Why is it that people get muscle weakness in hyperkalaemia and hypokalaemia?

Potassium critical: Potassium is a very integral part of the muscle contraction process. It is also extremely sensitive to being in too high or too low a concentration. It has a rather narrow window of normal range so that any deviation above or below can cause drastic changes to the effectiveness of muscle contraction. With potassium, when it is either too high or too low muscles often become weak.

Important K: Potassium is integral for muscle contraction - when too high or too low it causes problems like muscle cramps. As the heart is a muscle it is also very important for your heart function.