Fall Detection using Accelerometer
If you're going to do this in any sort of useful way, you're going to need to do something a lot more sophisticated than a simple threshold check on the raw accelerometer data.

First of all, accelerometers drift, and you're going to have to correct for that. Then, since you don't know the orientation of a device in someone's pocket, you're going to have to continuously compute the 3-D acceleration vector, and then evaluate that for significant events, such as changes in angle and/or magnitude.

Consider the consequences of errors &mdash; both "false positives" (creating an alert when no actual fall occurred) and "false negatives" (failing to detect an actual fall). If either of these occurs at a significant rate, your product will be a failure.

You're going to have to build a significant number of devices and simply collect data from a lot of different users doing a lot of different activities (including falling) and then determine whether you can reliably recognize the fall events among all of the other activity.