In this post, I am going to discuss why the optimal stance for skiing is dependent on the loading sequence of the new outside foot of turn, how this must start in the transition phase and why it is critical to the rocker impulse loading mechanism that engages the shovel and inside edge of the outside ski at edge change. This issue was introduced in THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: TIMING OF EDGE CHANGE. The rocker impulse loading mechanism and the ability to balance on and control the outside ski is dependent on the ability to rapidly tension the biokinetic chain that stiffens the forefoot and torsionally stiffens the ankle and knee joints. This process enables top down, whole leg rotational force, into the turn, to be effectively applied to the foot and ski from the pelvis.
A Middle Ground on Stance
Although there is much discussion in skiing on the subject of stance, it is rare for discussions to include, let alone focus on, the foot.
The red rectangle in the graphic below shows the mid stance phase in the 8 component Gait Cycle.
A common position amongst the various authorities in skiing on stance, is that it is represented by the mid stance phase of the Gait Cycle. The 8 component Gait Cycle is the universal standard for discussion and analysis of gait in human movement. During the turn phase, the sole the outside foot or stance foot is in substantially constant contact with the zeppa or boot board. Since the ski stance does not involve initial heel contact or terminal phases, it was reasonable to conclude that skiing must be a mid stance activity.
Assuming that stance skiing is a mid stance activity also meant that the joints of the foot are mobile and the foot is still pronating and dissipating the shock of impact. The fact that the foot is not yet fully tensioned in mid stance, while still pronating, appears to have led to the conclusion that the foot is unstable and in need of support. Towards this end, form fitting footbeds, liners and, more recently, form-fitted shells were introduced and soon became standard. I described what has become known as the Holy Grail of skiing; a perfect fit of the boot with the foot and leg; one that completely immobilizes the joints of the foot in my post, A CINDERELLA STORY: THE ‘MYTH’ OF THE PERFECT FIT. This objective, precipitated the premise that forces are best applied to the ski using the shaft of the ski boot as a handle with the leg acting as a lever. In this paradigm, the foot was relegated to a useless appendage.
The Missing Ninth Component – Late Stance
The problem with the assumption that mid stance is the defacto ski stance is that it has only recently been suggested that a critical ninth component, Late Stance, is missing from 8 components of the Gait Cycle.
Although it has been known for decades that the foot undergoes a sequential loading/tensioning process that transforms it from what has been described at initial contact as a loose sack of bones, into a rigid lever in terminal stance for propulsion, the effect of fascial tensioning on late stance has remained largely unexplored until recently when the exclusive focus on the rearfoot began to shift to the forefoot. I discuss this in BOOT-FITTING 101: THE ESSENTIALS – SHELL FIT.
As recently as 2004, Achilles/PA loading of the forefoot was poorly understood. Under Background, a 2004 study (2.) on the role of the plantar aponeurosis in transferring Achilles tendon loads to the forefoot states:
The plantar aponeurosis is known to be a major contributor to arch support, but its role in transferring Achilles tendon loads to the forefoot remains poorly understood.
The study found:
- Plantar aponeurosis forces gradually increased during stance and peaked in late stance.
- There was a good correlation between plantar aponeurosis tension and Achilles tendon force.
- The plantar aponeurosis transmits large forces between the hindfoot and forefoot during the stance phase of gait.
- The varying pattern of plantar aponeurosis force and its relationship to Achilles tendon force demonstrates the importance of analyzing the function of the plantar aponeurosis throughout the stance phase of the gait cycle rather than in a static standing position.
Changes in Muscle-tendon unit (MTU) and peak EMG increased significantly with increasing gait velocity for all muscles. This is the first in vivo evidence that the plantar intrinsic foot muscles function in parallel to the plantar aponeurosis, actively regulating the stiffness of the foot in response to the magnitude of forces encountered during locomotion. These muscles may therefore contribute to power absorption and generation at the foot, limit strain on the plantar aponeurosis and facilitate efficient foot to ground force transmission.
Transmits large forces and foot to ground force transmission means large downward forces directed at the ground or to a ski and from there to the snow.
Although I did not understand the esoteric details of fascial tensioning back in 1993, I was sufficiently aware of the relationship between peak tension in the plantar aponeurosis (PA), to be able to construct a simple model that illustrates how peak PA tension results in peak Achilles tension and how this causes the soleus muscle to go into isometric contraction, arresting further forward movement of the shank. I discuss this in detail in my series of posts on the SR Stance.
The photos below shows the simple model I made in 1993. Simple models of this nature are finding increasing use today to model what are called Anatomy Trains.
In late stance, the foot gets shorter in length and the arch gets higher and tighter as intrinsic tension transforms the foot from a mobile adapter in early stance into a rigid lever in late stance so it can apply the high force to the ground necessary for propulsion in the terminal stance phase that occurs at heel separation. The graphic below shows how the arch height h to foot length L ratio increases as the foot is getting shorter and the arch gets higher in late stance.
What has only recently being recognized is that the fascial tension that occurs in stance maximizes balance responses, neuromuscular efficiency and protection of the lower limbs through a process of foot to core sequencing; one that stiffens the forefoot and torsionally stiffens the joints of the ankle and knee.
Loading/Fascial Tensioning Speed
A 2010 study (4.) found:
Early-stance tension in the PA increased with speed, whereas maximum tension during late stance did not seem to be significantly affected by walking speed. Although, on the one hand, these results give evidence for the existence of a pre-heel-strike, speed-dependent, arch-stiffening mechanism, on the other hand they suggest that augmentation of arch height in late stance is enhanced by higher forces exerted by the intrinsic muscles on the plantar aspect of the foot when walking at faster speeds.
…… or, by more rapid, forceful impulse loading at ski flat – see SUPER PETRA VLHOVA’S EXPLOSIVE IMPULSE LOADING IN ASPEN SLALOM
A 2013 study (3.) found:
Although often showing minimal activity in simple stance, the intrinsic foot muscles are more strongly recruited when additional loads are added to the participant.
A 2015 study (5.) found:
Changes in Muscle-tendon unit (MTU) and peak EMG increased significantly with increasing gait velocity for all muscles. This is the first in vivo evidence that the plantar intrinsic foot muscles function in parallel to the plantar aponeurosis, actively regulating the stiffness of the foot in response to the magnitude of forces encountered during locomotion.
These muscles may therefore contribute to power absorption and generation at the foot, limit strain on the plantar aponeurosis and facilitate efficient (vertical) foot to ground force transmission.
…….. or foot to ski to snow force transmission.
The Optimal Ski Stance is Unique
While the optimal stance for skiing has the greatest similarity to the late phase of stance, I am not aware of any stance that has requirements similar to the ski the stance where a specific loading sequence precedes rocker impulse loading as the outside ski changes edges in the top of a turn.
As with the gait cycle, the movement pattern associated with a turn cycle also involves loading and swing phases.
Time To Cascade
There are two intertwined rocker mechanisms that impulse load the forefoot at ski flat between edge change. These rocker mechanisms rely on what the 3 components of what I refer to as the Time To Cascade which is only possible when the plantar aponeurosis is rapidly fascially tensioned.
- Time to Fascial Tension which affects,
- Time to Stabilization which affects
- Time to Protection which protects the lower limbs
In my next post, we will Meet the Rockers and continue with the discussion of the mechanics of balance on the outside ski.
- Dynamic loading of the plantar aponeurosis in walking –Erdemir A1, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA. J Bone Joint Surg Am. 2004 Mar;86-A(3):546-52.
- Dynamics of longitudinal arch support in relation to walking speed: contribution of the plantar aponeurosis – Paolo Caravaggi, Todd Pataky, Michael Gu¨ nther, Russell Savage and Robin Crompton – Human Anatomy and Cell Biology, School of Biomedical Sciences, University of Liverpool, Liverpool, UK – J. Anat. (2010) 217, pp254–261
- The foot core system: a new paradigm for understanding intrinsic foot muscle function – Patrick O McKeon1, Jay Hertel2, Dennis Bramble3, Irene Davis4 Br J Sports Med doi:10.1136/bjsports-2013-092690
- Active regulation of longitudinal arch compression and recoil during walking and running Kelly LA, Lichtwark G, Cresswell AG – J R Soc Interface. 2015 Jan 6;12(102):20141076.