John Schumacher's reply:
If by "difficulty reaching" you mean you have to bend your leg to hold your foot because of tightness in the back of your leg when it is straight, then you should use a strap. In this case, the tightness is most likely in your hamstring, though it may also be in your lower back, your gluteus muscles, the back of your knee, and/or your calf. If you keep your knee bent, you may have less difficulty, but you will not fully stretch the back of your leg and may actually strain the tendons of the hamstring as well as the soft tissue and nerves at the back of the knee.
To safely and effectively stretch the back of your leg, keep it fully lengthened without bending your knee and draw your quadriceps muscle from your knee toward your hip. Then place a strap around the ball of your foot near the arch and move into whatever extended-leg pose you are doing. You should also use a strap if having difficulty means that in order to hold your foot with your leg extended, you have to round the front of your body, causing discomfort in your lower back. The reasons for the discomfort are probably the same as those mentioned abovetightness in the back of your leg and/or lower back.
If you persist in holding your foot with your hand by collapsing the front of your spine and then try to move further into the pose, you may strain your lower back muscles and/or put pressure on your intervertebral disks. If you use a strap around your foot, you can keep the front of your body lengthened and the back of your leg extended when you move more deeply into the pose. This increases the flexibility in your leg and back without harm or undue discomfort.
John Schumacher, founder and director of Unity Woods Yoga Center in Washington, D.C., is a certified senior Iyengar Yoga teacher who has practiced for more than 30 years. He has studied in India with B.K.S. Iyengar many times over the past 25 years.