Muscle and flesh (or wood or equivalents) can be tapped to draw forth certain supernatural energies based on what the flesh or muscle is composed of. There are a few ways to tap into these powers:

Cannibalism - devouring flesh in certain rituals to gain powers
Woodcraft - shaping and crafting wood to invoke certain powers
Dasgal - exercises that strengthen the flesh and muscles to unlock certain powers within it
Kulit - tanning hides or leather-making or similar processes that unlock certain powers from the skin or flesh