Gregfits, if you would care to read this and give your opinion it would be appreciated. It was exerted from a conversation with David McPhail several years ago. It is right on topic here I think.
One of the best references I have for muscle function is 'The Physiology of the Joints - Vol. 1-3' by Kapandji. Volume 2 deals with the Lower Limbs.
P 186 THE EXTENSOR MUSCLES OF THE ANKLE: THE TRICEPS SURAE
Here is a quote from this section
"Thus when the knee is extended the gastrocnemius passively stretched works at its best advantage and this allows some power of the quadriceps to be transferred to the ankle. On the other hand, when the knee is flexed the gastrocnemius is maximally slackened and loses all its efficiency. Note that the gastrocnemius is not a knee flexor in spite of its position."
The best book I have on balance is called 'Understanding Balance; The Mechanics of Posture and Locomotion' by Tristan Roberts. I guess there is not a lot of interest in this subject because I could not find a copy of the book in North America within a year of it being published. I had to get a Podiatrist friend in London England to get me a copy from a medical bookstore there.
Roberts goes into great detail about how in upright postures the challenge of our balance system is to monitor and analyse our every movement to determine whether we are in danger of falling. The price we pay for being bipeds is that our brain is at the top of our vertical column where it is susceptible to damage should a fall occur. Said Roberts in the introduction to his boot:
" At the time of writing that book (his last book 'The Neurophysiology of Postural Mechanisms') I had come to the conclusion that everything to do with balance came down to a problem of recognition. For example, if one is to make corrective movements to avoid overbalancing, one must be able, on some level, to recognise that the moment has arrived at which corrective movements are called for."
It is interesting in that when beginner skiers get pitched forward and CoM begins to pass the balls of their feet their leg muscles will often fire to extend the ankle as if they were about to take a step. If you view this action from the perspective of the beginners balance system with limited data on moments in skiing taking a step is about the only pattern it an relate such a movement to.
The gastroc is integrated with soleus in a muscle group called the triceps surae. This group is comprised of 3 muscle bellies (soleus = 1, gastroc = 2). Both muscles are tied to the rear of the heel bone in a band that is integrated with a ligamentous sheet that runs from the heel to the balls of the feet. The soleus ties to the top of the tibia just below the knee. The gastroc ties to 2 shelves at the lower end of the femur just above the knee that form a 'slack mechanism' in the muscle that keeps the gastroc loose until the knee is almost fully extended. The hamstrings run from the pelvis through the 2 gastroc bellies to the top of the tibia. So there is a pull on either side of the knee by these intertwined muscles.
The triceps surae is most effective as a unit when the ankle is flexed (not too much) and the knee is almost fully extended. This is the essential combination of a strong ski stance. If the ankle is too flexed the knee can not extend enough to optimally tension the gastroc. Any significant flexion of the knee will render the gastroc ineffective.
The key in skiing is to set up a pull from pelvis to the balls of the feet through the hamstring/triceps bridge. Extension of the new outside leg at initiation tensions the soleus-gastroc-hamstring band. As the turn progresses the pull is maintained as the knee flexes by the soleus - hamstring band (no gastroc) but only if the skier keeps forward (i.e. keeps CoM in front of their ankle). Since the pull from below the knee to the pelvis is by the hamstrings only flexion must be maintained at the pelvis to maximise the effectiveness of this muscle group.
The 2 Stage Ski Stance
Efficient balance in upright stances requires a coordinated Press-Pull mechanism. In walking, extending at the knee moves CoM up and in front of the ankle to activate the support or stance leg. As CoM moves in front of the ankle the soleus shifts into isometric contraction resisting further forward movement of CoM. This causes the weight of CoM to compress (i.e.push down) on the arches of the foot tensioning them. This is the Press part. The tendency of CoM to move forward by rotating about the angle tensions the soleus. This is the Pull part.
In quiet standing the Press force on the arches is quite light because the Pull force which is primarily in the triceps surae (soleus + gastroc) is light. There is only a slight Pull by the pelvis on the hamstrings which are co-contracted with the quads. In this configuration there are 3 major muscle groups bridging the knee joint.
The basic extended stance in skiing is based on the same mechanism. Moving CoM forward by adjusting ankle, knee and hip joint angles on a level surface increases the force in the Press-Pull mechanism. Tilting the stance slightly forward increases the force more. The triceps surae is most effective and powerful when the ankle is flexed and the knee extended. In the extended stance the gastroc is more prominent in the Pull than the hamstrings.
As the turn progresses the knee and hip joints flex progressively for pressure control. There should be much more flexion change here than in the ankle which can not change very much because the soleus is the primary Pull mechanism in the leg. As knee flexion increases the gastroc will go slack and and drop out of the Pull loop. As it does the Pull mechanism will switch gears. As it does the hamstrings assume the task of the prime Puller in the thigh. For this reason the skier must increase hip flexion to maintain the tension in the hamstrings.
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