Trunk Forward Lean

Why this matters

A slight forward lean of the whole body lets gravity assist propulsion and helps the foot land closer under the centre of mass rather than out in front. The key word is whole body: the efficient lean comes from the ankles, keeping a straight line from ankle through hip to shoulder — not a bend at the waist. See Posture Hinge for the ankle-vs-hip distinction.

• Too upright or leaning back tends to land the foot ahead of the body and brake on every step — often paired with overstriding (see Stride Angle) and a heavier heel strike.
• Optimal (4°–8°) keeps momentum moving forward without over-reaching.
• Over-leaning (>12°) loads the lower back and shifts braking to the quadriceps. A big forward number is often really a hip-hinge rather than a true ankle lean.

Forward lean is measured here as trunk lean only, following the side-view forward-lean method: draw a line along the trunk and another vertical to the ground; the angle between them is the lean.

What good looks like

Lateral view, captured at initial contact. The trunk sits a few degrees ahead of vertical, and the whole body forms a near-straight line from the ankle up through the hip to the shoulder.

Common faults

• Upright / backward trunk — usually travels with overstriding and a braking footstrike.
• Excessive forward lean (>12°) — loads the lower back; check whether it is a true ankle lean or a bend at the hips.
• Bending at the hips — the trunk angle can look "forward" while the runner is actually folding at the waist. Cross-check Posture Hinge.

Coaching cues

• "Lean from the ankles, not the waist" — the single most useful cue for forward lean.
• Posterior-chain and core strength — holds posture as fatigue sets in late in a run.
• Hip-flexor mobility — tight hip flexors and anterior pelvic tilt push runners into a hip-hinge instead of an ankle lean.

Related measurements

• Posture Hinge — tells you where the lean comes from (ankles vs. hips).
• Stride Angle — an upright trunk and overstriding usually appear together.
• IC Knee Flexion — braking at footstrike shows up here too.