I know this is a track forum, but I am hoping someone can help me out. I want to know if there is a difference between sprinting on land versus in the water on an athlete’s CNS? In other words, can someone in the water sprint more and/or more often than someone on land? My guess would be no, that sprinting effects the CNS the same regardless. But since I have no evidence one way or the other I am turning to the knowledgeable folks on this board to help me out.
From my understading due to the resistence of the water swimming is less taxing on the CNS. Muscle contraction and limb movement can never be as fast as movements on the track. Likewise, the resistance of the water is not enough to tax the CNS to the same extent as sprinting or heavy resistance training.
However it is important to note that swimming is very taxing on the PNS and has a large effect on depletion of muscle glycogen.
Perhaps Pakewi could help with this… 
CNS fatigue has the elements of watts, Motor Units recruitment, and limb velocity. Swimming has less CNS stress but more general fatigue. Add in the fact that few programs have high velocity programs, swimmers find new speed-speed endurance programs draining.