Here, we present a novel protocol to measure positional stability at key events during the sit-to-stand-to-walk using the center-of-pressure to the whole-body-center-of-mass distance. This was derived from the force platform and three-dimensional motion-capture technology. The paradigm is reliable and can be utilized for the assessment of neurologically compromised individuals.
Individuals with sensorimotor pathology e.g., stroke have difficulty executing the common task of rising from sitting and initiating gait (sit-to-walk: STW). Thus, in clinical rehabilitation separation of sit-to-stand and gait initiation – termed sit-to-stand-and-walk (STSW) – is usual. However, a standardized STSW protocol with a clearly defined analytical approach suitable for pathological assessment has yet to be defined.
Hence, a goal-orientated protocol is defined that is suitable for healthy and compromised individuals by requiring the rising phase to be initiated from 120% knee height with a wide base of support independent of lead limb. Optical capture of three-dimensional (3D) segmental movement trajectories, and force platforms to yield two-dimensional (2D) center-of-pressure (COP) trajectories permit tracking of the horizontal distance between COP and whole-body-center-of-mass (BCOM), the decrease of which increases positional stability but is proposed to represent poor dynamic postural control.
BCOM-COP distance is expressed with and without normalization to subjects' leg length. Whilst COP-BCOM distances vary through STSW, normalized data at the key movement events of seat-off and initial toe-off (TO1) during steps 1 and 2 have low intra and inter subject variability in 5 repeated trials performed by 10 young healthy individuals. Thus, comparing COP-BCOM distance at key events during performance of an STSW paradigm between patients with upper motor neuron injury, or other compromised patient groups, and normative data in young healthy individuals is a novel methodology for evaluation of dynamic postural stability.
Clinical pathologies affecting the sensorimotor systems, for example upper motor neuron (UMN) injury following stroke, lead to functional impairments including weakness, loss of postural stability and spasticity, which can negatively affect locomotion. Recovery can be variable with a significant number of stroke survivors failing to achieve the functional milestones of safe standing or walking1,2.
The discrete practice of walking and sit-to-stand are common rehabilitative tasks after UMN pathology3,4, however transitional movements are frequently neglected. Sit-to-walk (STW) is a sequential postural-locomotor task incorporating sit-to-stand (STS), gait initiation (GI), and walking5.
Separation of STS and GI, reflective of hesitation during STW has been observed in patients with Parkinson's disease6 and chronic stroke7, in addition to older unimpaired adults8, but not in young healthy individuals9. Therefore sit-to-stand-and-walk (STSW) is commonly implemented within the clinical environment and is defined by a pause phase of variable length when standing. However, there are no published protocols to date defining STSW dynamics in a context suitable for patient populations.
Usually in STW studies the initial chair height is 100% of knee height (KH; floor-to-knee distance), foot-width and GI lead-limb are self-selected, arms are constrained across the chest and an ecologically meaningful task context is often absent5-9. However, patients find rising from 100% KH challenging10 and frequently adopt a wider foot position compared with healthy individuals11, initiate gait with their affected leg7, and use their arms to generate momentum7.
To initiate gait, a state change in whole-body movement in a purposeful direction is required12. This is achieved by uncoupling the whole-body center-of-mass (BCOM: the weighted average of all considered body segments in space13) from the center-of-pressure (COP: the position of the resultant ground reaction force (GRF) vector14). In the anticipatory phase of GI, rapid stereotypical posterior and lateral movement of the COP toward the limb to be swung occurs thereby generating BCOM momentum12,15. The COP and BCOM are thus separated, with the horizontal distance between them having been proposed as a measure of dynamic postural control16.
The calculation of COP-BCOM distance requires simultaneous measurement of the COP and BCOM positions. The standard calculation of COP is shown below in equation (1)17:
(1)
Where M and Force represent moments about the force platform axes and the directional GRF respectively. The subscripts represent axes. The origin is the vertical distance between the contact surface and the origin of the force platform, and is considered to be zero.
The kinematic method of deriving BCOM position involves tracking the displacement of segmental markers. A faithful representation of body-segment motion can be achieved by employing markers clustered on rigid plates placed away from bony landmarks, minimizing soft-tissue-artifact (CAST technique18). In order to determine BCOM position, individual body segment masses are estimated, based on cadaveric work19. Three-dimensional (3D) motion system proprietary software uses the coordinate positions of proximal and distal segment locations to: 1) determine segmental lengths, 2) arithmetically estimate segmental masses, and 3) compute segmental COM locations. These models are then able to provide estimates of 3D BCOM position at a given point in time based on the net summation of inter-segmental positions (Figure 1).
Thus, the purpose of this paper is first to present a standardized STSW protocol that is ecologically valid and includes rising from a high seat-height. It has been shown previously that STSW from 120% KH is biomechanically indistinct from 100% KH barring generation of lower BCOM vertical velocities and GRF's during rising20, meaning rising from 120% KH is easier (and safer) for compromised individuals. Second, to derive COP-BCOM horizontal distances to assess dynamic postural control during key milestones and transitions using 3D motion-capture. This approach, which in healthy individuals during STSW is independent of limb-lead20, offers the prospect of functional recovery evaluation. Finally, a preliminary STSW data set representative of young healthy individuals is presented, and intra and inter-subject variability in the group is defined in order to inform comparison with pathological individuals.
Figure 1. 2D BCOM calculation. For simplicity, the example is based on calculating whole-leg COM from a 3-linked mass in 2 dimensions, where coordinates of the respective COM positions (x,y), and segmental masses (m1, m2, m3) are known. Segment masses and location of segmental COM positions, with respect to the laboratory coordinate system (LCS; origin: 0, 0), are estimated by motion analysis system proprietary software using subject body mass and published anthropometric data (see main text). The x and y leg COM position, in this example of the 3-linked mass, is then derived using the formulae shown. Please click here to view a larger version of this figure.
The protocol follows the local guidelines for the testing of human participants, defined by London South Bank University research ethics committee approval (UREC1413/2014).
1. Gait Laboratory Preparation
Figure 2. Experimental Protocol. This example shows a left-leg lead: Subjects sit on an instrumented stool at 120% knee height (KH) with ankles 10° degrees in dorsiflexion and feet at shoulder width apart orientated forward. On a visual cue, subjects perform 5 trials of STSW leading with their non-dominant limb at self-selected pace terminated by switching off the light. Please click here to view a larger version of this figure.
Figure 3. L-Shaped Reference Structure and Wand for Camera Calibration. The L-shaped reference structure remains stationary and has 4 markers attached to it. The wand has two markers attached to it at a fixed distance and is moved, with respect to the reference structure, to create a 3-D calibrated volume of space that is sufficient enough for the intended marker set to pass through. Please click here to view a larger version of this figure.
2. Subject Preparation
Table 1: Subject Characteristics. Individual data and mean (±1 SD) across 10 subjects are shown.
Table 2: Marker-set placement. Markers (anatomical and tracking) based on a previously reported technical frame of reference23.
3. Static Capture
4. Familiarization
5. STSW Dynamic Trials
6. Proprietary Tracking Software Post Processing
7. Biomechanics Analysis Software Post Processing
Table 3a: Anatomical Coordinate System for Whole Body Model.
Table 3b: Joint Center Definitions for Whole Body Model.
(2a) Net medio-lateral force
(2b) Net anterior-posterior force
(2c) Net vertical force
(2d) Net platform moment about x-axis
(2e) Net platform moment about y-axis
(2f) x-Coordinate of net force application point (COPx)
(2g) y-Coordinate of net force application point (COPy)
Figure 4. Force Structure. Example of a rectangular force structure encompassing 4 force platforms in a right lead-limb orientation. Details of local COP application and dimensions with respect to a laboratory coordinate system (LCS) are shown for force platform 1 as an example. The x, y, z position of the platform reference system (PRS) is offset relative to the LCS where X1 and Y1 represent the mediolateral and anteroposterior distances from PRS, respectively. To calculate the individual platform moment about the x-axis, the vertical GRF is multiplied by the sum of the local y COP coordinate and the new PRS-LCS offset y coordinate (Y1+y1). The moment about the y-axis coordinate is similarly calculated by multiplying the vertical GRF by the negative sum of the local x COP coordinate and the new PRS-LCS offset x coordinate -(X1+x1). The total moment of force about the global force structure is equal to the sum of all of the moments of force, divided by the sum of the individual vertical forces. Net COP X and Y coordinates are thus produced for the force structure within the LCS (equations 2a-g). Please click here to view a larger version of this figure.
Table 4: Movement Event Definitions. GI – gait initiation; COP – center-of-pressure; HO1 – first heel-off; TO1 – 1st toe-off, IC1 – 1st initial contact.
8. Lab-specific Normative Value Calculations
All subjects rose with their feet placed on the twin force platforms, leading with their non-dominant limb as instructed. Normal gait was observed with subjects stepping cleanly onto the other platforms and 3D optical-based motion analysis successfully tracked whole body movement during 5 repeated goal-orientated STSW tasks rising from 120% KH. Simultaneous COP and BCOM mediolateral (ML) and anteroposterior (AP) displacements between seat-off and IC2 (100% STSW cycle) comprising: rise, pause, gait initiation (GI), step 1, and step 2 are shown respectively in Figure 5A and 5B for the first subject (left leg (non-dominant) lead). In the ML plane, there was negligible COP or BCOM displacement from seat-off to GI onset. However, after GI onset COP displaces leftward away from the standing limb toward the swing limb – separating from the BCOM, which displaces rightward. Then, the COP laterally displaces rightward toward the subsequent stance limb, passing beyond the BCOM rightward before toe-off. Thereafter, during steps 1 and 2, the BCOM follows a sinusoidal displacement, with the COP displacing further laterally during single limb stance (Figure 5A).
Figure 5. COP and BCOM Displacements. Panels show the first subject undertaking STSW from 120% KH with non-dominant limb-lead; in this case left-leg lead. The time axis is normalized to percentage time between seat-off and initial contact 2 (IC2). A) Mediolateral displacement. Y-axis direction labels with respect to the swing (left) leg. Lines show COP and BCOM data corresponding to each trial, the bold lines represents the mean, and shaded areas represent ±1SD around the mean. B) Anteroposterior displacements. Y-axis direction labels with respect to the swing (left) leg. Lines show COP and BCOM data corresponding to each trial, the bold lines represents the mean, and shaded areas represent ±1 SD around the mean. C) COP-BCOM horizontal distance. Lines show distance data corresponding to each trial, the bold line represents the mean, and shaded area represents ±1 SD around the mean. Seat-off and toe-off 1 events, and maxima during steps 1 and 2 are marked. Please click here to view a larger version of this figure.
In the AP plane, the COP at seat-off starts in front of the BCOM, and while they both move forward during rising; their separation diminishes steadily before merging at upright. After the pause phase the BCOM accelerates forwards through GI and steps 1 and 2. In contrast, the COP displaces backwards at GI onset and then forward after toe-off but remains behind the BCOM throughout step 1. The COP, however, passes in front of the BCOM during step 2 after initial contact 1 likely to correspond with the transition to single limb stance. COP forward displacement then slows and passes behind the BCOM again just before mid-stance/swing (Figure 5B).
The horizontal separation distance between COP and BCOM, throughout the STSW cycle, provides a composite of the planar description of COP and BCOM displacements. This approach simplifies the complex interaction of COP and BCOM displacement providing an index of positional stability (Figure 5C).
Intra-subject COP-BCOM separation distances were consistent at seat-off, TO1, and during step 1 and 2 by virtue of strong intraclass correlation coefficients at all 4 events. In addition, the measurement error (Table 5), or common standard deviation of repeated measures32, was small: 9 mm (seat-off) and 12 mm (TO1, step 1, step 2) across all subjects. Another useful way to present measurement error is the repeatability statistic (Table 5). It represents the magnitude of the expected difference between 2 repeated measures 95% of the time, and is between 24 mm and 34 mm for the 4 events.
Inter-subject COP-BCOM separation distances were consistent (Table 6) at seat-off and TO1, in addition to during step 1 and 2. In this homogenous, healthy adult group; subject leg-length range (0.803-0.976 m (Table 1))33 and variance was small (mean 0. 855 m; SD 0.051 m). Whilst it is not typical to normalize COP-BCOM distances to leg length and Figure 6 shows negligible differences between normalized and un-normalized inter-subject mean COP-BCOM data, normalization does reduce the coefficient of variance (COV; Table 6).
Table 5: COP-BCOM Distances. Intra (5 trials) and inter-subject mean ± 1 SD data is shown as actual distances and normalized to subject non-dominant leg length for discrete distances at seat-off and TO1, and maximum distances during step 1 and step 2.
Table 6: Intra-subject Variation. ICCs (95% confidence interval) and measurement error (mean intra-subject SD distance in m) and repeatability statistics32 are shown per event.
Figure 6. Within and Between-Subject COP-BCOM Distances. (A) Un-Normalized. Each line represents within-subject mean COP-BCOM distance. The bold line represents the between-subject mean distance. (B) Normalized to Dominant Leg Length. Each line represents within-subject mean COP-BCOM distance as a percentage of the subject's dominant leg length. The bold line represents the between-subject mean distance as a percentage of the subject's dominant leg length.
The sit-to-stand-and-walk (STSW) protocol defined here can be used to test dynamic postural control during complex transitional movement in healthy individuals or patient groups. The protocol includes constraints that are designed to allow subjects with pathology to participate, and the inclusion of switching off the light means it is ecologically valid and goal-orientated. As it has been shown previously that lead-limb and rising from a high (120% KH) seat does not fundamentally affect task dynamics during STSW20, the methods described here can be applied as a standard protocol. This STSW protocol has validity because compared to healthy individuals, patients find rising from low seat heights a challenge10, tend to generate less horizontal momentum7 and separate rising before initiating gait from a wide foot position11 with their affected leg7. This paper also describes how to calculate COP and BCOM displacement during STSW, from which the horizontal separation between COP and BCOM – an index of dynamic stability16 – can be derived between seat-off and the second step.
The results are dependent on a number of critical steps within the protocol. Firstly, the removal of artefactual light and optimal camera exposure settings is required to ensure the accuracy of optical 3D marker tracking. Secondly, attention to the capture volume when calibrating is an important consideration to further optimize motion capture accuracy. Thirdly, force plate synchronization with the motion capture system using an appropriate scale factor reduces the potential for error in the magnitude of the resultant ground reaction force vector. Fourthly, precise force plate identification in the 3D space is critical. Special care must be made when locating each plate's PRS, and validation of this accuracy must be a routine34. This ensures that force plate structure and rendering during post-processing is optimized for the presentation of high quality COP data. Finally, the main contributors to BCOM displacement estimation errors are inaccurate marker positioning, locating of joint centers and skin movement artifacts35. Thus, experience in anatomical palpation and adoption of the CAST method18 should be considered prerequisites. Other techniques involve using fewer markers or even a singular estimator of BCOM position during gait such as sacral inertial sensors. However, this technique requires validation36 and is of limited utility when body segment orientations deviate from those when upright i.e., during rise37. Thus, multiple camera quantification of BCOM remains the gold standard technique for STSW.
With these steps considered in a healthy population, intra-subject variability during STSW is low, justifying averaging across trials with a high degree of confidence. Furthermore, low (healthy) inter-subject variability suggests comparison with such (lab specific) normative data would provide high sensitivity to differences induced by pathology. Whilst, inter-subject variability was low, reduced COV can be achieved by normalizing for leg length. One aspect that warrants further investigation is the STSW pause phase. Healthy subjects self-selected a mean (± SD) pause phase of 0.84 sec (± 0.07). Whether this differs in pathological groups, and if so whether there is any effect upon stability during transition remains to be determined.
The degree of COP-BCOM separation varies during the different phases of STSW. The largest COP-BCOM distances were at seat-off, TO1, and just prior to foot contact during steps 1 and 2. These represent the biggest challenge to the postural control systems and are therefore defined as the events of interest. Decreased COP-BCOM separation is associated with increased positional stability, but indicates reduced postural stability31. At seat-off as the body transitions from a stable to an unstable base of support, positional stability is accomplished either by posterior positioning of the feet or anterior positioning of the trunk relative to the seat, both of which are commonly seen in functionally impaired patients38,39. After pause, BCOM-COP distances increase during GI; incorporating the anticipatory, postural "release" and "unloading" sub-phases15, and a locomotor swinging limb phase. The end of GI and start of step 1 occurs at TO1; where a relative increase in COP-BCOM separation is associated with BCOM forward acceleration caused by the combined GI phases, the outcome of which is higher walking velocity40. Therefore, COP-BCOM distance at seat-off and TO1 represent candidate dynamic postural stability variables to be tested in pathological groups.
In addition, maximum COP-BCOM distance peaks occur consistently during steps 1 and 2 at the end of single support. These are important events to measure because steps 1 and 2 represent the period where steady-state gait is realized. Larger mean COP-BCOM distances during step 1 compared to step 2 in all but one healthy subject using the protocol were observed. Step 1 remains part of the locomotor acceleration phase before steady-state gait is reached at the end of step 212. Therefore, step 1 is subject to both postural and locomotor control demands and is more positionally unstable than subsequent steps in gait; a feature supported by the increased risk of falling during every day transitional movements41. Step 2 is no less important as it represents the commencement of steady-state gait. Therefore, maximum COP-BCOM distances during both steps 1 and 2 phases are indicated in STSW analysis.
In conclusion, this STSW protocol extends the use of COP-BCOM horizontal separation to STSW and our preliminary results provide an initial normative data set for healthy individuals. COP-BCOM distances normalized to leg length at seat-off, TO1, and step 1 and 2 maxima during performance of a goal-orientated STSW paradigm is a novel methodology for evaluation of dynamic postural stability. It offers the possibility of deriving highly consistent normative global or local data sets that can be compared with UMN injured patients or other compromised patient groups.
The authors have nothing to disclose.
The authors would like to thank Tony Christopher, Lindsey Marjoram at King's College London and Bill Anderson at London South Bank University for their practical support. Thank you also to Eleanor Jones at King's College London for her help in collecting the data for this project.
Motion Tracking Cameras | Qualysis (Qualysis AB Gothenburg, Sweden) | Oqus 300+ | n=8 |
Qualysis Track Manager (QTM) | Qualysis (Qualysis AB Gothenburg, Sweden) | QTM 2.9 Build No: 1697 | Proprietary tracking software |
Force Platform Amplifier | Kistler Instruments, Hook, UK | 5233A | n=4 |
Force Platform | Kistler Instruments, Hook, UK | 9281E | n=4 |
AD Converter | Qualysis (Qualysis AB Gothenburg, Sweden) | 230599 | |
Light-Weight Wooden Walkway Section | Kistler Instruments, Hook, UK | Type 9401B01 | n=2 |
Light-Weight Wooden Walkway Section | Kistler Instruments, Hook, UK | Type 9401B02 | n=4 |
4 Point "L-Shaped" Calibration Frame | Qualysis (Qualysis AB Gothenburg, Sweden) | ||
"T-Shaped" Wand | Qualysis (Qualysis AB Gothenburg, Sweden) | ||
12mm Diameter Passive Retro reflective Marker | Qualysis (Qualysis AB Gothenburg, Sweden) | Cat No: 160181 | Flat Base |
Double Adhesive Tape | Qualysis (Qualysis AB Gothenburg, Sweden) | Cat No: 160188 | For fixing markers to skin |
Height-Adjustable Stool | Ikea, Sweden | Svenerik | Height 43-58cmwith ~10cm customized height extension option at each leg |
Circular (Disc) Pressure Floor Pad | Arun Electronics Ltd, Sussex, UK | PM10 | 305mm Diameter, 3mm thickness, 2 wire |
Lower Limb Tracking Marker Clusters | Qualysis (Qualysis AB Gothenburg, Sweden) | Cat No: 160145 | 2 Marker clusters, lower body with 8 markers (n=2) |
Upper Limb Tracking Marker Clusters | Qualysis (Qualysis AB Gothenburg, Sweden) | Cat No: 160146 | 2 Marker clusters, lower body with 6 markers (n=2) |
Self-Securing Bandage | Fabrifoam, PA, USA | 3'' x 5' | |
Cycling Skull Cap | Dhb | Windslam | |
Digital Column Scale | Seca | 763 Digital Medical Scale w/ Stadiometer | |
Measuring Caliper | Grip-On | Grip Jumbo Aluminum Caliper – Model no. 59070 | 24in. Jaw |
Extendable Arm Goniometer | Lafayette Instrument | Model 01135 | Gollehon |
Light Switch | Custom made | ||
Visual3D Biomechanics Analysis Software | C-Motion Inc., Germantown, MD, USA | Version 4.87 |