Exercise Anatomy

• ✓Muscle-activation apps colour a drawing per exercise from a table. Here one rigged figure moves through the exercise and every muscle is coloured frame by frame — a lunge colours the front leg and the back leg differently, a side plank the lower side, and a selected muscle stays visible from every angle through the others.
• ✓It shows lengthening as well as work. A muscle working while it lengthens — the glutes lowering into a squat — reads purple; one held on stretch, the lats at the bottom of a hang, reads teal.
• ✓Every curve declares its provenance, and the panel never prints a number it can’t back: qualitative by role, estimated by static optimisation on a published musculoskeletal model, or measured EMG with the conditions attached. None are measured yet, and the page says so.

I wanted to see which muscles work through a movement, not a chart of which muscles an exercise targets. That meant a figure that actually does the exercise, a rig underneath it, and a colour per muscle per frame — and a rule about what a colour is allowed to claim, because it would be trivial to fake precision with a number that looked measured.

A bodyweight squat from a free motion-capture clip on a rigged écorché built from the open Z-Anatomy atlas, 21 named muscles coloured by role bands, scrubbable, with a focus mode. A procedural capsule figure came first and was replaced the same day it was judged “still quite bad.” Live on 2026-09-08 as a static site: no server, the whole app a folder and a 4.7 MB model.

• A skeleton the clips can drive

  The first pipeline retargeted captured motion joint by joint onto my own rig, and the pull-up and the swimmer came out wrong. The figure was rebound to the Mixamo skeleton instead, so a clip plays bone for bone with no retargeting. Thirty-five exercises now: twenty from free capture, the rest designed by kinematics — a strict pull-up, an L-sit, a muscle-up, a handstand kicked up from standing, a planche, a front lever, five yoga holds — because no free capture of them exists, and each page says designed.

• Sides apart, and the scene from the clip

  One mesh per muscle per side, so alternating movements colour each limb on its own turn. The wall climb’s left-right timing was read off the clip’s hand and foot contacts, and the wall’s holds are placed wherever a hand or foot rests still in it — so a reconverted clip moves its own holds.

• A neck that nods where a neck nods

  The skull stretched in a crow and a warrior III. The rig’s neck bone ran from T1 to C7, two centimetres, and the head bone from C7 to the crown — so the whole cervical spine was welded to the skull. The joint moved to the atlas, and the viewer now takes only rotations from a clip, so a joint can move without reconverting every clip.

The hard part was never the rendering — it was deciding what a colour is allowed to claim.

35 exercises across seven categories, 50 named muscles, each pose reviewed from several angles and fixed where a real body would not do that. Next is the estimate made credible, and measured EMG with the conditions attached.