Vertical Lunge Straight Question

So, I've got a question about the vertical lunge straights.  On both the right and left legs, I am so stiff that I cannot maintain an upright posture with my front leg straight, even if I am using stretch blocks standing upright.  My question is should I just find something taller (like bar chairs) or am I allowed to bend the front leg until my quad/hip flexor mobility improves?  Sorry if this is a dumb question.

 

Cheers, Nick

This is the "Money Stretch" of the series, as long as you understand 

 

1) Where you should end up

2) Where you are now

3) Where you need to work on to improve (if you can't stay up right it's hip flexors flexibility

4) Only to work the depth to 'mild' discomfort

 

set up supports as needed to assist the process.

Thank you, Cole!  I will definitely focus on those hip flexors then (and take it slow).