Spine Positioning — "S" Shape, Never "C"

Positioning and stabilizing the spine requires holding it in a semi-isometric contraction, to keep it in an "S" shape. Never allow a "C"-shape spine during exercise.

This principle reappears in the five-step evaluation of proper resistance exercise, under body positioning: proper spinal alignment is essential — the "S" shape is opposed to the "C." Maintaining the spine's natural curves recruits the erector spinae to protect the discs and vertebrae against compressive and shearing loads.

• Hold the spine in a semi-isometric contraction to keep its natural "S" shape.
• Never allow a "C"-shape spine during exercise.
• Proper spinal alignment is essential to the body-positioning step of safe exercise evaluation.

This "S, never C" phrasing is a simplified teaching cue from the source text, not a literal, universal rule — flagged here so a coach doesn't over-apply it as absolute. What still holds broadly: under a real external load (a barbell, a loaded carry, anything adding meaningful compressive/shear force to the spine), holding the neutral "S" curve is the safer default, and it's the right cue to give a novice lifter by default.

What the blanket phrasing misses: spinal flexion tolerance is load- and task-dependent, not a fixed binary. Some controlled, low-load flexion (a bodyweight cat-cow, a deep unloaded stretch, rounding at the bottom of an unloaded sit-up) is a normal, healthy range-of-motion demand on the spine, not an injury waiting to happen — and some individuals (e.g. certain deadlift/kettlebell-swing populations under coach supervision) train with a degree of controlled flexion intentionally. The real risk this cue is protecting against is loaded flexion under fatigue or heavy resistance, not spinal flexion in general.

Coaching takeaway: use "S, never C" as the default cue for loaded lifting and for teaching a new client's first rep pattern — it's simple, safe, and correct in that context. Don't extend it to every unloaded movement or present it to an athlete as a universal anatomical law; that's a defensible but stronger claim than the evidence supports.