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.
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.