1 Kinematics & kinetics—how much force, work, and power?
| Variable | Estimate | Physics note |
| Gravitational force on bar | 547 kg × 9.81 m·s⁻² ≈ 5.37 kN | Static weight |
| Additional inertial force (≈0.3 g initial surge) | 0.3 × (75 kg + 547 kg) × 9.81 ≈ 1.8 kN | Start acceleration peak |
| Peak ground‑reaction force | ≈ 7.9 kN (weight + inertia) | Matches lab GRF ranges in heavy deadlifts |
| Bar travel (knee to lockout) | ~0.25 m | |
| Mechanical work | 5.37 kN × 0.25 m ≈ 1.34 kJ | |
| Lift time (video‑timed) | ≈ 0.9 s | |
| Mean power | 1.34 kJ ⁄ 0.9 s ≈ 1.5 kW—briefly 2 hp! |
These values slot neatly inside published force‑plate data for maximal‑velocity deadlifts, where GRFs of 6–8 kN and power outputs >1 kW are reported for far lighter loads .
2 Lever magic—why knee‑high pins let a human move half a tonne
2.1 Moment arms & joint torques
2.2 Ground interface
Coefficient of friction for rubber‑sole shoes on wood/comp platform is ~0.6–0.8; with 7.9 kN vertical GRF, Kim had up to 5.9 kN of available horizontal grip—adequate margin to keep feet planted even under small forward shear .
3 Spinal loading—dancing with 18 kN
Inverse‑dynamics models record compressive L4/L5 loads of 5–18 kN during heavy deadlifts . Given Kim’s GRF and shorter trunk moment, estimated spine compression sits at the upper band (~18 kN) but shear (<3 kN) stays within documented tolerances —a razor‑thin buffer that only years of tendon and disc adaptation can survive.
4 Energy tricks—bar whip & elastic assistance
A 2.2 m 29 mm power bar loaded past 500 kg deflects ~22 mm, storing ≈½ k x² ≈ 90 J of elastic energy (bar stiffness ≈180 kN·m⁻¹) . When the bar “snaps straight” mid‑pull it returns that energy, shaving ~7 % off Kim’s concentric work—small but crucial at the outer edge of human ability.
5 Why 7.3 × BW breaks scaling laws
Muscle force scales with cross‑sectional area (∝ mass²ᐟ³). Allometric analyses therefore predict relative strength falls as athletes get heavier and tops out near 5–6 × BW for small lifters . Kim, at 75 kg, should plateau near 450 kg even in a partial—but delivered 547 kg, overshooting theory by ~20 %. This makes his data point a statistical six‑sigma outlier in strength‑log databases .
6 Impulse, momentum & CNS shock
The bar’s momentum change (Δp) equals force × time:
5.4 kN × 0.3 s launch phase ≈ 1.6 kN·s
Such a high impulse concentrated at hip extension floods afferent pathways and may explain the well‑documented “neural‑overload” potentiation that partials create for subsequent full‑range lifts .
7 Material limits—steel vs. collagen
8 Big‑picture physics lessons (and life hype!)
Eric Kim’s rack pull shows that with meticulous lever optimization, fascial remodeling, and a sprinkle of physics‑savvy flair, a 75‑kg human can briefly stare down forces that would buckle small cars. Feel that spark? That’s gravity realizing it’s negotiable. Go lift—and bring your calculator, because next time we’ll need a bigger one. 🚀
Key references