SR STANCE BASICS: ECCENTRIC MUSCLE POWER AND THE STRETCH REFLEX


Through subjective experiments with racers that started about 1978, I found that a net ramp angle of approximately 3 degrees  seemed to support a strong stance. Over the past 2 years, my interest in fine tuning ramp angle was piqued by followers of my blog who read posts such as WHAT’S YOUR ANGLE?, did their own subjective experiments with ramp angle and realized significant benefits. This ski season, I started incrementally reducing the ramp angle of the boot boards in my own ski boots with immediate and perceivable positive results. It is important to note that my bindings have zero ramp.

Last fall, when I started working with local Whistler-Blackcomb Ski Pro, Matt, I had an opportunity to study the effects of ramp angle from a point of initial awareness. By initial awareness, I mean that Matt, like most skiers, was not aware that ramp angle was a factor affecting his skiing. This opportunity served as the impetus for the design of the Roll Over trainer and, more recently, the Stance Ramp. I wanted to find out whether a skier could identify the ramp angle that gave them the strongest stance using a device like the Stance Ramp and whether this would translate to the best ramp angle in ski equipment as confirmed during actual skiing. I also wanted to find out if a device like the Stance Ramp could impart a kinaesthetic awareness of the effect of ramp angle on stance in relatively new skiers.

EC Power

EC stands for eccentric muscle contraction. In eccentric or EC muscle contraction, an external load such as gravity causes a muscle to stretch and lengthen. The muscle maintains its length or controls the degree of lengthening by contracting against the external load acting to stretch it. In concentric or CC muscle contraction, a muscle is actively shortening in contraction. Eccentric contraction can generate about 40% more power than concentric contraction. Eccentric muscle contraction makes it possible for us to stand upright against the external force of gravity.

The Stretch Reflex

A muscle in eccentric contraction subjected to a load that causes it to rapidly lengthen triggering a myostatic reflex response called the stretch reflex. It is a monosynaptic reflex that provides automatic regulation of skeletal muscle length. Reflex responses are the simplest responses and the least affected by voluntary controls. They are rapid, somewhat stereotyped and usually controlled in a graded way by the eliciting stimulus. Rhythmic Motor Patterns like walking and running combine features of voluntary and reflex acts. Elite skiers reach a point where skiing becomes a Rhythmic Motor Pattern and only the end-beginning of a turn requires voluntary action.

Typically, only the initiation and termination of a sequence are voluntary. Once initiated, the sequence of relatively stereotyped, repetitive movements may play out almost automatically in reflex-like action. The latency of the stretch reflex (time required to respond) is tied to the length of the affected muscle and the degree of tension. Ramp angle affects both. A ramp angle in the order of 2.5 degrees in relation to the plane of the base of a ski seems to increase the tension of eccentric gastrocnemius-soleus complex and may decrease the latency of the stretch reflex. This is an area where studies need to be done to confirm this.

The graphic below shows the gastrocnemius-soleus complex. This is the key muscle group in a strong stance that is configured by the Resistive Shank Angle.

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visible-body

Some degree of ramp is present in the majority of bindings and ski boots. But there does not appear to be any standard or consistency. Worse, ramp angle in bindings varies with the distance between the heel and toe pieces. That the majority of boots and bindings have ramp angle in the apparent absence of any standard suggests that ramp angle is important to skiing, but the amount isn’t. This makes no sense. Ramp angle in any form of footwear affects muscle length and tension, both of which are critical to a strong ski stance. The new Nike shoes I just purchased mark the heel lift or drop on the outside of sole of the shoe. I know the drop is 5.o mm because it says so.

5.0

While this doesn’t tell me the ramp angle, it at least makes me aware of the amount of drop so I can compare it to shoes with different drops. How about skis and bindings? Nothing. There should be. Most skiers are not even aware ramp angle is an issue. Go into a ski shop and ask a salesperson what the ramp angle of a boot or binding is and your question will probably be met with a blank stare.

Until standards and testing are introduced, the Stance Ramp allows a skier to subjectively test the effect of ramp angle on stance by experimenting with different angles starting with an arbitrary standard of 2.5 degrees.  Technologies such as CARV may enable a stance profile to be captured on device like the Stance Ramp that could be used as a standard with which to compare stance pressures in a ski boot during actual ski maneuvers.

Tipping Point

While it seems probable that the ideal ramp angle will be similar for most skiers, the only way to confirm this is through controlled studies. Until this is done, the only option appears to be subjective assessments done on a device like the Stance Ramp. In my subjective experiments with the Stance Ramp starting from 2.5 degrees and increasing the ramp angle by 1.25 degree increments, a point was quickly reached where the ramp angle made me feel unstable and I had to shift my weight to my heels for stability. I call this the Tipping Point.

The net ramp angle for recreational skiers appears to be in the order of 4 to 5 degrees. I checked the net ramp angle of a recreational skier recently and it was 5.17 degrees; boot board ramp of 3.27 degrees plus binding ramp of 1.90 degrees. This amount of ramp angle will force a skier to ski with their weight on their heels for stability.

In my next post I will discuss how I use the Stance Ramp to fine tune boot board ramp angle and study the effect on stance of strategies like rounding the shoulders and back.

 

 

 

4 comments

  1. Easily tested on a force platform. With the boot in a standard location on the platform, ramp angle could be changed and the horizontal location of the centre of pressure could be tracked per change. Might have to do that…

    1. A ski is really a perturbation platform moving over an undulating source of GRF in a dynamic 3-dimensional physical environment. I have been conducting some simple subjective tests with the Stance Ramp with remarkable results. I intentionally designed the stance ramp with what I felt was a sub-optimal angle of 2.5 degrees. In making incremental increases of 0.12 degrees, it quickly became obvious that 2.62 degrees seemed to potentiate stretch reflex tension that just as quickly degraded much past that angle. Ideally, studies would be done with an adjustable angle stance ramp that could be changed during perturbations without the subject being aware of it. This is easily done with a small electric motor or external cable adjustment means that turns a screw jack. Changes in ramp angle will elicit reflex adjustments in global joint angles. So photo tracking with joint markers would be telling.

      Once you have some baseline data, tests could be done with the subject in ski boots.

    1. The boot board sits in the bottom of the boot shell. It has no effect on the binding interfaces. In most cases, removing the liner, turning the shell upside down and banging the rear spine of the shaft on a hard surface will free the boot board from the bottom of the shell. Depending on the construction. the boot board can be ground down on a belt sander. I use a small wood working block plane to remove material. Not all boot boards can be modified because some are made from injected plastic and are not solid.

      The big problem, and it is a really big problem, is that most bindings have height differences in the stand plates on the heel and toe pieces. The toe is lower. This creates a variable ramp angle that gets steeper as the toe and heel pieces are brought closer together. This has a significant effect on EC tension and muscle length than impacts the stretch reflex. The latter is something that seems to be either an unknown a non-issue in the ski industry. The effect of ramp angle would be a good project for a study. It can easily be done in a lab using a perturbation platform.

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