https://www.medicalnewstoday.com/articles/320718.php
"Myositis ossificans usually occurs where a person has experienced a single traumatic injury, such as sustaining a hit while playing football or soccer that causes a deep muscle bruise.
It can also happen when there is a repetitive injury to the same area, such as in the thighs of horseback riders. Very rarely, myositis ossificans can occur after a severe muscle strain."
As both may occur after trauma Just recall : *Synovial Cyst (ganglion cyst ) - forms a fluctuant mass
*Myositis ossificans - as in the question - forms a " very firm " mass
submitted by bunye(11)
Mesenchymal tissue can also undergo metaplasia.
A classic example is myositis ossificansmin which connective tissue within muscle changes to bone during healing after trauma - Pathoma Ch 1 pg 2