Yoga teacher, Melina, recently asked me, "Why can't my student straighten her knees in Paschimottasana (seated forward fold) when she can in Uttanasana (standing forward fold)? Her hamstrings seem pretty open. What are the mechanics behind this?"
While it is difficult to say exactly what is going on here without seeing the actual student and her alignment in the poses, I can provide a good educated guess. My guess would be based on the mechanics involved, so let's take a look at the anatomy of the hamstrings.
Hamstring Anatomy
Each hamstring muscle is made up of three muscles, the biceps femoris, semimembranosus and semitendinosus. The hamstrings' attachments give us our first clue of what is going on with our forward folding yogi. The hamstrings are what we call multi-joint muscles, that is they cross more than one joint. Taking a look at the picture of the left hamstrings we can see that they attach to the ischial tuberosity (sit bone), cross the hip joint, attach to the back of the femur (big thigh bone), cross the knee, and then attach to both leg bones, (tibia and fibula).
We can see that the hamstrings can affect both the hip joint and the knee joint because the muscles pull across both the joints. Taking some tension out of the hamstrings at the knees lets you flex more at the hips and vice versa. This is why bending the knees in a forward fold lets you fold deeper.
So, getting back to our question. What could be happening to our forward folding yogi? My guess based purely on anatomy would be that while standing, our yogi is doing her forward fold mostly from the low back rather than flexing at the hips where it is intended. If the hips are not flexing as much, the hamstrings will have more room to lengthen at the knee, so the legs can be straightened. While sitting, the yogi would need to flex at the hips more to bring her body forward in order to not fall backwards. Since the hips would now be flexed more, this would stretch out the hamstrings at the hip and there would be less length available at the knees, forcing them to bend.
We can see that the hamstrings can affect both the hip joint and the knee joint because the muscles pull across both the joints. Taking some tension out of the hamstrings at the knees lets you flex more at the hips and vice versa. This is why bending the knees in a forward fold lets you fold deeper.
So, getting back to our question. What could be happening to our forward folding yogi? My guess based purely on anatomy would be that while standing, our yogi is doing her forward fold mostly from the low back rather than flexing at the hips where it is intended. If the hips are not flexing as much, the hamstrings will have more room to lengthen at the knee, so the legs can be straightened. While sitting, the yogi would need to flex at the hips more to bring her body forward in order to not fall backwards. Since the hips would now be flexed more, this would stretch out the hamstrings at the hip and there would be less length available at the knees, forcing them to bend.
Conclusion
Based only on the anatomy and assuming our yogi doesn't have any other conditions affecting forward folding, this would be my guess as to what is happening. I would need to see the actual yogi to figure out what is happening for sure. However, this question is a great way to review hamstring anatomy and how it affects movement at the hip and knees. Also, movement at the hips and pelvis affect the low back because of lumbopelvic rhythm. It's all connected! Just remember, when looking at forward folds always consider what is happening at the
- low back
- hips
- and knees.
Thanks for the question!
Namasté
Namasté
Forward fold image from Yoga Journal. Hamstring graphic from Thieme Atlas and Textbook of Human Anatomy General Anatomy and Musculoskeletal System.
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