Which of the following occurs during muscle relaxation? 

During muscle relaxation, the  Ca⁺⁺ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments. This causes the return of ‘Z’ lines back to their original position, i.e., relaxation. A bands retain their length during contraction. During muscle relaxation, thin filaments move away from M line.