Muscle outlines from Body Muscles by Ivan Vulović, Apache 2.0 — regrouped and recoloured for Muscle Train.
Stand with feet shoulder-width apart, holding a dumbbell in each hand with a neutral grip. Keep elbows close to your torso and back straight. Curl one dumbbell upward, keeping the upper arm stationary, until your forearm is vertical. Pause, then slowly lower. Alternate arms.
Maintain a controlled motion, avoiding momentum. Keep wrists straight and elbows close to your body. Exhale on the lift, inhale on the lower.
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