What happens to slow twitch muscle fibers during immobilization?

During immobilization, slow twitch muscle fibers can undergo a transformation that leads to them developing characteristics typically associated with fast twitch muscle fibers. Slow twitch fibers, which are primarily designed for endurance activities and have a high capacity for aerobic energy production, may adapt to a lack of use by changing their metabolic properties and contractile characteristics.

When a muscle is immobilized, the absence of regular activity reduces the demand placed upon it. In response, the body may initiate a process of adaptation that results in an increase in muscle fiber size or morphological changes that allow these fibers to act more like fast twitch fibers, which are associated with shorter bursts of more explosive activity. This can include aspects such as an increase in glycogen storage and an alteration in the oxidative capabilities of the slow twitch fibers.

As such, during immobilization, the inability to engage in regular use leads to a shift that effectively deteriorates the endurance capabilities typical of slow twitch fibers, making them more similar to fast twitch fibers in terms of their characteristics. This reflects the profound influence that activity—and the lack thereof—has on muscle fiber composition and function.