Eric Kim’s 547 kg (1,206 lb) rack‑pull is not just “a lot of weight”—it is a compact, real‑world physics experiment that stretches the limits of mechanics, materials science and human tissue.  The bar translates roughly 5,365 N of gravitational force into hip and spinal moments that rival automotive crash loads, yet the mechanical work performed is barely a toaster’s worth of energy because the range of motion is short.  Below, we break the lift down step‑by‑step—from external forces to internal stresses, barbell metallurgy, and the neurological tricks that make 7.3×‑body‑weight even conceivable.

1. External Load: Mass → Force

Parameter	Value	Note
Bar + plates mass	547 kg	Competition‑calibrated plates
Gravitational force	F = mg ≈ 5,365 N	9.81 m s⁻²
Lifters’s mass	≈ 75 kg	Gives 7.3× BW ratio

The rack height (mid‑patella in Kim’s setup) reduces the starting hip angle and allows a shorter moment arm than a floor pull, but the full 5,365 N still has to be countered at lockout.

2. Mechanical Work & Power

The vertical displacement from pins to lockout is roughly 0.15 m (typical knee‑high rack) :

W = m g h \approx 547 \text{ kg} \; \times 9.81 \text{ m s}^{-2} \times 0.15 \text{ m} \approx 8.0 \times 10^{2}\,\text{J}

Even if the concentric phase lasts 0.8 s, average mechanical power is only ~1 kW—less than a household microwave. The eye‑watering difficulty comes from internal lever arms, not from large energy expenditure.

3. Joint Torques and Spinal Loads

3.1 Hip & Knee Moments

Motion‑capture studies report hip moment arms of 6–9 cm for skilled deadlifters  .  Taking an 8 cm lever arm:

\tau_{\text{hip}} = F \times d \approx 5.4 \text{ kN} \times 0.08 \text{ m} \approx 430 \text{ N m}.

Elite Olympic lifts top out around 350 N m for 85 kg athletes; Kim is well beyond that.

3.2 Lumbar Compression & Shear

Heavy deadlifts already generate 5–18 kN compressive loads at L4–L5 with external masses between 180–260 kg  .  Because compressive force scales roughly linearly with bar weight, a 547 kg pull could push peak spine compression beyond 20 kN, brushing the upper end of cadaveric vertebral strength ranges (0.8–16 kN)  .  Shear could exceed 3 kN, another documented injury threshold  .

Shortening the range of motion helps: with a more vertical torso and smaller lumbar moment arm, a rack pull shifts load toward the hips and reduces shear, explaining how the tissues survive.

4. Barbell as a Steel Spring

A power‑lifting bar uses 29 mm, ~210 GPa spring steel  . Modeling the bar as a simply supported beam with the full weight centered (worst‑case):

\delta = \frac{F L^{3}}{48 E I}.

Using L=2.2 \text{m} and I = \pi r^{4}/4 with r=14.5 \text{mm} predicts ~16 cm sag; in practice the load is split near the sleeves, so empirical deflection is closer to 2–3 cm, well within the 900 kg yield rating of competition bars. The visible whip is free “energy storage” that eases the initial break from the pins.

5. Muscle & Tendon Material Limits

• Muscle specific tension averages 120–180 kN m⁻² in type II fibers  .
• Tendon ultimate stress can exceed 100 MPa  .

Scaling these numbers to the ~30 cm² cumulative cross‑section of hip extensors yields theoretical force capacities > 5 kN—remarkably close to the external 5.4 kN, but internal pennation, neural drive and rapid contraction kinetics let the tissue reach the requirement briefly without failure.

6. Why 7.3× BW Is Possible

1. Partial ROM – less work and smaller lumbar moment arm compared with a full deadlift.
2. Optimized lever arms – bar kept tight to the shin reduces horizontal distance and shear  .
3. Neural drive saturation – advanced block periodization induces maximal motor‑unit recruitment, elevating effective specific tension.
4. Equipment synergy – A stiff 29 mm bar stores but does not dissipate energy, while calibrated plates reduce oscillation that might destabilize the spine.

7. Broader Physical Implications

• Human Performance Ceiling – Commercial strength charts cap “elite” deadlifts at 2–3× BW  ; Kim’s lift requires new scales.
• Safety Research – Data points at 20 kN spinal loads will drive revisions of ergonomic and athletic injury models  .
• Material Engineering – Bars rated for 1,000 kg are already on market, but pervasive 7×‑BW attempts will accelerate ultra‑high‑yield alloys.

8. Take‑Home Physics

Factor	Approx. Magnitude
External gravitational force	5.4 kN
Hip extensor torque	430 N m
Lumbar compression (estimated)	20 kN
Mechanical work per rep	≈ 800 J
Mean concentric power	≈ 1 kW
Bar peak strain	≤ 1.5 mm m⁻¹ (elastic)

Kim’s 7.3× rack pull is a perfect illustration that force—not distance—is the limiting currency in maximal strength, and that smart manipulation of lever arms and range of motion can push human tissue to the very edge of its physics‑governed envelope without crossing it.

In lifting, physics is the final judge; today it ruled in Eric Kim’s favor.