At the moment, our gold standard way of measuring CNS fatigue is to assess the voluntary activation of the worked muscle, using an interpolated twitch during and after maximum voluntary isometric contractions. There are likely muscle-specific and also non-local mechanisms that cause CNS fatigue, so measuring another muscle will only capture a portion of the CNS fatigue that is present in a previously-trained muscle group. This is why measuring handgrip strength doesn’t help us figure out whether there is CNS fatigue of (say) the leg muscles.