A treadmill ergometer for three-dimensional ground reaction forces measurement during walking.

This report describes new treadmill ergometer designed to measure the vertical and horizontal ground reaction forces produced by the left and right legs during walking. It was validated by static and dynamic tests. Non-linearity was from 0.2% (left vertical force) to 1.4% (right antero-posterior force). The resonance frequency was from 219 (right vertical direction) to 58 Hz (left medio-lateral direction). A calibration "leg", an air jack in series with a strain gauge, was developed and used to produce force signals comparable to those obtained during human locomotion. The mean differences between the force measured by the calibration leg and treadmill ergometer at 5 km h(-1) were 3.7 N (0.7%) for the left side and 6.5 N (1.2%) for the right. Measurements obtained during human walking showed that the treadmill ergometer has considerable potential for analysing human gait.     

Surgical treatment of scoliosis: a review of techniques currently applied

Background Context

Research employing gait measurements indicate asymmetries in ground reaction forces and suggest relationships between these asymmetries, neurological dysfunction and spinal deformity. Although, studies have documented the use of centre of pressure (CoP) and net joint moments in gait assessment and have assessed centre of mass (CoM)-CoP distance relationships in clinical conditions, there is a paucity of information relating to the moments about CoM. It is commonly considered that CoM is situated around S2 vertebra in normal upright posture and hence this study uses S2 vertebral prominence as reference point relative to CoM.

Purpose

To assess and establish asymmetry in the CoP pattern and moments about S2 vertebral prominence during level walking and its relationship to spinal deformity in adolescents with scoliosis.

Patient sample

Nine Adolescent Idiopathic Scoliosis subjects (8 females and 1 male with varying curve magnitudes and laterality) scheduled for surgery within 2–3 days after data collection, took part in this study.

Outcome measures

Kinetic and Kinematic Gait assessment was performed with an aim to estimate the CoP displacement and the moments generated by the ground reaction force about the S2 vertebral prominence during left and right stance during normal walking.

Methods

The study employed a strain gauge force platform to estimate the medio-lateral and anterior-posterior displacement of COP and a six camera motion analysis system to track the reflective markers to assess the kinematics. The data were recorded simultaneously.

Results

Results indicate wide variations in the medio lateral direction CoP, which could be related to the laterality of both the main and compensation curves. This variation is not evident in the anterior-posterior direction. Similar results were recorded for moments about S2 vertebral prominence. Subjects with higher left compensation curve had greater displacement to the left.

Conclusion

Although further longitudinal studies are needed, results indicate that the variables identified in this study are applicable to initial screening and surgical evaluation of scoliosis.     

Controlling propulsive forces in gait initiation in transfemoral amputees

During prosthetic gait initiation, transfemoral (TF) amputees control the spatial and temporal parameters that modulate the propulsive forces, the positions of the center of pressure (CoP), and the center of mass (CoM). Whether their sound leg or the prosthetic leg is leading, the TF amputees reach the same end velocity. We wondered how the CoM velocity build up is influenced by the differences in propulsive components in the legs and how the trajectory of the CoP differs from the CoP trajectory in able bodied (AB) subjects. Seven TF subjects and eight AB subjects were tested on a force plate and on an 8 m long walkway. On the force plate, they initiated gait two times with their sound leg and two times with their prosthetic leg. Force measurement data were used to calculate the CoM velocity curves in horizontal and vertical directions. Gait initiated on the walkway was used to determine the leg preference. We hypothesized that because of the differences in propulsive components, the motions of the CoP and the CoM have to be different, as ankle muscles are used to help generate horizontal ground reaction force components. Also, due to the absence of an active ankle function in the prosthetic leg, the vertical CoM velocity during gait initiation may be different when leading with the prosthetic leg compared to when leading with the sound leg. The data showed that whether the TF subjects initiated a gait with their prosthetic leg or with their sound leg, their horizontal end velocity was equal. The subjects compensated the loss of propulsive force under the prosthesis with the sound leg, both when the prosthetic leg was leading and when the sound leg was leading. In the vertical CoM velocity, a tendency for differences between the two conditions was found. When initiating gait with the sound leg, the downward vertical CoM velocity at the end of the gait initiation was higher compared to when leading with the prosthetic leg. Our subjects used a gait initiation strategy that depended mainly on the active ankle function of the sound leg; therefore, they changed the relative durations of the gait initiation anticipatory postural adjustment phase and the step execution phase. Both legs were controlled in one single system of gait propulsion. The shape of the CoP trajectories, the applied forces, and the CoM velocity curves are described in this paper.     

Prediction of ground reaction forces during gait based on kinematics and a neural network model

Kinetic information during human gait can be estimated with inverse dynamics, which is based on anthropometric, kinematic, and ground reaction data. While collecting ground reaction data with a force plate is useful, it is costly and requires regulated space. The goal of this study was to propose a new, accurate methodology for predicting ground reaction forces (GRFs) during level walking without the help of a force plate. To predict GRFs without a force plate, the traditional method of Newtonian mechanics was used for the single support phase. In addition, an artificial neural network (ANN) model was applied for the double support phase to solve statically indeterminate structure problems. The input variables of the ANN model, which were selected to have both dependency and independency, were limited to the trajectory, velocity, and acceleration of the whole segment's mass centre to minimise errors. The predicted GRFs were validated with actual GRFs through a ten-fold cross-validation method, and the correlation coefficients (R) for the ground forces were 0.918 in the medial–lateral axis, 0.985 in the anterior–posterior axis, and 0.991 in the vertical axis during gait. The ground moments were 0.987 in the sagittal plane, 0.841 in the frontal plane, and 0.868 in the transverse plane during gait. The high correlation coefficients(R) are due to the improvement of the prediction rate in the double support phase. This study also proved the possibility of calculating joint forces and moments based on the GRFs predicted with the proposed new hybrid method. Data generated with the proposed method may thus be used instead of raw GRF data in gait analysis and in calculating joint dynamic data using inverse dynamics.     

Human-Robot Interaction: Does Robotic Guidance Force Affect Gait-Related Brain Dynamics during Robot-Assisted Treadmill Walking?

In order to determine optimal training parameters for robot-assisted treadmill walking, it is essential to understand how a robotic device interacts with its wearer, and thus, how parameter settings of the device affect locomotor control. The aim of this study was to assess the effect of different levels of guidance force during robot-assisted treadmill walking on cortical activity. Eighteen healthy subjects walked at 2 km.h^-1 on a treadmill with and without assistance of the Lokomat robotic gait orthosis. Event-related spectral perturbations and changes in power spectral density were investigated during unassisted treadmill walking as well as during robot-assisted treadmill walking at 30%, 60% and 100% guidance force (with 0% body weight support). Clustering of independent components revealed three clusters of activity in the sensorimotor cortex during treadmill walking and robot-assisted treadmill walking in healthy subjects. These clusters demonstrated gait-related spectral modulations in the mu, beta and low gamma bands over the sensorimotor cortex related to specific phases of the gait cycle. Moreover, mu and beta rhythms were suppressed in the right primary sensory cortex during treadmill walking compared to robot-assisted treadmill walking with 100% guidance force, indicating significantly larger involvement of the sensorimotor area during treadmill walking compared to robot-assisted treadmill walking. Only marginal differences in the spectral power of the mu, beta and low gamma bands could be identified between robot-assisted treadmill walking with different levels of guidance force. From these results it can be concluded that a high level of guidance force (i.e., 100% guidance force) and thus a less active participation during locomotion should be avoided during robot-assisted treadmill walking. This will optimize the involvement of the sensorimotor cortex which is known to be crucial for motor learning.     

Compliant bipedal model with the center of pressure excursion associated with oscillatory behavior of the center of mass reproduces the human gait dynamics

Although the compliant bipedal model could reproduce qualitative ground reaction force (GRF) of human walking, the model with a fixed pivot showed overestimations in stance leg rotation and the ratio of horizontal to vertical GRF. The human walking data showed a continuous forward progression of the center of pressure (CoP) during the stance phase and the suspension of the CoP near the forefoot before the onset of step transition. To better describe human gait dynamics with a minimal expense of model complexity, we proposed a compliant bipedal model with the accelerated pivot which associated the CoP excursion with the oscillatory behavior of the center of mass (CoM) with the existing simulation parameter and leg stiffness. Owing to the pivot acceleration defined to emulate human CoP profile, the arrival of the CoP at the limit of the stance foot over the single stance duration initiated the step-to-step transition. The proposed model showed an improved match of walking data. As the forward motion of CoM during single stance was partly accounted by forward pivot translation, the previously overestimated rotation of the stance leg was reduced and the corresponding horizontal GRF became closer to human data. The walking solutions of the model ranged over higher speed ranges (~1.7 m/s) than those of the fixed pivoted compliant bipedal model (~1.5 m/s) and exhibited other gait parameters, such as touchdown angle, step length and step frequency, comparable to the experimental observations. The good matches between the model and experimental GRF data imply that the continuous pivot acceleration associated with CoM oscillatory behavior could serve as a useful framework of bipedal model.     

Effects of arm swing on mechanical parameters of human gait

The aim of this study was to investigate the influence of the upper limb swing on human gait. Measurements were performed on 52 subjects by using the Elite system with two cameras and a Kistler force platform. The recording of trajectories of characteristic body points on the subjects and the measurement of ground reaction forces have been performed at normal walking and at walking with emphasised rhythmic upper limb swing. The trajectory of the whole body mass centre, central dynamic moments of inertia and ground reaction forces have been calculated for every subject and mean curves of the entire group have been determined for walking with the natural and the emphasised upper limb swing. The determined mean values of normalised mechanical parameters have been compared and differences between the gait with the natural and the emphasised upper limb swing have been described.     

Effect of surgery to implant motion and force sensors on vertical ground reaction forces in the ovine model

Activities of daily living (ADLs) generate complex, multidirectional forces in the anterior cruciate ligament (ACL). While calibration problems preclude direct measurement in patients, ACL forces can conceivably be measured in animals after technical challenges are overcome. For example, motion and force sensors can be implanted in the animal but investigators must determine the extent to which these sensors and surgery affect normal gait. Our objectives in this study were to determine (1) if surgically implanting knee motion sensors and an ACL force sensor significantly alter normal ovine gait and (2) how increasing gait speed and grade on a treadmill affect ovine gait before and after surgery. Ten skeletally mature, female sheep were used to test four hypotheses: (1) surgical implantation of sensors would significantly decrease average and peak vertical ground reaction forces (VGRFs) in the operated limb, (2) surgical implantation would significantly decrease single limb stance duration for the operated limb, (3) increasing treadmill speed would increase VGRFs pre- and post operatively, and (4) increasing treadmill grade would increase the hind limb VGRFs pre- and post operatively. An instrumented treadmill with two force plates was used to record fore and hind limb VGRFs during four combinations of two speeds (1.0 m/s and 1.3 m/s) and two grades (0 deg and 6 deg). Sensor implantation decreased average and peak VGRFs less than 10% and 20%, respectively, across all combinations of speed and grade. Sensor implantation significantly decreased the single limb stance duration in the operated hind limb during inclined walking at 1.3 m/s but had no effect on single limb stance duration in the operated limb during other activities. Increasing treadmill speed increased hind limb peak (but not average) VGRFs before surgery and peak VGRF only in the unoperated hind limb during level walking after surgery. Increasing treadmill grade (at 1 m/s) significantly increased hind limb average and peak VGRFs before surgery but increasing treadmill grade post op did not significantly affect any response measure. Since VGRF values exceeded 80% of presurgery levels, we conclude that animal gait post op is near normal. Thus, we can assume normal gait when conducting experiments following sensor implantation. Ultimately, we seek to measure ACL forces for ADLs to provide design criteria and evaluation benchmarks for traditional and tissue engineered ACL repairs and reconstructions.     

Estimation of unmeasured ground reaction force data based on the oscillatory characteristics of the center of mass during human walking

To enhance the wearability of portable motion-monitoring devices, the size and number of sensors are minimized, but at the expense of quality and quantity of data collected. For example, owing to the size and weight of low-frequency force transducers, most currently available wearable gait measurement systems provide only limited, if any, elements of ground reaction force (GRF) data. To obtain the most GRF information possible with a minimal use of sensors, we propose a GRF estimation method based on biomechanical knowledge of human walking. This includes the dynamics of the center of mass (CoM) during steady human gait resembling the oscillatory behaviors of a mass-spring system. Available measurement data were incorporated into a spring-loaded inverted pendulum with translating pivot. The spring stiffness and simulation parameters were tuned to match, as accurately as possible, the available data and oscillatory characteristics of walking. Our results showed that the model simulation estimated reasonably well the unmeasured GRF. Using the vertical GRF and CoP profile for gait speeds ranging from 0.93 to 1.89 m/s, the anterior-posterior (A-P) GRF was estimated and resulted in an average correlation coefficient of R = 0.982 ± 0.009. Even when the ground contact timing and gait speed information were alone available, our method estimated GRFs resulting in R = 0.969 ± 0.022 for the A-P and R = 0.891 ± 0.101 for the vertical GRFs. This research demonstrates that the biomechanical knowledge of human walking, such as inherited oscillatory characteristics of the CoM, can be used to gain unmeasured information regarding human gait dynamics.     

Development of an advanced mechanised gait trainer, controlling movement of the centre of mass, for restoring gait in non-ambulant subjects.

The study aimed at further development of a mechanised gait trainer which would allow non-ambulant people to practice a gait-like motion repeatedly. To simulate normal gait, discrete stance and swing phases, lasting 60% and 40% of the gait cycle respectively, and the control of the movement of the centre of mass were required. A complex gear system provided the gait-like movement of two foot plates with a ratio of 60% to 40% between the stance and swing phases. A controlled propulsion system adjusted its output according to patient's efforts. Two eccenters on the central gear controlled phase-adjusted the vertical and horizontal position of the centre of mass. The patterns of sagittal lower limb joint kinematics and of muscle activation of a normal subject were similar when using the mechanised trainer and when walking on a treadmill. A non-ambulatory hemiparetic subject required little help from one therapist on the gait trainer, while two therapists supported treadmill walking. Gait movements on the trainer were highly symmetrical, impact-free, and less spastic. The weight-bearing muscles were activated in a similar fashion during both conditions. The vertical displacement of the centre of mass was bi-instead of mono-phasic during each gait cycle on the new device. In conclusion, the gait trainer allowed wheelchair-bound subjects the repetitive practice of a gait-like movement without overstraining therapists.     

Gait synchronized force modulation during the stance period of one limb achieved by an active partial body weight support system

Our purpose was to demonstrate the ability of an actively controlled partial body weight support (PBWS) system to provide gait synchronized support during the stance period of a single lower extremity while examining the affect of such a support condition on gait asymmetry. Using an instrumented treadmill and a motion capture system, we compared gait parameters of twelve healthy elderly subjects (age 65-80 years) during unsupported walking to those while walking with 20% body weight support provided during only the stance period of the right limb. Specifically, we examined peak three-dimensional ground reaction force (GRF) data and the symmetry of lower extremity sagittal plane joint angles and of time and distance parameters. A reduction in all three GRF components was observed for the supported limb during modulated support. Reductions observed in the vertical GRF were comparable to the desired 20% support level. Additionally, GRF components examined for the unsupported limb during modulated support were consistently similar to those measured during unsupported walking. Modulated support caused statistically significant increases in asymmetry for knee flexion during stance (increased 5.9%), hip flexion during late swing (increased 9.1%), and the duration of single limb support (increased 2.8%). However, the observed increases were similar or considerably less than the natural variability in the asymmetry of these parameters during unsupported walking. The ability of the active PBWS device to provide unilateral support may offer new and possibly improved applications of PBWS rehabilitation for patients with unilateral walking deficits such as hemiparesis or orthopaedic injury.     

Contributions of muscles to mediolateral ground reaction force over a range of walking speeds

Impaired control of mediolateral body motion during walking is an important health concern. Developing treatments to improve mediolateral control is challenging, partly because the mechanisms by which muscles modulate mediolateral ground reaction force (and thereby modulate mediolateral acceleration of the body mass center) during unimpaired walking are poorly understood. To investigate this, we examined mediolateral ground reaction forces in eight unimpaired subjects walking at four speeds and determined the contributions of muscles, gravity, and velocity-related forces to the mediolateral ground reaction force by analyzing muscle-driven simulations of these subjects. During early stance (0-6% gait cycle), peak ground reaction force on the leading foot was directed laterally and increased significantly (p<0.05) with walking speed. During early single support (14-30% gait cycle), peak ground reaction force on the stance foot was directed medially and increased significantly (p<0.01) with speed. Muscles accounted for more than 92% of the mediolateral ground reaction force over all walking speeds, whereas gravity and velocity-related forces made relatively small contributions. Muscles coordinate mediolateral acceleration via an interplay between the medial ground reaction force contributed by the abductors and the lateral ground reaction forces contributed by the knee extensors, plantarflexors, and adductors. Our findings show how muscles that contribute to forward progression and body-weight support also modulate mediolateral acceleration of the body mass center while weight is transferred from one leg to another during double support.     

A probabilistic method to estimate gait kinetics in the absence of ground reaction force measurements

Human joint torques during gait are usually computed using inverse dynamics. This method requires a skeletal model, kinematics and measured ground reaction forces and moments (GRFM). Measuring GRFM is however only possible in a controlled environment. This paper introduces a probabilistic method based on probabilistic principal component analysis to estimate the joint torques for healthy gait without measured GRFM. A gait dataset of 23 subjects was obtained containing kinematics, measured GRFM and joint torques from inverse dynamics in order to obtain a probabilistic model. This model was then used to estimate the joint torques of other subjects without measured GRFM. Only kinematics, a skeletal model and timing of gait events are needed. Estimation only takes 0.28 ms per time instant. Using cross-validation, the resulting root mean square estimation errors for the lower-limb joint torques are found to be approximately 0.1 Nm/kg, which is 6–18% of the range of the ground truth joint torques. Estimated joint torque and GRFM errors are up to two times smaller than model-based state-of-the-art methods. Model-free artificial neural networks can achieve lower errors than our method, but are less repeatable, do not contain uncertainty information on the estimates and are difficult to use in situations which are not in the learning set. In contrast, our method performs well in a new situation where the walking speed is higher than in the learning dataset. The method can for example be used to estimate the kinetics during overground walking without force plates, during treadmill walking without (separate) force plates and during ambulatory measurements.     

Can treadmill walking be used to assess propulsion generation?

Instrumented treadmills offer significant advantages for analysis of human locomotion, including recording consecutive steady-state gait cycles, precisely controlling walking speed, and avoiding force plate targeting. However, some studies of hemiparetic walking on a treadmill have suggested that the moving treadmill belt may fundamentally alter propulsion mechanics. Any differences in propulsion mechanics during treadmill walking would be problematic since recent studies assessing propulsion have provided fundamental insight into hemiparetic walking. The purpose of this study was to test the hypothesis that there would be no difference in the generation of anterior/posterior (A/P) propulsion by performing a carefully controlled comparison of the A/P ground reaction forces (GRFs) and impulses in healthy adults during treadmill and overground walking. Gait data were collected from eight subjects walking overground and on a treadmill with speed and cadence controlled. Peak negative and positive horizontal GRFs in early and late stance, respectively, were reduced by less than 5% of body weight (p<0.05) during treadmill walking compared to overground walking. The magnitude of the braking impulse was similarly lower (p<0.05) during treadmill walking, but no significant difference was found between propulsion impulses. While there were some subtle differences in A/P GRFs between overground and treadmill walking, these results suggest there is no fundamental difference in propulsion mechanics. We conclude that treadmill walking can be used to investigate propulsion generation in healthy and by implication clinical populations.     

Experimental/analytical analysis of human locomotion using bondgraphs

A new vectorial bondgraph approach for modeling and simulation of human locomotion is introduced. The vectorial bondgraph is applied to an eight-segment gait model to derive the equations of motion for studying ground reaction forces (GRFs) and centers of pressure (COPs) in single and double support phases of ground and treadmill walking. A phase detection technique and accompanying transition equation is proposed with which the GRFs and COPs may be calculated for the transitions from double-to-single and single-to-double support phases. Good agreement is found between model predictions and experimental data obtained from force plate measurements. The bondgraph modeling approach is shown to be both informative and adaptable, in the sense that the model resembles the human body structure, and that modeled body segments can be easily added or removed.     

Biomechanical gait analysis of pigs walking on solid concrete floor

Inappropriate floors in pig pens and slippery floor conditions may cause leg problems that reduce animal welfare. Therefore the objectives of the present study were to characterise the walk of pigs on dry concrete solid floor, to evaluate whether pigs modify their gait according to floor condition, and to suggest a coefficient of friction (COF) that ensures safe walking on solid concrete floors for pigs. Kinematic (50 Hz video recordings in the sagittal plane) and kinetic (1 KHz force plate measuring three perpendicular ground reaction forces) data were collected from four strides of both the fore- and hindlimbs of 30 healthy pigs walking on dry, greasy and wet concrete floor with 10 pigs on each floor condition. The COF of the floor conditions were tested in a drag-test. The results from the gait analysis showed that the pigs adapted their gait to potentially slippery floors by lowering their walking speed and reducing their peak utilised COF on greasy and wet (contaminated) floors compared with dry floors. Moreover, the pigs shortened their progression length and prolonged their stance phase duration on greasy floor compared with dry and wet floors. Thus the greasy floor appeared the most slippery condition to the pigs, whereas the wet floor was intermediate to the other two conditions. The pigs walked with a four-beat gait, and the limbs differed biomechanically, as the forelimbs carried more load, received higher peak vertical forces and had longer lasting stance phases than did the hindlimbs. The utilised COF from the gait analysis indicated that a high floor COF (>0.63) is needed to prevent pigs from slipping and thus to ensure safe walking on dry concrete floors.     

A force plate study of avian gait

OBJECTIVE: To test the force plate as a gait analysis system for broilers and to determine how the ground reaction force (GRF) patterns change in these birds with growth and administration of analgesia. MATERIALS AND METHODS: Thirty-three male Ross 308 chicks were raised on either an ad libitum or restricted-feeding regime, and subsequently treated with carprofen or a placebo. Vertical, craniocaudal and mediolateral GRFs were measured as the birds walked across a standard force plate. RESULTS: The data were easy to collect, and peak vertical forces of an equivalent percentage of bodyweight as seen in human walking were identified. Mediolateral forces were 2-3 times greater than those demonstrated in other species. GRF patterns showed significant changes during growth, but analgesia did not have a significant effect on the speed of walking, or GRF patterns. CONCLUSIONS AND CLINICAL RELEVANCE: The force plate is a suitable research tool for recording GRFs from avian bipeds. The large mediolateral forces identify a particularly inefficient aspect of avian gait; however, the role of pain remains to be determined.     

Measured and estimated ground reaction forces for multi-segment foot models

Accurate measurement of ground reaction forces under discrete areas of the foot is important in the development of more advanced foot models, which can improve our understanding of foot and ankle function. To overcome current equipment limitations, a few investigators have proposed combining a pressure mat with a single force platform and using a proportionality assumption to estimate subarea shear forces and free moments. In this study, two adjacent force platforms were used to evaluate the accuracy of the proportionality assumption on a three segment foot model during normal gait. Seventeen right feet were tested using a targeted walking approach, isolating two separate joints: transverse tarsal and metatarsophalangeal. Root mean square (RMS) errors in shear forces up to 6% body weight (BW) were found using the proportionality assumption, with the highest errors (peak absolute errors up to 12% BW) occurring between the forefoot and toes in terminal stance. The hallux exerted a small braking force in opposition to the propulsive force of the forefoot, which was unaccounted for by the proportionality assumption. While the assumption may be suitable for specific applications (e.g. gait analysis models), it is important to understand that some information on foot function can be lost. The results help highlight possible limitations of the assumption. Measured ensemble average subarea shear forces during normal gait are also presented for the first time.     

Simple and complex models for studying muscle function in walking

While simple models can be helpful in identifying basic features of muscle function, more complex models are needed to discern the functional roles of specific muscles in movement. In this paper, two very different models of walking, one simple and one complex, are used to study how muscle forces, gravitational forces and centrifugal forces (i.e. forces arising from motion of the joints) combine to produce the pattern of force exerted on the ground. Both the simple model and the complex one predict that muscles contribute significantly to the ground force pattern generated in walking; indeed, both models show that muscle action is responsible for the appearance of the two peaks in the vertical force. The simple model, an inverted double pendulum, suggests further that the first and second peaks are due to net extensor muscle moments exerted about the knee and ankle, respectively. Analyses based on a much more complex, muscle-actuated simulation of walking are in general agreement with these results; however, the more detailed model also reveals that both the hip extensor and hip abductor muscles contribute significantly to vertical motion of the centre of mass, and therefore to the appearance of the first peak in the vertical ground force, in early single-leg stance. This discrepancy in the model predictions is most probably explained by the difference in model complexity. First, movements of the upper body in the sagittal plane are not represented properly in the double-pendulum model, which may explain the anomalous result obtained for the contribution of a hip-extensor torque to the vertical ground force. Second, the double-pendulum model incorporates only three of the six major elements of walking, whereas the complex model is fully 3D and incorporates all six gait determinants. In particular, pelvic list occurs primarily in the frontal plane, so there is the potential for this mechanism to contribute significantly to the vertical ground force, especially during early single-leg stance when the hip abductors are activated with considerable force.     

A novel design for an instrumented stairway

Kinetics during stair ambulation is currently studied via either the use of sensing elements embedded in the steps of the stairway or simple rigid blocks of different height positioned on top of existing force platforms, typically embedded in a walkway for gait analysis. Neither of these approaches is truly satisfactory for gait analysis laboratories. The first one is expensive and requires setting up a dedicated space. The second approach is limited by the number of platforms utilized in the laboratory for evaluating level walking. This communication proposes a novel design, referred to as "interlaced stairway", that allows one to measure ground reaction force and position of the center of pressure (CoP) for four foot contacts during stair ambulation using only two force platforms embedded in a walkway. Accuracy and precision of the CoP estimates and natural frequency of the stairway structure were derived from experimental data. Test results indicate that the interlaced stairway structure does not appreciably reduce the quality of the measures gathered by the existing force platforms. Specifically, the estimated CoP coordinates show good agreement with the horizontal coordinates of the geometric center of the calibration object utilized to assess accuracy and precision of the CoP estimates (max difference < 6 mm). The natural frequency of the stairway structure is lower than the one for the unloaded force platform but higher than the frequency components of interest in stair ambulation analysis.