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The possible peripheral and/or central origin in the mechanisms responsible for day-time fluctuation in maximal torque of the triceps surae muscle were investigated with a special emphasis on antagonist muscle coactivation. Eleven healthy male subjects (physical education students) took part in this investigation. The electromechanical properties of the plantar flexor muscles were recorded at two different times of day: between 06:00 h and 08:00 h in the morning and between 17:00 h and 19:00 h in the evening. To investigate peripheral mechanisms, the posterior tibial nerve was stimulated at rest, using percutaneous electrical stimuli, to evoke single twitch, double twitch, and maximal tetanic contraction (100 Hz). Maximal voluntary contraction of the plantar flexors was also assessed by means of the relative electromyographic activity of respective agonist and antagonist muscles (soleus, gastrocnemius medialis, gastrocnemius lateralis, and tibialis anterior). A double twitch was delivered during maximal voluntary plantar flexion to record muscle activation (i.e., interpolated twitch technique). The coactivation level of the tibialis anterior muscle during plantar flexion was calculated. The results indicated a significant decrease in maximal voluntary muscle torque of triceps surae in the evening as compared with the morning (-7.0 %; p < 0.05). Concerning the central command, when extrapolated by the twitch interpolation technique, the decrease in mean activation level of -6.8 % was consistent with the fluctuation in torque (-7.0 %). Soleus muscle electromyographic activity (normalized to the M-wave) showed a significant decline (21.6 %; p < 0.001). Moreover, individual changes in MVC percentage were significantly related to those of normalized electromyographic activity of the soleus muscle (r = 0.688; p < 0.01). Thus, it indicated that the subject's capacity to activate the soleus muscle was affected by the time of day. The coactivation level in the tibialis anterior muscle during plantar flexion did not change significantly in the evening. Concerning peripheral mechanisms, we observed a decrease in maximal M-wave amplitude for soleus and gastrocnemii, associated with unchanged single twitch and tetanus torque. To conclude, impairment in soleus muscle central command seemed to be the mechanism in the origin of torque failure. Such information would be of importance in the investigation of day-time fluctuations in complex motor task performances implicating the triceps surae muscle.

Laser radiation: ocular risks and safety standards. The risk for biological tissues to be damaged by a laser beam is mainly determined by their optical properties. Laser radiations emitted in ultraviolet and infrared B and C represent a hazard for the anterior segment of the eye. Depending the wavelength, laser radiation is absorbed by the cornea, the lens or both. The lower damage thresholds are reported at the retinal level, in the spectral range of 400 to 1 400 nm. The exposure limit values, recommended by laser safety standards, are evaluated with wavelength, exposure duration, pulse repetition frequency and the size of the visual angle subtending the source. The fast evolution of laser products induces a continuous revision of guidelines. Next revision of European standard EN 60825-1/A2 will include subnanosecond limit values, changes in the continuous wave exposure limits and a new classification of laser products susceptible to be in agreement with next ANSI and FDA/CDRH laser safety standards.

Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. This effect is a reduction in the threshold energy per pulse relative to the single pulse threshold. The best fit to experimental data predicts that the threshold energy per pulse decreases in proportion to the fourth root of the number of pulses n. This model has been reported to be in agreement for exposures ranging from picoseconds to seconds. However, the very limited data reported in the sub-nanosecond domain of time were obtained with a low pulse ratio frequency. No data exists in the litterature concerning the effect on the retina of ultrashort pulses delivered with a very high repetition rate. Also, it has been hypothesized that one cannot extrapolate from longer pulses width because the ultrashort pulse (single or multiple) induce effects which may involve fundamentally different mechanisms of damage. Thus, some laser safety guidelines do not specifically provide exposure limits for a single-pulse duration less than 1 ns because of a lack of biological data1. If exposure limits are given, the limit values for these exposure times has been derived by maintening the irradiances applicable to nanosecond pulses2. With the goal in mind to obtain adequate data base for single and multiple ultrashort pulses, an experimental study has been directed toward determining the retinal damage thresholds induced by picosecond pulses emited in the visible spectrum.Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. This effect is a reduction in the threshold energy per pulse relative to the single pulse threshold. The best fit to experimental data predicts that the threshold energy per pulse decreases in proportion to the fourth root of the number of pulses n. This model has been reported to be in agreement for exposures ranging from picoseconds to seconds. However, the very limited data reported in the sub-nanosecond domain of time were obtained with a low pulse ratio frequency. No data exists in the litterature concerning the effect on the retina of ultrashort pulses delivered with a very high repetition rate. Also, it has been hypothesized that one cannot extrapolate ...

A mechanical device for studying changes in mechanical properties of human muscle as a result of spaceflight is presented. Its main capacities are to allow during a given experiment investigation of both contractile and visco-elastic properties of a musculo-articular complex using respectively isometric contractions, isokinetic movements, quick-release tests and sinusoidal perturbations. This device is a motor driven ergometer associated to an experimental protocol designed for pre- and post-flight experiments. As microgravity preferentially affects postural muscles, the apparatus was designed to test muscle groups crossing the ankle joint. Three subjects were tested during the Euromir '94 mission. Preliminary results obtained on the european astronaut are briefly reported. During the next two years the experiments will be performed during six missions.

The current laser safety standards do not define the exposure limit values for pulsewidth less than one nanosecond. It has been hypothesized that one cannot extrapolate from longer pulse widths because the ultrashort pulse contains high peak power and may induce non linear effects. These effects may involve fundamentally different mechanisms of damage particularly for repetitively ultrashort pulses1. Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. No data exists in the litterature concerning the histological effects on the retina of ultrashort pulses delivered with a high repetition rate.The current laser safety standards do not define the exposure limit values for pulsewidth less than one nanosecond. It has been hypothesized that one cannot extrapolate from longer pulse widths because the ultrashort pulse contains high peak power and may induce non linear effects. These effects may involve fundamentally different mechanisms of damage particularly for repetitively ultrashort pulses1. Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. No data exists in the litterature concerning the histological effects on the retina of ultrashort pulses delivered with a high repetition rate.