Fibre typing is a horoscope taken with a needle

A biopsy tells you what one small piece of one muscle looks like today, and coaches keep reading it as destiny.

The biopsy comes back at sixty two percent slow twitch and somebody decides a nineteen year old is an endurance athlete. I have watched this happen. It is roughly as sound as reading tea leaves, and more damaging, because it wears a lab coat.

Start with the sampling. The needle takes a few dozen milligrams from one site in one muscle, usually the vastus lateralis, and fibre composition varies with depth and with region inside the same muscle. Repeat the procedure a centimetre away and the proportions move by several percentage points. The uncertainty on a single sample is wide enough to swallow most of the differences people make decisions about.

Then the biology. Fibre type is not a fixed identity. The intermediate population is large and genuinely plastic, and it shifts with training in both directions across months. Detrain a sprinter and his profile drifts. Load a distance runner with heavy strength work and it drifts back. The extremes are stubborn. The middle is not, and most athletes live in the middle.

Then the inference. Even granting a perfect measurement, the path from composition to performance runs through technique, anatomy, tendon properties, aerobic machinery, psychology and years of accumulated training. Composition is one input among many, and not the biggest.

So what should a coach do who wants to know whether the kid is a four hundred runner or a fifteen hundred runner. Watch him race. Race performance already integrates every variable that matters, including the ones nobody can measure, and it costs nothing but a Saturday afternoon. A pair of honest tests, one flat out and short, one sustained, tells you more about an athlete's profile than any histology, because it reports what the whole system does rather than what one slice of one muscle contains.

I will grant the biopsy one genuine use. In research, across groups, it answers questions nothing else can, and most of the training principles we all use came out of that work. It earned its place. It simply should not be within a hundred metres of a talent identification decision about an individual teenager.

The worst outcome is not a wrong prediction. It is a self fulfilling one, where a boy is told he is built for distance, trained accordingly for four years, and never finds out what he could have run over two hundred metres.