You might add to your set of parameters the posture and goal states available to the organism so manipulated, as these present novel texture in the once thought to be straightforward stimulus-to-motor-response reflex arc.  I regret that I don't have more answers than added questions, but you may find that posture and goal-state may matter quite a bit.

Thank you Damian for your added point. My questions are just a few pieces of a slightly bigger picture. I am trying to find if there is a way to evoke predefined motor actions that could lead to a head and neck posture adjustments on a car occupants, in a short window time between the moment of the impact and the load that will be applied to the neck. The window time is so short to make those adjustments that only localized and precise reflex arcs do manage to fulfill, far sooner than integrated answers from cortex.

I hope being more specific now. The neck and head initial posture are a key factor when referred to the level and type of injuries and trauma sustained by the neck, in a car accident. And with slight adjustments before the load could mean big differences on the consequences.

This paper may take you a step further:

Brain Stimul. 2013 Jul;6(4):624-30. doi: 10.1016/j.brs.2012.09.009. Epub 2012 Oct 12.

Interaction between simultaneously applied neuromodulatory interventions in humans.

Schabrun SM, Chipchase LS, Zipf N, Thickbroom GW, Hodges PW.

Indeed, another step forward. Thank you very much Béatrice. I feel that I'm diggin in a very soft soil. Meantime I hope not be buried.

This is a complicated matter with no simple answer, eventually you can shake off the soil:)