Reports of "muscle splitting open" usually describe a full-thickness rupture that requires emergency surgical debridement and reconstruction, similar to procedures used for high-speed trauma or complete muscle ruptures. Context on Sean Costin

Sean Costin has publicly documented his fitness journey, including his recovery from surgeries such as . In the bodybuilding community, discussions regarding his more severe injuries often focus on the limits of human physiology and the potential dangers of pushing muscle growth beyond what the structural integrity of the body can support.

In extreme cases of muscle loading and hypertrophy—often accelerated by exogenous substances—muscle fibers can actually branch or split. While small-scale splitting can be a physiological response to training, "splitting open" suggests a catastrophic failure of the muscle and its surrounding connective tissue (fascia and aponeuroses).

Re-innovation of split lateral gastrocnemius muscle flap ... - PMC