Stefano Rossi

Feasibility Study of a Wearable Exoskeleton for Children: Is the Gait Altered by Adding Masses on Lower Limbs?

We are designing a pediatric exoskeletal ankle robot (pediatric Anklebot) to promote gait habilitation in children with Cerebral Palsy (CP). Few studies have evaluated how much or whether the unilateral loading of a wearable exoskeleton may have the unwanted effect of altering significantly the gait. The purpose of this study was to evaluate whether adding masses up to 2.5 kg, the estimated overall added mass of the mentioned device, at the knee level alters the gait kinematics. Ten healthy children and eight children with CP, with light or mild gait impairment, walked wearing a knee brace with several masses. Gait parameters and lower-limb joint kinematics were analyzed with an optoelectronic system under six conditions: without brace (natural gait) and with masses placed at the knee level (0.5, 1.0, 1.5, 2.0, 2.5 kg). T-tests and repeated measures ANOVA tests were conducted in order to find noteworthy differences among the trial conditions and between loaded and unloaded legs. No statistically significant differences in gait parameters for both healthy children and children with CP were observed in the five “with added mass” conditions. We found significant differences among “natural gait” and “with added masses” conditions in knee flexion and hip extension angles for healthy children and in knee flexion angle for children with CP. This result can be interpreted as an effect of the mechanical constraint induced by the knee brace rather than the effect associated with load increase. The study demonstrates that the mechanical constraint induced by the brace has a measurable effect on the gait of healthy children and children with CP and that the added mass up to 2.5 kg does not alter the lower limb kinematics. This suggests that wearable devices weighing 25 N or less will not noticeably modify the gait patterns of the population examined here.

Two calibration procedures for a gyroscope-free inertial measurement system based on a double-pendulum apparatus

This paper presents a novel calibration algorithm to be used with a gyro-free inertial measurement unit (GF-IMU) based on the use of linear accelerometers (AC). The analytical approach can be implemented in two calibration procedures. The first procedure (P-I) is articulated in the conduction of a static trial, to compute the sensitivity and the direction of the sensing axis of each AC, followed by a dynamic trial, to determine the AC locations. By contrast, the latter procedure (P-II) consists in the calculation of the previously indicated calibration parameters by means of a dynamic trial only. The feasibility of the two calibration procedures has been investigated by testing two GF-IMUs, equipped with ten and six bi-axial linear ACs, with an ad hoc instrumented double-pendulum apparatus. P-I and P-II were compared to a calibration procedure used as a reference (P-REF), which incorporates the AC positions measured with an optoelectronic system. The experimental results we present in this paper demonstrate that (i) P-I is able to determine the calibration parameters of the AC array with a higher accuracy than P-II; (ii) consequently, the errors associated with translational (a0 ? g) and rotational () acceleration components for the two GF-IMUs are significantly greater using P-II than P-I and (iii) the errors in (a0 ? g) and obtained with P-I are comparable with the ones obtainable by using P-REF. Thus, the proposed novel algorithm used in P-I, in conjunction with the double-pendulum apparatus, can be globally considered a viable tool in GF-IMU calibration.

Centre of pressure in dynamic posturography: a comparison among systems based on a pressure matrix and a force platform

The centre of pressure (CoP) measurement is a very important evaluation tool in posturography. Several papers have been focused on the estimation of CoP accuracy with a pressure matrix (PM) and a force platform (FP) in static conditions, while a few works reported analyses in dynamic conditions. Here, the previous sensors were compared in terms of relative and absolute error during two dynamic sessions: with a rigid bipod and with a healthy adult. Trials were conducted placing the PM and FP in series over a three-axial rotating base, programmed with sinusoidal trajectories of amplitude 10° and frequencies 0.2 Hz and 0.5 Hz. In order to compute the absolute errors of the PM and FP, a further estimation of the bipod CoP was done from the moving base rotation and bipod position gathered by an optoelectronic system. The CoP relative error between the two sensors was about 7–8 mm. Because the relative errors obtained during the bipod session were comparable with the ones obtained during the subject sessions, the absolute errors estimated with the bipod, <7 mm, can be reasonably extended to the dynamic posturography tests performed with the subject. The accuracies were estimated checking all the possible error sources, such as the sensors alignment and the inertial artefacts; because these are rarely considered parameters in clinical routine, they can decrease the overall CoP accuracy if not carefully controlled.

Compensation to whole body active rotation perturbation

The aim of the present study is the exploration of the compensation mechanisms in healthy adults elicited by superimposing a horizontal perturbation, through a rotation of the support base, during a whole body active rotation around the participants own vertical body axis. Eight healthy participants stood on a rotating platform while executing 90° whole body rotations under three conditions: no concurrent platform rotation (NP), support surface rotation of ± 45° in the same (45-S) and opposite (45-O) directions. Participants kinematics and CoP displacements were analyzed with an optoelectronic system and a force platform. In both 45-S and 45-O conditions, there was a tendency for the head to be affected by the external perturbation and to be the last and least perturbed segment while the pelvis was the most perturbed. The observed reduced head perturbation in 45-S and 45-O trials is consistent with a goal-oriented strategy mediated by vision and vestibular information, whereas the tuning of lumbar rotation is consistent with control mechanisms mediated by somato-sensory information.

Shoulder motor performance assessment in the sagittal plane in children with hemiplegia during single joint pointing tasks

BackgroundPointing is a motor task extensively used during daily life activities and it requires complex visuo-motor transformation to select the appropriate movement strategy. The study of invariant characteristics of human movements has led to several theories on how the brain solves the redundancy problem, but the application of these theories on children affected by hemiplegia is limited. This study aims at giving a quantitative assessment of the shoulder motor behaviour in children with hemiplegia during pointing tasks.MethodsEight children with hemiplegia were involved in the study and were asked to perform movements on the sagittal plane with both arms, at low and high speed. Subject movements were recorded using an optoelectronic system; a 4-DOF model of children arm has been developed to calculate kinematic and dynamic variables. A set of evaluation indexes has been extracted in order to quantitatively assess whether and how children modify their motor control strategies when perform movements with the more affected or less affected arm.ResultsIn low speed movements, no differences can be seen in terms of movement duration and peak velocity between the More Affected arm (MA) and the Less Affected arm (LA), as well as in the main characteristics of movement kinematics and dynamics. As regards fast movements, remarkable differences in terms of strategies of motor control can be observed: while movements with LA did not show any significant difference in Dimensionless Jerk Index (JI) and Dimensionless Torque-change Cost index (TC) between the elevation and lowering phases, suggesting that motor control optimization is similar for movements performed with or against gravity, movements with MA showed a statistically significant increase of both JI and TC during lowering phase.ConclusionsResults suggest the presence of a different control strategy for fast movements in particular during lowering phase. Results suggest that motor control is not able to optimize Jerk and Torque-change cost functions in the same way when controls the two arms, suggesting that children with hemiplegia do not actively control MA lowering fast movements, in order to take advantage of the passive inertial body properties, rather than to attempt its optimal control.

Experimental Measurement of the Ski Boot Stiffness in Sagittal and Frontal Planes

In the present paper a novel device and a post-processing procedure are proposed to evaluate the mechanical behavior of a ski boot stiffness. The originality of the methodology consists in the following elements: (1) the stiffness is evaluated not only in plantar/dorsi flexion direction, but also in inversion/eversion one; (2) the loads are applied automatically and along programmable paths; and (3) the stiffness is calculated as the full jacobian of the applied moments. The testing apparatus is a XY Cartesian robot equipped with two linear potentiometers, a 6-axes load cell and a cardanic/prismatic joint for resembling the behavior of the knee complex. The experiments consisted of a cyclic horizontal displacement applied to the knee, with a period of 1 s in variable directions, and the measurement of the consequent boot reaction force. The chosen directions of load were straight lines passing through the neutral position of the boot; the lines changed their angle in respect to the ski when three displacement cycles were completed. The range of variation of flexion and inversion angles were 7° and 5°, respectively. The device showed a high level of repeatability, in comparison with the level of accuracy typical for the biomechanical evaluations, that permitted estimation of the stiffness of the tested boot as a 3rd order polynomial. The results achieved could be useful for the estimation of the reaction forces corresponding to ankle joint angles measurable during the skiing.Copyright