We use Uttasana or Stand Forward Fold multiple times throughout our class practice. It is used during Sun Salutation, moving from floor to standing or standing to floor and to move from pose to pose. I was taught to fold at the hips with legs straight like the person on the left in the following illustration. When I took some teacher training specifically geared to anatomy and yoga, I learned to practice Forward Folds with knees bent like the person on the right of the illustration. I find it interesting that even leading journals and web sites like Yoga Journal teach the Forward Fold with straight legs. This illustration from www.squencewiz.org, a website for yoga teachers, illustrates the differences between the two positions. The bent knee position is safer on the sacroiliac joint. From my own practice, I know that the straight leg position puts stress on the fascia that run down the back of the legs. These fascia also connect to the fascia along the bottom of the feet. As we stress the leg fascia, the feet fascia also become stressed. If we already have plantar fasciitis it may be inflamed. Even if we do not, we may find the fascia of our feet complaining with some pain.
I teach the bent knee position. I have found it less stressful on my legs/feet and my lower back. I also feel that more of my students are able to maintain the bent knee position and therefore feel more successful. Hope this helps your personal practice.
Feel free to give me feedback.
I enjoy yoga as it brings peace to my life. Teaching yoga has brought joy to my life and given me a chance to encourage others in learning yoga.