Model of a new temperature-compensated optical current sensor using Bi(12)SiO(20).

A new optical bulk current sensor is presented with Bi(12)SiO(20) as a sensing crystal. Through the use of a mirror in the setup and the reciprocity of the optical activity of Bi(12)SiO(20), the sensor becomes insensitive to intrinsic linear birefringence and birefringence that is due to the enclosure of the crystal. Therefore the sensor is also insensitive to temperature, which affects the total linear birefringence. By making a computer model of the Bi(12)SiO(20) sensor, it was proved that the output signal of the sensor, which has a sinusoidal response, has a maximum relative error of 0.05%, apart from the variation of the Verdet constant, for a temperature change of approximately 100 °C.

Differences in the activation of m. biceps brachii in the control of slow isotonic movements and isometric contractions.

We have compared muscle activation in the control of slow isotonic movements and isometric contractions. Specific attention has been given to the contribution of the two force-grading mechanisms, the recruitment of motor units and the modulation of firing frequency in motor units that have already been recruited. The recruitment order of the m. biceps motor units under study was the same during isometric contractions and slow isotonic movements. However, the recruitment thresholds of the m. biceps units were considerably lower for both isotonic flexion and extension movements, even at velocities as low as 2 deg/s, than for isometric contractions. Furthermore, firing frequency at recruitment was found to depend on the motor task: at recruitment the motoneurone starts firing with a higher firing frequency during isotonic flexion movements and a lower firing frequency during isotonic extension movements than during isometric contractions. Two main conclusions can be drawn from these results. First of all, the concept of one single activation parameter (total synaptic drive?) cannot account for the motor-unit behaviour observed during our experiments: the relative contribution of the two force-grading mechanisms is different for different tasks. Secondly, the distribution of activity among flexor motoneurone pools is different for isometric contractions and isotonic movements.