Find exact time step

I have two column one is time column and another is speed column. The speed is constant but when a subject starts running the speed increase. I want to know the exact time point when a subject start run using a MATLAB code. I collected five trials in one file. I want to know the five time steps from one file. Attached is the excel sheet.

Answer 1

[KSSV]

 clc; clear all ;
 data = xlsread('Book1.xlsx') ;  % Read the data
 t = data(:,1) ; x = data(:,2) ; % Seperate columns 
 start_t = [];                   % Initialize needed time 
 start_idx = [] ;                % Initialize needed indices 
 for i = 1:length(t)-1
     dx = x(i+1)-x(i) ;
     if dx ~=0 && dx == x(i+1)
        start_idx = [start_idx ; i ] ;
        start_t = [start_t ; t(i)] ;
     end
 end
 plot(t,x) ;
 hold on
 plot(t(start_idx),x(start_idx),'.r') ;

[Mahdi Hassan]

Thank you Dr. Kolukula

Answer 2

[Jos (10584)]

Let S be your vector of speeds, and T your time points

% Determine, for each time point, is the subject running or not
  IsRunning = S > 0 ;
% For each time point, did the subject change from not-running to running
  TF = [false ; diff(IsRunning(:))==1] ;
% At what indices did this happen
  index = find(TF)
% and the time points are:
  T_startedToRun = T(index)
% check with a plot
  plot(T,S,'b-', T_startedToRun,0,'rs') ;

[Mahdi Hassan]

Thanks JOS