FATIGUE FACTOR. “It is well established that human mimetic musculature is dominated by fast-twitch myosin fibers. Our facial muscles are able to contract quickly and spontaneously (think of how quickly and automatically we smile at the sight of a familiar friend or a funny joke) but it is difficult to hold that contraction longer than a few seconds before fatigue sets in.”
http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full