I-C-E: SR


These letters are not some sort of secret code. They are abbreviations for Isotonic, Concentric and Eccentric and Isometric.

  • Isotonic is a muscular contraction in which the length of the muscle changes
  • Concentric is an isotonic contraction where the length of the muscle shortens
  • Eccentric is an isotonic contraction where the length of the muscle lengthens
  • Isometric is when a muscle contracts but does not change length

Tonus is state of light muscle contraction. When there is no weight on a leg (i.e. – the foot is off the ground as it is in the swing phase of gait), there is no tonic activity.

When we are standing erect on a flat, level, hard, stable surface in the ideal anatomic plumb alignment position, the degree of muscle effort required to maintain an upright posture is minimal and tonus is light. But skiing involves maintaining balance in a dynamic physical environment characterized by constantly changing 3-dimensional forces made more complex by asperities (variations in the snow surface) and undulations in terrain. This places enormous demands on the balance system. Maintaining balance in such a challenging enviroment, requires minimal latency (time for the balance system to respond) and maximal refex corrective action.

Despite the importance of eccentric contraction in balance in skiing, suprising little, if any, recognition or consideration is given to the type of muscle action associated with a movement such as knee angulation, let alone any consideration of the effect on skier balance and the physiologic processes that protect against injury. I could not find the words concentric or eccentric listed in the Index to LeMasters’s Ultimate Skiing even though he refers to specific muscles in his description of knee angulation. An online search for ‘stretch reflex alpine skiing’, garnered only one hit (I will discuss the results in a future post). There doesn’t even appear to be universal agreement among the various authorities on what constitutes balance in skiing let alone any discussion of how the balance process works. Balance seems to be whatever the authorities declare it to be.

In order to understand how to build a strong stance based on the SR, one has to possess at least a basic knowledge of muscle actions, the role of EC in postural responses and how the static preload enables the SR.

The postural responses that maintain an upright posture are mediated by muscles in eccentric contraction; primarily the soleus-gastrocnemius-hamstring chain. What is seldom mentioned is that postural muscle action relies on reaction force resulting from compression loading of the plantar ligament that supports vault of the 3 arches of the foot. Compression of the arches sets up tension in the plantar ligament that creates shear force driven reaction force for EC-SR contraction.

In my next post, I will explain how this works. For now, the graphic below shows a simple model of the foot that I made in 1992 to illustrate how a static preload is set up.

In the LH graphic, the model is suspended in the air (unweighted). So there is no load on the base and no tension in the arches.  The black arrows show the reaction forces. The blue lines show tensile forces. The red arrows show the action force arising from muscle contraction. The plumb bob, representing CoM, is pulling from the front ends of the two struts (balls of the foot) to the top of the Tibia where it acts vertically over the arches of the model of the foot.

Think about how this works.

sr-tripod-demo