The static preload shank angle that enables the SR results from the simultaneous peaking of tension in the Achilles tendon and the sheet-like ligament called the plantar  aponeurosis (PA). The PA is the primary source of support for the arch of the foot and the Achilles tendon. It  is the white membrane shown in the graphic below that connects the toes to the heel bone.


The graphic below shows the Achilles tendon as an extension of the plantar aponeusosis through a common interface at the posterior aspect of the calcaneus (heel bone).


While the principle of the simultaneous peaking of tension in the Achilles tendon  and the plantar aponeurosis is simple, the surrounding mechanics responsible for this event are complex.

The graphic below is from the previous post, I-C-E: SR ( This is the model that I fabricated to validate my theory that simultaneous peaking of the Achilles tendon  and plantar aponeurosis and tension sets up the static preload that enables the

While I have yet to find evidence that the mechanism of the SR is known and understood in application to skiing, it has been observed for many years that elite skiers tend to maintain substantially the same angle of the shank of the 0utside leg in the high load phase that occurs in the bottom of a turn. Racers such as Shiffrin have commented on the need to keep the shin (shank) in contact with the inside of the tongue of the boot. But this is result of a boot that is set up so that what I have termed the Reference Shank Angle is be maintained by the SR. The comments below excerpted from a recent paper reflect this characteristic.

“……….. experienced skiers tend to keep a constant lower leg posture (shank angle) using boots with varying stiffness [16, 17]. – Flexural behavior of ski boots under realistic loads – The concept of anvimproved test method: Michael Knye*, Timo Grill, Veit Senner, Technical University of Munich (TUM), 2016.

The Stretch Reflex is a Protective Mechanism

The SR is an important mechanism that maintains balance and protects the lower limbs from injury by maintaining the joint angles associated with the static preload shank angle as observed in the studies referenced in the paper cited above.

The SR is a spinal reflex. Spinal reflexes are reflexes in which sensory input arises from receptors in joints, muscles and skin. The neural circuitry responsible for the reflex motor response is contained entirely in the spinal cord. The SR is the most basic and most rapid form of reflex.

In Application to Skiing, the SR Controls the Angles of Joints

The authors of the previously cited paper further note:

“The FAP (Force Application Point) and COP (Center of Pressure) movement is not only controlled by the body position of the skier as plantar flexion or dorsal extension, moments at the ankle joint will also contribute to the shift of the FAP/COP. For various skiing situations, high activation levels for the m. (muscle) triceps surea and m. (muscle) gastrocnemius were measured. Hintermeister et al. argued that the skier is using the medial gastrocnemius to maintain a stable position, increase the (vertical) pressure on the ski and resist (attenuate) vibrations.

“In alpine skiing, high levels of ground reaction (vertical) forces (GRF) were measured for various skiing situations reaching from 1500 N up to 3000 N for professional skiers [27–29]. These forces are causing high external flexion moments on ankle-knee-and hip-joints which need to be absorbed (attenuated) by the athlete and the ski boot.

Fabricating a simple working model that illustrates the mechanism that sets up a static preload on the shank was relatively easy compared to trying to provide a simple explanation with text and graphics. I spent several hours attempting to generate a simple schematic to illustrate the complex mechanism that exponentially tensions the arch spring-energy return system that assists walking by releasing energy at the end of the stance phase. In skiing, the same basic mechanism is used to set up the SR.

The graphic below is a schematic of the working model. It is simplest version I could come up with that illustrates the forces at work.


It is not necessary to possess an intimate understanding of the esoteric details of the static shank preload that results from the triceps surae in isometric contraction or the SR in order to develop an SR based stance. But it is necessary to acquire sufficient appreciation of the potential value of the SR in order to override the currently ingrained and widely accepted narrative that the foot works best in skiing when its joints are immobilzed in neutral and that the foot should be prevented from elongating and the forefoot prevented from spreading, both of which will prevent the SR from engaging.

In my next post, I will explain how simultaneous achilles tendon – plantar  aponeurosis (PA) tension peaking occurs.




  1. Dear David,

    This is not a comment for this post but has to with a contemplation I have been undertaking since becoming acquainted with your work.

    I was wondering if you could put some thought into the mechanics of skiing on one ski where there is no inside or outside ski. It stands to reason in one ski skiing that the center of pressure on the first metatarsal while effective in initiating a turn toward the person’s midline would not be effective in initiating a turn latterly on the same leg.

    In one legged skiing to simplify the shifting of the center of pressure to initiate a medial turn would be accomplished by firing the flexor hallucis longus. By comparison a turn laterally would shift the center of pressure laterally to the 4th and 5th metatarsal by firing the flexor digitorum longus. In either case the center of mass has to shift to “get over it” and on edge as in the Mikaela Shiffrin analogy.

    Hope this finds you well as I have enjoyed your blog, thomas

  2. Dave,
    Probably better to think in terms of a single ground reaction force vector, rather than splitting it and applying it to the heel and Mt heads in some proportion. You could represent pressure (F/area) that way, but the vertical ground reaction force vector would be more or less aligned with the vertical vector representing gravity acting on the CoM. As the shank rotates forward, and the CoM moves forward, and more pressure is applied under the Mt heads, the GRF and Centre of Pressure would shift forward.
    Also, although the Achilles tendon does generate a moment (torque) around the tarsal joints, and does transfer load to the plantar aponeurosis, some of the flattening of the foot that occurs under load occurs as a direct effect of the sagittal location of the Centre of Mass of the superincumbent body. The effect is the same as simply putting a weight on the dorsum of the foot.

    1. You mean average GRF because vector emanating CoM does apply a physical force. At one point I showed CoP aligned vertically with the vector from CoM. But since my working model and schematic are a tripod structure, I elected to show GRF 1 amd GRF 2 at the points of contact and fudge the compression force.

      I agree with what you said. But the central issue is how does the position of CoM compress the arch? Let’s start by asking, “What aspect of the arch is restrained? Which part(s) can move?”
      The posterior aspect (back end) of the arch is substantially restrained.
      The anterior aspect (front end) of the arch is substantially unrestrained.
      The connection of the bottom of the tibia with the arch the arch is substantially unrestrained vertically in that it can be depressed.

      What we have is a 3rd class lever with an inverted pivot at the back of the arch with a load at the base of the tibia and with an effort (applied force) acting on the lever created by the vertical column supporting CoM. As CoM advances towards the front of the arch, the moment arm increases and applies increasing load on the base of the tibia which acts to compress the arch. The length of the Achilles doesn’t increase that much. But muscle tension increases to resist the torque created by the lever supporting CoM.

      I drew the force diagram about 10 years ago. I’ll post it as an addendum tomorrow.

      1. “But muscle tension increases to resist the torque created by the lever supporting CoM.” True to an extent, in that rapid forward rotation of the shank over the ankle, even to a minor degree, sets up a corrective stretch reflex, even if the shank is buttressed against the tongue of the boot. Muscle tension (= Achilles tension) IS the torque creating the lever effect onto the metatarsal heads in the forefoot. The digital flexors, coming from the back of the shank and running around the medial side of the ankle and into the sole of the foot are also involved in that postural reflex, in that they will also transfer load onto the metatarsal heads and toes, so moving the CoP forward, but they have the additional effect of holding up the medial arch. One of the problems I see in even intermediate skiers with poorly fitting boots is they curl their toes in their boots as if they were trying to hang onto their ski using their toes. This stresses the importance, as you’ve said many times, that effective load transfer onto the forefoot depends on the arch of the foot being constrained vertically in the region of the instep with an effective restraint system that ensures all of that post-tibial muscle force is directed into allowing some degree of pronation of the foot by allowing some flattening of the medial arch.

      2. We are basically on the same page.

        There are several significant problems especially in the boots of recreational skiers; excessive boot board-binding ramp, often over 6 degrees which will prevent the SR from being effective and boots with too little forward shaft angle and/or too much padding in the liner and/or the shaft buckles and especially the power strap cinched tightly about the leg so it splints it to the rear spoiler of the shaft, or all of the above.

        The good news? The bad aspect of skiing seems to have finally bottomed out. I am starting to see more racers and recreational skiers with solid technique. Perhaps they are reading my blog!

      1. That make sense David about my feeling. And you are just putting words onto it ! What is great is that you are brillant to explain with words and schématics the “feeling”. Schématics Help a lot, sin don’t stop using it and perhaps more 😉

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