muscle fiber types
Not all muscle is built the same. Inside every muscle there is a mix of two basic kinds of fibre, like a workforce of patient marathoners and explosive sprinters. The proportions differ from muscle to muscle and from person to person, and they shape what that muscle is good at.
Type I fibres (slow-twitch) contract slowly, resist fatigue, and are rich in mitochondria and blood supply, so they hum along all day using oxygen — they dominate the postural muscles that hold you upright. Type II fibres (fast-twitch) contract quickly and powerfully but tire fast; they lean on the anaerobic systems and come in subtypes ranging from moderately fatigue-resistant to purely explosive. When a muscle needs to act, it recruits the small, fatigue-resistant slow units first and calls in the powerful fast units only as the demand grows — a tidy ordering known as the size principle.
This biology explains a lot of what clinicians see. Fast-twitch fibres are the first and the hardest hit by ageing, illness, and disuse, which is why deconditioned and older patients lose power and speed — the ability to catch themselves from a fall — even faster than they lose endurance. It also explains why strength training and endurance training produce different results: you can shift fibre properties somewhat with training, but you largely cannot convert a person wholesale from one fibre profile to the other.
An 80-year-old can still walk a kilometre but cannot rise quickly from a low chair or step off a curb without wobbling. Her endurance (slow-twitch) is fairly preserved, but her fast-twitch fibres — the source of quick, powerful movement — have faded with age.
Fast-twitch power fades first with age and disuse, which is why fall-saving quickness is often lost before endurance.
The popular idea that you are 'born a sprinter or an endurance type' is only partly true: genetics set a tendency, but training, age, and disuse meaningfully reshape fibre properties across a lifetime.