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86 m a s s a g e & b o d y w o r k s e p t e m b e r / o c t o b e r 2 0 1 4 STORY 1 ON THE BUS Skeletal muscle tissue can perform its functions because it possesses four properties: • Excitability. • Contractility. • Extensibility. • Elasticity. Singularly amazing, these attributes, when combined, lend muscle tissue an almost superhero- like quality seen in no other tissue of the body. Found in both muscle and nerve cells, excitability is the capacity to respond to stimuli. Chemical, electrical, or mechanical stimuli can all lead to muscle activation (Image 1). For instance, when standing on a crowded bus, you reach up to grasp the overhead railing. This is possible because your shoulder and arm muscles respond to the electrochemical stimuli sent from your somatic motor nerves. As the fl exors of your fi ngers curl your digits around the rail, they demonstrate contractility, a muscle's ability to develop tension when stimulated (Image 2). It may seem counterintuitive, but a muscle's contractions may result in a muscle shortening, elongating, or staying the same length. In order for your fi ngers to grasp the bar, your fl exor muscles need to shorten, which means other muscles have to lengthen. This property is extensibility, the capacity of muscle tissue to stretch without being damaged. On the bus, this is displayed by the extensors of your fi ngers, which passively lengthen to allow the fl exors to shorten (Image 3). Extensibility allows a muscle to maintain functionality while adopting a range of lengths and shapes. As you release your grip, both groups of muscles display elasticity, a muscle's tendency to return to its original length after being stretched. Your hand fl exors are stretched and lengthened while your fi nger extensors contract and shorten (Image 4). When you relax the fl exors, the elastic property of the stretched extensors returns both muscle groups to their resting lengths. Excitability as you reach for the rail. 1 Extensibility as your extensors stretch. Elasticity as your muscle tissues return to their original length after releasing the rail. 3 4 Muscle tissue is an elastic material—its length can vary. Your hamstrings, for instance, may shorten to 12 inches or elongate to 20 inches. Somewhere between these extremes is a normal resting length. This is a muscle's length when it is unstimulated, with no forces acting upon it. Due to physiological processes, a muscle's normal resting length will change continually with posture, exercise, health and age. 2 Contractility as your fl exors shorten.