Human beings are born with only two fears - that of loud noises and of falling and in this short video of their respective dugouts, the New York Yankees and the Boston Red Sox give a great example of the former with a "Startle Reflex". Often confused with body language, it is neither. This reflex occurs at below the level of either the conscious or the subconscious. As with many things however, this is not always as clear cut as it sounds - for the reflex rapidly gives way to emotion.
Although it is not possible to see all the fine detail through the rain covered camera lens, variations of the startle reflex are clearly seen here. This is characterized by a rapid configuration to the fetal-like position occurring 250-500 milliseconds after the thunder (or whatever event/sound). The hands rapidly cover the head/face with additional flexing of the head and neck in an effort to protect the throat, face and chest. The arms and shoulders pull inwards - closer to the body's center and the back, hips, knees and ankles flex and contract closer to the central axis as well. If greater detail were possible a very tight closure of the eyelids could be seen here (another protection mechanism).
Intriguingly, after the noise, there is almost always a "looking towards the source" to discern a cause (not as much with thunder as the cause is of course known, yet it is still often seen).
In the video, note how the second Yankee from the camera is hyper-reactive. This reaction is a manifestation of surprise-fear. Indeed in the image below, although it is relatively blurred - widely opened eyelids and elevated brows are captured on the third Yankee, while the second, third and fourth all display the wide open, vertically elongated mouth opening highly indicative of surprise. More can be seen within the dynamics of the video.
In conclusion, the startle reaction is a reflex (and is not a type of body language) and lasts from 250 to 500 milliseconds (1/4 to 1/2 second). Thereafter it is rapidly replaced by surprise and surprise-fear and then fear (bottom image, Red Sox closest to camera) and the displays of these nonverbals can be clearly seen.
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Surprise on face of Yankee 2, 3 & 4
Fear on this closest Red Sox player's face