15 Apr 2026
A frame-by-frame animated free-body diagram of one full overhead dumbbell triceps extension repetition — from deep behind the head through full extension overhead and back down. Each frame pairs a force diagram with a torque/capacity chart and coaching text. The animation uses finer 5° steps through the hypertrophy zone (20–90°) and 10° steps through the shortened range (90–150°), giving more dwell time where it matters most.
▶ Open the AnimationThe animation cycles through four contraction phases in order:
| Control | Action |
|---|---|
| Play / Pause button | Auto-advance through all frames |
| « / » | Jump to first / last frame |
| ‹ / › | Step one frame backward / forward |
| Slider | Drag to scrub to any point in the rep |
| Speed dropdown | 0.5×, 1×, 1.5×, 2×, or 4× playback |
| Space | Play / pause |
| ← / → | Step backward / forward |
| Home / End | Jump to first / last frame |
Below the slider, a frame label shows the current frame number and elbow angle (e.g., "Frame 08/50 — 40.0°"), and a phase badge displays the current contraction phase:
The animation uses an elbow extension angle where 0° = fully extended (straight arm) and 180° = fully flexed. This is the supplement of the conventional anatomical flexion angle. To convert: flexion angle ≈ 180° − displayed angle. A goniometer reading of 160° flexion corresponds to 20° in this animation.
The upper panel is a 2D schematic of the elbow joint, upper arm, forearm, and the forces acting on them.
| Element | Appearance | What It Is |
|---|---|---|
| Vertical bar | Dark gold | Upper arm — fixed vertical (arm is raised overhead) |
| Rotating segment | Gold | Forearm holding the dumbbell — the lever that rotates about the elbow |
| Circle at the junction | Dark with white center | Elbow joint (pivot point) |
| Arrow | Color | Label | Meaning |
|---|---|---|---|
| Straight down from forearm tip | Red | Fload | Gravity pulling the dumbbell downward — always vertical regardless of forearm angle |
| Near elbow, along tendon line | Green | Ftriceps | Triceps muscle force — pulls to extend the forearm, inserting at the olecranon process ~2 cm from the elbow pivot (~6% of upper arm length) |
| Element | Color | Meaning |
|---|---|---|
| Dashed line from pivot to the gravity line | Cyan | The moment arm (d⊥) — the perpendicular distance from the elbow pivot to the dumbbell's line of gravity. At 90° (forearm horizontal) it equals the full forearm length (maximum torque). Near lockout it approaches zero. |
| Numeric readout below the pivot | Teal box | The current moment arm value |
| Readout | Location | What It Tells You |
|---|---|---|
| Elbow ≈ XX° | Top-left | Current elbow extension angle. Low values (20–60°) = deep behind head; high values (>120°) = near lockout. |
| Torque ~ 0.XX (∝ sin θ) | Below angle | Rotational load the triceps must resist, normalized 0–1. At 90° this is 1.00. |
| F_triceps ≈ X N (Y kg) | Below torque | Computed internal triceps force at the current angle — shows how the ~17:1 mechanical disadvantage amplifies a light dumbbell into enormous internal muscle forces. |
| Phase badge | Top-right | Current contraction phase (CONCENTRIC / ECCENTRIC / ISOMETRIC) |
| STRETCHED ZONE (20–60°) | Bottom-center | Appears when the forearm is deep behind the head — long head maximally stretched across both joints |
| HIGH TENSION ZONE (20–90°) | Bottom-center | Appears throughout the hypertrophy zone when outside the stretched sub-zone |
| HOLD | On the forearm | Appears during isometric phases |
Color legend (printed at the bottom of the diagram): upper arm (dark) · forearm (gold) · green = triceps pull · red = load · cyan = moment arm · blue arc = angle
Two curves plotted against elbow angle:
| Curve | Color | What It Represents |
|---|---|---|
| Torque Demand | Red | How much torque gravity imposes at each angle — follows sin(θ), peaking at 90° and dropping toward zero at lockout |
| Force-Length Capacity | Teal | The triceps' ability to produce force at each muscle length — peaks around 95° and drops off at extremes |
A vertical green marker tracks the current frame's angle. The background is divided into three zones:
Phase-specific biomechanical coaching that changes with every frame:
Torque from a dumbbell in the overhead position is proportional to sin(θ). Between 20° and 90° of elbow extension (forearm behind the head through horizontal), sin(θ) ranges from 0.34 to 1.00 — the triceps is under substantial to maximum mechanical tension. The 20–60° sub-zone is especially valuable because the long head is also under maximum stretch.
The overhead position is unique among triceps exercises. The long head crosses both the shoulder and elbow. When the arm is overhead and the elbow is deeply flexed, the long head is stretched at both joints simultaneously. No other triceps exercise — pushdowns, dips, kickbacks, close-grip bench — achieves this. Recent research shows lengthened-position training produces greater hypertrophy.
At lockout (~140–150°), the forearm is nearly vertical and the moment arm approaches zero. There is almost no load on the triceps here. Don't chase full lockout — stop just before full extension to keep continuous tension. The growth stimulus lives in the bottom half of the movement.
The triceps inserts roughly 2 cm from the elbow joint, while the dumbbell acts at ~35 cm. This ~17:1 mechanical disadvantage means a 10 kg dumbbell can demand 175 kg of internal triceps force at 90°. This is why overhead extensions are so effective even with moderate weights.
| Phase | Time | Rationale |
|---|---|---|
| Concentric (extend) | 1–2 s | Controlled speed, recruit motor units |
| Isometric (top hold) | 0.5–1 s | Brief contraction — low tension at lockout |
| Eccentric (lower) | 3–4 s | Maximize tension and stretch-mediated growth |
| Bottom stretch | 1–2 s | Hold the stretch for long head stimulus |
Target ecc:con ratio ≥ 2.0 (e.g., 1.5 s up, 3–4 s down).