Thomas’ comments on the effect of the reptilian brain on stance created a perfect segue to discuss a recent paper on the effect of ski boots on skier balance.
Influence of a nine-day alpine ski training programme on the postural stability of people with different levels of skills (April 2016, Biomedical Human Kinetics (DOI: 10.1515/bhk-2016-0004) – Michał Staniszewski, Przemysław Zybko and Ida Wiszomirska, Józef Piłsudski University, Warsaw, Poland.
The subject paper is an important source of information for any party with a serious interest in the mechanics, biomechanics and physics of skiing, especially academics and researchers.
The aim of the study, which recognised that balance is one of the key elements that determine the effectiveness of the ride in alpine skiing, was to determine to what extent a few days of skiing activities and the level of technical skills affected the skiers’ level of postural stability; i.e. balance. While recognizing the importance of balance in alpine skiing, the authors commented:
Publications on issues related to the biomechanics of a descent, with particular emphasis on the balance parameters, are rare in the literature on alpine skiing.
Authors of a 2014 study, Changes in the Balance Performance of Polish Recreational Skiers after Seven Days of Alpine Skiing – Beata Wojtyczek, Małgorzata Pasławska, Christian Raschner, similarly commented:
Our results were in agreement with the scarce information available regarding balance changes during or after a ski training camp.
The conclusion of the subject study is that skiing had a positive effect on postural stability. But the authors qualified this conclusion as valid only in measurement conditions that were similar to those in which this stability was practiced, i.e. in measurements involving ski boots and in the trials where the participants stood on one lower limb (monopedal stance) and mainly in the beginners. In the trials done with ski boots on both feet (bipedal stance), balance was worse than when the subjects were barefoot.
The fact that the improvment occured mainly in beginners provides a vital clue.
The restriction of mobility within the ankle joint significantly influenced the training-induced changes in the postural stability of both beginner and advanced alpine skiers.
Wearing Ski Boots Weakens Balance
Wearing ski boots for a few hours can lead to a weakening of the muscles that operate within the ankle joint. This works as though one joint was excluded from the locomotive function.
Dudek et al. demonstrated that the stabilising function in the process of maintaining balance was significantly weakened after an injury to the ankle joint which excluded it from locomotion for some time. Also, according to Caplan et al. , the muscle groups that determine strength and are responsible for the function of stability in the ankle joint are very sensitive to changes caused by immobilisation. They found that immediately after immobilising the ankle joint for a week, the balance parameters were 50% lower than before the immobilisation.
In summary, it can be stated that the restriction on the mobility within the ankle joints had a significant (detrimental) impact on the training-induced changes in postural stability while the participants were standing on two feet, in both beginner and advanced alpine skiers. However, in the trials where the participants stood on one lower limb, balance improved mainly in the beginners.
The most plausible explanation of this effect is that the biomechanics of the human limbs and the human torso depend on the cooperative actions of various groups of muscle.
A number of researchers have stated that, in addition to endurance and strength components, coordination and sensorimotor performance are of the utmost importance in alpine skiing.
The apparent paradox of the improvement seen in the balance of beginners on one limb when in ski boots is explained by the fact that the ski boot acts as a balance stabilizing frame as shown in Figure 58 show below from my US Patent No. 5,265,350.
In monopedal stance, inversion-eversion oscillation of the subtalar joint occurs about the tuberosity of the calcaneus (heel bone) as shown Figure 58B. Stabilizing the ankle joint within the confines of the rigid shell of ski boot effectively doubles the proximate width of the tuberosity of the calcaneus while the shaft acts as a steadying cage.
Clues to the Real Balance Issue, Torque
Scott et al.  demonstrated that during ski turns, the angular changes in the knee and hip joints can reach 50°, while in the ankle joint the oscillation is only a few degrees.
The authors of the paper, Flexural behavior of ski boots under realistic loads – The concept of an improved test method (Michael Knye*, Timo Grill, Veit Senner) commented:
Usually boots with high flex indices (stiff boots) are used by more experienced and skilled skiers whereas for beginners softer boots are recommended.
Coincidentally experienced skiers tend to keep a constant lower leg posture using boots with varying stiffness.
Isometric Contraction – The SR Stance
Three types of postural reactions to the loss of the body’s balance can be observed. The first reaction is the myotatic stretch reflex, which appears in response to changes in the position of the ankle joints, and is recorded in the triceps surae muscles. This is the earliest mechanism, which increases the activity of the muscles surrounding the joint that is subject to destabilisation. The reflex caused by a myotatic stretch causes its contraction, which then results in the stiffening of the surrounding joints as a response to the stimulus that has disturbed the balance.
The observations of the authors are confirming that experienced skiers have learned a stance that places the calf muscles in isometric contraction; an SR Stance.
The authors of the subject paper further commented:
In this situation, a torque is created between the force of gravity and the force of the reaction of the snow on the skis, which results in a descent in a curve and a loss of balance.
There are actually 3 torques that affect balance. Two of the torques are coupled through the subtalar joint as shown by the bridged rotations of Lateral-Medial Axial Rotation/Inversion-Eversion in the graphic below.
It is the outset of force applied to the outside foot through load transfer from the pelvis offset to the ground reaction force acting along the entire length of the inside edge in contact with the snow, that creates a torque that inverts the base of the ski and the foot. The torque is translated through the subtalar joint to vertical lateral axial rotation of the tibia. While the translation is somewhat restricted by confines of the shaft of the ski boot, the moment of force is not significantly diminshed,
Both sets of torques must be balanced across the ski equipment/snow interface. The question is which torque must be balanced first? Once again, the authors of the subject paper provide a clue.
This is the earliest mechanism, which increases the activity of the muscles surrounding the joint that is subject to destabilisation.
In my next post, I will describe the mechanism by which the remaining (coupled) torque is balanced.