The Structure of Skeletal Muscles, and How The Force They Generate Results in Human Motion
Skeletal muscles are surrounded by three layers of connective tissue: the epimysium, the perimysium, and the endomysium, which fuse together to form a string of tough connective tissue termed tendon. The tendon in turn is attached to the outmost of the bones (the periosteum). During muscular contraction the force generated by the muscle fibres (active elements), is transmitted to the muscle connective tissues (passive elements) and, in turn via tendon to the bones, which results in a moment about the corresponding joint. The muscle fibres are composed of numerous sarcomeres (the smallest functional unit). The sarcomere exhibits two important myofilaments, actin and myosin, as well as regulatory and other structural proteins.
The contraction of skeletal muscles is a classic example of a biological structure-function relationship. In fact, at both macro and micro-scopic level, skeletal muscles are exquisitely tailored for force production. There are different kinds of muscular contraction: isometric, dynamic (or isotonic), and isokinetic. During isometric contraction the muscle contracts without changing its length, and therefore does not produce any observable movement, e.g. when carrying bags biceps brachii contracts, but no movements occurs. Isokinetic contraction refers to contraction at constant speed, and this can only be achieved with the use of an isokinetic piece of equipment. However, the most used within life is the dynamic contraction (also known as isotonic). The dynamic muscular contraction is composed of two phases, concentric contraction and eccentric contraction, respectively.
Concentric and eccentric contraction
During concentric contraction, the tension generated by the muscle is superior than the resistance to shortening (load), and the muscle shortens during tension. This results in a “pull” about the tendons, which in turn cause a pull about the bone where the tendon is attached generating movements such as walking, running, or lifting a weight. Whereas, during eccentric contraction the tension generated by the muscle is less than the resistance to shortening, and the muscle lengthens during tension; for example running downhill or lowering a weight.
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