Muscle outlines from Body Muscles by Ivan Vulović, Apache 2.0 — regrouped and recoloured for Muscle Train.
Hands and knees, arms straight, hands under shoulders. Extend legs back to plank (straight line head to heels). Hop one foot to chest, quickly switch legs (run in place). Keep plank, hips low; alternate.
Tight abs for core burn. Straight back (no hunch/high hips). Arms straight, hands shoulder-width. Toes touch floor on knee drive (full range). Breathe: inhale one knee, exhale other.
MuscleTrain is actively being built. If something looks broken, or you have a suggestion, let us know — it really helps shape the app.