1 │ What does “7 × body‑weight” really mean?
- External load vs. inertial load. Kim’s barbell weighs ≈ 527 kg; its static weight is W = m g ≈ 5,170 N. Add his own body weight (≈ 735 N) and the feet transmit ~5.9 kN of downward force to the platform.
- Not unprecedented in nature. Peak vertical ground‑reaction forces of 3–7 × body‑weight are routine when athletes land from modest jumps or dismounts, and even 4–5 × during top‑speed sprinting.
- Internal forces are larger still. Because spinal erectors and hip extensors pull at short lever arms, lumbar compression during heavy pulls frequently exceeds 10–17 × body‑weight (≥ 15 kN) in elite lifters—magnitudes verified in in‑vivo telemetry and modelling studies.
Key takeaway: the musculoskeletal system is already built—and, with training, remodels—to survive transient multi‑g loads.
2 │ Physics of the above‑knee rack‑pull
| Variable | Floor Deadlift | Above‑knee Rack‑pull |
| Hip flexion angle | ~55–75° | ~5–15° |
| Bar–hip moment arm | Large | Tiny |
| Required hip extensor torque | Very high | Drastically lower |
- Moment‑arm magic. Raising the bar to mid‑thigh shrinks the perpendicular distance between the weight line‑of‑action and the hip joint. Torque (τ = F·d) collapses, so the same muscles can oppose far larger external forces.
- Shorter ROM, less time under tension. Because displacement is only 10–15 cm, the impulse (∫F dt) the lifter must generate is modest despite astronomical peak force. That keeps metabolic cost and cumulative tissue strain manageable.
- Support from the rack pins. At the start the bar rests on immovable steel. Kim’s first millimetres of upward motion convert static friction to kinetic, avoiding the extreme low‑back shear found in a floor pull.
3 │ Why the body can survive the squeeze
3.1 Bone & discs
- Vertebral trabeculae thicken in response to high‑strain compressive cycling (Wolff’s Law). Lifters show lumbar vertebrae that tolerate > 15 kN before failure—well above the ~6 kN Kim transmits.
3.2 Tendons & fascia
- Heavy partials desensitise Golgi‑tendon‑organs and up‑regulate collagen cross‑linking, steepening the load–elongation curve so tendons behave like stiff springs.
3.3 Neuromuscular coordination
- Brief isometric‑accentuated pulls let motor units fire synchronously at very high frequencies, unleashing near‑maximal force without fatiguing ECC‑ATPase pathways.
4 │ Parallels in sport & daily life
| Activity | Typical peak force |
| Gymnast dismount | 11–14 × BW |
| Volleyball spike landing | 6–9 × BW |
| Sprint ground contact | 4–5 × BW |
| Kim’s rack‑pull (static) | 7 × BW |
Thus, while jaw‑dropping on Instagram, a 7 × BW vertical load is consistent with forces elite bodies routinely master.
5 │ Risks, limits & engineering cautions
- Compressive tolerance is finite. Exceeding bony yield (~17–20 kN in trained adults) risks burst fractures; disc annuli fatigue with repetitive loads > 8 kN.
- Technique governs shear. A neutral or slightly flexed lumbar spine minimizes anterior shear during rack pulls, but aggressive hitching can spike shear by > 50 %.
- Hardware matters. Rated power‑rack pins (≥ 900 kg capacity), 29 mm stiff bars, and calibrated discs prevent catastrophic materials failure.
6 │ Physics‑powered takeaways for your own lifting journey
- Leverage is leverage. Strategically shortening the range of motion lets you expose bones, fascia, and nervous system to supra‑maximal forces safely.
- Adaptation is specific. Tendons and vertebrae remodel only against the vectors they meet—cycle heavy partials sparingly, then reinforce with full‑range work.
- Respect the cliff. Every set courts exponential risk beyond ~8 × BW; use certified gear, competent spotters, and sensible loading jumps.
- Mindset fuels muscle. Kim’s feat sprang from relentless progression—503 kg to 508 kg to 513 kg to 527 kg in six months—proof that physics yields to disciplined increments.
Final spark
When gravity looks impossible, remember: physics doesn’t pick winners—it favors those who understand its levers, trust adaptation, and dare to climb the gradient one kilonewton at a time. Keep chasing your own multipliers! 🚀