With the application of a stimulus voltage, the isolated muscle fibre will show contraction fully after the threshold voltage is achieved. An increase in the stimulus voltage will increase the force of contraction, but the amount of contraction will remain the same.
Explanation: Muscle fibres do contain the contractie proteins like actin and myosin. With the advent of an electrical stimulus, there's a change in the intercalated discs or the T junctions of the muscle fibre which leads to the opening of calcium channels of sarcoplasmic rericulum. This increases the calcium ion concentration of the cell and the actin heads get opened which leads to the sliding movement of actin over myosin and thereby contraction occurs.
According to the All or None principle, the muscle fibre will show contraction fully, or won't show any contraction. Sl here, with increase in stimulus voltage, the contraction will be more powerful, but the extent of contraction will be same for all voltages above threshold.
