You're holding a pose for a photograph.Your cheek muscles fatigue quickly from smiling,meanwhile your neck muscles feel fine from holding your head up.This is because your cheek muscles are predominantly type 2 muscle fibers.

Question

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Type 2 muscle fibers are associated with quick, intense bursts of activity and tire more easily than type 1 muscle fibers. The muscles in the cheeks are small and not used as frequently as the neck muscles, so they are more likely to be composed mainly of type 2 muscle fibers, which tire quickly.

You're Holding a Pose for a Photograph