Cardiac Muscle Physiology I. Cardiac MUSCLE cells: Stable RMP, not self-excitable. Cardiac muscle cells operate ONLY via twitches – they can never reach tetany, unlike skeletal muscle. RECALL: Skeletal muscle – timing of twitch: Cardiac muscle – timing of twitch: Heartbeats are just one twitch after another. How to make contractions stronger? Increase calcium levels – cardiac muscle contractions are graded (similar to smooth muscle), and when there’s more calcium available, the contraction is stronger. Skeletal : at optimal length, at tetany;