Complex sentence: Under normal circumstances, the tropomyosin sterically obstructs binding sites for myosin on the thin filament; once calcium binds to the troponin C and causes an allosteric change in the troponin protein, troponin T allows tropomyosin to move, unblocking the binding sites.
Split into simpler sentences: Under normal circumstances, the tropomyosin sterically obstructs binding sites for myosin on the thin filament. Once calcium binds to the troponin C, it causes an allosteric change in the troponin protein, troponin T allows tropomyosin to move, unblocking the binding sites.