What pressures are required for the best systemic response?

The pressure to produce the best systemic response is one that creates a robust metabolic crisis in the working muscles for a substantial (minutes) period of time. So for example, if the pressure and/or load are so high that only a few repetitions of an exercise are possible, there is not enough time for that metabolic crisis to develop in the muscle and the signal is not sent. On the other hand, if the pressure and/or load are so low that the metabolic crisis takes a long time or never happens, the alarm signal to the brain is not sent, (or sent as if there were no BFR), minimizing a systemic response.