It is well known that shaking babies can cause serious brain damage, but what exactly constitutes shaking? I've seen some videos that demonstrate shaking on dolls, and I can say with certainty that I don't do that. But at the same time, the motion of a rocking chair is not nearly enough to soothe my 2 month old when she's worked up, and larger amplitude or higher frequency movements are useful to calm (or at least quiet) her.
Some motions that are effective include something like a bicep curl, various types of bouncing, and going over a rough gravel trail that jostles her stroller a lot. In all these cases, her head and neck are supported, which is a big difference from the shaking I've seen in videos. At the same time, the accelerations are not trivial. If someone subjected me to the same accelerations, I'd find some of them unpleasant. Even with the head and neck supported, there must be some level of acceleration that is harmful.
We also have a snoo, and some of its motions seem pretty aggressive. One setting can cause her head to wag side-to-side by a fair amount.
The fact that these motions are calming may suggest that they're safe. However, I worry that it's like brain injuries in football: concussions get all the headlines, but the cumulative effect of a lot of small impacts is just as dangerous.
Danger often exists on a continuum, but experts often come up with some safety threshold to stay below. Where is that threshold for baby motion? What exactly constitutes shaking?