Much research exists on neural performance and fatigue in high intensity and endurance athletes.   In a nutshell, the consensus of articles tends to value that the nervous system needs more time to recover from intense exercise than does the muscular and cardiovascular systems.  Pushing into fatigue on hard days in a training regimen is beneficial, only if you also have your ‘easier’ or ‘off’ days.  This allows all the systems in the body the time to recover and reset before optimal output is once again attempted.