You should initialize the graphics objects outside the loop and then modify the properties inside the loop.
The dynamic expansion of F could still be a bottleneck though. Also note that you are using j as an index there, so you can speed this code up by only executing the last frame iteration.
F(num_frames) = struct('cdata',[],'colormap',[]);
cla(ax)
hold(ax ,'on')
grid(ax, 'on')
xlim(ax, [-axlim axlim])
ylim(ax, [-axlim axlim])
h_line1=plot(ax, [0, x(1, 1)], [0, y(1, 1)]);
for i = 1:3
h_line_i(i)=plot(ax, [x(1, i), x(1, i+1)], [y(1, i), y(1, i+1)]);
end
for frame = 1:240
set(h_line1,'XData',[0, x(frame, 1)],'YData',[0, y(frame, 1)])
for i = 1:3
set(h_line_i(i),...
'XData',[x(frame, i), x(frame, i+1)], ...
'YData',[y(frame, i), y(frame, i+1)])
end
drawnow
F(j) = getframe(f);
end
