Viscoelasticity and Joint Biomechanics

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The previous article reviewed that, in pure fitness training, movement is used to meet the goal of applying stress to the muscle, which develops the muscle’s performance. This articles explains the importance of viscoelasticity and joint biomechanics, how fitness training should be properly practiced to preserve muscle health. The red fleshy part of the muscle is called the ‘muscle belly’, the part that contracts (shortens and then lengthens).  The amount of force that the muscle can generate is proportionate to its cross-sectional area.  The range of joint movement a muscle can produce is proportionate to its fiber length and pattern.  The muscle belly blends into tendon at the ‘myotendinous junction’, a frequent location of injury with inappropriate application of force.  The tendon is the rope that transfers the force of muscle contraction into joint movement.  To do this, the tendon must cross over the joint.  Some muscle-tendon units cross only one joint, others cross more than one joint.  Most joints primarily rotate around a pivot point.  The muscle-tendon’s efficiency at moving the joint is determined by the distance from the pivot point to where the tendon passes by and is called the ‘moment arm’.  The longer the moment arm is, the more efficient the tendon at rotating the joint.

During fitness training, resistance to movement is intentionally added by gravity, free weights, bands, or cables. The force of contraction in the muscle is used to move the joint against the added resistance.  But because each individual joint is rotating (even though the weight may be moving in a straight line), the quantity being measured is really torque, not linear force.

Torque = force x distance (moment arm of the tendon from the center of joint rotation)

The formula for work performed by the muscle is similar:

Work = force x distance (distance referring to movement)

Some fitness analysts state that during an isometric contraction, the muscle isn’t doing any work because the weight isn’t in motion.  But ask the man holding the weight in mid-air contraction and he will say he is doing plenty of work.  Of course he is, and the reason is that the muscle is exerting a constant torque across its moment arm relative to the joint.

Torque on the joint applies stress to the connective tissues: cartilage, tendon, and ligament.  Basic levels of regular daily stress keep these tissues healthy.  Overload causes repetitive or singular injury.  Because these connective tissues are viscoelastic (their stress-strain behavior depends on how rapidly force is applied to them), sudden or violent movements cause greater damage than the controlled application of force.  This leads us to next month’s topic of the different types of muscle contraction and how muscle groups work together.

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Dr. Mark Henry, MD
After more than twelve years in this sub-specialty, Doctor Mark Henry cares for more patients with these conditions each year than many surgeons see in an entire career. Having earned the Top Medical Graduate award from Duke University in 1992 (ranked a top-five medical school) and the nation's highest score on the 10-year maintenance of certification examination in 2011, Dr. Henry is considered an international expert in hand surgery. Ranging from relatively simple procedures such as carpal tunnel release, all the way to the most complex microsurgical reconstructions, Dr. Henry has more than 90 peer-reviewed publications and presentations in the world's leading textbooks, journals, and professional societies.

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