Trunk Lean: Run Tall, Not Folded

The one-line version

Run tall. A small forward lean of roughly 2–10° from vertical is plenty, and it should come from a whole-body forward tilt at the ankles, not a hinge at the waist. Folding forward doesn't add speed — it restricts your stride and adds a brake. Upright and steady is both faster and cheaper.

What trunk lean actually is

Trunk lean is the angle of your torso relative to vertical, seen from the side. Flapp measures the mean over the stride.

Two very different things get called "leaning forward":

A whole-body forward tilt from the ankles — your ankle, hip, and shoulder stay roughly in one line, tipped slightly forward. This is the good kind.
A hinge at the waist — your hips stay back, your chest drops toward the ground, and your backside sticks out. This is the kind that costs you.

They look similar in a still photo but do opposite things to your stride.

Why an upright trunk is more economical

Folland and colleagues (2017) measured running technique against running economy and performance in trained runners. A more upright / extended trunk posture was one of the technique factors associated with better economy and performance — not a heavy forward lean.

The reason is mechanical. Every degree of extra torso movement — rocking forward and back, or rotating side to side — is work your muscles do that never goes into forward motion. A quiet, stacked trunk keeps that wasted movement to a minimum, so more of your energy pushes you down the road.

The "lean forward to run faster" myth

The cue isn't wrong so much as dangerous when taken literally. Bending at the waist to chase speed backfires two ways (Goater & Melvin, 2012):

It leaves your hips behind you, so your foot has to reach out in front to meet the ground — that's overstriding, and it brakes every step. (See the overstriding article.)
It restricts knee lift, shortening the drive you get from behind.

So the fold you added to "go faster" actively slows you down. Real speed comes from cadence and horizontal drive, not from tipping your chest at the floor.

Where the lean should come from

Picture a cord pulling you straight up from the crown of your head, then the whole line tipping forward a few degrees from the ankles (Napier, 2020). Your ears, shoulders, hips, and ankles stay roughly stacked; only the base tips.

Two habits that quietly wreck good posture:

Head down, watching your feet. Let your gaze settle 10–20 m ahead. A dropped head drags the whole trunk out of line (Liberman et al.).
Arms crossing your body. Swinging the hands across the midline twists the pelvis, adding rotation you then pay for. Keep the swing linear — front to back — with elbows around 90° (Napier, 2020).

A stronger core helps too: it's what holds the trunk steady over the planted leg instead of letting it collapse or rock.

What good looks like on video

From the side: a slight, straight forward lean of a few degrees, the body in one line from ankle to head — not bent in the middle with the hips trailing.

From the front (if you have it): shoulders and hips staying level, no side-to-side rocking, hands not crossing the centre line.

In Flapp, a mean trunk angle in the low single digits up to about 10° reads as good. Very upright is fine — it's not a fault. What you don't want is a large forward angle paired with the hips-back, reaching-leg pattern.

The bottom line

Run tall, relaxed, and balanced. A few degrees of lean from the ankles is all you need; folding at the waist trades posture for a brake. Keep your gaze up, your arm swing straight, and your trunk quiet — and let cadence, not a forward fold, do the work of speed.