Answer: The concentration of calcium in the myofibril must decrease.
Explanation:s soon as the stimulus which causes myofibril to contract stops, its time for muscle to relax.
A low calcium level in the protoplasm is the utmost requirement of muscle relaxation otherwise sliding of myosin will continue and muscle will keep on showing contraction. In order for muscle contraction, calcium ion i.e. Ca²⁺ must come out of the protoplasmic reticulum so for relaxation reverse process occurs i.e. Ca²⁺ is pushed back to the protoplasmic reticulum. There are many pumps known as "calcium pumps" which facilitate this process. Now, the concentration of Ca²⁺ becomes comparatively low in the protoplasm which causes troponin C to detach from Ca²⁺. Their detachment causes actin and myosin to detach from each other too which finally leads to muscle relaxation.