Hi Farzaneh - the following journal article may help;

Foster, R.J., De Asha, A.R., Reeves, N.D., Maganaris, C.N. & Buckley, J.G. (2014). Stair-specific algorithms for identification of touch-down and foot-off when descending or ascending a non-instrumented staircase. Gait and Posture, 39, 816-821.

Regards,

Rich