I’m talking legs. In fact I’m looking at what you’re doing with your rear leg in sparring, pad work, or indeed any paired drills.
Every martial arts system, whether it be predominantly grappling or striking based, or whether it hails from China, the Philippines, Okinawa, Japan, India, Malaysia, New Zealand or Europe, has a number of different foot and leg positions. These are often taught to beginners as ‘stances’ and in many systems we are conditioned at an early stage into thinking in particular ways about how to employ them.
I’m going to come out of a little karate closet, I believe that biomechanically (and therefore tactically) in almost every situation bent is better than straight.
Among the most important elements required to dominate a standing situation are the ability to move and effectively employ power or weight against another person. A bent rear leg achieves this quicker and with more power than a straight leg. The obvious ‘counter-argument’ to this is that a bent leg generally achieves the above by straightening, but there is a huge difference between straightening and becoming straight, and between thrusting and straightening.
Once the rear leg has thrust and initiated a process of power transference there is a moment of choice. The leg can continue to straighten: this effectively jams against any active return resistance (such as momentum of the target towards you) and checks forward momentum by placing the heel on the ground, but provides no further ability to drive forwards without give since a fully straightened leg has to bend in order to thrust again. Alternatively the leg can remain bent once sufficient thrust has been generated to drive forward or rotate the hips; if the foot stays where it is that gives less counter stability in the case of active resistance in the opposite direction, but a comparatively greater degree of hip rotation and arm extension which should transfer greater power. Another option is to carry the foot forward (not necessarily stepping through) post thrust with the momentum of the hip, then all the advantages of the bent leg are retained combined with the stability that easy heel placement with minimum give in a short deep stance can bring. The little elephant in the room being that when we push against a resisting object or a heavy object (think about pushing a car), in order to move we naturally take our heels off the ground anyway, so whether bent or straight the heel on the ground isn’t part of optimum forward power transference.
In many Traditional Martial Arts we see straight rear leg postures. Don’t think of these as wrong, instead try to view them in context. A straight leg can be an exaggerated example of thrust, codified into ‘good form’ for aesthetic purposes. It can also, due to the linked foot and heel placement, be a result of postures designed for employment in traditional inflexible flat or platform footwear.
The depth of a stance will affect the ‘need’ to bend or straighten the leg (or lift the heel) to gain power transference in strikes, but at close quarters against an actively resisting person the higher the stance (and therefore the straighter the leg) the more vulnerable you are to being taken off balance. That naturally leads to the question as to whether the spine should be upright or angled, ramrod straight or hunched.