Trunk Bend

Why this matters

When the foot strikes and the runner loads the stance leg, the contralateral side of the pelvis is unsupported and tends to drop (Pelvic Drop). The trunk has a couple of options to keep the body's centre of mass over the foot:

• Lean the trunk toward the stance leg (small, controlled trunk bend). Brings the torso closer to the foot, reduces the moment arm the gluteus medius has to overcome, and is the natural, efficient compensation when hip strength is adequate.
• Don't lean — relies entirely on the glute medius to hold the pelvis level. Works for well-trained runners.

Either pattern can be efficient; both are normal up to a point. The pathological pattern is a large trunk bend (>8°) paired with a large pelvic drop (>5°). That combination is the Trendelenburg gait signature: the runner is leaning excessively because the glute medius can't hold the pelvis level on its own — bigger drop forces bigger compensation.

Reading the value with pelvic drop

Trunk bend	Pelvic drop	Reading
≤ 3°	≤ 3°	Strong, neutral pattern. Glute medius doing its job.
3°–6°	≤ 3°	Small natural compensation. Common in fast runners using forward lean.
≤ 3°	4°–6°	Pelvis drops but trunk holds — borderline. Hip strength is adequate but the brake is the glute medius alone.
> 6°	> 5°	Trendelenburg pattern. Glute medius can't hold the pelvis level; the trunk overcompensates.
> 10°	> 8°	Severe — recommend strength/loading assessment.

What good looks like

Posterior view, captured at the same frame as pelvic drop (mid-stance of the stance leg). The neck and mid-pelvis form a near-vertical line. A small lateral lean toward the stance leg is acceptable.

Common faults

• Excessive lateral lean (>8°) — the trunk drops sharply toward the stance side. Usually pairs with weak gluteus medius and contralateral pelvic drop.
• Asymmetric lean (L vs R differs by >3°) — typically a unilateral hip stability issue.
• Forward lean confused for lateral bend — Trunk Bend measures lateral lean. The posterior camera makes forward lean look small; if you have side video, cross-check with the lateral view.

Coaching cues

• Strength work — single-leg bridges, side planks with hip abduction, monster walks. The compensation patterns mostly trace back to glute medius capacity.
• "Run tall" — gross cue that helps runners notice their trunk lean. Pairs well with cadence-up cues that shorten ground contact time.

Related measurements

• Pelvic Drop — the partner measurement. Trunk Bend is the trunk's response to drop; read them together.
• Midstance Knee Flexion — the third leg of the same compensation pattern; collapsing knee + dropping pelvis + leaning trunk all point to the same gluteus medius weakness.