The relations between motor unit global firing rates and established quantitative measures for processing the surface electromyogram (EMG) signals were explored using a simulation approach. Surface EMG signals were simulated using the reported properties of the first dorsal interosseous muscle in man, and the models were varied systematically, using several hypothetical relations between motor unit electrical and force output, and also using different motor unit firing rate strategies. The utility of using different EMG processing parameters to help estimate global motor unit firing rate was evaluated based on their relations to the number of motor unit action potentials (MUAPs) in the simulated surface EMG signals. Our results indicate that the relation between motor unit electrical and mechanical properties, and the motor unit firing rate scheme are all important factors determining the form of the relation between surface EMG amplitude and motor unit global firing rate. Conversely, these factors have less impact on the relations between turn or zero-crossing point counts and the number of MUAPs in surface EMG. We observed that the number of turn or zero-crossing points tends to saturate with the increase in the MUAP number in surface EMG, limiting the utility of these measures as estimates of MUAP number. The simulation results also indicate that the mean or median frequency of the surface EMG power spectrum is a poor indicator of the global motor unit firing rate.
Since the demonstration of the possibility of high amplification of soft X-rays in laser-produced plasmas,'`' many different approaches to the improvement of X-ray laser characteristics, i.e. shorter wavelengths and higher efficiency, have been pursued.3)There are two pumping schemes for soft X-ray lasers, the electron collisional excitation scheme and the recombining plasma scheme. The main results of recent experiments are listed in Table I. It was believed that it is important to produce a large ion density at a higher ionization stage to get a high gain amplification and then it is necessary to use a high power laser with more than several hundred joules 'pulse for the pumping of soft X-ray lasers. As the electron collisional excitation scheme really needs very high input power for the pumping of soft X-ray lasers, the practical limit of short wavelength lasers was 106.4 A of Mo:2+ for the Ne-like ion series even in the largest laser facility.After those experiments, Ni-like ions were found to be more efficient in producing a short wavelength laser compared to Ne-like ions, and a 0 50.3 A laser was obtained in the Ni-like ion of Yb42+. Experiments for the Ni-like ion series are being carried out to produce shorter wavelength lasers.On the other hand, the recombining plasma scheme needs less input energy than the electron collisional excitation scheme. In the recombination scheme, a laser is focused onto a solid target and creates a laser-produced plasma which includes a large ion density at a higher ionization stage. Next, rapid cooling of the plasma creates favorable conditions for the population inversion between the two excited levels of ions at low ionization stages through recombination.As the recombination rate depends strongly on the electron temperature, Te, of the plasma, faster cooling leads to a larger population inversion. This greatly improves the efficiency of the recombination laser. In the present study, amplified spontaneous emission from Al ions is observed in a recombining plasma, produced by a low power driving laser for the first time.Our experimental set-up is shown in Fig. 1. We use a focus line of narrow width for rapid cooling of the plasma by free expansion. A single beam of a Nd-doped glass laser was operated at a wavelength of 1.05 µm in a 5 ns FWHM pulse. A 40 µm wide by 12 mm long line of focus was produced using a cylindricalspherical lens system. The expanding plasma was produced by a laser beam of 6 J. The power density at the target surface was approximately 2.5x 1011 W/cm`'. The thickness of all aluminum targets used was approximately 1 mm.
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