Which electrolyte imbalances at the cellular level cause changes in the normal conduction and contractile function of the myocardium?

Magnesium, potassium and calcium

Explanation: This is evident in the roles these cations play at the intercellular levels on muscle contractions. Potassium is vital to the contractions of the heart and skeletal muscles, Calcium mediates muscular contractions and regulates the activity of the CNS, Magnesium regulates the Flux of calcium. So it is evident that any change in their concentrations would significantly affect the activities of the heart muscles especially.

exercising at a moderate intensity, and slowly increasing the duration.