Compensatory processes during unilateral loss of vestibular function]

Changes in the compensation of the sequences of the unilateral loss of the labyrinthine function were studied in rabbits. Destruction of the labyrinth was accompanied by ocular nystagmus, increase of frequency of respiration and heart contractions, and EEG-activation. Investigations carried out showed these reactions to be extinguished at different time. At the late periods of labyrinthectomy a considerable asymmetry of nystagmus reaction to the angular accelerations equal in intensity, but opposite in direction was revealed. Stimulation of an intact otolith labyrinth was accompanied by the appearance of positional nystagmus. The results obtained indicated imperfection of the compensatory mechanisms during complete unilateral loss of the vestibular function.     

Interaction of the angular and linear components of the horizontal vestibulo-ocular reflex in the pigeon

Pigeons were exposed to centric and eccentric horizontal rotations in darkness by velocity trapezoid. Different in sign the duration alterations of the opposite directed horizontal eye nystagmus occurred during otolith membrane shifts in sagittal as well as frontal planes. A direct dependence was found between the duration alterations of the primary nystagmus phase and the peak value alterations of its slow phase velocity under increased (but not decreased) centrifugal force. In the both cases, if duration of the primary nystagmus phase was enlarged, duration of its secondary phase was diminished and vice versa. It suggests the otolith component does not decay up to zero by constant velocity and at once after rotation; by deceleration it is biphasic. In affirms the own hypothesis that the linear component is asymmetric central neuronal activity that modifies the canal component even if this activity by itself is not enough for eye movement initiation.     

Fragmental regulation of vestibuloocular responses

Intact pigeons were exposes to whole body centric and eccentric horizontal rotations in darkness during angular velocity trapezoids. The overall nystagmus alteration patterns were analysed. In 10 pigeons, all nystagmus alterations may be explained on the basis of the dynamics of peripheral otolith activity and central effects that are the same for all combinations of interacting inputs (type 1 patterns), whereas in other pigeons part of the nystagmus alterations were connected with some central effects that were individually specific and touch upon the responses to separate combinations of interacting input (type 2 patterns). It was observed the transformation of type 2 to type 1-patterns during the reiterated rotational trails and light nembutal anesthesia. A fragmentary control of the vestibulo-ocular responses seems to exist. This implies that the CNS is able to discern numerous kinds of bilaterally organized interacting inputs arising during different otolith membrane shifts.     

Habituation of horizontal eye nystagmus in pigeon during alternation of central and eccentric rotations

Intact pigeons were rotated in the horizontal plane in the dark in different positions relatively to the rotation axis. At central rotations, the pigeon's head was in the rotation axis whereas at eccentric rotations it was displaced from the axis. Series of central and eccentric rotations were alternated. Each series consisted of 2-5 rotations using angular velocity trapezoids. All stimuli producing habituation were used at most 14 times each. Eccentric rotations did not prevent a gradual decrease of peak velocities of the slow component of primary nystagmus on transition from one series of central rotations to another in 17 pigeons (group 1). The increase of peak velocities was observed in 2 pigeons (group 2). In group 1, a direct dependence among alterations of this parameter of primary nystagmus, modifications of its duration, and variations of peak velocities of secondary nystagmus, were observed. If two identical stimuli did not follow in sequence directly, the effect of the second one produced same nystagmus changes as were observed in present pigeon by comparison of the first and last responses in the series of the central rotations. If they follow one by one, in many cases the second stimulus in the pair produced an increase of peak velocity of primary and secondary nystagmus and rise of delay of the point of primary nustagmus peak velocity. These variations were not random (probability, > 95%).     

Muscular coordination of knee motion during the terminal-swing phase of normal gait

Children with cerebral palsy often walk with diminished knee extension during the terminal-swing phase, resulting in a troublesome "crouched" posture at initial contact and a shortened stride. Treatment of this gait abnormality is challenging because the factors that extend the knee during normal walking are not well understood, and because the potential of individual muscles to limit terminal-swing knee extension is unknown. This study analyzed a series of three-dimensional, muscle-driven dynamic simulations to quantify the angular accelerations of the knee induced by muscles and other factors during swing. Simulations were generated that reproduced the measured gait dynamics and muscle excitation patterns of six typically developing children walking at self-selected speeds. The knee was accelerated toward extension in the simulations by velocity-related forces (i.e., Coriolis and centrifugal forces) and by a number of muscles, notably the vasti in mid-swing (passive), the hip extensors in terminal swing, and the stance-limb hip abductors, which accelerated the pelvis upward. Knee extension was slowed in terminal swing by the stance-limb hip flexors, which accelerated the pelvis backward. The hamstrings decelerated the forward motion of the swing-limb shank, but did not contribute substantially to angular motions of the knee. Based on these data, we hypothesize that the diminished knee extension in terminal swing exhibited by children with cerebral palsy may, in part, be caused by weak hip extensors or by impaired hip muscles on the stance limb that result in abnormal accelerations of the pelvis.     

Kinetic chain of overarm throwing in terms of joint rotations revealed by induced acceleration analysis

This study investigated how baseball players generate large angular velocity at each joint by coordinating the joint torque and velocity-dependent torque during overarm throwing. Using a four-segment model (i.e., trunk, upper arm, forearm, and hand) that has 13 degrees of freedom, we conducted the induced acceleration analysis to determine the accelerations induced by these torques by multiplying the inverse of the system inertia matrix to the torque vectors. We found that the proximal joint motions (i.e., trunk forward motion, trunk leftward rotation, and shoulder internal rotation) were mainly accelerated by the joint torques at their own joints, whereas the distal joint motions (i.e., elbow extension and wrist flexion) were mainly accelerated by the velocity-dependent torques. We further examined which segment motion is the source of the velocity-dependent torque acting on the elbow and wrist accelerations. The results showed that the angular velocities of the trunk and upper arm produced the velocity-dependent torque for initial elbow extension acceleration. As a result, the elbow joint angular velocity increased, and concurrently, the forearm angular velocity relative to the ground also increased. The forearm angular velocity subsequently accelerated the elbow extension and wrist flexion. It also accelerated the shoulder internal rotation during the short period around the ball-release time. These results indicate that baseball players accelerate the distal elbow and wrist joint rotations by utilizing the velocity-dependent torque that is originally produced by the proximal trunk and shoulder joint torques in the early phase.     

Angular acceleration,compensatory head movements and the halteres of flies (Lucilia serricata)

Summary Tethered flies were subjected to accelerations about their vertical axes while flying or walking. These accelerations were applied either suddenly to stationary animals or continuously by oscillating the animal from side to side. Head and wing movements resulting from the imposed angular accelerations were photographed with a camera and a stroboscopic flash.Analysis of the photographs shows that the wing movements act to counter the imposed angular accelerations and that during sinusoidal oscillations about the vertical axis, head turns are in antiphase with angular acceleration.Head turns do not occur when the halteres are absent or present and not oscillating. When oscillating, the halteres detect high values of angular acceleration, outside the known capabilities of the visual movement detection system.     

Direct and indirect effects of joint torque inputs during an induced speed analysis of a swinging motion

This study proposed a method to quantify direct and indirect effects of the joint torque inputs in the speed-generating mechanism of a swinging motion. Linear and angular accelerations of all segments within a multi-linked system can be expressed as the sum of contributions from a joint torque term, gravitational force term and motion-dependent term (MDT), where the MDT is a nonlinear term consisting of centrifugal force, Coriolis force and gyroscopic effect moment components. Direct effects result from angular accelerations induced by a joint torque at a given instant, whereas indirect effects arise through the MDT induced by joint torques exerted in the past. These two effects were quantified for the kicking-side leg during a rugby place kick. The MDT was the largest contributor to the foot centre of gravity (CG)’s speed at ball contact. Of the factors responsible for generating the MDT, the direct and indirect effects of the hip flexion-extension torque during both the flight phase (from the final kicking foot take-off to support foot contact) and the subsequent support phase (from support foot contact to ball contact) were important contributors to the foot CG’s speed at ball contact. The indirect effect of the ankle plantar-dorsal flexion torque and the direct effect of the knee flexion-extension torque during the support phase showed the largest positive and negative contributions to the foot CG’s speed at ball contact, respectively. The proposed method allows the identification of which individual joint torque axes are crucial and the timings of joint torque exertion that are used to generate a high speed of the distal point of a multi-linked system.     

Decrease of the slow phase velocities of postrotatory nystagmi as a result of the otolith organs stimulation

Intact pigeons were rotated in darkness in a horizontal plane at various orientations relative to axis of rotation, which passed between labyrinths, or was displaced relative to them. Trapezoidal (tests 1) and triangular (tests 2) rotation programs were used. In the tests 1, positive and negative angular accelerations were separated by two-minute periods of rotation with constant angular velocity. Such periods were absent in the tests 2. Results of the canal-otolith interactions in the tests 1 and 2 were different only in postrotatory nystagmi: the peak velocities of the slow phases decreased in both postrotatory nystagmi in the tests 1, but only in one of them in the tests 2. Apparently, at oppositely directed postrotatory nystagmi, decrease of peak velocities in the tests 1 is provided with different mechanisms. At one of them the decrease of nystagmus velocity reflects a result of a summation of canal and otolithic signals on the interneurones of the semicircular canal reflex arches, whereas at another one it is related with long-lasting activity imbalance of these interneurones which is supported by otolithic afferentation during rotation at constant angular velocity.     

Effect of angular velocity on soleus and medial gastrocnemius H-reflex during maximal concentric and eccentric muscle contraction

At rest, the H-reflex is lower during lengthening than shortening actions. During passive lengthening, both soleus (SOL) and medial gastrocnemius (MG) H-reflex amplitudes decrease with increasing angular velocity. This study was designed to investigate whether H-reflex amplitude is affected by angular velocity during concentric and eccentric maximal voluntary contraction (MVC). Experiments were performed on nine healthy men. At a constant angular velocity of 60°/s and 20°/s, maximal H-reflex and M-wave potentials were evoked at rest (i.e., H_max and M_max, respectively) and during concentric and eccentric MVC (i.e., H_sup and M_sup, respectively). Regardless of the muscle, H_max/M_max was lower during lengthening than shortening actions and the H_sup/M_sup ratio was higher than H_max/M_max during lengthening actions. Whereas no action type and angular velocity effects on the MG H_sup/M_sup were found, the SOL H_sup/M_sup was lower during eccentric than concentric MVC and this depression was increased with higher angular velocity. Our findings indicate that the depression of the H-reflex amplitude during eccentric compared to concentric MVC depends mainly on the amount of inhibition induced by lengthening action. In conclusion, H-reflex should be evoked during both passive and active dynamic trials to evaluate the plasticity of the spinal loop.     

Crouched postures reduce the capacity of muscles to extend the hip and knee during the single-limb stance phase of gait

Many children with cerebral palsy walk in a crouch gait that progressively worsens over time, decreasing walking efficiency and leading to joint degeneration. This study examined the effect of crouched postures on the capacity of muscles to extend the hip and knee joints and the joint flexions induced by gravity during the single-limb stance phase of gait. We first characterized representative mild, moderate, and severe crouch gait kinematics based on a large group of subjects with cerebral palsy (N=316). We then used a three-dimensional model of the musculoskeletal system and its associated equations of motion to determine the effect of these crouched gait postures on (1) the capacity of individual muscles to extend the hip and knee joints, which we defined as the angular accelerations of the joints, towards extension, that resulted from applying a 1N muscle force to the model, and (2) the angular acceleration of the joints induced by gravity. Our analysis showed that the capacities of almost all the major hip and knee extensors were markedly reduced in a crouched gait posture, with the exception of the hamstrings muscle group, whose extension capacity was maintained in a crouched posture. Crouch gait also increased the flexion accelerations induced by gravity at the hip and knee throughout single-limb stance. These findings help explain the increased energy requirements and progressive nature of crouch gait in patients with cerebral palsy.     

Viscoelastic deformation response of red blood cells under conditions of oscillating centrifugal field

The red cell deformation under the conditions of oscillating centrifugal fields was studied. Experiments were carried out with a modified Cell-Elastometer operating in oscillating mode (0.02 to 0.30 Hz). Gravitational acceleration was sinusoidally modulated between 620 g and 2250 g. At low frequencies (below 0.08 Hz), native red cells followed the applied stress without delay. At 0.09 Hz and up, the cellular deformation was still periodical and included an additional perturbation due to intracellular movements. This perturbation was analysed and quantified. The influence of alterations on the erythrocyte membrane by diamide was analysed to verify the sensitivity of this method. On increasing the membrane stiffness with low concentrations of diamide, the response to oscillatory centrifugal stress was impaired characteristically in terms of amplitude deformation. Based on tangential and centrifugal accelerations, a physical model was developed that describes the basic observable changes on varying the oscillation frequency. From the data it can be concluded that viscoelastic properties of red cells can be analysed and quantified using oscillatory centrifugal accelerations. The described method can become a valid tool to differentiate between membrane alterations or intracellular viscous modifications.     

Measurement of six degrees of freedom head kinematics in impact conditions employing six accelerometers and three angular rate sensors (6aω configuration)

The ability to measure six degrees of freedom (6 DOF) head kinematics in motor vehicle crash conditions is important for assessing head-neck loads as well as brain injuries. A method for obtaining accurate 6 DOF head kinematics in short duration impact conditions is proposed and validated in this study. The proposed methodology utilizes six accelerometers and three angular rate sensors (6aω configuration) such that an algebraic equation is used to determine angular acceleration with respect to the body-fixed coordinate system, and angular velocity is measured directly rather than numerically integrating the angular acceleration. Head impact tests to validate the method were conducted using the internal nine accelerometer head of the Hybrid III dummy and the proposed 6aω scheme in both low (2.3?m/s) and high (4.0?m/s) speed impact conditions. The 6aω method was compared with a nine accelerometer array sensor package (NAP) as well as a configuration of three accelerometers and three angular rate sensors (3aω), both of which have been commonly used to measure 6 DOF kinematics of the head for assessment of brain and neck injuries. The ability of each of the three methods (6aω, 3aω, and NAP) to accurately measure 6 DOF head kinematics was quantified by calculating the normalized root mean squared deviation (NRMSD), which provides an average percent error over time. Results from the head impact tests indicate that the proposed 6aω scheme is capable of producing angular accelerations and linear accelerations transformed to a remote location that are comparable to that determined from the NAP scheme in both low and high speed impact conditions. The 3aω scheme was found to be unable to provide accurate angular accelerations or linear accelerations transformed to a remote location in the high speed head impact condition due to the required numerical differentiation. Both the 6aω and 3aω schemes were capable of measuring accurate angular displacement while the NAP instrumentation was unable to produce accurate angular displacement due to double numerical integration. The proposed 6aω scheme appears to be capable of measuring accurate 6 DOF kinematics of the head in any severity of impact conditions.     

Isokinetic elbow flexion and coactivation following eccentric training.

The influence of an eccentric training on torque/angular velocity relationships and coactivation level during maximal voluntary isokinetic elbow flexion was examined. Seventeen subjects divided into two groups (Eccentric Group EG, n = 9 Control Group CG, n = 8) performed on an isokinetic dynamometer, before and after training, maximal isokinetic elbow flexions at eight angular velocities (from - 120 degrees s(-1) under eccentric conditions to 240 degrees s(-1) under concentric conditions), and held maximal and submaximal isometric actions. Under all conditions, the myoelectric activities (EMG) of the biceps and the triceps brachii muscles were recorded and quantified as the RMS value. Eccentric training of the EG consisted of 5x6 eccentric muscle actions at 100 and 120% of one maximal repetition (IRM) for 21 sessions and lasted 7 weeks. In the EG after training, torque was significantly increased at all angular velocities tested (ranging from 11.4% at 30 degrees (s-1) to 45.5% at - 120 degrees s(-1)) (p < 0.05). These changes were accompanied by an increase in the RMS activities of the BB muscle under eccentric conditions (from - 120 to - 30 degrees (s-1)) and at the highest concentric angular velocities (180 and 24 degrees s(-1)) (p < 0.05). The RMS activity of the TB muscle was not affected by the angular velocity in either group for all action modes. The influence of eccentric training on the torque gains under eccentric conditions and for the highest velocities was attributed essentially to neural adaptations.     

On the potential of lower limb muscles to accelerate the body's centre of mass during walking

Quantification of lower limb muscle function during gait or other common activities may be achieved using an induced acceleration analysis, which determines the contributions of individual muscles to the accelerations of the body's centre of mass. However, this analysis is reliant on a mathematical optimisation for the distribution of net joint moments among muscles. One approach that overcomes this limitation is the calculation of a muscle's potential to accelerate the centre of mass based on either a unit-force or maximum-activation assumption. Unit-force muscle potential accelerations are determined by calculating the accelerations induced by a 1 N muscle force, whereas maximum-activation muscle potential accelerations are determined by calculating the accelerations induced by a maximally activated muscle. The aim of this study was to describe the acceleration potentials of major lower limb muscles during normal walking obtained from these two techniques, and to evaluate the results relative to absolute (optimisation-based) muscle-induced accelerations. Dynamic simulations of walking were generated for 10 able-bodied children using musculoskeletal models, and potential- and absolute induced accelerations were calculated using a perturbation method. While the potential accelerations often correctly identified the major contributors to centre-of-mass acceleration, they were noticeably different in magnitude and timing from the absolute induced accelerations. Potential induced accelerations predicted by the maximum-activation technique, which accounts for the force-generating properties of muscle, were no more consistent with absolute induced accelerations than unit-force potential accelerations. The techniques described may assist treatment decisions through quantitative analyses of common gait abnormalities and/or clinical interventions.     

Effect of the distance of optokinetic stimuli from the eyes on certain parameters of the optokinetic nystagmus.

The above effect was studied in 65 subjects with normal vision (mean age 20 years) in investigations in which the following factors were successively changed: distance of optokinetic stimuli from the eyes; this distance and angular velocity of stimuli; distance and frequency of stimuli or finally distance and accommodation level. The angular velocity of the pursuit nystagmus phase was found to be by far the highest and simultaneously the closest to the angular velocity of optokinetic stimuli when the latter are 1.5m from the eyes. With shorter distances, the velocity of the pursuit movements lags steadily behind that of stimulus velocity. This change is conditioned by changes in OKN amplitude since its frequency as a whole does not change. Even though the accommodation level significantly affects the velocity of the pursuit nystagmus phase, the dependence on the distance of optokinetic stimuli from the eyes persists even after atropinization. The interpretation of these findings must take into account sepcifically the demands on accommodation, convergence, and on visual attention which are increased with shorter distances.     

Modelling the maximum voluntary joint torque/angular velocity relationship in human movement

The force exerted by a muscle is a function of the activation level and the maximum (tetanic) muscle force. In "maximum" voluntary knee extensions muscle activation is lower for eccentric muscle velocities than for concentric velocities. The aim of this study was to model this "differential activation" in order to calculate the maximum voluntary knee extensor torque as a function of knee angular velocity. Torque data were collected on two subjects during maximal eccentric-concentric knee extensions using an isovelocity dynamometer with crank angular velocities ranging from 50 to 450 degrees s(-1). The theoretical tetanic torque/angular velocity relationship was modelled using a four parameter function comprising two rectangular hyperbolas while the activation/angular velocity relationship was modelled using a three parameter function that rose from submaximal activation for eccentric velocities to full activation for high concentric velocities. The product of these two functions gave a seven parameter function which was fitted to the joint torque/angular velocity data, giving unbiased root mean square differences of 1.9% and 3.3% of the maximum torques achieved. Differential activation accounts for the non-hyperbolic behaviour of the torque/angular velocity data for low concentric velocities. The maximum voluntary knee extensor torque that can be exerted may be modelled accurately as the product of functions defining the maximum torque and the maximum voluntary activation level. Failure to include differential activation considerations when modelling maximal movements will lead to errors in the estimation of joint torque in the eccentric phase and low velocity concentric phase.     

A model of the nystagmus induced by off vertical axis rotation

A three-dimensional model is proposed that accounts for a number of phenomena attributed to the otoliths. It is constructed by extending and modifying a model of vestibular velocity storage. It is proposed that the otolith information about the orientation of the head to gravity changes the time constant of vestibular responses by modulating the gain of the velocity storage feedback loop. It is further proposed that the otolith signals, such as those that generate L-nystagmus (linear acceleration induced nystagmus), are partially coupled to the vestibular system via the velocity storage integrator. The combination of these two hypotheses suggests that a vestibular neural mechanism exists that performs correlation in the mathematical sense which is multiplication followed by integration. The multiplication is performed by the otolith modulation of the velocity storage feedback loop gain and the integration is performed by the velocity storage mechanism itself. Correlation allows calculation of the degree to which two signals are related and in this context provides a simple method of determining head angular velocity from the components of linear acceleration induced by off-vertical axis rotation. Correlation accounts for the otolith supplementation of the VOR and the sustained nystagmus generated by off-vertical axis rotation. The model also predicts the cross-coupling of horizontal and vertical optokinetic afternystagmus that occurs in head-lateral positions and the reported effects of tilt on vestibular responses.     

Static cervical-ocular reflex and optokinetic reflex interaction in rabbits

Summary The effect that tonic eye deviations, induced by angular deviation of the torso, have on the characteristics of optokinetic (OK) nystagmus was studied in rabbits. When the slow component of the OK nystagmus moved in the direction of the tonic eye deviation, the amplitude of the slow and fast components of the nystagmus was decreased and their frequency was increased, whereas when the slow component moved in the opposite direction, the amplitude and the frequency of the nystagmus were not different from those when the head and torso were aligned.Under the influence of neck reflexes, the total range of eye movements was double that when the torso was aligned with the head. The place in the orbit where the fast-component is initiated — the so-called fast-component threshold — was deviated in the direction of the neck-reflex-induced tonic eye deviation. The characteristics of the fast component, however, except for its amplitude, were not affected by the change of location of the fast-component threshold.These data indicate that the OK reflex function, as judged by measurement of the slow component velocity, is not affected by neck-vestibular reflexes. They also show that the fast-component threshold is dependent on parameters other than the actual orbital position and that there must be an internal representation of the range of possible eye movements within the brain to regulate the production of fast components.Abbreviations OK optokinetic - CW clockwise - CCW counterclockwise - CNS central nervous system This work was supported by grants NS07059, NS09823, and NS08335 from the National Institutes of Health     

Transformation of angular accelerations by the semicircular canal system of the vestibular apparatus]

Transformation of angular accelerations by the system of three semicircular channels is considered which takes into account mutual influence of vertical channels through a common crus.