Table 2. Modification of articulatory control parameters and related phonetic output.

Parameter	Parameter description and phonetic output
(a) Target	A change in the underlying target involves changes in the peak velocity in proportion to the target value (target undershoot), while the duration of the movement remains unchanged [30]. Articulatory output: A reduction in target in fast speech involves smaller and slower, but not shorter movements (target undershoot). Acoustic output: Reduction in quality, e.g. spirantization or centralisation.
(b) Stiffness	Stiffness is an abstract control parameter related to the relative speed of the movement. It is calculated as the ratio of peak velocity to the maximum displacement, a temporal-spatial measure [27,31–33] Articulatory output: Increasing a gesture’s underlying stiffness in fast speech leads to faster and shorter movements, i.e., the target is achieved in a shorter time. Acoustic output: Shorter durations
(c) Rescaling	Rescaling involves a proportional change in target and stiffness modifications. It affects the acceleration and deceleration phases [34] Articulatory output: Movements are shorter and smaller in fast speech, while the peak velocity remains the same. Acoustic output: Shorter duration and reduction in quality, e.g., spirantization or centralisation
(d) Phasing	Phasing affects the overlap between two gestures, e.g., the timing between a closure and a release. When the release gesture is timed earlier with respect to the closure, the overlap between the gestures increases and the closure will be truncated [27,35,36]. Articulatory output: In fast speech, the closing gesture becomes shorter (truncation, especially of the deceleration phase) and the target is reduced (undershoot), while the peak velocity remains the same. Acoustic output: Shorter duration and reduction in quality, e.g., spirantization or centralisation