I'm looking for the same. I've tried to use pedometer to deduce velocity and by the distance to calculate power of some tests as stand up and go test, but it is very imprecise

Maybe: SpeedClock 

https://itunes.apple.com/it/app/speedclock-video-radar/id400876654?mt=8

Hello Jose,

Counts are usually proprietary, so it is difficult to compare acceleration data using different devices. Researchers have published accelerometer data in g and thresholds exist for triaxial accelerometers using raw acceleration data. Van Hees and Hildebrand have completed work in this area using two different accelerometers so the data processing method may be useful for you depending on your research questions.

Many thanks,

Lynne

Thanks for your answer, Lynne.

Van Hees and Hildebrand work seems interesting. Could you give me the document reference?

Thanks in advance.

I need to compare a validate accelerometer with smartphone, so I'd like to find some app to use for it,but I've found only app for horizontal measure such as pedometer. It could be interesting read the Van Hees and Hildebrand's document 

Finally I found it:

Hildebrand, M., Van Hees, V. T., Hansen, B. H., & Eekelund, U. (2014). Age group comparability of raw accelerometer output from wrist- and hip-worn monitors. Medicine & Science in Sports & Exercise, 46(9), 1816-1824. doi:10.1249/mss.0000000000000289