Muscle outlines from Body Muscles by Ivan Vulović, Apache 2.0 — regrouped and recoloured for Muscle Train.
Lie on your back, knees bent 90 degrees, feet flat. Hands behind your head, elbows wide. Engage glutes/hamstrings (partner assist or band). Exhale, curl torso towards knees, contracting abs. Pause, then slowly lower with control.
Use abs to lift, avoid neck/arm pull. Keep lower back pressed to floor. Control movement, synchronize breathing (exhale up, inhale down). Adjust form if neck/back strain occurs.
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