Final answer:
Calcium ions are responsible for initiating the sliding of filaments in muscle contraction, which occurs when calcium released from the sarcoplasmic reticulum binds to troponin, exposing actin-binding sites for the myosin heads.
Step-by-step explanation:
The element that initiates the sliding of the filaments in muscle contraction is calcium ions (Ca++). During the process of muscle contraction, an action potential causes the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm. These calcium ions then bind to troponin, causing a change in the troponin-tropomyosin complex, resulting in the exposure of the actin-binding sites. This allows myosin heads to form cross-bridges with actin, leading to the sliding of filaments and muscle contraction, powered by ATP.