Extra Torque Research Articles

Twitch interpolation of the elbow flexor muscles at high forces.

We investigated factors affecting maximal voluntary torque and the assessment of the level of voluntary drive in the elbow flexor muscles. First, the effective compliance of the system was tested by using single, paired, and trains of four stimuli to measure voluntary activation. At high voluntary torques the responses to all these stimuli were identical, suggesting that single stimuli are adequate for estimating voluntary drive. Second, the contribution of torque from synergist elbow flexor muscles was assessed. In attempted maximal voluntary contractions (MVCs), the voluntary activation of brachioradialis (median 91.5%, range 68.9-100%) was lower than for biceps brachii (median 99.1%, range 78.5-100%; P < 0.01). This suggests extra torque may be generated by brachioradialis during elbow flexion, beyond the torque where biceps brachii is maximally activated. Finally, lengthening of the elbow flexors occurred during MVCs, due to slight shoulder movements. This would allow force to increase independently of an increase in voluntary drive.

Extent of muscle inhibition as a function of knee angle

The present study was aimed at assessing muscle inhibition (MI) of the quadriceps muscles of healthy subjects as a function of knee angle. The extent of muscle activation and of twitch potentiation following maximal contractions at different knee angles were investigated. Six males and four females (mean age 29.5 ± 6.2 yr) performed three maximal isometric knee extensions on a KinCom dynamometer with the right and left legs at 15, 30, 45, 60 and 90 ° from full extension. MI was assessed using the interpolated twitch technique. This technique requires surface stimulation of the femoral nerve during the maximal contractions. MI was estimated by the amount of extra torque evoked by the superimposed twitch. Electromyographical (EMG) activity of the vastus lateralis muscle was measured in order to estimate muscle activation. Twitch potentiation was assessed at 5, 20, 60 and 120 s after the maximal contractions by applying a single electrical twitch to the relaxed muscle. Results revealed a strong dependency of MI on the knee angle tested; MI increased with increasing muscle lengths and was almost three times higher at 60 ° than at 15 ° of full extension. Muscle activation measured as the root mean square (RMS) values of the vastus lateralis EMG was the same for four out of the five muscle lengths measured; at a knee angle of 90 °, the RMS values were significantly higher. Twitch potentiation was observed at all knee angles following maximal contractions. Potentiation was highest immediately after contraction (i.e. 5 s after), and the amount of potentiation did not depend on the knee angle. Muscle activation and twitch potentiation only accounted for a small percentage of the differences in MI as a function of knee angle. It is suggested that increased patellofemoral pressure and increased ligament strain, which are highest between 45 and 60 ° of full extension, might be responsible for the high MI measured at these knee angles. It is further hypothesized that the differences in MI as a function of muscle length are also associated with the shortening of the contractile elements during contractions, and the concomitant loss in force potential; a phenomenon which is probably more pronounced at short compared to long muscle lengths.

Near-Minimum Time Feedback Controller for Manipulators Using On-Line Time Scaling of Trajectories

A near-minimum time feedback controller for robotic manipulators with bounded input torques is developed. Since the bang-bang input torque obtained from the timeoptimal control theory leaves little or no room for the extra torque of the feedback control action, it is difficult to combine a minimum time open-loop controller with an additional feedback controller. A simple solution to this problem has been to solve the minimum time problem using arbitrarily reduced torque bounds so that a torque head room is created for the feedback control action. Such a scheme, however, wastes considerable input torque potential and gives significantly larger execution time of the trajectory than the theoretical minimum time calculated from the time-optimal control theory. A stable feedback controller is developed in this paper which applies a time scaling method to move a manipulator in near-minimum time using the allowable input torques efficiently. This new feedback controller algorithm adapts to an uncertain environment and automatically adjusts the desired speed along a specified path to be as fast as possible while avoiding the velocity saturation condition. Numerical examples of the near-minimum time feedback controller are provided using a two-link SCARA manipulator.

Are humans able to voluntarily elicit maximum muscle force?

Previous investigators have used electrical stimulation superimposed on voluntary efforts to show that humans are capable of maximum muscle activation. In the present study this notion was tested using the interpolated twitch technique enhanced by triggered averaging and doublet stimulation of the human biceps brachii. It was found that the decline in extra torque evoked by percutaneous stimulation with increasing levels of voluntary effort was nonlinear and that none of the twelve subjects was able to elicit a voluntary effort large enough to prevent extra torque of elbow flexion. The exponential nature of the declining extra torque indicated that an extrapolated maximum muscle force could be considerably larger than that to which subjects were able to elicit voluntarily.

Corrections to the Stokes drag near thermodynamic critical points

Low Reynolds number flow is considered in a pure fluid near the thermodynamic critical point and in a binary mixture near the critical mixing ’’point.’’ It is shown that, due to the breakdown of the assumption of imcompressibility, a sphere rotating with steady angular velocity Ω in a stream U experiences an extra force proportional to Ω×U, but no extra torque.

Torque Reduction in Finite Amplitude Secondary Flows of Dilute Polymer Solutions

When a dilute polymer solution is sheared between concentric cylinders with the inner cylinder rotating and the outer fixed, the onset of cellular secondary flows (Taylor vortices) is usually delayed relative to a Newtonian liquid of the same kinematic viscosity, and the extra torque in the polymer solution following instability is less than that for the Newtonian liquid at the same relative Taylor Number. This torque reduction is explained here by means of a finite amplitude analysis of the secondary motion in a second-order fluid using the energy equilibrium approach of Landau and Stuart. The theoretical calculations are in general agreement with data of Denn and Roisman for a 0.5% solution of polyisobutylene in decalin. Using these calculations and the linearized stability theory it is possible to estimate the three rheological parameters for a second-order fluid from a single set of torque-rotational speed data in a Couette viscometer.

Magnetic Anisotropy of Some Nickel Zinc Ferrite Crystals

Single crystals of Ni1−pZnpFe2O4 (p=0.20 and 0.65) were prepared from a melt in a PbO:B2O3 flux. The anisotropy constant K1 was deduced from torque measurements in a (100) plane. | K1 | increases linearly with p at low temperatures. Combining the results with data on p=0 and p=0.33 from the literature, one can represent K1 at 4°K as the sum of contributions from tetrahedral Fe3+ (+0.04 cm−1 per ion) and octahedral Fe3+ (−0.07 cm−1) in agreement with data on other iron spinels. The temperature, dependence of K1 for p=0.65 agrees with Wolf's expectations for ferrimagnets with a weak exchange. Above room temperature the torque curves for this sample show a spurious uniaxial term, increasing with temperature. Both K1 and this extra torque are reduced by increasing the external field.