In healthy human muscles, a molecule called phosphocreatine moves around at a specific speed when there's nothing blocking it — and scientists use that speed to understand how energy gets shuttled inside cells.

In healthy adult muscles at rest, a molecule called phosphocreatine moves more easily along the length of muscle fibers than across them—like sliding along a hallway instead of pushing through walls—because the muscle's internal structure guides its movement.

In resting muscles, the way energy molecules move isn't slowed down by how far they have to travel — they can get where they need to go just fine.

In healthy adult muscles, a molecule called phosphocreatine doesn't move freely — it's kind of boxed in by cell walls, and the size of those boxes matches the size of the muscle fibers themselves.

In resting human muscles, a molecule called phosphocreatine can travel far enough to deliver energy where it's needed, even between distant parts of the muscle cell — kind of like a tiny energy shuttle making the rounds.