Effect of microwave electromagnetic field on skeletal muscle fibre activity.

The aim of the present study was to investigate the influence of microwave irradiation on fatiguing activity of isolated frog skeletal muscle fibres. The changes in the electrical and mechanical activity were used as criteria for the exposure effects. Repetitive suprathreshold stimulation with interstimulus interval of 200 ms for 3 min was applied. Intracellular (ICAP) and extracellular (ECAP) action potentials and twitch contractions (Tw) of muscle fibres after 1 hour microwave exposure (2.45 GHz, 20 mW/cm( 2) power density) were compared with those recorded after one hour sham exposure (control). The duration of uninterrupted activity in the trial (endurance time; ET) was not significantly affected by microwave field exposure. After microwave irradiation, the ICAP amplitude was higher, the rising time was shorter, and the resting membrane potential was more negative compared to controls. There was a slower rate of parameters changes during ET in potentials obtained from irradiated fibres. Microwave exposure increased the propagation velocity of excitation, the ECAP and Tw amplitudes, as well as shortened their time parameters. We concluded that a 2.45 GHz microwave field possesses a stimulating effect on muscle fibre activity, which is in part due to its specific, non-thermal properties. The microwave induced-changes in muscle fibre activity may reduce development of skeletal muscle fatigue.

Muscle fatigue assessment during sustained high isometric contractions.

The myoelectrical fatigue manifestation was assessed during sustained isometric maximal and submaximal (75% of maximal (MVC)) isometric contractions by means of a level-trigger averaging of the surface electromyogram (EMG) recorded from m. biceps brachii by branched bipolar electrodes. The conduction velocity (CV) of excitation was calculated from the time shift of the negative peaks of the averaged potentials (APs). The amplitude and the duration of the negative phase of APs were also measured. The conduction velocity was the most appropriate EMG-derived parameter for muscle fatigue assessment. The duration of the endurance time and the time pattern of CV changes during submaximal sustained contractions were found to resemble very closely those during maximal contraction efforts sustained to 30 percent reduction of the initial torque. Thus the recruitment of additional motor units (MUs) during sustained high contractions does not affect essentially the calculated values of CV.

Long-lasting (fatiguing) activity of isolated muscle fibres influenced by microwave electromagnetic field.

The study aims to clarify the effect of exposure to microwave electromagnetic field (MMW) on muscle fibre fatigue. Repetitive stimulation with interstimulus interval of 200 ms was applied on isolated frog muscle fibre to evoke intracellular action potentials and twitch contractions. After their recording muscle fibre preparation was moved in a Petri dish with radius of 28 mm on open air for one hour exposure to continuous MMW with frequency of 2.45 GHz and power density of 20 mW/cm2. Then it was again moved in the chamber with non irradiated Ringer's solution at controlled temperature for the repeated records. After MMW exposure the changes in amplitude and time parameters characterizing fatigue were attenuated and delayed vs. controls. The twitch amplitude curve described an drastic fall in the first 5 sec followed by an increase and next decrease. MMW (2.45 GHz) have a specific, non-thermal influence on muscle fibre activity resulting in some resistance to fatigue.

Excitability changes during the recovery cycle of human myelinated motor and sensory axons in normal case and in amyotrophic lateral sclerosis.

The excitability changes of human axons during 100-ms recovery cycles are investigated by paired stimulations. Using double cable models of normal axons and of three simulated types amyotrophic lateral sclerosis (termed as ALS1, ALS2 and ALS3, respectively), the recovery cycles are presented in the case of action potential propagation and in the case of a uniformly polarized fibre. Following the testing pulse, human axons go through an oscillating sequence of excitability changes, the recovery cycle: they are initially inexcitable (absolute refractory period), than excitable with a raised threshold (early subnormal period) and after about 2-ms they become more excitable than normal (superexcitable period). For uniformly polarized axons in the normal and ALS1 types, the superexcitable period usually followed by a late subnormal period. With increasing the degree of ALS, the length-dependent potentials propagate with complex, desynchronized amplitudes and velocities, and abnormalities in the recovery cycles are obtained for the ALS3 axonal types.

Surface EMG recorded by branched electrodes during sustained muscle activity.

The purpose of the present investigation is to use surface interference EMG recorded by branched electrodes for assessment of muscle fatigue during sustained voluntary isometric contractions at different levels. Level-trigger averaging and turn/amplitude analysis have been applied. The conduction velocity (CV) of excitation was calculated from the time shift of the negative peaks of the averaged potentials (AvPs) derived from the EMG recorded by two electrodes placed along the muscle fibers. The recruitment of new motor units affects the negative amplitude (NA) of AvPs, the number of turns per second and the mean amplitude of turns in a different way depending on the level of sustained contractions. In contrast, the CV declined at all levels of sustained contractions and was the most appropriate parameter for the muscle fatigue assessment. There was a good correlation between CV decrease and torque reduction during sustained maximal efforts. The level-trigger averaging technique of the interference EMG recorded by surface branched electrodes is easy and non-invasive, thus being very convenient for routine application.