The refractory period is an interval during which it is more difficult to elicit an action potential. Absolute refractory period is the interval during which another action potential cannot be elicited, regardless of the strength of stimulus. It begins at the start of action potential upstroke and extends into the downstroke. It lasts the duration of the action potential in all myocardial fibres. Relative refractory period is the interval in which a record of action potential can be elicited if the stimulus is sufficient. The given figures shows a record of the contraction of the muscle, the lower figure shows the varying excitability of the muscle to stimuli.