According to sliding filament theory of muscle contraction, 

According to sliding filament theory, muscle contraction occurs because myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line . As a result, the thin filaments slide inward and meet at the centre of a sarcomere. Active sites on actin filament are exposed when ${\mathrm{Ca}}^{++}$ bind to troponin. During muscle contraction, I-bands get reduced whereas A-bands retain the length. Cross-bridge is broken when ATP binds to myosin.