There was a rumour going around that the BKS in BKS Iyengar stood for â€˜Both Knees Straightâ€™, such is the insistence on straight legs in Iyengar yoga. Why is this?
In most poses the legs are the foundation of the asana. The legs are used to feed the energy from the foundation into the hips and the base of the spine in order to create its extension. In other words the legs act as conduits. In straight legged poses, and in particular straight legged standing poses, the most efficient conduit is a straight one, so it makes sense for the legs to be absolutely straight. The foot, or feet, press into the floor, and the energy is drawn up from the sole of the foot through the bone structure of the leg into the hips to help feed the extension of the spine from its base.
It may seem harsh for the unseasoned practitioner to be demanded to work through the dullness that often resides at the backs of the knees and thighs. This work can take time. In the beginning the practitioner may get momentary jerks of engagement interspersed with long periods of non responsiveness, and the work and focus required for such fleeting results can seem hardly worthwhile. However it is important to persist, as the constant refocusing and recharging will, over time, start to bear better results, and the effort required to create these results will become more effortless.
A common issue that seems to occur with practitioners is that the straightening of the shin bone tends to happen more quickly than the straightening of the thigh bone, which can tend to cause an over stretching at the top of the calf muscle and at the back of the knee. It is important to learn to press the foot into the floor correctly in samasthiti, and bring this same action into the other straight legged poses, especially those where the leg is angular to the floor, such as trikonasana and parsvottanasana. It is also necessary to learn to engage the musculature of the leg when straightening it, so that the drawing of the muscle (and tissue) to the bone, is happening in concordance with the straightening action. The calf muscle needs to draw up and almost act as a resistance to the the shin bone working back into it, whereas the quadriceps need to engage and draw up in order for the thigh to work back more from the top (toward the hip), rather than at the bottom (toward the knee).