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Calcium plays a crucial role in cardiac muscle contraction by enabling the process of excitation-contraction coupling. When an action potential reaches cardiac muscle cells, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm. This influx of calcium ions binds to troponin, a regulatory protein on the actin filaments. The binding of calcium to troponin causes a conformational change that moves tropomyosin away from the active sites on actin, allowing for the interaction between actin and myosin. This interaction is what generates force and results in muscle contraction.

Moreover, the strength of contraction can be modulated by the amount of calcium present: higher levels of calcium correlate with stronger contractions. In addition to this role during contraction, calcium is also involved in relaxation by being actively pumped back into the sarcoplasmic reticulum and extracellular space after contraction, but its primary and significant role is in increasing contraction strength. Hence, characterizing calcium as essential for enhancing contraction strength accurately reflects its pivotal function in this physiological process.