This event is commonly studied under two different conditions Isometric Contraction, where both tendons are firmly fixed, the muscle develops force or tension, but does not shorten significantly. Isotonic Contraction, where one tendon is fixed, the other is permitted to shorten against a constant tension or load. If you now reapply a brief single stimulus to nerve-muscle preparation connected to an isometric force transducer, you can obtain a mechanogram of the muscle twitch contraction. If the stimulus is delivered repetitively using a low frequency,you see a process called twitch summation , the peak of each twitch is greater than the previous one, up to some equilibrium value. I'll talk more about the mechanism later in the series.
Repetitive stimulation is called tetanic stimulation.
Conversely, the stimulus frequency required for a fused tetanus is called the fusion frequency for a particular muscle type. Let's take a closer look at this piece of tissue that we are stimulating. The muscle is made up of parallel elongated multinucleated cells known as muscle fibres. In some muscles these fibres extend the entire length of the muscle. At either end the fibre fuses with tendinous tissue.