Muscle outlines from Body Muscles by Ivan Vulović, Apache 2.0 — regrouped and recoloured for Muscle Train.
Lie supine, legs straight, hands under low back. Keep legs straight, raise to forehead contracting abs/exhaling. At full contraction (legs just above parallel), lower slowly to start. Repeat.
Exhale hard (like blowing candles), hold contraction 1 sec for mind-muscle connection. Back pain? Switch to anti-extension/rotation moves. No hands behind head (strains neck).
MuscleTrain is actively being built. If something looks broken, or you have a suggestion, let us know — it really helps shape the app.