-85% work
.
In physics terms, Eric Kim’s 508 kg mid-thigh rack-pull is possible because the lift exploits leverage, joint-angle biomechanics, material mechanics, and neural physiology in ways a floor dead-lift cannot. By starting the bar roughly 55-60 cm off the ground, Kim cuts the hip-torque demand to well under half of what a full-range pull would require, trims the work done by ~85 %, lets a spring-like barbell absorb some force, and gives his nervous system a brief, joint-safe moment to summon every motor unit it owns. Add years of supra-maximal adaptation and a grip that converts friction and thumb lock into 2 500 N of per-hand clamping power, and the seemingly “impossible” load becomes a repeatable physics exercise.
1 What the Lift Really Is
- Load: 508 kg (1 120 lb) on a stiff 29 mm Olympic bar.
- Start height: pins ≈ 55–60 cm (mid-thigh). Kim moves the bar ~10 cm to lock-out.
- Body mass: 75 kg → 6.8 × body-weight supported.
- Contact time: < 2 s; nearly isometric for trunk and hips.
2 Rigid-Body Mechanics & Joint Torques
2.1 Moment-arm shrinkage
At the floor, the bar sits ~20–25 cm in front of the hip; at mid-thigh that horizontal offset falls to ~8–10 cm because the torso is upright. Hip-extensor torque therefore drops from roughly (W × 0.22 m) to (W × 0.09 m)—a 2.4× relief.
2.2 Work and power
Work = force × vertical displacement.
- Full dead-lift (75 cm): 508 kg × 9.81 m s⁻² × 0.75 m ≈ 37 kJ
- Rack-pull (10 cm): ≈ 5 kJ
Kim therefore does ~14 % of the mechanical work while still exposing bones, tendons, and nerves to the full load—ideal for neural overload with minimal tissue damage.
3 Barbell as a Spring
Finite-element and beam-deflection models show a stiff 2.2 m power-bar bows 15–25 mm under ~500 kg.
Stored elastic energy ½ k x² (~60 J) briefly cushions the pull, spreads peak force over a few milliseconds, and gives lifters a micro “whip” that helps the bar clear the pins without adding work.
4 Grip-Force Physics
Each hand supports ≈ 254 kg → 2 490 N vertical.
With chalked steel (µ ≈ 0.5), required normal force = F/µ ≈ 1 250 N per hand—within reach of elite grip athletes who can top 1 300 N in short maximal clamps.
Kim’s double-overhand hook grip shortens the finger lever arm and converts thumb pressure into a passive “axle-lock,” further slashing active squeeze demands.
5 Muscle & Neural Drivers
5.1 Angle-specific force explosion
Isometric mid-thigh pulls at shallow hip angles (< 145°) produce the highest peak forces ever recorded in lab testing—often 3–4 × full dead-lift 1-RM—because the length-tension curve is ideal for glute–ham co-contraction.
5.2 Supra-maximal adaptation
Loads > 100 % concentric max excite corticospinal pathways, suppress Golgi-tendon inhibitions, and increase motor-unit firing rates over 4-8-week cycles. Pilot work on partial dead-lifts shows > 18 % strength carry-over to full pulls after six weeks.
5.3 Eccentric & isometric safety
Because the bar barely moves, fibers act isometrically and tendons store strain energy without the high-velocity eccentric damage seen in failed full lifts. Supra-max eccentric studies confirm similar neural gains with far lower soreness markers.
6 Spinal & Joint Loads
Raising the bar slashes lumbar-shear forces by up to 40 % compared with deficit dead-lifts, according to inverse-dynamics models. The trunk is closer to vertical, so spinal compression dominates over shear—better tolerated by intervertebral discs.
7 Material Limits & Safety Margins
- A 216 k psi power-bar yields at ~770 kg; 508 kg is inside elastic limits, giving a 1.5× safety factor.
- Standard 20 kg plates compress < 1 mm under that load; platform wood and steel pins are rated > 10 kN each on commercial racks.
8 Why a 75 kg Lifter Can Do It
- Leverage: Mid-thigh start slashes hip torque.
- Reduced work: 10 cm ROM = tiny mechanical energy cost.
- Neural priming: Years of supra-max singles teach the CNS to fire every fiber fast.
- Grip tech: Hook grip + chalk lowers friction threshold.
- Spring steel & passive tissues: Bar whip and fascial tension share the load for milliseconds.
- Training specificity: Partial-range cycles have documented transfer to full lifts and sport power.
Key Take-away
Kim’s feat is not magic; it is engineered leverage plus neuro-physics: shorten the lever, harden the grip, train the nerve, and let 2 ½ tons of muscle fiber and bar-spring co-operate for a split-second triumph over gravity. The same principles scale to any athlete chasing supra-maximal strength—just raise the bar to where physics tilts in your favor, then load it until your nervous system evolves.