[Torque forces applied through rectangular wires].
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Humans↗
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Dantrium--torque induced motion ("TIM"). Analysis in normal subjects.
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Adult↗
Wavelet analysis of electromyography for back muscle fatigue detection during dynamic constant-torque exertions.
The fatigue of the back muscles appears to be strongly implicated as a risk factor for acquisition of low back pain, which is one of the leading ills of our industrial society. Previously, researchers have successfully measured the level of muscular fatigue by using the Fourier transform to analyze the frequency content of the electromyogram (EMG). However, due to the requirement that the EMG signal be stationary, the Fourier transform is suitable only for the analysis of static muscle exertions in which the muscle is held at constant length and tension. Because the majority of industrial work tasks are not static in nature, new methods for quantifying fatigue during dynamic work are needed. The wavelet transform is a novel, although mathematically well developed, technique for analyzing non-stationary signals that has only recently been applied to the study of EMG. Consequently, the main objective of this project is to develop techniques, using the wavelet transform, for the quantification of back muscle fatigue during dynamic repetitive working conditions.
Back Pain↗
A postural model of balance-correcting movement strategies.
The patterns of joint torques and movement strategies underlying human balance corrections were examined using a postural model. Two types of support-surface perturbation, dorsiflexion rotation (ROT) and rearward translation (TRANS), were employed. These two perturbations were adjusted to produce similar profiles of ankle dorsiflexion in order to obtain information on the role of lower leg proprioceptive inputs on triggering balance corrections. In addition, the dependence of balance control on head angular and linear accelerations was investigated by comparing the responses of normal and vestibularly deficient subjects under eyes-closed and eyes-open conditions. Differences in ROT and TRANS movement strategies were examined in three ways First, the amplitude and polarity of active joint torques were analysed. These were obtained by altering joint torques applied to a postural model until movements of the model accurately duplicated those of measured responses. Second, the pattern of body-segment angular movements depicted by stick figures moving in response to the computed joint torques was investigated. Third, the peak amplitude and patterns of crosscorrelations between joint torques were measured. Active ankle, knee, and hip joint torques computed for normal subjects rotated the body forward for ROT. In the case of TRANS, computed active torques in normals were of opposite polarity to those of ROT and reversed the forward motion of the body. Subjects with vestibular deficits had lower amplitude torques for ROT and failed to counter the platform rotation. Hip torques for TRANS in vestibular deficient subjects were of opposite polarity to those of normal subjects and resulted in excessive forward trunk rotation. Normally, neck torques acted to stabilize the head in space when trunk angular velocity peaked. Vestibular deficient subjects displayed head movements in response to ROT similar to those generated when neck torques were absent. For TRANS, these same subjects exhibited overcompensatory neck torques. Stick figures of normal responses indicated a stiffening of the body into a leg and a trunk-head link for ROT and a flexible multilink motion for TRANS. Likewise, normal response strategies, defined by using crosscorrelations of joint torques, differed for ROT and TRANS. All joint torque crosscorrelations were significant for TRANS. Neck torques led those of all other joint torques by 40 ms or more, and hip joint led ankle torques by 30 ms. Joint torque correlations for ROT were organised around hip and ankle torques without a major correlation to neck torques. Fundamental changes in all torque crosscorrelations occurred for vestibularly deficient subjects under both eyes-open and eyes-closed conditions.(ABSTRACT TRUNCATED AT 400 WORDS)
Biomechanical Phenomena↗
Effects of inactivation of the anterior interpositus nucleus on the kinematic and dynamic control of multijoint movement.
We previously showed that inactivating the anterior interpositus nucleus in cats disrupts prehension; paw paths, normally straight and accurate, become curved, hypometric, and more variable. In the present study, we determined the joint kinematic and dynamic origins of this impairment. Animals were restrained in a hammock and trained to reach and grasp a cube of meat from a narrow food well at varied heights; movements were monitored using the MacReflex analysis system. The anterior interpositus nucleus was inactivated by microinjection of the GABA agonist muscimol (0.25-0.5 microgram in 0.5 microliter saline). For each joint, we computed the torque due to gravity, inertial resistance (termed self torque), interjoint interactions (termed interaction torque), and the combined effects of active muscle contraction and passive soft tissue stretch (termed generalized muscle torque). Inactivation produced significant reductions in the amplitude, velocity, and acceleration of elbow flexion. However, these movements continued to scale normally with target height. Shoulder extension was reduced by inactivation but wrist angular displacement and velocity were not. Inactivation also produced changes in the temporal coordination between elbow, shoulder, and wrist kinematics. Dynamic analysis showed that elbow flexion both before and during inactivation was produced by the combined action of muscle and interaction torque, but that the timing depended on muscle torque. Elbow interaction and muscle torques were scaled to target height both before and during inactivation. Inactivation produced significant reductions in elbow flexor interaction and muscle torques. The duration of elbow flexor muscle torque was prolonged to compensate for the reduction in flexor interaction torque. Shoulder extension was produced by extensor interaction and muscle torques both before and during inactivation. Inactivation produced a reduction in shoulder extension, primarily by reduced interaction torque, but without compensation. Wrist plantarflexion, which occurred during elbow flexion, was driven by plantarflexor interaction and gravitational torques both before and during inactivation. Muscle torque acted in the opposite direction with a phase lead to restrain the plantarflexor interaction torque. During inactivation, there was a reduction in plantarflexor interaction torque and a loss of the phase lead of the muscle torque. Our findings implicate the C1/C3 anterior interpositus zone of the cerebellum in the anticipatory control of intersegmental dynamics during reaching, which zone is required for coordinating the motions of the shoulder and wrist with those of the elbow. In contrast, this cerebellar zone does not play a role in scaling the movement to match a target.
Animals↗
Compensation for intrinsic muscle stiffness by short-latency reflexes in human triceps surae muscles.
The incremental torque resisting rotation of the foot about the ankle joint was studied in normal seated subjects. Prior to each rotation, subjects were required to activate triceps surae (TS) muscles and maintain a constant plantar flexion torque (range 6-14 N X m) on a platform whose position was controlled by a torque motor. Subjects were instructed to increase torque as rapidly as possible once rotation commenced. Rotations ranged from 0.5 to 14 degrees amplitude and from 20 to 300 degrees/s maximum velocity. The torque in response to rotations stretching TS muscles and releasing tibialis anterior (TA) muscles increased steeply and then rapidly decreased with stretch velocity. At approximately 60 ms from stretch onset, the torque reduction terminated, torque then increased again until it began to level off at approximately 120 ms. A further large increase in torque occurred at 180 ms. A burst of short-latency (SL) electromyographic (EMG) activity in soleus (SOL) commenced at 40 ms, and was followed by a second burst at approximately 68 ms, provided that stretch deceleration started later than 20 ms after stretch onset. A period of sustained EMG activity in SOL commenced at approximately 130 ms (long-latency (LL) activity). Incremental torque in response to stretch of TA and release of TS muscles initially showed a step decrease followed by a reversal of the torque trajectory back toward base line. This change was arrested at 60 ms and torque then remained approximately constant until a large increase in torque at 180 ms. Ischemia was used to reduce SL EMG reflexes without significantly modifying the background EMG activity. A comparison between torque curves under control and ischemic conditions indicated that SL EMG activity in TS muscles recruited the force responsible for terminating the torque reduction coincident with decreasing stretch velocity. The torque response prior to the onset of force recruited by SL activity was attributed to the intrinsic properties of active muscle fibers. Thereafter, until the onset of LL activity, the torque response was attributed to intrinsic and reflex-recruited force. Torque in these two time periods was compared under a variety of stretch conditions in order to test the hypothesis that force recruited by segmental reflexes compensates for the non-linear stretch properties of active TS muscles. The relationships of SL EMG amplitudes and areas to stretch velocity and acceleration were also examined.(ABSTRACT TRUNCATED AT 400 WORDS)
Ankle Joint↗
Understanding movement control in infants through the analysis of limb intersegmental dynamics.
One important component in the understanding of the control of limb movements is the way in which the central nervous system accounts for joint forces and torques that may be generated not only by muscle actions but by gravity and by passive reactions related to the movements of limb segments. In this study, we asked how the neuromotor system of young infants controls a range of active and passive forces to produce a stereotypic, nonintentional movement. We specifically analyzed limb intersegmental dynamics in spontaneous, cyclic leg movements (kicking) of varying intensity in supine 3-month-old human infants. Using inverse dynamics, we calculated the contributions of active (muscular) and passive (motion-dependent and gravitational) torque components at the hip, knee, and ankle joints from three-dimensional limb kinematics. To calculate joint torques, accurate estimates were needed of the limb's anthropometric parameters, which we determined using a model of the human body. Our analysis of limb intersegmental dynamics explicitly quantified the complex interplay of active and passive forces producing the simple, involuntary kicking movements commonly seen in 3-month-old infants. our results revealed that in nonvigorous kicks, hip joint reversal was the result of an extensor torque due to gravity, opposed by the combined flexor effect of the muscle torque and the total motion-dependent torque. The total motion-dependent torque increased as a hip flexor torque in more vigorous kicks; an extensor muscle torque was necessary to counteract the flexor influences of the total motion-dependent torque and, in the case of large ranges of motion, a flexor gravity torque as well. Thus, with changing passive torque influences due to motions of the linked segments, the muscle torques were adjusted to produce a net torque to reverse the kicking motion. As a consequence, despite considerable heterogeneity in the intensity, range of motion, coordination, and movement context of each kick, smooth trajectories resulted from the muscle torque, counteracting and complementing not only gravity but also the motion-dependent torques generated by movement of the linked segments.
Journal Article↗
Nature, reliability, and predictive value of muscle performance measures in patients with hemiparesis following stroke.
The purposes of this investigation were to examine among a sample of hemiparetic stroke patients the nature and reliability of selected muscle performance measures and the correlation between the measures and gait speed. Fourteen subjects who met specific entry criteria participated. The Lido Active Rehabilitation System was used to obtain bilaterally all muscle performance measures of knee extension (velocity, isometric torque-peak and time specific, and time to peak torque and 90% peak torque). Gait speed was measured under both "comfortable" and "maximum safe" conditions. In addition to lower torques, the subjects demonstrated slower knee extension speeds and longer times to peak torque and 90% peak torque on the paretic than on the nonparetic side. High intrasession reliabilities were demonstrated for all muscle performance measures except the torque at 0.1 sec on the nonparetic side and the time to peak torque and 90% peak torque measures. Significant correlations were demonstrated between both gait speeds and paretic knee extension torque (except at 0.1 sec), nonparetic knee extension torque at 0.1 sec and 0.2 sec, and time to nonparetic peak torque. Muscle performance measures other than peak torque appeared to offer little advantage (alone) over paretic peak torque for predicting gait speed. Over 74% of the variance in gait speed, however, could be predicted by the peak isometric peak torque of the paretic side and the time to peak torque on the nonparetic side, together. Thus, there may be benefits to testing the muscle performance of both lower extremities when examining patients with stroke.
Aged↗
Multijoint arm movements in cerebellar ataxia: abnormal control of movement dynamics.
In cerebellar ataxia, kinematic aberrations of multijoint movements are thought to originate from deficiencies in generating muscular torques that are adequate to control the mechanical consequences of dynamic interaction forces. At this point the exact mechanisms that lead to an abnormal control of interaction torques are not known. In principle, the generation of inadequate muscular torques may result from an impairment in generating sufficient levels of torques or from an inaccurate assessment and prediction of the mechanical consequences of movements of one limb segment on adjacent joints. We sought to differentiate the relative contribution of these two mechanisms and, therefore, analyzed intersegmental dynamics of multijoint pointing movements in healthy subjects and in patients with cerebellar degeneration. Unrestrained vertical arm movements were performed at three different target movement velocities and recorded using an optoelectronic tracking system. An inverse dynamics approach was employed to compute net joint torques, muscular torques, dynamic interaction torques and gravitational torques acting at the elbow and shoulder joint. In both groups, peak dynamic interaction forces and peak muscular forces were largest during fast movements. In contrast to normal subjects, patients produced hypermetric movements when executing fast movements. Hypermetric movements were associated with smaller peak muscular torques and smaller rates of torque change at elbow and shoulder joints. The patients' deficit in generating appropriate levels of muscular force were prominent during two different phases of the pointing movement. Peak muscular forces at the elbow were reduced during the initial phase of the movement when simultaneous shoulder joint flexion generated an extensor influence upon the elbow joint. When attempting to terminate the movement, gravitational and dynamic interaction forces caused overshooting extension at the elbow joint. In normal subjects, muscular torque patterns at shoulder and elbow joint were synchronized in that peak flexor and extensor muscular torques occurred simultaneously at both joints. This temporal pattern of muscular torque generation at shoulder and elbow joint was preserved in patients. Our data suggest that an impairment in generating sufficient levels of phasic muscular torques significantly contributes to the patients' difficulties in controlling the mechanical consequences of dynamic interaction forces during multijoint movements.
Adult↗
Characteristics of synergic relations during isometric contractions of human elbow muscles.
We studied the patterns of EMG activity in elbow muscles in three normal human subjects. The myoelectrical activity of 7-10 muscles that act across the human elbow joint was simultaneously recorded with intramuscular electrodes during isometric joint torques exerted over a range of directions. These directions included flexion, extension, varus (internal humeral rotation), valgus (external humeral rotation), and several intermediate directions. The forces developed at the wrist covered a range of 360 degrees, all orthogonal to the long axis of the forearm. The levels of EMG activity were observed to increase with increasing joint torque in an approximately linear manner. All muscles were active for ranges less than 360 degrees and most were active for less than 180 degrees. The EMG activity was observed to vary in a systematic manner with changes in torque direction and, when examined over the full angular range at a variety of torque levels, is simply scaled with increasing torque magnitude. There were no torque directions or torque magnitudes for which a single muscle was observed to be active alone. In all cases, joint torque appeared to be produced by a combination of muscles. The direction for which the EMG of a muscle reached a maximum value was observed to correspond to the direction of greatest mechanical advantage as predicted by a simple mechanical model of the elbow and relevant muscles. Muscles were relatively inactive during varus torques. This implies that the muscles were not acting to stabilize the joint in this direction and could have been allowing ligaments to carry the load. Plots of EMG activity in one muscle against EMG activity in another demonstrate some instances of pure synergies, but patterns of coactivation for most muscles are more complicated and vary with torque direction. The complexity of these patterns raises the possibility that synergies are determined by the task and may have no independent existence. Activity in two heads of triceps brachii (medial head--a single-joint muscle and long head--a two-joint muscle) covaried closely for a range of torque magnitudes and directions, though shoulder torque and hence the forces experienced by the long head of the triceps undoubtedly varied. The similarity of activation patterns indicates that elbow torque was the principal determining factor. The origins of muscle synergies are discussed. It is suggested that they are best understood on the basis of a model which encodes limb torque in premotor neurons.(ABSTRACT TRUNCATED AT 400 WORDS)
Elbow Joint↗
Organizing principles for single-joint movements. IV. Implications for isometric contractions.
1. Normal human subjects made isometric pulse and step contractions about the elbow to visually defined target torques of different amplitudes and at different rates. We measured joint torque and electromyograms (EMG) from two agonist and two antagonist muscles. 2. When the task specification requires that the subject explicitly alter the rate at which torque is increased, the rates of rise of the agonist and antagonist EMG bursts covary with the rate of rise of the torque. For pulses of torque the duration of motoneuron excitation varies with the duration of the task-defined contractile event. 3. When a subject is asked to generate torques of different amplitudes without specifying a time interval, torque amplitude is positively correlated with how long, and therefore how high, the EMG rose. Subjects usually proportionately covary the strength of the agonist and antagonist contractions but are not constrained to do so. Some subjects use a strategy of varying the antagonist inversely with the agonist contraction. 4. We extend the organizing principles for the control of movement about a single joint to the control of isometric torque. These rules state that control of torque about a single joint is exercised by one of two strategies: the speed-sensitive strategy modulates the rate at which contraction rises by varying the intensity of motoneuron-pool excitation. The speed-insensitive strategy varies the duration over which contraction rises but does not change the rate. These two respective patterns of torque emerge from pulse-height and pulse-width modulation of motoneuron-pool excitation. 5. The rules defining speed-sensitive and speed-insensitive strategies for movements are broadened for isometric contractions because of the wider range of torque patterns that we observe under these conditions. We propose a step-excitation component for prolonged isometric step contractions and slowly rising ramp patterns of excitation for contractions that develop over several hundreds of milliseconds. 6. The choice of strategies is based on task-specific torque requirements. The same two strategies that control torque to produce movement apply to the control of isometric torque. Unlike movements, however, isometric tasks are more often controlled by a blending of the two patterns. Possible reasons for this are discussed.
Electromyography↗
Adaptive control for backward quadrupedal walking. IV. Hindlimb kinetics during stance and swing.
1. Hindlimb step-cycle kinetics of forward (FWD) and backward (BWD) walking in adult cats were assessed. The hindlimb was modeled as a linked system of rigid bodies and inverse-dynamics techniques were used to calculate hip, knee, and ankle joint kinetics. For swing, net torque at each joint was divided into three components: gravitational, motion dependent, and a generalized muscle torque. For stance, vertical and horizontal components of the ground-reaction force applied at a point on the paw (center of pressure) were added to the torque calculations. Muscle torque profiles were matched to electromyograms (EMGs) recorded from hindlimb muscles. 2. Torque profiles for BWD swing were the approximate time reversal of those for FWD swing. At each joint, the net torque during swing was small because the mean motion-dependent and muscle torque components counteracted each other. At the hip a flexor muscle torque persisted except for a brief extensor muscle torque late in FWD swing and at the onset of BWD swing. At the knee the muscle torque was relatively negligible except for a peak flexor muscle torque late in FWD swing and early in BWD swing. At the ankle there was a midswing transition from a flexor to an extensor muscle torque during FWD swing and the reverse was true for BWD swing. 3. The vertical ground-reaction force was greater for the forelimbs than the hindlimbs during FWD stance; the reverse was true for BWD stance. Thus the hindlimbs bore a greater percentage (66%) of body weight than the forelimbs during BWD stance, and the forelimbs bore a greater percentage (59%) during FWD stance. For most of FWD stance, the hindlimb exerted a small propulsive ground-reaction force, but for BWD stance the hindlimb first exerted a braking force and then a propulsive force, with the transition occurring after midstance (59% of stance). 4. At the hip the ground-reaction force vector was oriented anteriorly and then posteriorly to the estimated joint center with a midstance transition during FWD stance. The muscle torque and joint power patterns showed similar transitions, changing from extensor and power generation to flexor and power absorption, respectively. For most of BWD stance the ground-reaction force vector was oriented anteriorly to the joint center and was counter-balanced by a large extensor muscle torque; nonetheless, power was absorbed because the hip flexed.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Modeling three-dimensional velocity-to-position transformation in oculomotor control.
1. A considerable amount of attention has been devoted to understanding the velocity-position transformation that takes place in the control of eye movements in three dimensions. Much of the work has focused on the idea that rotations in three dimensions do not commute and that a "multiplicative quaternion model" of velocity-position integration is necessary to explain eye movements in three dimensions. Our study has indicated that this approach is not consistent with the physiology of the types of signals necessary to rotate the eyes. 2. We developed a three-dimensional dynamical system model for movement of the eye within its surrounding orbital tissue. The main point of the model is that the eye muscles generate torque to rotate the eye. When the eye reaches an orientation such that the restoring torque of the orbital tissue counterbalances the torque applied by the muscles, a unique equilibrium point is reached. The trajectory of the eye to reach equilibrium may follow any path, depending on the starting eye orientation and eye velocity. However, according to Euler's theorem, the equilibrium reached is equivalent to a rotation about a fixed axis through some angle from a primary orientation. This represents the shortest path that the eye could take from the primary orientation in reaching equilibrium. Consequently, it is also the shortest path for returning the eye to the primary orientation. Thus the restoring torque developed by the tissue surrounding the eye was approximated as proportional to the product of this angle and a unit vector along this axis. The relationship between orientation and restoring torque gives a unique torque-orientation relationship. 3. Once the appropriate torque-orientation relationship for eye rotation is established the velocity-position integrator can be modeled as a dynamical system that is a direct extension of the one-dimensional velocity-position integrator. The linear combination of the integrator state and a direct pathway signal is converted to a torque signal that activates the muscles to rotate the eyes. Therefore the output of the integrator is related to a torque signal that positions the eyes. It is not an eye orientation signal. The applied torque signal drives the eye to an equilibrium orientation such that the restoring torque equals the applied torque but in the opposite direction. The eye orientation reached at equilibrium is determined by the unique torque-orientation relation. Because torque signals are vectors, they commute.(ABSTRACT TRUNCATED AT 400 WORDS)
Abducens Nerve↗
Eddy-current induction in extended metallic parts as a source of considerable torsional moment.
PURPOSE: To examine eddy-current-provoked torque on conductive parts due to current induction from movement through the fringe field of the MR scanner and from gradient switching. MATERIALS AND METHODS: For both cases, torque was calculated for frames of copper, aluminum, and titanium, inclined to 45 degrees to B0 (maximum torque case). Conditions were analyzed in which torque from gravity (legal limit, ASTM F2213-02) was exceeded. Experiments were carried out on a 1.5 T and a 3 T scanner for copper and titanium frames and plates (approximately 50 x 50 mm2). Movement-induced torque was measured at patient table velocity (20 cm/second). Alternating torque from gradient switching was investigated by holding the specimens in different locations in the scanner while executing sequences that exploited the gradient capabilities (40 mT/m). RESULTS: The calculations predicted that movement-induced torque could exceed torque from gravity (depending on the part size, electric resistance, and velocity). Two experiments on moving conductive frames in the fringe fields of the scanners confirmed the calculations. For maximum torque case parameters, gradient-switching-induced torque was calculated to be nearly 100 times greater than the movement-induced torque. Well-conducting metal parts located off center vibrated significantly due to impulse-like fast alternating torque characteristics. CONCLUSION: Torque on metal parts from movement in the fringe field is weak under standard conditions, but for larger parts the acceptable limit can be reached with a high static field and increased velocity. Vibrations due to gradient switching were confirmed and may explain the sensations occasionally reported by patients with implants.
Aluminum↗
In vivo rotatory knee stability. Ligamentous and muscular contributions.
Active and passive components of torsional stability of the knee were measured with an instrumented clinical knee-testing apparatus. Torque-versus-rotation response curves were recorded in the non-weight-bearing condition with muscles relaxed for twenty normal subjects who were tested at 20 and 90 degrees of knee flexion with the hips flexed and extended. At applied torque levels as high as +/-10 newton-meters, tibial rotation averaged approximately one-half the foot rotation. The mean algebraic right-left rotation difference for the group was nearly zero; however, sizable standard deviations for this difference indicated considerable right-left variations between individuals in the test group. Maximum isometrically generated tibial torques were measured by asking the subjects to twist with an explosive effort against a locked torque-cell. No significant differences in generated torque were measured between preferred and non-preferred lower limbs, with only one minor exception. Subjects generally were able to generate greater internal torque than external torque. When the foot was locked in a position of internal or external rotation, an individual was able to generate increased tibial torque in the direction that would tend to return the foot to the neutral position. Flexion of the knee from 20 to 90 degrees increased externally generated torque, while internal torque was affected to a lesser degree. Flexion of the hip had little effect on generated torque. Six cadaver knees without menisci that were tested to failure in external rotation showed torque levels for ligament failure to be similar in magnitude to the maximum generated isometric torque that acts to protect the knee ligaments.
Adolescent↗
Assessment of cold welding properties of the internal conical interface of two commercially available implant systems.
STATEMENT OF PROBLEM: The cone-screw abutment has been shown to diminish micromovement by reducing the burden of component loosening and fracture. However, anecdotal concern for cold welding of cone-screw joints in implant design has been identified as a potential source for lack of retrievability. PURPOSE: This comparative study evaluated the loosening torque, as a percentage of tightening torque, for the ITI Straumann and Astra Tech (3.5 and 4.0 mm diameters) implant systems, which use an 8-degree and 11-degree internal cone, respectively. MATERIAL AND METHODS: Implants and abutments from each system were mounted in a torque device, and a range of tightening torques was applied. Loosening torques were then measured, and the influence of conus angle, interfacial surface area, saliva contamination, and time delay to loosening were all assessed. RESULTS: The loosening torque only exceeded tightening torque at the highest levels, just before component failure, when plastic deformation was expected. For all clinically relevant levels of torque, both in a dry environment and with components bathed in artificial saliva at 37 degrees C, loosening torque was always seen to be 80% to 90% of tightening torque, demonstrating that cold welding does not occur. There was a high correlation between loosening and tightening torque for all systems tested, but no statistical difference when comparing wet versus dry or comparing individual data for each system. CONCLUSIONS: It can be concluded that for clinically relevant levels of tightening torque, no problems are anticipated with respect to retrievability.
Cold Temperature↗
Directional control of planar human arm movement.
We examined the patterns of joint kinematics and torques in two kinds of sagittal plane reaching movements. One consisted of movements from a fixed initial position with the arm partially outstretched, to different targets, equidistant from the initial position and located according to the hours of a clock. The other series added movements from different initial positions and directions and >40-80 cm distances. Dynamic muscle torque was calculated by inverse dynamic equations with the gravitational components removed. In making movements in almost every direction, the dynamic components of the muscle torques at both the elbow and shoulder were related almost linearly to each other. Both were similarly shaped, biphasic, almost synchronous and symmetrical pulses. These findings are consistent with our previously reported observations, which we termed a linear synergy. The relative scaling of the two joint torques changes continuously and regularly with movement direction. This was confirmed by calculating a vector defined by the dynamic components of the shoulder and elbow torques. The vector rotates smoothly about an ellipse in intrinsic, joint torque space as the direction of hand motion rotates about a circle in extrinsic Cartesian space. This confirms a second implication of linear synergy that the scaling constant between the linearly related joint torques is directionally dependent. Multiple linear regression showed that the torque at each joint scales as a simple linear function of the angular displacement at both joints, in spite of the complex nonlinear dynamics of multijoint movement. The coefficients of this function are independent of the initial arm position and movement distance and are the same for all subjects. This is an unanticipated finding. We discuss these observations in terms of the hypothesis that voluntary, multiple degrees of freedom, rapid reaching movements may use rule-based, feed-forward control of dynamic joint torque. Rule-based control of joint torque with separate dynamic and static controllers is an alternative to models such as those based on the equilibrium point hypotheses that rely on a positionally based controller to produce both dynamic and static torque components. It is also an alternative to feed-forward models that directly solve the problems of inverse dynamics. Our experimental findings are not necessarily incompatible with any of the alternative models, but they describe new, additional findings for which we need to account. The rules are chosen by the nervous system according to features of the kinematic task to couple muscle contraction at the shoulder and elbow in a linear synergy. Speed and load control preserves the relative magnitudes of the dynamic torques while directional control is accomplished by modulating them in a differential manner. This control system operates in parallel with a positional control system that solves the problems of postural stability.
Arm↗
An evaluation of fit in osseointegrated implant components using torque/turn analysis.
PURPOSE: The accurate and passive fit of dental prostheses supported by endosseous implants is of primary importance in securing long-term restorative success. In the clinical setting, adequate visual and radiographic assessment of joined implant components can be limited. Mechanical engineering principles show a linear relationship between tightening and the degree of rotation of a precision bolted assembly. At a constant torque, with certain variables controlled, a threaded fastener should return to the same rotational end position on repeated tightenings. This study evaluated the terminal screw positions of joined implant components as a potential aid to the clinician in confirming the fit of a fixed and removable prosthesis. There were three areas of experimental inquiry: (1) How reproducible are the various clinical means by which torque is applied to the fastening screws, both in absolute and relative value? (2) How reproducible are the rotational end positions of the gold (attachment) and titanium (center) screws when a controlled torque is applied? (3) Do changes in screw position occur as a function of the magnitude of artificially introduced discrepancies? MATERIALS AND METHODS: Three different torque delivery devices were evaluated: a hand-held screwdriver (DIB 048; NobelpharmaUSA, Chicago, IL), a manual torque wrench (DIA 250; NobelpharmaUSA), and an electronic Torque Controller (DEA 020; NobelpharmaUSA), using a calibrated torque measuring dynamometer (Magtrol, Inc, Buffalo, NY). The reproducibility of turning limits were determined for both the titanium and gold screws contained in five Brånemark implant assemblies. Each assembly was subjected to six trials, tightening to recommended torque. The position of each screw head was recorded with a special scribe on acetate sheets and transferred to graph paper. Five implant assemblies were invested in dental stone within a die form mold. A casting was made supported by three implant analogues. Stainless steel shims of 12.7-microns, 25.4-microns, 38.1-microns, and 50.8-microns thickness were used to create impingement and space discrepancies. Controlled trials were conducted, and changes in rotational limits for each screw were recorded. RESULTS: The following values were measured, intending to achieve a torque of 10 Ncm, based on 10 trials for each implement: hand driver, 6.48 (+/- 0.85) Ncm; torque wrench, 7.77 (+/- 0.56) Ncm; and the Torque Controller, 8.54 (+/- 0.19) Ncm. The electronic Torque Controller proved to be the most reproducible instrument and was selected as the delivery vehicle for the remainder of the study. The titanium center screws had a rotational limit that was reproducible to within 0.6 degrees (+/- 0.2 degrees). For the gold screws, it was found that at least two trials had to be conducted for each assembly before the rotational limits conformed to a reproducible position within 1.85 degrees (+/- 1.87 degrees). A linear relationship of approximately 0.9 degrees/micron was observed between the changes in rotational limit and each subsequent shim thickness. CONCLUSIONS: This study suggests that screw position can be an indicator of fit in dental implant prostheses if the end point of screw rotation is adequately indexed, specific to each assembly and screw.
Analysis of Variance↗