Robert Riener

Stair ascent and descent at different inclinations.

The aim of this study was to investigate the biomechanics and motor co-ordination in humans during stair climbing at different inclinations. Ten normal subjects ascended and descended a five-step staircase at three different inclinations (24 degrees, 30 degrees, 42 degrees ). Three steps were instrumented with force sensors and provided 6 dof ground reactions. Kinematics was analysed by a camera-based optoelectronic system. An inverse dynamics approach was applied to compute joint moments and powers. The different kinematic and kinetic patterns of stair ascent and descent were analysed and compared to level walking patterns. Temporal gait cycle parameters and ground reactions were not significantly affected by staircase inclination. Joint angles and moments showed a relatively low but significant dependency on the inclination. A large influence was observed in joint powers. This can be related to the varying amount of potential energy that has to be produced (during ascent) or absorbed (during descent) by the muscles. The kinematics and kinetics of staircase walking differ considerably from level walking. Interestingly, no definite signs could be found indicating that there is an adaptation or shift in the motor patterns when moving from level to stair walking. This can be clearly seen in the foot placement: compared to level walking, the forefoot strikes the ground first--independent from climbing direction and inclination. This and further findings suggest that there is a certain inclination angle or angular range where subjects do switch between a level walking and a stair walking gait pattern.

Identification of passive elastic joint moments in the lower extremities

Musculotendon actuators produce active and passive moments at the joints they span. Due to the existence of bi-articular muscles, the passive elastic joint moments are influenced by the angular positions of adjacent joints. To obtain quantitative information about this passive elastic coupling between lower limb joints, we examined the passive elastic joint properties of the hip, knee, and ankle joint of ten healthy subjects. Passive elastic joint moments were found to considerably depend on the adjacent joint angles. We present a simple mathematical model that describes these properties on the basis of a double-exponential expression. The model can be implemented in biomechanical models of the lower extremities, which are generally used for the simulation of multi-joint movements such as standing-up, walking, running, or jumping.

Biomechanical model of the human knee evaluated by neuromuscular stimulation

A detailed model of the human knee was developed to predict shank motion induced by functional neuromuscular stimulation (FNS). A discrete-time model is used to characterize the relationship between stimulus parameters and muscle activation. A Hill-based model of the musculotendon actuator accounts for nonlinear static and dynamic properties of both muscle and tendon. Muscle fatigue and passive muscle viscosity are modeled in detail. Moment arms are computed from musculotendon paths of 13 actuators and from joint geometry. The model also takes nonlinear body-segmental dynamics into consideration. The simulated motion is visualized by graphic animation. Individual model parameters were identified by specific procedures such as anthropometric measurements, a passive pendulum test, and specific open-loop stimulation experiments. Model results were compared with experimental data obtained by stimulating the quadriceps muscle of paraplegic patients with surface electrodes. The knee moment, under isometric conditions, and the knee angle, under conditions of freely swinging shank, were measured. In view of the good correspondence obtained between model predictions and experimental data, we conclude that a biomechanical model of human motion induced by FNS can be used as a mathematical tool to support and accelerate the development of neural prostheses.

A physiologically based model of muscle activation verified by electrical stimulation

Abstract A physiological and mathematical model of muscle activation is presented which accounts for major effects that occur (during artificial stimulation of human muscle under isometric conditions. The model is based on the biophysical processes underlying excitation and activation of human muscle. Inputs of the model include the time pattern of the stimulation interpulse interval, pulse width, and pulse amplitude. Total muscle force depends on the force developed in the single motor units (temporal summation) and the number of units recruited (spatial summation). Different motor units with different contraction, recruitment, and fatigue properties are distinguised. The model was identified and evaluated by comparing the isometric force output with experimental data obtained by stimulating different muscles of different subjects. It was shown to be able to predict the output to an arbitrary stimulation input. Thus, it is proposed for the design of effective functional electrical stimulation (FEMS) control strategies. Furthermore, it provides significant insight into muscle processes during FES.

Biomechanical analysis of sit-to-stand transfer in healthy and paraplegic subjects

OBJECTIVEnAn experimental study of the sit-to-stand transfer in healthy adults with/without arm-support and in paraplegic patients with/without electrical stimulation of the quadriceps muscles was performed. The study was aimed to compare the joint torques, momentum transfer hypothesis, and stability of the sit-to-stand transfer in the healthy and paraplegic subjects.nnnMETHODSnA planar 3-linkage rigid body model was used to compute the body-segmental linear momentum and the reaction forces and torques at the joints from measured data.nnnRESULTSnIn healthy subjects the arm-support enlarged the support base of the body and thus, increased the postural stability. Strong arm-assistance reduced the maximum hip and knee joint torques by more than 50%. It was observed that the healthy participants rising with arm-support used momentum transfer to facilitate the transition from sitting to standing. The paraplegic participants did not apply the momentum transfer strategy and the sit-to-stand transfer was accomplished in a quasi-static manner. Stimulating the quadriceps, the legs could participate partly in the movement dynamics.nnnCONCLUSIONnOur results indicate that some significant differences exist between the maneuver applied by the paraplegic patients to stand up and the strategies used by the healthy adults rising with arm-support.nnnRELEVANCEnAnalysis of the biomechanical factors underlying the sit-to-stand activity is essential in the design of competent closed-loop neuroprosthesis controllers which assist paraplegic patients during rising.

Inverse dynamics as a tool for motion analysis: arm tracking movements in cerebellar patients

Kinematic analysis of limb movements can be used to evaluate motion of patients with movement disorders. Those with clinically mild to moderate impairment, however, often show only small, insignificant deviations in the measured trajectories compared to those of healthy controls. Furthermore, kinematic data alone do not give sufficient information about internal quantities such as muscle activation or joint moments. In order to improve the sensitivity of motion analysis of limb movements, we propose the use of inverse dynamics, since it allows biomechanical quantities to be determined without restricting movement. We developed an inverse dynamic model of the upper limb with 9 degrees of freedom. Spatial positions (Cartesian coordinates) of anatomical landmarks, which were recorded by an infrared video-based three-dimensional motion analysis system, are transformed into body-related Cardan angles. The model determines joint moments and powers at the shoulder, elbow, and wrist. Arm tracking movements in a patient with a mild cerebellar ataxia and a healthy control demonstrate that the model allows a clear differentiation between normal and abnormal limb movements, even if no significant differences are noted in the recorded trajectories. We conlude that inverse dynamic modeling can be an effective tool for motion analysis in patients with cerebellar disorders. It also gives further insight into the parameters that may be controlled by the central nervous system.

Control aspects of a robotic haptic interface for kinesthetic knee joint simulation

Abstract A new 6 DOF (degrees of freedom) haptic device is presented for display of the dynamic properties of a virtual human knee in orthopaedic training. In order to achieve the high impedance requirements in this application an industrial robot has been selected and extended with a PC-based high performance controller hardware and redundant safety features. Two control architectures—a force-command and motion-command control—are presented and both implemented in a 1 DOF application. With a new method for evaluation and comparison of controller performance in terms of impedance error the experimental results reveal that motion-command provides better accuracy for display of the high impedances specific to the human knee application.

ON THE COMPLEXITY OF BIOMECHANICAL MODELS USED FOR NEUROPROSTHESES DEVELOPMENT

The use of mathematical models has the potential to enhance the development of lower extremity neuroprostheses (NP) based on Functional Electrical Simulation (FES). The choice of model complexity is not trivial when building a model for FES control design. On the one hand, a comprehensive model might be useful to account for the many different biomechanical and neurophysiological effects that can be observed during FES-induced movements. On the other hand, too complex models are difficult to be utilized in and identified for NP applications. In this paper we discuss the disadvantages of too complex models, and propose potential simplifications on the basis of existing models that are commonly used to describe muscle activation, muscle contraction and body-segmental motion. The obtained model approach is simple enough to be identified, and sufficiently comprehensive to describe most of the relevant effects that occur during FES-induced locomotion.

A NOVEL SLIDING MODE CONTROLLER FOR FUNCTIONAL ELECTRICAL STIMULATION

Abstract This article describes a model-based development of a new nonlinear controller for control of Functional Electrical Stimulation, which can be used to restore movement in paralyzed individuals. The control design is based on the theory of sliding mode control. The controller is mathematically derived and shown to provide asymptotic stability of knee joint angle tracking by electrical stimulation of knee extensor muscle group only, or by electrical stimulation of knee extensor and flexor muscle groups. Its behaviour was evaluated in simulations with artificial and physiological knee joint angle reference trajectories. The controller was able to track trajectories with a period of 2 s with a root-mean-square error of approximately 2 degrees, which is considered a good performance. It was also shown to be robust to parameter variations of the model. This is important as models for different persons will differ considerably.

Knee Joint Simulator Based on Haptic, Visual and Acoustic Feedback

Abstract A knee joint simulator that comprises the properties of a healthy or pathological knee can support medical education and training. In this paper a mechatronic system is presented that allows a user to touch and move a virtual shank and simultaneously observe the generated movement, feel the contact force, and hear sounds produced by the joint movement or the patient due to pain. These features enable the user to assess the properties of the knee by testing the joint laxity and end-point stiffness in six degrees-of-motion (DOF). It was possible to approximate the elastic knee joint properties by simple non-linear algebraic functions based on experimental data. Thus, the haptic feedback could be performed at a high rate (1000 Hz), which yielded considerably realistic knee behaviour. The performance was further improved by implementing a realistic leg shape, different contact stiffness for shin and calf, and contact friction. For additional tests the knee tendon reflex can be activated and the pulse can be palpated in the hollow of the knee.