Figure 1.

The effect of superimposed twitches on incremental voluntary quadriceps femoris (QF) contractions using the twitch interpolation technique. During a voluntary contraction, 1-Hz electrical twitches are delivered via surface electrodes applied over the motor nerve or muscle motor points. At low levels of voluntary activation (on the y-axis), a large proportion of muscle fibres remain unactivated, so that superimposed stimulation can induce large interpolated twitches. As the level of voluntary activation increases, the proportion of yet unactivated fibres decreases and so the height of the stimulation-induced interpolated twitches decreases until, near or at maximum voluntary contraction (MVC), twitches can no longer be seen (adapted from [32]).