Horizontal force is a phase, not a quality

Sprint testing has fallen in love with the horizontal force profile, and treats as a trait what is really a moment in a race.

The profiling craze gave coaches a tidy vocabulary. This athlete is force-dominant. That one is velocity-dominant. Train the missing end. It is neat, it fits on a slide, and it quietly smuggles in an assumption that does not hold, which is that horizontal force production is a stable property of a person.

It is not. It is what a body does in a particular part of a run, at a particular speed, at a particular body angle.

Think about why the horizontal share collapses as the sprint develops. Early on, the athlete is leaning hard, the foot lands well behind the centre of mass, and a large fraction of the total impulse points forward. By top speed, the athlete is upright, contact is short, and the leg is mostly resisting gravity while switching direction. The physical capacity did not change between second one and second five. The geometry did. Horizontal force is therefore best understood as the output of a posture and a speed, not as a bank account the athlete carries around.

Which matters because of what people do with the profile. An athlete flagged as velocity-deficient gets sent off to do more flying runs; an athlete flagged as force-deficient gets sled pushes and hills. Sometimes that is the right prescription. Often the profile is really telling you the athlete has a technical fault in acceleration posture and no amount of resisted work will fix a body that stands up too early.

I would also flag a measurement problem that gets waved through. Most of these profiles come from a radar or a timing-gate model fitted to a single maximal run. One run. On the day. With a start that may or may not have been representative, and a model whose assumptions about drag were built for somebody else's body shape. The output has three decimal places and roughly one meaningful digit.

Use it as a conversation starter and it earns its keep.

What I would not do is put an athlete into a category and train them there for a season, because the categories move. Test the same athlete in November and April and the profile shifts more than the athlete did, which should tell you something about how much of it was the athlete in the first place.