No loop necessary —
LD = load(websave('V1','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1172553/V1.mat'));
V1 = LD.V1;
x = 0:numel(V1)-1;
V1int = cumtrapz(x,V1);
figure
yyaxis left
plot(x,V1)
ylabel('V_1')
yyaxis right
plot(x,V1int)
ylabel('V_1 Integrated')
.