You Run on One Leg at a Time.

An athlete could back squat a hundred and twenty kilos. Genuinely strong, no question about it.

I asked him to do a split squat with twenty kilos in each hand. His left side was fine. His right knee travelled inward on every repetition and he could manage four before his form went.

He had been squatting for three years and had no idea, because for three years his left leg had been quietly doing more of the work and the bar had never told him.

A barbell across your back averages your two legs. Running does not average anything.

Why Bilateral Work Hides Things

When both feet are on the floor, your body will find the easiest path to moving the weight. If one side is stronger or better coordinated, it takes more. You cannot feel it and the bar does not care, because the bar only knows whether it went up.

Force plate work has shown asymmetries of fifteen or twenty percent in athletes who report feeling perfectly balanced. That is not a rare pathology, it is common, and most of it goes undetected for years.

Running exposes it immediately, because running is a series of single leg landings. Every stride is one leg absorbing your bodyweight several times over, with nothing to share the load with. There is no averaging available.

What the Asymmetry Actually Costs

Not much at kilometre five. A great deal at kilometre thirty.

The weaker side fatigues first. As it fatigues, the pelvis starts dropping on that side, the knee falls inward, and the whole load path through that leg changes. That is the mechanism behind IT band pain, patellofemoral pain and a good proportion of stress reactions, and it explains why those injuries almost always appear late in long runs rather than at the start.

It also costs you efficiency, because a leg that is compensating is doing more work than it needs to.

And critically, it is progressive. The stronger side takes more, gets stronger still, and the gap widens. Bilateral training does not close it. It usually makes it worse.

How to Find Yours

Three tests, no equipment, ten minutes.

Single leg calf raise to failure, each side, through full range. A difference of more than about ten percent is worth acting on, and this is the one that catches the most people.

Single leg squat to a box, watching the knee from the front. If it tracks inward on one side and not the other, you have found a hip control difference.

Single leg bridge. If one hamstring cramps and the other does not, the glute on the cramping side is not contributing.

Film it rather than trusting how it feels. Almost everybody reports feeling symmetrical, including the ones with the largest differences.

What I Programme

Rear foot elevated split squat, which I use more than any other lower body exercise for endurance athletes. It loads one leg heavily while the back foot provides just enough balance that you can actually push, so it is a strength exercise rather than a balance exercise.

Single leg Romanian deadlift, which is a hinge and a balance challenge at once and is brutally honest about hip control.

Step ups, from a height that puts the working thigh at roughly parallel, driving through the heel.

And single leg calf raises, always, because the calf asymmetry is the most common and the most consequential for running.

The Rule for Correcting It

Start with the weak side, and match the strong side to it.

If your left can do eight and your right can do twelve, you do eight on both. Not twelve on the right and eight on the left, which maintains the gap you are trying to close.

That feels wasteful to competitive people and it is the whole intervention. Over eight to twelve weeks the weak side catches up, and then you progress both together.

I do not remove bilateral work entirely, because heavy hinging and squatting build force in a way unilateral work cannot match at the same loads. But roughly half the lower body volume goes single leg for anybody with an asymmetry, and that is most people.

The Bottom Line

Do the three tests. Film them. If there is more than about ten percent between sides, that is your limiter and it is not your engine.

Then work one leg at a time, start with the weak side and match the strong one to it, and accept that the numbers will look worse on paper for a couple of months while something real gets fixed.