Calsequestrin is a calcium-binding protein primarily found in the sarcoplasmic reticulum of muscle cells. It plays a crucial role in regulating calcium levels, which are essential for muscle contraction and relaxation. By storing and releasing calcium ions, calsequestrin helps ensure that muscles function properly during physical activity. This protein is particularly important in skeletal muscle and cardiac muscle, where precise control of calcium is vital for effective contraction. Abnormalities in calsequestrin can lead to muscle disorders and affect overall muscle performance, highlighting its significance in muscle physiology.