The syntax o f function title is
title(obj,txt)
where variable obj must be an axes object or a legend object, but in your code, you are trying to put the output of the plot command (which are chart line objects).
Then, a solution is:
ax(1) = subplot(1,3,1) %Thorax Joint
for n = 1:10
plot(SF.T{n,1}(:,1),'g'); %right side trials
hold on
end
for n = 1:10
plot(SF.T{n,2}(:,1),'b'); %left side trials
hold on
end
title(ax(1),'Thorax')
