I am not sure how or why this division came about, but whether or not to bend the knees during a hamstring stretch is a fiercely contested topic in the yoga community. Perhaps it is because injury at the proximal tendon (yellow part of photo below) is quite common among yoga teachers and students. When I lead biomechanics workshops for yoga teachers, I often ask who in the room has experienced, or knows someone who has experienced, proximal hamstring tendon injuries. The show of hands is staggering.
We are keenly aware that something about our practice may be contributing to this mild (and not well documented) epidemic. But our solution is not so keen, “It must be the knee joint angle!”
I have heard teachers argue that a bent knee forward fold is the only way to protect the hamstring tendons from injury. I have heard other teachers argue the opposite, that straight legs is the only way to promote a practice free from tendon tears. Likewise, I have heard both sides argue that their way is the only way to heal an already injured tendon. I have chased these claims for over a decade, looking for data to support the anecdotes, and have yet to come up with anything.
It’s possible that the great yoga debate exists because it is really a matter of biomechanics (load), but is being argued in the language of anatomy (joint position). So let’s put aside anatomy for a moment in order to review some tissue mechanics.
Your hamstring tendons are elastic, meaning they yield under a tensile load/stress (stretch) and return to resting length when the load is removed. Your hamstring tendons have an internal tensile strength, meaning there are limits to the load/stress (stretch) they can withstand.
Tendons also adapt to healthy doses of loading. If you’re new to the blog, pause for a moment here and review my Progressive Overload post. To summarize that post, tendons get stronger with heavy loads and not with light loads.
So how do you load a tendon? With a muscle contraction, of course – as determined by the real estate they occupy (positioned to transmit force between muscle proteins and bone). The muscle contractions associated with passive stretching do not provide a stress great enough to improve the tensile strength of your tendons [1, 2]. This is why I encourage deliberate contractions upwards of 80% of the target muscle, particularly if resistance training is not part of your movement profile.
During hip flexion (forward folding), rather than celebrating one knee joint angle and criticizing the other, I prefer to improve the strength of my muscle contractions in all knee joint angles.
If you’re thinking that a bent knee creates a greater contraction of the lower hamstring fibers, you’re thinking of concentric contractions and forgetting that muscles produce force at varying lengths. Sometimes even greater forces after a stretch! Here’s a past blog on how muscles actually work .
I only bring up the contraction conversation here because we recently wrapped my 300 hour teacher training and we probably reviewed this subject every single day, some days even twice per day! One of my major themes was to encourage the teachers to think beyond concentric contractions and to take a bird’s eye view, instead of a literal view, of the anatomy book. And this meant climbing on my soap box over and over and over again.
But I didn’t mind. In fact, I was grateful. Those teachers really showed me which topics are needed in yoga teacher trainings and biomechanics is a clear winner.
Back to the knee joint angle during hip flexion…
One of the best ways, in my opinion, to evaluate stretch positions is to look at the orthopedic testing procedures, since most sports rehab professionals use methods of treating and training that mimic testing methods. You won’t be surprised to learn that there are three common positions used for testing hamstring
tightness extensibility – one bent knee and two straight knee positions.
When determining which test to use, researchers consider the merits of these tests. They consider factors such as reliability and validity [4, 5, 6]. Protecting the hamstrings tendons from injury based on knee joint angle is not a consideration because hamstring tendon strength is not dependent on knee joint angle. Hamstring tendon strength is dependent on the loading history of those tissues.
Researchers do, however, consider that in some individuals, the Straight Leg Raise (SLR) test, which looks an awful lot like a straight leg supta padangusthasana, may stretch neurological tissue and thus interfere with the ability for the hamstrings to yield . In this case, the Active Knee Extension (AKE) test, where the hip is flexed and fixed to 90° in a supine position while the subject tries to actively extend her knee, is preferable over the SLR. (This stretch can also be performed passively with assistance, photo below left.) The third test, the Sit and Reach (SR) which resembles paschimottanasana, is generally considered the least reliable and valid for obvious compensatory contributions of the spine (photo below right). Researchers defend superiority of these tests in terms of effectiveness (reliability and validity) based on data rather than oral traditions.
(images by Shutterstock)
Regarding effectiveness measured by improvement in flexibility, researchers utilize bent knee stretches [7, 8, 9, 10], straight knee stretches , or both [12, 13]. Clearly, if one of these methods were considered injurious, it would not be approved for testing on human subjects.
As leaders and educators in the yoga community, we can advance conversation beyond debating the merits and consequences of bent or straight leg forward folds* (anatomy) by including the promotion of connective tissue health (biomechanics). Basically the topic of my last post.
*This post discusses the effect of forward folds on the hamstring tendons, not vertebral discs or any other structures which would require an entirely different post.
 Kubo, K., Kanehisa, H., & Fukunaga, T. (2002). Effect of stretching training on the viscoelastic properties of human tendon structures in vivo. Journal of Applied Physiology, 92(2), 595–601. http://doi.org/10.1152/japplphysiol.00658.2001
 Kubo, K., Kanehisa, H., Miyatani, M., Tachi, M., & Fukunaga, T. (2003). Effect of low-load resistance training on the tendon properties in middle-aged and elderly women. Acta Physiologica Scandinavica, 178(1), 25–32. http://doi.org/10.1046/j.1365-201X.2003.01097.x
 Minozzo, F., & Lira, C. (2013). Muscle residual force enhancement: a brief review. Clinics, 68(2), 269–274. http://doi.org/10.6061/clinics/2013(02)R01
 Gajdosik, R., & Lusin, G. (1983). Hamstring muscle tightness. Reliability of an active-knee-extension test. Physical Therapy, 63(7), 1085–1090.
 Hamid, M., Ali, M., & Yusof, A. (2013). Interrater and Intrarater Reliability of the Active Knee Extension (AKE) Test among Healthy Adults. Journal of Physical Therapy Science, 25(8), 957–61. http://doi.org/10.1589/jpts.25.957
 Kane, Y., & Bernasconi, J. (1992). Analysis of a modified active knee extension test. The Journal of Orthopaedic and Sports Physical Therapy, 15(3), 141–146.
 Davis, D., Ashby, P. E., Mccale, K. L., Mcquain, J. A., & Wine, J. (2005). The effectiveness of 3 stretching techniques on hamstring flexibility using consistent stretching parameters. Journal of Strength and Conditioning Research, 19(1), 27–32.
 Freitas, S. R., Vilarinho, D., Vaz, J. R., Bruno, P. M., Costa, P. B., & Mil-Homens, P. (2014). Responses to static stretching are dependent on stretch intensity and duration. Clinical Physiology and Functional Imaging. http://doi.org/10.1111/cpf.12186
 Magnusson, S. P., Simonsen, E. B., Aagaard, P., Gleim, G. W., McHugh, M. P., & Kjaer, M. (1995). Viscoelastic response to repeated static stretching in the human hamstring muscle. Scandinavian Journal of Medicine & Science in Sports, 5(6), 342–347.
 Magnusson, S. P., Simonsen, E. B., Aagaard, P., & Kjaer, M. (1996). Biomechanical Responses to Repeated Stretches in Human Hamstring Muscle in Vivo. The American Journal of Sports Medicine, 24(5), 622–628.
 Borman, N. P., Trudelle-Jackson, E., & Smith, S. S. (2011). Effect of stretch positions on hamstring muscle length, lumbar flexion range of motion, and lumbar curvature in healthy adults. Physiotherapy Theory and Practice, 27(2), 146–154. http://doi.org/10.3109/09593981003703030
 Fasen, J., O’Connor, A., Schwartz, S., Watson, J., Plastaras, C., Garvan, C., … Akuthota, V. (2009). A randomized controlled trial of hamstring stretching: comparison of four techniques. The Journal of Strength & Conditioning Research, 23(2), 660–667.
 Meroni, R., Cerri, C. G., Lanzarini, C., Barindelli, G., Morte, G. Della, Gessaga, V., … De Vito, G. (2010). Comparison of active stretching technique and static stretching technique on hamstring flexibility. Clinical Journal of Sport Medicine, 20(1), 8–14. http://doi.org/10.1097/JSM.0b013e3181c96722