It is relatively meaningless to discuss muscle action or training without considering the role of the connective tissues associated with muscle. All muscle comprises a contractile actin-myosin component and a non-contractile connective tissue component. The contractile component is in series with a series elastic component (SEC), consisting of elements such as the tendons and muscle cross-bridges, and in parallel with a parallel elastic component (PEC), which involves units such as the fasciae and epimysium. The SEC can store considerable elastic energy after ballistic actions such as running, jumping or throwing, thereby increasing the thermodynamic efficiency of movement. For example, it has been found that much of the muscle activity in running is associated with the tensioning of tendons, which thereby store energy for successive phases of movement. This is why resistance training methods chosen to enhance SEC strength may improve running performances at all distances. Moreover, increased connective strength of ligaments, tendons and joint capsules can improve resistance to injury.

Besides its role in ballistic and rapid movements, connective tissue can also increase the overall bulk of the muscle complex and aid it in producing usable strength. Recently, it has been found that increases in the quality (or type) and quantity of connective tissue may improve the transmission of force from individual muscle fibres to the skeletal system. For example, an insufficiently strong or extensive sheath of connective tissue will allow the muscle to dissipate some of its force in a direction which is perpendicular to its line of action and thereby lower its efficiency in overcoming a load.

Analysis of elastic energy mechanisms and the stretch reflexes has led to development of a training method known as plyometrics, which involves involuntarily stimulating the muscles with eccentric contraction and explosive isometric action followed by a sudden stretch preceding the voluntary concentric effort1. Characteristically it involves explosive depth jumps and medicine ball rebounding, with the primary aim of enhancing reactive ability, explosive strength and speed-strength Plyometrics can be a highly effective strength training means, but applied by the unprepared or unskilled, it can readily produce musculoskeletal injury.

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