The effects of electrical stimulation on muscle are not fully known, particularly muscle fatigue is a limiting factor in functional electrical stimulation (FES).

Muscle activation patterns have been investigated in several experimental studies. The results show that individuals have a very stabile selective EMG pattern for cyclic movements. On the basis of these results, there probably is the possibility to significantly reduce muscle fatigue during stimulating the muscle. This could lead to a physiologically based FES mode. Therefore it is essential to identify electrical sensitive points in the muscle. It was published that there are indications for such areas in the muscle. In the presented study we started to identify intramuscular electrical sensitive points in M. triceps brachii of rats.

For the stimulation we used our self-developed flexible and atraumatic full silicon coated electrode array. Two array electrodes (9 channels each, array with equal spacing between all adjacent electrodes) were applied into the M. triceps with one in the C. longum and the other one in the C. laterale. An intermittent stimulation was used and applied to the muscle with a positive rectangular shape signal of 1–7 mA at a frequency of 40 Hz with a pulse wide of 200 ms. Two seconds of stimulation were followed by a period of a two seconds stimulation pause. Rising and declining flanks took 0.5 seconds each.

Electrical stimulation was applied over all adjacent electrodes one after another. In vivo the resulting force was measured simultaneously to the electrical stimulation. The stimulation of two adjacent electrodes ended when the force went down to zero. Additionally the EMG (10–700 Hz, sampling rate 4000 Hz, 0.488 mV/bit) was measured over the same electrodes between cyclic movements to identify the position of motor end plates.

After finishing the stimulation procedures the stimulated muscle was stained with Sihler's staining to locate the electrode positions in relation to the nerve branches. Through this kind of staining the nerve branches can be presented in high resolution.

Our results show that the potential stimulation duration of the C. longum is longer than that of the C. laterale. While the position of the longest excitability is located proximally in the C. longum, it is found more distally in the C. laterale. The duration of stimulation varied between a few seconds and several hours. The longest stimulation duration was 4 hours 45 minutes, but even longer duration would have been theoretically possible. But in consideration of the long anaesthesia of the animal the stimulation was broken off. Overall, the positions of the motor end plates do not correspond to the positions of the longest excitability.