Chapter 3: Limbs & Joints — Elbows, Knees, Wrists, Ankles

Created by Sarah Choi (prompt writer using ChatGPT)

Limbs & Joints — Elbows, Knees, Wrists, Ankles

Limbs are where posture turns into action. For character concept artists on both the concepting and production sides, knowing the skeletal mechanics, muscular overlays, and surface landmarks of the elbow, knee, wrist, and ankle prevents impossible poses, protects readability, and keeps costumes/gear from fighting animation. This article distills design‑critical anatomy, practical mobility rails, and packaging habits for indie and AAA pipelines.

1) Why Joints Matter to Concept Art

  • Readability: Clean hinge logic creates believable silhouettes at 5–20 m and survives motion blur.
  • Construction: Joints define where to split armor and route seams; they control where folds accumulate and where straps must float.
  • Gameplay: Start/hold/impact silhouettes depend on predictable joint ranges; misdesigned gear causes clipping and delays.
  • Production: Orthos and callouts with joint centers, capsule guides, and “no‑twist” zones save rig and vendor time.

2) Elbow (Humeroulnar + Humeroradial + Proximal Radioulnar)

Skeletal landmarks.

  • Medial & lateral epicondyles (inside/outside elbow bumps) frame the hinge; surface‑obvious in lean builds.
  • Olecranon (ulna tip) forms the posterior point; sits within the triceps tendon and moves slightly during flexion.
  • Ulna ridge (posterior forearm) and radius head (lateral forearm, near elbow) define rotation behavior.

Muscular overlays.

  • Triceps tendon inserts at olecranon (posterior). Biceps inserts on the radial tuberosity (anterior/medial); its line informs forearm rotation. Brachioradialis is the lateral forearm cable—great for silhouette and strap placement.

Mobility rails (approx.). Flexion ~140–150°; extension ~0° (–5° hyper). Pronation/supination ~80–90° each (radius rolls around ulna).

Design notes.

  • Avoid hard bracer plates across the cubital crease—split or float pieces to allow flexion.
  • Forearm devices (bracers, gauntlet UIs) should respect radius roll: place rotating dials on the ulnar side to keep them visually stable.
  • Sleeve patterns should angle with delto‑pec rhythm into the biceps; set a relief dart over the olecranon.

Camera emphasis. In FPP, show styloid bumps at the wrist and the brachioradialis cable; in TPP, silhouette the triceps mass and olecranon point.

3) Wrist (Radiocarpal + Midcarpal)

Skeletal landmarks.

  • Radial styloid (thumb side) and ulnar styloid (pinky side) create the asymmetrical cuff profile.
  • The carpal arch forms the palm base; metacarpal heads shape the knuckle line.

Muscular/Tendon reads. Extensor tendons create dorsal wrist cords; thenar (thumb) and hypothenar (pinky) pads define palm wedges.

Mobility rails. Flexion ~80°; extension ~70°; radial/ulnar deviation ~20°/~30°; coupling with forearm rotation is common.

Design notes.

  • Gauntlet cuffs should flare more ulnar‑side and notch around styloids; keep watch/bracelets on the radial side minimal to avoid clash with weapon grips.
  • For casting/tech gestures, enlarge glove seam geometry over thenar pad for clearer reads; place small emitters on flat dorsal planes between tendons.
  • Weapon handles should align with index knuckle axis; oversized pommels may force radial deviation—balance for reload animations.

Camera emphasis. FPP favors clear finger landmarks and glove seam logic; avoid patterns that shimmer under bloom on dorsal hand.

4) Knee (Tibiofemoral + Patellofemoral)

Skeletal landmarks.

  • Patella (kneecap) floats within the quadriceps tendon; glides superior during extension.
  • Femoral condyles form the hinge bulges; tibial plateau supports weight; the tibial crest is the sharp shin ridge.
  • Fibular head (lateral below knee) is a strap anchor caution zone.

Muscular overlays.

  • Quadriceps (rectus femoris crosses hip and knee) define anterior mass; hamstrings form posterior cords; IT band runs lateral thigh to tibia—reads as a tight strap.

Mobility rails. Flexion ~130–150°; extension 0°. Slight axial rotation (~10–15°) when flexed.

Design notes.

  • Knee armor should hinge around the patella with a floating cap; split greaves along the tibial crest for believable plate breaks.
  • Long coats/skirts require rear slits aligned with hamstring path; add gussets for kneel/crouch.
  • Holsters and thigh plates must clear the ASIS and not bite when sitting; keep lateral clear over fibular head.

Camera emphasis. In TPP/ISO, emphasize patella cap and tibial crest highlights; avoid muddying with micro‑patterns.

5) Ankle (Talocrural + Subtalar)

Skeletal landmarks.

  • Medial malleolus (tibia) and lateral malleolus (fibula) create ankle knob asymmetry (lateral sits lower/posterior).
  • Calcaneus (heel), navicular and cuboid shape midfoot; toe box follows metatarsal splay.

Muscular/Tendon reads.

  • Achilles tendon defines posterior ankle; tibialis anterior cords the front during dorsiflexion; peroneals ride the lateral side.

Mobility rails. Dorsiflexion ~20°; plantarflexion ~45–55°; inversion/eversion ~30°/~20°.

Design notes.

  • Boot cuffs must notch around malleoli (lower lateral cut). Add toe‑spring to soles for gait; stiff soles demand increased rocker.
  • Greaves should float over the ankle with a bell‑shaped flare; spats need elastic gussets for stairs/sprint.
  • Spur or heel guards seat on the calcaneus; avoid blocking Achilles glide.

Camera emphasis. Readable heel geometry and toe‑spring silhouette sell stance at small scales.

6) Foreshortening & Overlap at Joints

  • Use tapered cylinders with aligned ellipses to show limb depth; widen foreground segments.
  • Overlap decisively at hinges (forearm over biceps in full flex; calf over posterior thigh in squat).
  • Cast shadows should foreshorten with the form to preserve depth cues in busy comps.

7) Cloth, Armor, and Strap Routing Around Joints

  • Elbows: Route seam over delto‑pec groove into biceps; split sleeve at cubital crease; keep armor plates floating over olecranon with lateral hinges.
  • Knees: Place darts along vastus medialis and IT band; rear panels stretch over hamstrings; armor straps avoid popliteal fossa.
  • Wrists: Cuff scallops mirror styloid bumps; gussets on palm side; fasteners on dorsal flats.
  • Ankles: Bell‑flare greaves; tongue gussets; laces align with metatarsal arcs; straps clear malleoli.

8) Indie vs AAA: Owning Joint Decisions

Indie. The generalist concept artist incorporates joint logic into silhouettes, pose sheets, and callouts; quick engine tests validate crouch/reload/stairs. Expect to do paintovers on block‑ins to correct cuff and armor pivots.

AAA. Exploration sets stylization of joint reads (sharper patella caps, heroic forearm tapers). Production concept authors orthos with joint centers and callouts specifying hinge arcs and cloth/armor breaks; tech art and animation enforce ranges. Vendor packs include “no‑pinch” zones and cuff notch diagrams.

9) Deliverables That Encode Joint Truths

  • Joint Center Ortho: Front/side with humeral/ulnar axes, patella track, ankle malleoli asymmetry; include capsule guides.
  • Mobility Pose Sheet: Start/hold/impact silhouettes for reload, block, sprint, kneel; angular annotations.
  • Armor/Cuff Break Map: Color‑coded rigid/semi‑rigid/flex zones; bevel widths; floating plate callouts.
  • Hand/Foot Function Sheets: Grips (power, precision, pinch) and gait frames (heel‑strike, mid‑stance, toe‑off) with boot last notes.

Package with naming/versioning and change logs.

10) Failure Modes & Fast Fixes

  • Bracer pins elbow. Split across cubital crease; add floating elbow cup; bevel over olecranon.
  • Sleeve twist distortion. Realign seam to biceps/forearm rotation; move graphics off high‑shear zones.
  • Knee plate stabs thigh. Hinge the cap; add clearance above patella; split greave at tibial crest.
  • Boot locks stairs. Lower medial cuff; notch around malleoli; add tongue gusset and toe‑spring.
  • Wrist UI drifts in reloads. Move readouts to ulnar side; design rotating bezels that track radius roll.

11) Drills (Daily/Weekly)

Daily (20–30 min): Landmark tracings for one joint from front/side/back; redraw from memory; overlay seam/armor splits.

Weekly (60–90 min): Mobility sheet for crouch → kneel → sprint; build an armor/cuff break map for forearm + shin with photo tests.

12) Collaboration Map

Design: Validates tells (block, reload, cast) and readability at distance.

Animation/Tech Anim: Confirms joint ranges and cuff/armor clearances; flags clipping.

Character/Tech Art: Approves seam routing, plane breaks, texel density at cuffs; sets shader stretch maps.

VFX/Audio: Places emitters on stable bony flats; syncs audio to joint extremes.

UI/UX: Tests glove/boot icons and wrist devices for usability; checks HUD occlusion.

Cinematics/Marketing: Uses clear hinge reads for hero frames and slow‑mo shots.

Production/Outsourcing/QA: Uses joint center orthos and break maps for acceptance; tracks deviations.

13) Final Thought

Great limb design looks inevitable because it respects hinges. If you anchor on bony landmarks, route seams and plates along muscle rhythms, and package mobility rails, your elbows, knees, wrists, and ankles will read cleanly, animate smoothly, and ship without drama—across cameras, platforms, and seasons.