Asymmetry is not a diagnosis

Left-right differences on a force plate get treated as injuries waiting to happen, and the evidence is thinner than screening companies admit.

Ten per cent. That is the figure that gets quoted in return-to-play meetings, and I have never been able to find out who decided on it. If the injured limb produces within ten per cent of the healthy one, the athlete is cleared. If it does not, the athlete does another block of rehab.

The number is arbitrary and, worse, it is applied to athletes who were asymmetric before they ever got hurt.

Sprinters are not symmetrical machines. They turn on bends, they push off blocks with a preferred front leg, they have spent fifteen years developing a takeoff side if they ever did any jumping. Measure a healthy squad on a dual plate and you will find people sitting at eight, twelve, fifteen per cent difference who have never missed a session. Their asymmetry is a signature, not a fault. Chasing it back towards zero means deliberately weakening the strong side or drilling the athlete out of a movement pattern that has been working.

What actually carries information is change. An athlete who has been at four per cent for two seasons and reads eleven this month is telling you something. The same athlete arriving at eleven and having always been there is telling you nothing except that legs differ. Without a healthy baseline the whole screen is a coin flip with a spreadsheet attached, and most programmes do not have that baseline because they only start testing after somebody gets hurt.

I would also push back on how the test is chosen. Bilateral squat and countermovement jump numbers are convenient and slow. Injuries happen in fast, single-leg, horizontally loaded contacts, which is a completely different demand. An athlete can pass a jump symmetry screen comfortably and still be unable to tolerate the moment in late swing when the hamstring is doing eccentric work at length. If that is where the tissue failed, that is where the test should live.

None of this means stop measuring. It means stop reporting the difference as though it were a verdict.

The most useful thing I have seen a physio do with a force plate was ignore the headline entirely and look at the shape of the curve, because the athlete's injured side was producing the same total force by a different route. Same number. Different leg. That is the one you keep back another fortnight.