Stretching may not be so important - at least for tendons.

     One of the reasons that flexibility may not be as important as we once thought, is that tendons that are adequately stiff are more energy efficient than overly flexible ones. When you are walking and plant your right foot on the ground, your body continues to advance over and past it. As it does, the Achilles tendon becomes increasingly taut. Like a wound up rubber band propeller in a toy airplane, it is storing energy - "kinetic energy."  When stretched to a certain length of tension at push off, the Achilles tendon literally snaps into contraction helping push your foot into plantarflexion (i.e. your foot pushes off the ground). A study by  Maganaris and Paul (2002, see Evidence page on PrimeforLife.net under "Achilles Tendinopathy") determined that in walking this "catapult action" of the Achilles tendon  provided about 6% of the energy in walking in 6 men! Hopping on one foot in a study by Lichtwark and Wilson (2005), provided in elastic recoil 16% of the total work!
     The degree of elasticity, or ability of the tendon to spring back to its original length is not perfect, though. The inability to return immediately to the original length/tension results in lost kinetic energy - a phenomenon known as hysteresis. The lost kinetic energy is, instead, heat energy. If excessive, this heat production may be injurious to the tendon and contribute to Achilles tendon degeneration. Animal studies have shown that the heat produced may exceed 108 degrees F. - more than enough to cause tendon damage.  More flexible tendons tend to not spring back as well as adequately stiff ones, which means they produce more heat then stiffer tendons. Indeed, the Lichtwark/Wilson study found a variable amount of hysteresis (or lost energy) of 17 to 30%. More flexible individuals tended to be more inefficient at using the elastic recoil. Several other studies showed that individuals with stiffer lower extremities ran and walked with significantly more energy efficiency than more flexible people. It makes sense. A flexible person with long, less stiff Achilles tendons, never even feels a stretch of the tendon as they advance past the planted foot. There is no or little kinetic energy to provide recoil and help push off. So this, looser person, must compensate by adding an additional 6% of energy to maintain the same pace as the stiffer person. In most sports the elite athletes are not the gumbies. They are stiff, coiled springs.
     Stretch appropriately, but don't do so with the goal of being super flexible. Doing so may increase the odds of tendon problems. Read the Flexibility chapter in Prime for Life to assess whether you need to stretch and which areas need added range.

    
Posted by Randy Raugh at 6/26/2009 9:13 PM
Categories: Flexibility
Tags: Randy Raugh
 

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