Denis Rancourt

Increased risk for falling associated with obesity: mathematical modeling of postural control

Recent epidemiological studies report that obesity is positively related to fracture incidence. In the present experiment, a model of postural control was used to examine the impact of an abnormal distribution of body fat in the abdominal area upon postural stability. Obese and lightweight humanoids were destabilized by imposing a small initial angular speed from a neutral standing position. To avoid a loss of stability yielding a stepping reaction or a fall, an ankle torque is necessary to counteract the perturbation. Three torque parameters-ankle torque onset, time to peak torque, and muscular ankle torque-were entered in a program to simulate the intrinsic variability of the human postural control system. A loss of stability was detected when the center of pressure exceeded stability margins. The most striking observation is the nonlinear increase of torque needed to stabilize the humanoid when the motor response was chararterized by delayed temporal parameters. The effect was more pronounced when an anterior position of the center of mass was included in the simulations. This suggests that, when submitted to daily postural stresses and perturbations, obese persons (particularly those with an abnormal distribution of body fat in the abdominal area) may be at higher risk of falling than lightweight individuals.

Modelling liver tissue properties using a non-linear visco-elastic model for surgery simulation

In this work, we introduce an extension of the linear elastic tensor-mass method allowing fast computation of non-linear and visco-elastic mechanical forces and deformations for the simulation of biological soft tissue. We aim at developing a simulation tool for the planning of cryogenic surgical treatment of liver cancer. Percutaneous surgery simulation requires accurate modelling of the mechanical behaviour of soft tissue, and previous experimental characterizations have shown that linear elasticity is only a coarse approximation of the real properties of biological tissues. We first show that our model can simulate different types of non-linear and visco-elastic mechanical behaviours at speeds which are compatible with real-time applications. Then an experimental setup is presented which was used to characterize the mechanical properties of deer liver tissue under perforation by a biopsy needle. Experimental results demonstrate that a linear model is not suitable for simulating this application, while the proposed model succeeds in accurately modelling the axial load measured on the needle.

Stability in force-production tasks.

Abstract Exerting a force on a mechanical system can induce mechanical instability. To overcome that instability, humans may take advantage of their upper limb mechanical impedance (e.g., hand stiffness). The authors investigated what stiffness is required to maintain static stability and how humans can achieve that stiffness in the context of the task of pushing on a pivoting stick. Results showed that the stiffness required is in the range of measured human upper limb stiffness. To avoid an ill-posed problem, one can better express the requirements for stability as a simple geometrical criterion related to the curvature of the potential energy field at the hand. A planar model of the upper limb revealed that individuals can use both hand rotational and translational stiffness to stabilize a stick. Although hand rotational stiffness does not participate in producing the axial force on the stick, it can significantly contribute to achieving a limb stiffness appropriate for maintaining static stability. Hand rotational stiffness can be important for the design of hand tools, because humans can increase it only by augmenting grip force, a biomechanical factor associated with cumulative trauma injuries of the upper extremities.

Using orientation statistics to investigate variations in human kinematics

This paper applies orientation statistics to investigate variations in upper limb posture of human subjects drilling at six different locations on a vertical panel. Some of the drilling locations are kinematically equivalent in that the same posture could be used for these locations. Upper limb posture is measured by recording the co‐ordinates of four markers attached to the subjects hand, forearm, arm and torso. A 3×3 rotation characterizes the relative orientation of one body segment with respect to another. Replicates are available since each subject drilled at the same location five times. Upper limb postures for the six drilling locations are compared by one‐way analysis‐of‐variance tests for rotations. These tests rely on tangent space approximations at the estimated modal rotation of the sample. A parameterization of rotations in terms of unit quaternions simplifies the computations. The analysis detects significant differences in posture between all pairs of drilling locations. The smallest changes, less than 10° at all joints, are obtained for the kinematically equivalent pairs of locations. A short discussion of the biomechanical interpretation of these findings is presented.

Influence of smoothing window length on electromyogram amplitude estimates

A systematic, experimental study of the influence of smoothing window length on the signal-to-noise ratio (SNR) of electromyogram (EMG) amplitude estimates is described. Surface EMG waveforms were sampled during nonfatiguing, constant-force, constant-angle contractions of the biceps or triceps muscles, over the range of 10%-75% maximum voluntary contraction. EMG amplitude estimates were computed with eight different EMG processor schemes using smoothing length durations spanning 2.45-500 ms. An SNR was computed from each amplitude estimate (deviations about the mean value of the estimate were considered as noise). Over these window lengths, average /spl plusmn/ standard deviation SNRs ranged from 1.4/spl plusmn/0.28 to 16.2/spl plusmn/5.4 for unwhitened single-channel EMG processing and from 3.2/spl plusmn/0.7 to 37.3/spl plusmn/14.2 for whitened, multiple-channel EMG processing (results pooled across contraction level). It was found that SNR increased with window length in a square root fashion. The shape of this relationship was consistent with classic theoretical predictions, however none of the processors achieved the absolute performance level predicted by the theory. These results are useful in selecting the length of the smoothing window in traditional surface EMG studies. In addition, this study should contribute to the development of EMG processors which dynamically tune the smoothing window length when the EMG amplitude is time varying.

Cyclic traction machine for long-term culture of fibroblast-populated collagen gels.

AbstractOur research group has been investigating the effect of cyclic deformations on the evolution of fibroblast populated collagen gels (FPCG). Since existing traction machines are not designed for such an application, we had to design a cyclic traction machine adapted to tissue culture inside an incubator over an extended period of time. Biocompatible materials were used for fabrication to allow for easy sterilization and to prevent any adverse reaction from the tissue. The traction machine is based on a computer-controlled stepping motor system for easy adjustment of the deformation amplitude and frequency. The maximum stretching speed achieved is around 1mm/s. The traction machine can measure FPCG mechanical properties and perform rupture tests to determine its ultimate strength. Several FPCGs have been successfully cultured with the machine for up to four weeks without any adverse reaction.

Dynamics of pushing

Abstract A standing individual can use several strategies for modulating pushing force magnitude. Using a static model, researchers have shown that the efficacy of those strategies varies considerably. In the present article, the authors propose a human motor control dynamic model for analyzing transients that occur when an individual is asked to modulate force magnitude. According to the model, the impedances of both the upper and the lower limbs influence the time course of force variations and foot placement has a profound effect on pushing force dynamics. With a feet-together posture, the center of pressure has a limited range of motion and changes in force may be preceded by initial changes in the opposite direction; that is, to decrease force, an individual must first increase force. When the feet are placed apart, individuals can move the center of pressure over a much larger range, thereby modulating pushing force magnitude, without reversing behavior, over a larger range of force magnitudes. Therefore, the best way to control pushing force at the hand may be by using the foot.

Electromyogram (EMG) amplitude estimation and joint torque model performance

The relationship between surface EMG and torque about a joint has been the focus of many studies. Some of these studies however, have utilized conventional processors to obtain the EMG amplitudes. Recent advances (multiple channel combination and whitening) have demonstrated significant improvement in EMG amplitude estimation accuracy. The purpose of this study was to investigate the influence of these EMG amplitude advances into EMG-torque estimation. EMG from biceps/triceps muscles and torque about the elbow joint were collected from fifteen subjects producing constant-posture, nonfatiguing, force-varying contractions. EMG amplitudes were obtained using processors with and without the advances, and then they were related to torque using a linear FIR model. Results demonstrated that both whitening and multiple-channel combination reduced EMG-torque errors and their combination provided an additive benefit Specifically, the EMG-torque prediction errors were reduced to an average of 8% of maximum voluntary contraction of flexion (MVC/sub F/) when incorporating a four-channel, whitened processor.

Intrinsic properties of the optical coupling between axisymmetric Gaussian beams

On the basis of the overlap integral method, an approximate analytical model is derived to estimate the coupled optical power between axisymmetric Gaussian beams when transverse, axial, and angular misalignments simultaneously exist in three dimensions. Seven optical properties are derived from a detailed analysis of the model. Because the model is an approximate analytical solution to the overlap integral method, the existence of each property is also investigated by a numerical solution. Results show that all seven properties are intrinsic to the optical coupling phenomenon between Gaussian beams. Because numerous single-mode device-to-fiber coupling systems can be well described by use of Gaussian beams, the seven properties provide a solid basis to develop model-based algorithms for single-mode device-to-fiber alignment automation.

The Biomechanics of Force Production

To interact mechanically with the world and especially to use hand tools we exert force. However, the biomechanical consequences of force production can be challenging. In particular, due to the nonlinear kinematics of the mammalian skeleton and of typical hand tools, exerting force can destabilize posture, compromising the ability to control force. In this chapter we present a simplified analysis of this phenomenon that shows how the destabilizing effect of force production varies with pose and tool geometry, and how it may be offset by neuro-muscular stiffness. We also show that in some circumstances the limits of force production may, in fact, be due to a limited ability to produce stiffness rather than a limited ability to produce force. An experimental confirmation of these predictions is presented.