Confusion using cumtrapz for acceleration

I have data for a person swinging their leg back and forth, I have this data in the form of acceleration. I have tried using cumtrapz to turn this data into displacement.

ie Vel = vel0 + cumtrapz(t,Acc)
   Pos = pos0 + cumtrapz(t,Vel)
however I get a graph that is increasing only, no osccilations, I am confused, is cumtrapz dealing with the negative accelerations accurately?

any ideas about how to tackle this problem?

[A Jenkins]

If you would provide some sample data it would help people reproduce your problem.

I made up my own data but your code seems to work just fine:

t=0:0.1:10;
Acc=sin(t);
vel0=-1;
pos0=0;

Vel = vel0 + cumtrapz(t,Acc)
Pos = pos0 + cumtrapz(t,Vel)

plot(t,Acc,t,Vel,t,Pos)
legend('Acc','Vel','Pos')