximum, before the leading swinging limb touches the floor (heel strike).
The measurement started with quiet standing at ease in a bipedal standing position and ended after the subject walked at least two steps away from the force plate. The position of the feet on the force plate was self-selected. The TF subjects performed two trials with the sound limb as leading limb and two trials with the prosthetic limb as leading limb. In the AB group the temporal and spatial parameter values of the right and left limb was assessed and averaged, and used in the data analysis. This method was chosen to minimize the influence of asymmetry between the limbs. In this way we analyzed two conditions in the AB group: the sound leading limb and the sound trailing limb.
The leading limb preference was determined from video images of four trials, in which the subjects had to walk over an eight meters long walkway. They had to stop and start walking halfway the walkway. The leading limb was self-selected. To obtain a leading limb preference score in the amputee group, the amount of trials in which the prosthetic and sound limb were used as leading limb was expressed as a percentage of the total number of trials. In the AB group the number of right and left leading limb trials was expressed as a percentage as well.

Outcome Parameters

The forces and moments were used to calculate the final outcome parameters, similar to Michel and Chong (2004). The CoM acceleration vector was directly calculated from Newton’s second law: m *⃗a = ⃗W + ⃗F, where m is the subject’s mass, ⃗a is the CoM acceleration, ⃗W is the subject’s gravity vector and ⃗F is the GRF vector. The instantaneous CoM velocity was obtained by integration of the acceleration.
The duration of the two phases (dApa, dExe), the slope of the CoMy velocity (sApa, sExe), the CoMy velocity at Apa phase - Exe phase transition before foot off (VFO), the gain of CoMy velocity during step execution (G), the duration of the gait initiation (tVm), and the VmHor were calculated. Also the VmVert was calculated at the end of the Exe phase.

Statistical Analysis

For each subject, individual means of the trials for the leading and trailing (prosthetic and sound) limb were calculated. Leading limb preference was investigated by a one-group t-test. A paired t-test was performed in which the leading limb preference score of the TF and AB groups was compared to zero. The level of significance was set to p 0.05.

Results

The results of the outcome parameters are presented in table 2. The leading limb preferences are presented in table 3. Schematic diagrams of gait initiation characteristics of the TF and AB subjects are presented in figure 2. This figure contains the CoPy trajectory, forces and velocities and a stick figure representation of the gait initiation. The stick figure representation is divided in 5 sub phases, the start of the Apa phase, the middle of the Apa phase, the Apa phase - Exe phase transition, the middle of the Exe phase and the end of the Exe phase. The CoPy position during these sub phases is represented by a triangle. The stick figures are based on video images.
The trajectory of CoPy, the force curves (GRFy; GRFz), the CoM velocity curves, and the Apa phase duration and Exe phase duration from typical AB subjects and typical TF subjects during gait initiation are presented in figure 3.
Table 2 shows that some significant differences were found between and within the TF and AB groups in the properties of the phases, dApa, SApa, VFO, dExe and G. Within the TF group no significant differences were found for the VmHor and tVm when leading with the sound limb or with the prosthetic limb. During gait initiation, the TF group reached a lower VmHor than the AB group. A tendency was found for differences in VmVert for the two different leading limb conditions in the TF group (p = 0.06). The VmVert when leading with the sound limb was higher in downward direction compared to leading with the prosthetic limb. The tVm needed to for gait initiation was not longer in the TF group compared to the AB group. The force curves and CoM velocity curves had characteristic signatures in both the TF group and the AB group. The subjects who deviate from these characteristic signatures are described at the end of the results section.


Table 2: Calculated gait initiation phases for AB subjects and TF subjects and their characteristics: the duration of the two phases (dApa, dExe) and the slope of the CoMy velocity (sApa, sExe), the CoMy velocity at Apa phase - Exe phase transition before foot off (VFO), the gain of the CoMy velocity during step execution (G), the duration of the gait initiation (tVm), and the CoMy end velocity (VmHor) calculated similar to Michel and Chong (2004), supplemented with the vertical end velocity (VmVert). The end velocities were calculated at the end of the Exe phase.
Calculated ItemAB Sound limbTF Sound limbTF Prosthetic limb




dApa (sec) 0.46 0.11 0.68 0.19 0.23 0.25 *
SApa (m∕s2) 0.51. 0.10 0.52. 0.9 0.19 0.10 *
VFO (m∕s) 0.23 0.06 0.35 0.10 0.06 0.08 *
dExe (sec) 0.51 0.04 0.35 0.04 0.72 0.12 *
SExe (m∕s2) 1.54 0.32 1.02 0.24 0.94 0.24
G (m∕s) 0.77 0.11 0.36 0.10 0.66 0.12 *
tVm (sec) 0.97 0.10 1.03 0.17 0.95 0.18
VmHor (m∕s) 1.00 0.16 0.71 0.16 0.71 0.15
VmVert (m∕s) -0.13 0.04 -0.16 0.06 -0.09 0.05




Statistically significant P-values (p 0.05) of differences between the TF group and the AB group are marked with .
Statistically significant P-values (p 0.05) of differences between the TF - sound limb leading group and the TF - prosthetic limb leading group are marked with *.



Table 3: Leading limb preference score for the prosthetic (P) and the sound (N) limb in TF subjects and for the right (R) and left (L) limb in AB subjects in gait initiation.
Group limbLeading limb preference (%)



TF (n=7)P 71.4 39
N 28.6 39
AB (n=8)R 47.2 23
L 52.8 23



No significant differences were found.


TF, sound limb leading

Figure 2 and figure 3a show that during the relative long Apa phase, the CoPy moves to posterior in the first part of the Apa phase and to anterior in the second part. At the end of the Apa phase a translation of the CoPy to posterior is seen. This posterior translation continues at the beginning of the Exe phase, but changes in anterior direction shortly afterwards. During the posterior translation at the end of the Apa phase, a drop in GRFy is seen. Also the GRFz drops. At the same time the CoMz velocity increases in upward direction. This increase of the CoMz velocity is preceded and followed by a decrease of the CoM velocity, resulting in a downward - upward - downward motion of the CoM. Both the CoM velocity and GRFy increase at the end of the Exe phase. The CoMy velocity increases during the total gait initiation. These typical curves were found in five out of seven TF subjects. In two TF subjects a double peak in the GRFy was seen at the end of the Apa phase.

TF, prosthetic limb leading

Figure 2 and figure 3b show that during a relative short Apa phase and during the first half of the Exe phase the CoPy moves in a posterior direction. During the last part of the Exe phase an anterior translation of the CoPy is seen. Compared to TF, sound limb leading, similar GRFz curves are found. The CoMz velocity curve differs from the TF, sound limb leading, group. No decrease in CoMz velocity and therefore no downward motion were found before the Apa phase - Exe phase transition. Also the GRFy curve differs from the GRFy curves in TF, sound limb leading. A continuous increasing of the GRFy is seen. The increase of the GRFy diminishes halfway the Exe phase. Combined to this diminution in the GRFy, a stationary position of the CoPy was found during the first part of this diminution. The second part was accompanied by a motion of the CoPy in anterior direction. These curves were found in six of the seven TF subjects. The curves of the deviating subject are described at the end of this section.

AB, sound limb leading

Figure 2 and figure 3c show that in the AB group, the dApa and dExe are almost equal. During gait initiation the CoPy moves to posterior during most of the Apa phase. At the end of the Apa phase the CoPy moves slightly to anterior. At the beginning of the Exe phase the CoPy moves to posterior again. These translation shifts were seen in at least three of four trials per subject. The rest of the Exe phase the CoPy moves to anterior again. The GRFz curves are quite similar to the curves of the TF group in both conditions. The GRFy curve also shows resemblances with the TF group, when leading with the prosthetics limb. The only difference is the more pronounced bump at the beginning of the Exe phase. The CoMz velocity curve shows no upward velocity. These curves were found in seven out of eight subjects. The curves of the deviating subject are described at the end of this section.

Leading limb preference

The TF subjects showed a tendency towards a preference for the prosthetic limb as the leading limb. The prosthetic limb was the leading limb in 71.4 percent of the total trials.

Deviators

Figure 4 and figure 5 show the data of two deviating subjects, who initiated gait differently as the majority above. Both subjects, one from the TF, prosthetic limb leading, group and one from the AB group, initiated gait while using a toe-standing strategy in the Exe phase. Before the Apa phase - Exe phase transition the most posterior position of the CoPy was reached. During the transition the CoPy translates in anterior direction. The vertical velocity increases and moves the CoM in upward direction. A decrease in GRFy was found. The GRFz curves and the CoMy velocity curves were similar to the curves of the typical subjects above.


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Figure 2: Schematic overview of typical CoMy and CoMz velocities, the forces (GRFy; GRFz) and the CoPy motion during gait initiation of TF subjects leading with their sound limb or the prosthetic limb and AB subjects. The stick figures show five sub phases of gait initiation: 1) the start of the Apa phase, 2) the middle of the Apa phase, 3) the Apa phase - Exe phase transition, 4) the middle of the Exe phase and 5) the end of the Exe phase. The CoP position during these sub phases is represented by a triangle. The stick figures are based on video images. Two typical GRFy curves were found In the TF - sound limb leading group. These curves are represented by the solid and the dotted line.



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Figure 3: Typical time courses of GRFz and GRFy, CoPy motion and the resulting velocities of typical TF subjects (a: sound limb leading; b: prosthesis limb leading) and c: AB subjects). The time course is divided in an Apa phase and an Exe phase. The start of the Apa phase is defined as the moment in time that GRFy is bigger than one percent of the body weight in Newton. The start of the Exe phase is defined as the moment in time the CoPx reaches the highest velocity when moving from the leading limb towards the trailing limb (not in the figure). The end of the Exe phase is defined as the as the moment in time that GRFz is at its maximum, before the leading swinging limb touches the floor (heel strike). The end velocities at the moment the leading limb touches the floor are marked with the dashed lines. To fit GRFz in the figure a scaling factor of 0.1 is used.



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Figure 4: Deviator from TF amputee group - prosthetic limb (a) leading and AB group(b). The time course of GRFz and GRFy, CoPy motion and the resulting velocities of the deviating subjects, divided in an Apa phase and an Exe phase. To fit GRFz in the figure a scaling factor of 0.1 is used.



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Figure 5: Schematic overview of typical CoMy and CoMz velocities, the forces (GRFy; GRFz) and the CoPy motion during gait initiation of TF amputee deviators leading with their prosthetic limb and AB deviators. The stick figures show five sub phases of gait initiation: 1) the start of the Apa phase, 2) the middle of the Apa phase, 3) the Apa phase - Exe phase transition, 4) the middle of the Exe phase and 5) the end of the Exe phase. The CoP position during these sub phases is represented by a triangle. The stick figures are based on video images.


Discussion

The aim of this study was to identify modifications in CoP and CoM movement control strategies in TF prosthesis users in the Apa phase and the Exe phase during gait initiation. In this section we discuss the CoPy motions, the applied forces and the resulting CoM velocities separately to get more insight in gait initiation strategies used by the TF subjects.

CoP

As stated earlier in the introduction, the lack of ankle strategy, which contributes to the CoPy displacement in posterior direction at gait initiation and thereby creating a forward momentum 4; 9 on the CoM has to be compensated with other strategies. Our data show that TF subjects adopt a strategy which is mainly depending on the sound limb enabling them to move their CoPy in posterior direction while initiating gait.
The absence of active ankle function in the prosthetic limb is compensated with the active ankle function in the sound limb. When the TF subjects are leading with their prosthetic limb, the Apa phase is relative short. The prosthetic ankle does not contribute to keeping the CoPy in posterior position, while there is a need to terminate the swing phase before the next stance on the prosthesis. During the first part of the relative long Exe phase the trailing sound limb keeps the CoPy in posterior position. When the TF subjects are leading with the sound limb the TF subjects change the relative durations of the Apa phase and Exe phase, making the Apa phase longer at the cost of the Exe phase, shifting the transition more towards the end of gait initiation. This timing shift enables them to produce sufficient propulsive GRF by positioning the CoPy in posterior position. This shift is based on the use of active ankle function of the sound limb as long as possible while still being able to move the leading sound limb forward during a short swing phase.
The dependence on the sound limb active ankle function can be seen quite clearly during the Apa phase - Exe phase transition in the TF - sound limb leading group. Just before the transition, the CoPy moves to anterior as a result of active plantar flexion muscle activity and tibia rotation of the sound limb. At the moment that the leading sound limb leaves the ground, the CoPy moves to posterior. This motion of the CoPy is the result of repositioning the CoPy under the trailing prosthetic foot. A similar CoPy motion can be seen in the AB group, when repositioning the CoPy under the trailing sound foot. In the TF - prosthetic limb leading condition this CoPy motion is not present, because the trailing sound limb is controlling the CoPy motion.
The two deviators differ from the strategy above, due to their preference to use a toe-standing strategy in the Exe phase when initiating gait. Already in the Apa phase the CoPy moves in anterior direction. This anterior motion is caused by the preference to stand on the toes while swinging the leading limb forward.
Analysis of the CoP of the TF subject shows not only that the CoPy moves forward, but also that the CoPx does not move towards the sound limb during the first part of the Apa phase. That means that both the sound foot and the prosthetic foot are equally responsible for the CoPy movement. Therefore no active ankle plantar flexion is applied with the sound limb during this first phase. The CoPy motion is only possible with a prosthetic ankle, which is loose enough to allow the subject to fall forward, but is stiff enough to imitate the CoPy movement from under the sound foot. Analysis of the video images of the TF subject show that the heel of the prosthetic foot is lifted of the group during the end of the Apa phase. At the end of the Apa phase the CoPx moves towards the sound limb, because of the plantar flexion of the sound ankle which is necessary for the toe standing. Analysis of the video images of the TF subject on the walkway also showed that the subject used a toe-walking strategy during gait. A possible reason for this strategy is that the TF deviator tried to gain more clearance for the leading prosthetic limb during the swing phase.
Analysis of the video images of the AB subject showed that during gait, the subject did not use the toe walking strategy. Only during gait initiation the toe-standing strategy was used in the Exe phase. A possible reason for this strategy is that the AB subject tried to gain more clearance for the leading limb.

Forces

The GRFz curves that were applied by our groups did not differ much between the groups. The shapes of the curves were quite similar, although the position of the maximum and minimum of the GRFz curves are located differently in gait initiation. In the TF group, the maximum is around the Apa phase - Exe phase transition, while in the AB group the maximum is after the Apa phase - Exe phase transition.
The GRFy curves showed greater differences, especially when comparing the TF - sound limb leading with the TF - prosthetic limb leading and the AB group.
A drop in the GRFy curves is seen at the moment the sound limb leaves the ground during the Apa phase - Exe phase transition in the TF - sound limb leading group. Again, this drop shows that the prosthetic user uses a strategy in which the sound limb has an important role in gait initiation. Another difference which was found is the shape of the GRFy curve after the Apa phase - Exe phase transition in the TF - prosthetic limb leading group and the AB group. The TF group had a less evident increase and decrease of GRFy, compared to the AB subjects, although both groups had a sound active ankle function in their trailing limb. An explanation for this difference can be found in the acceleration and deceleration of the swing limb.
In the AB group the swing limb is the sound limb, which has considerable higher inertia than the swinging prosthetic limb in the TF group. Also, the sound limb has to be moved forward in a relative shorter time. Since the CoPy shows a comparable profile, it must the inertia of the swing limb and the duration of the swing phase that causes this difference.
The first of the double peak which is found in half of the TF subjects - sound limb leading group, coincides with the latest moment in time the center of pressure is in the posterior position. The second peak coincides with the final push off of the sound limb. The valley in between can be explained by loss in force due to the anterior displacement of the center of pressure, coming closer to a position under the CoM. In the AB group and the group with the other TF subjects these two peaks are fused, since the remaining sound limb is able to keep the center of pressure in a posterior position even when the leading limb is making a roll over movement.
The force curves of the two deviators showed a drop in the applied forces compared to the other subjects. This drop is partly the result of the CoPy shift in anterior direction.

CoM

The CoMy velocity curves of the groups are all alike. The curvatures show a non-linear increase in velocity. A small difference can be seen in the curve of the TF - sound limb leading group compared to the other groups. The curve shows a small velocity change at Apa phase - Exe phase transition, as a result of the diminished GRFy as the sound limb leaves the ground. After that, the velocity increases again.
On the other hand, the vertical velocities show a clear difference. Although the shape of the curves is quite similar, the maximum velocity which can be found in the Exe phase of the TF group shows a positive (upward) velocity, while in the AB group the maximum stays below zero, showing no upward velocity. In other words, the TF group uses CoM displacement in upward direction during gait initiation. When the TF subject is leading with the prosthetic limb this CoM motion can be the consequence of several actions: 1) raising of the swing limb, 2) plantar flexion of the trailing stance ankle, and 3) elevation of the swing hip. All these motions provide extra clearance in case the prosthetic limb is the swing limb. When the subject is leading with the sound limb, the clearance issue is not the biggest concern. However, a positive CoMz velocity is found during the Exe phase. In the TF - sound limb leading group the Apa phase - Exe phase transition occurs more at the end of the gait initiation and is therefore combined with a relative short Exe phase. The TF subject has to move his sound limb upwards and forwards relatively quickly. These fast motions also result in a more pronounced CoM displacement.
The relative short Exe phase is the result of a passive ankle function of the prosthetic limb. During the relative long Exe phase when leading with the prosthetic limb the subjects make use of the active ankle function of the sound trailing limb. This strategy enables them to 1) gain more time for the swing limb, 2) maintain more height with their CoM and 3) end with less negative CoMz velocity at the end of the gait initiation.
The CoMz velocity curves from the two deviators showed relative higher velocities compared to the other subjects. A big upward velocity was found as a result of the toe-standing preference of the subjects. In the CoMy velocity a drop in the curve was found. This drop in velocity is the result of the CoPy shift in anterior direction and the decrease in GRFy.

Overall

Similar to the findings of Michel and Chong (2004), the data show that although the duration of the gait initiation is the same in both the TF group and the AB group, the VmHor is lower in the TF group whether the subjects are leading with their sound limb, or with their prosthetic limb. These differences in VmHor may be caused by the absence of active ankle function in the prosthetic limb.
Although the VmHor and tVm are the same for the two leading limb conditions in the TF group, a tendency is found for the VmVert to be different between the conditions. When the TF group is leading with the sound limb, the VmVert at the end of the Exe phase in downward direction is relatively higher compared to the condition in which the TF subjects are leading with the prosthetic limb. This relatively higher VmVert is probably the result of the absence of an active ankle function in the trailing prosthetic limb. During the Exe phase the TF subjects act more like an inverted pendulum without an active ankle function when leading with the sound limb and trailing with the prosthetic limb.
Michel and Chong (2004) stated that the absence of ankle and knee muscles does not affect the CoMy velocity. The study by Kerrigan et al. (2000) implies however that the CoMz velocity is influenced by the absence of the ankle musculature. Although we did not find a significant difference in VmVert, but only a tendency, we agree with Kerrigan et al.. Not only there was a tendency of a VmVert difference, also the motion of the CoPy was clearly affected by the absence of an active ankle function in the prosthetic limb. The subjects adopt a strategy in which they depend on the functioning of the healthy ankle.
Our subjects preferred to initiate gait with their prosthetic limb on the walkway. From a gait initiation view point possible reasons for that are: 1) they are already standing with most of their weight on the sound limb before they start the gait initiation. Therefore it is easier to start the gait initiation without the necessary weight shift which occurs at the Apa phase - Exe phase transition in AB subjects who have distributed their weight evenly over both their limbs; 2) the Apa phase - Exe phase transition takes place at the beginning of the gait initiation; their VFO is still low, which might give them the impression that the risk of falling is less; 3) moving their CoM in upward direction is easier when standing still straight up; and 4) that’s the way they were taught to initiate gait during rehabilitation.
When considering the next step after gait initiation, there is another reason to initiate gait with the prosthetic limb. The choice of the leading limb in gait initiation also has consequences for the next step in which the leading swing limb becomes the stance limb. Gait initiation with the sound limb in the TF groups gives the penalty of a higher downward CoMz velocity just before heel strike, which needs to be compensated by the lifting action of the sound limb when on the ground again. The prosthetic limb without the active ankle function can not contribute actively to regaining CoM height.
When initiating gait with the prosthetic limb, a lower downward CoMz velocity needs less catching action of the prosthetic limb, due to the possibilities the active ankle function of the trailing sound limb offers.
We hypothesized that because of the absence of the active ankle function in the prosthetic limb a relative smaller range of motion of the CoPy is shown within the prosthesis users during gait initiation when standing on the prosthetic limb compared to standing on the sound limb. Also, we hypothesized that this posterior positioning of the CoPy is only possible as long as the sound limb is still in contact with the ground.
We found that the range of motion of the CoPy is mainly influenced by the sound limb. The figures show that when the sound limb is in contact with the ground a clear towards posterior - towards anterior translation of the CoPy is seen.
Due to the lack of the active ankle function in the prosthetic limb, our subjects compensated the disability to move the CoPy and generate the GRF with the active ankle function of the sound limb. They used their two limbs as a functional unit resulting in the same VmHor, whether they were leading with their prosthetic limb or their sound limb. It seems that as long as there are no adequate active prosthetic ankles, more symmetry within a TF subject during gait initiation is impossible.
We also hypothesized that because of the absence of active ankle function in the prosthetic limb, the CoMz velocity during gait initiation may be different when leading with the prosthetic limb compared to leading with the sound limb. We found a tendency in vertical end velocities. When leading with the sound limb a bigger downward VmVert was found. Also, the composition of the velocity curves during the gait initiation showed differences.
The finding in this paper can be useful for professionals who work with amputee patients. The information can help the professionals to advance their instructions about gait initiation for new amputees that just learn to walk with a prosthesis or for amputees who have trouble initiating gait. The gait initiation instructions are based on gait initiation strategies from experienced prosthetic users who have advanced their technique during common daily activity.
From a biomechanical perspective, the results of experienced prosthesis users show that it makes no difference whether gait is initiated with the sound limb leading or with the prosthetic limb leading. In both cases the same forward end velocity is reached in the same amount of time. Therefore, patients can be trained with their preferred limb leading. When a patient wants to initiate gait with the prosthetic limb, the patient should be instructed to lift his prosthetic feet of the ground as early as possible and then move the prosthetic limb forwards, while keeping the pressure under the heel of the sound foot. When the prosthetic limb is moved forward sufficiently, the CoP will move forward automatically under the sound foot. When the heel of the prosthetic limb is almost on the ground, the sound foot starts with the push off. This strategy is clinically known as the way patients tend to initiate gate. When the patient prefers to initiate gait with the sound limb, the patient should be instructed to lean forward with both feet on the ground and the hips and limbs straight. This motion will create forward velocity. As soon as the patient feels sufficient pressure under the sound forefoot, the sound limb has to be moved forward, while accepting weight on the prosthetic limb.

Conclusions

During gait initiation TF amputees generate anterior - posterior and vertical forces that take them from a state of stable dynamic balance to an unstable single supported posture during walking. The VmHor within the TF subjects is the same, whether the TF subjects are leading with the prosthetic limb or the sound limb. Also, the duration of the gait initiation is the same, irrespective of which limb is chosen to be the leading limb. A tendency was found for differences in VmVert, depending on which limb was trailing.
TF subjects make use of a gait initiation strategy in which they control their two limbs as a functional unit. This strategy has a great dependency on the sound limb with the active ankle function. The Apa phase - Exe phase transition shift strategy is adapted to whether the sound limb is leading or trailing. The sound limbs active ankle function is used to move the CoPy, to manipulate the forces and to produce the end velocities.

Competing Interests

There are no competing interests related to this study.

Acknowledgement

The authors wish to acknowledge the OIM foundation, Beatrixoord foundation and Anna Foundation for their financial support.

Ethical Approval

The medical ethics committee of the University Medical Center Groningen approved the study protocol (reference number 2004.176). All subjects signed an informed consent before testing.

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