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R N Stiles

Publications and source records attributed to R N Stiles.

13 recordsLinked to original sources
PubMed1991

A comparison of static and dynamic characteristics between rectus eye muscle and linear muscle model predictions.

The characteristics of a muscle model are analyzed using rectus eye muscle parameter values and compared to rectus eye muscle data. The muscle is modeled as a viscoelastic parallel combination connected to a parallel combination of active state tension generator, viscosity element, and length tension elastic element. Each of the elements is linear and their existence is supported with physiological evidence. The static and dynamic properties of the muscle model are compared to rectus eye muscle data. The length-tension characteristics of the model are in good agreement with the data within the operating region of the muscle. With the muscle model incorporated into a lever system to match the isotonic experiment paradigm, simulation results for this linear system yield a nonlinear force-velocity curve. Moreover, the family of force-velocity curves generated with different stimulus rates reported in the literature match the predictions of the model without parametric changes. The results of this paper are important in studies involving the oculomotor plant and oculomotor neural networks. Additionally, these results may be applicable to other muscles.

Isometric Contraction↗
PubMed1990

Factors influencing the anterior component of occlusal force.

We hypothesized that the anterior component of occlusal force (ACF) generated by mandibular molars was a function of molar inclination, height of the transverse condylar axis above the occlusal plane, steepness of the occlusal plane, gape, molar root dimensions, interproximal tooth contact force when not biting, and bite force. Our research aim was to identify those biomechanical factors which determine ACF. Mandibular second molars were axially loaded with a 90 N force (10 mm second molar gape) in 15 subjects, and the resulting ACF was measured at the mandibular first molar-second premolar contact using a recording technique based on interproximal frictional forces. Morphologic measurements were obtained from lateral cephalometric radiographs of each subject and included: Frankfort mandibular plane angle, occlusal plane angle, angles formed by the longitudinal axis of the second molar and the occlusal and mandibular planes, perpendicular distance from the top of the condyle to the occlusal plane, and second molar root width and root length. For ten subjects, ACF resulting from axial loads of 50, 100, 150, and 200 N was measured. For ten subjects, ACF resulting from an axial load of 50 N and second molar gapes of 10 mm, 14 mm, 18 mm, and 22 mm were measured. ACF increased with increasing gape and increased proportionally to increasing bite force. Correlation and stepwise regression analyses revealed that ACF varies with interproximal tooth contact force when not biting (contact 'tightness') and molar root width (model R2 = 0.71, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Additional developments in oculomotor plant modeling.

A new oculomotor plant is presented in this study using an updated third-order linear muscle model. The lateral and medial rectus muscle is modeled as a viscoelastic parallel combination connected to a parallel combination of active state tension generator, viscosity element and length tension elastic element. The eyeball is modeled as a sphere, connected to two parallel viscoelastic elements, connected in series. Each of the elements is linear. The static and dynamic properties of the muscle model are in good agreement with rectus muscle data. The length-tension characteristics of the model match the data within the operating region of the muscle. Simulation results for the muscle model yield hyperbolic shaped force-velocity curves that match the data very well. All parameters and initial conditions are estimated or directly measured from physiological data. The oculomotor plant is derived through direct programming state-space representation by Laplace variable analysis about the operating point or initial eye position. The form of the oculomotor plant makes this representation even more ideal than previous models for use in the development of more sensitive tests of oculomotor pathology and in the description of normal oculomotor function.

Elasticity↗
PubMed1989

Buckle muscle tension transducer: what does it measure?

The question is considered whether the strain of a buckle transducer attached to a muscle tendon provides a proportional measure of the force of the muscle acting directly on that tendon. It is shown that if muscle contains elastic and/or viscous elements in parallel with the force generator, the transducer strain may, under certain conditions, reflect other applied forces acting on the load (limb) in addition to the muscle force.

Animals↗
PubMed1989

A linear muscle model predicts the hyperbolic force-velocity relationship.

A variety of different schemes have been reported in the literature for linearizing the force-velocity relationship observed in muscle, a dominant element in the muscle. This report extends assertions that a linear muscle system has force-velocity characteristics as described by Hill's hyperbolic equation, and that no linearization whatsoever is required. The muscle is modeled as a parallel combination of passive elasticity, and series elasticity connected to the parallel combination of active state tension generator, viscosity and length tension elasticity. Each of the elements are linear. Simulation solutions of this third-order system yield a hyperbolic shaped force-velocity curve using physiologically derived estimates, based on the oculomotor system, for the parameters of the muscle model.

Elasticity↗
PubMed1989

Agonist and antagonist muscle tension during horizontal saccadic eye movements.

Agonist and antagonist muscle tension simulations are reported for a fourth-order model of the oculomotor plant and active state tensions generated by a neural feedback model during horizontal saccadic eye movements. The lateral and medial rectus muscles are modeled as a parallel combination of passive elasticity, and series elasticity connected to a parallel combination of active state tension generator, viscosity element and elastic element. The eyeball is modeled as a sphere with moment of inertia connected to a viscosity element and an elastic element. The active state tension is generated by a low-pass filtered output from the neural burst circuit. The saccade generator is first-order time optimal and located in the superior colliculus. Agonist muscle tensions simulated with TUTSIM match the data extremely well. Antagonist muscle tension simulated with TUTSIM have an initial drop in tension, consistent with microelectrode predictions, and then a rise in muscle tension. The initial drop in antagonist muscle tension has not been reported in the literature because of band limitations of the force transducer used to record muscle tension.

Computer Simulation↗
PubMed1977

Involuntary ankle oscillations from normal subjects.

Spectral analysis of ankle tremor records obtained from normal seated subjects during continuous elevation of the heel for 10-45 min revealed that the root-mean-square (rms) displacement amplitude of the tremor increased from minimum values of about 4 micronm to values as large as 4,000 micronm. Associated with this increase in the displacement amplitude was a systematic decrease in the tremor frequency from values of 7-8 Hz to values of 5-6 Hz. Spectral analysis of demodulated soleus EMG records indicated that the rms value of this EMG (calculated at the tremor frequency) and the rms displacement of the tremor are related by a power function, with the rms value of the EMG increasing over a range of about 4-40 micronV as the tremor displacement increased from about 4 to 4,000 micronm. The negative relation between frequency and rms displacement amplitude values for postural ankle tremor was similar to that found previously for postural hand tremor.

Ankle↗
PubMed1976

Frequency and displacement amplitude relations for normal hand tremor.

Spectral analysis of hand tremor records obtained from normal subjects during continuous extension of the hand for 15-45 min revealed that the root-mean-square (rms) displacement amplitude of the tremor increased from control levels of about 30 mum to levels on the order of 100-1,000 times control. Associated with this increase in the displacement was a systematic decrease in the hand tremor frequency from control values of 8-9 Hz to values of 4-6 Hz. Spectral analysis of demodulated extensor EMG records indicated a consistent relation between EMG modulation amplitude at the tremor frequency and the tremor displacement amplitude for tremor records with rms displacement above about 100 mum. No consistent relation was found between these two variables for tremor records with displacements below 100 mum. Consideration of both mechanical and neural reflex effects indicated that a viscoelastic-mass mechanism primarily determined the small-amplitude (less than 100 mum) tremors, while the large displacement tremors may have involved both mechanical and neural feed back factors.

Computers↗
PubMed1976

A mechanical-reflex oscillator hypothesis for parkinsonian hand tremor.

Spectral analysis was performed on postural hand tremor records obtained from 22 parkinsonian subjects. Of these 22 subjects, 18 had postural hand tremor that occurred primarily at a single frequency during any one 16-s period. In general, this tremor occurred at different steady-state frequencies (each calculated over 16 s) between about 4 Hz and 8-9 Hz. This frequency decreased approximately 1 Hz for each 10-fold increase in displacement amplitude (root-mean-square, rms, amplitude determined at 16 cm from the wrist), decreasing from 8-9 Hz at about 30 mum to 3.75-4.0 Hz at about 30,000 mum. The major finding was that the frequency of parkinsonian hand tremor was nearly the same as that for hand tremor from normal subjects when these frequenceis were compared at similar rms displacement levels. This comparison, plus a comparison between other aspects of these two kinds of tremor, indicate that the mechanism for parkinsonian hand tremor is similar to that for large-displacement (greater than 100 mum) hand tremor of normal subjects, i.e., a mechanical-reflex oscillator mechanism.

Hand↗
PubMed1975

Acceleration time series resulting from repetitive extension-flexion of the hand.

Power, or variance, spectra of acceleration records obtained from normal subjects during extension-flexion oscillations of a hand at frequencies between 0.5-5.0 Hz generally contained two or three frequency bands. Partial separation of these oscillations in the time domain was obtained using the method of digital filtering (smoothing). In general, the peak frequency of the lowest frequency band occurred at, or nearly at, the frequency that the subject attempted to maintain during a 16-s digitization period. Consideration of absolute frequencies and the effect of mass on frequency indicated that one of the higher frequency bands was the result of normal hand tremor. For low frequencies of voluntary oscillation (0.5-1.25 Hz), a second, higher frequency band occurred with a peak frequency (3.0-4.25 Hz) within the range (3-6 Hz) usually reported for abnormal hand tremor. For voluntary oscillation frequencies above about 1.5 Hz, this second, higher frequency oscillation generally occurred at twice the frequency of the voluntary oscillation.

Adolescent↗