Fast-twitch (Type II) muscle fibers generate force quickly and fatigue quickly. They're the engine of sprinting, jumping, lifting heavy.
Mechanism: Type II fibers atrophy preferentially with age. By age 80 a sedentary adult can lose 50% of their fast-twitch fiber count. They power explosive movement, fall recovery, and rapid response.
Use case: Heavy lifting (>80% 1RM), jumping, sprinting, plyometrics. The single most age-protective training quality you can keep.
Caveats: Higher injury risk per session. Earn the right with progressive loading. Older adults benefit enormously but need slower ramp-up.