At the onset of a muscle contraction, the presence of calcium triggers the opening of the Na-K channels in the membrane. Potassium is a calcium inhibitor, so as the potassium flows out of the cell, it eventually reduces the presence of calcium. This causes the closure of the Na-K channel (negative feedback mechanism). In a hypokalemic state, the lack of sufficient K doesn't inhibit the calcium channel, and in turn doesn't properly terminate the muscle contraction at the cellular level.
