Muscle tissue is unique in its ability to generate tensile forces. There are three types of muscle found within the body: smooth muscle, which is found as part of many organs including blood vessels, functioning to constrict vessels and so regulate blood flow. Cardiac muscle forms the bulk of the heart, and its powerful contractions are responsible for the movement of blood around the hearts chambers, and the circulatory system. Finally, skeletal muscle is attached to the skeleton via connective tissues, and utilises its tensile forces to pull against bones to provide the body with strength, and movement.
1.9.2 Gross anatomy of skeletal muscle
At the gross anatomical level skeletal muscles have a fibrous appearance, and can take on a variety of different shapes and sizes. As muscle tissue generates
contractile force, it is intuitive that a larger muscle produces more force. Size is not the only determinant of a muscle’s force, in fact fibre orientation, or muscle
architecture, is the primary indicator of a muscle’s functional capacity (Lieber and Ward 2011). The contractile units of skeletal muscle are arranged in series along the length of muscle fibres. Thus the orientation of a fibre determines the axis of force generated (Lieber and Fride´n, 2000).
43 Across skeletal muscles the range in fibre architecture is almost as varied as the number of muscles themselves, but broad terms can be used to describe common arrangements (Fig.1.12). Concentric rings of muscle fibres can provide a sphincter function, as seen in orbicularis oculi and orbicularis oris, guarding the eye and mouth respectively, this arrangement is known as a circular muscle. In some muscles the fibres are arranged parallel to each other and the axis of force generation, these are termed parallel or strap muscles as seen in sartorius and pronator quadratus. Fibres can be arranged at a single angle relative to the axis of force generation, these are unipennate muscles. Many skeletal muscles have fibres orientated at two or more angles relative to the axis of force generation and these are termed bipennate and multipennate muscles respectively (Lieber and Fride´n, 2000).
Figure 1.12 Examples of different muscular architectures. Skeletal muscles have large variations in the arrangements of their muscle fibres, some common arrangements being parallel, circular, as found in orbicularis oris, multipennate, as found in deltoid, parallel as found in sartorius, bipennate as found in rectus femoris and unipennate and found in extensor digitorum longus. Figure modified from Marieb and Hoehn (2007).
44 The pennation angle of a fibre is the angle at which it is orientated with respect to the axis of force generation. Often an increase in pennation angle reflects a decrease in contractile force as a smaller proportion of tensile force is transmitted along the tendon (Lieber and Fride´n, 2000). Despite this, many muscles exhibit highly pennate architectures, as a shift in pennation angle will also influence the length of muscle fibres (Lieber and Ward 2011) and so the excursion and velocity of a muscle (Bodine et al. 1982). A muscle with a highly pennate architecture will have shorter fibres than one of equal size without pennation. Shorter fibres consume proportionally less energy than longer fibres (Biewener and Roberts 2000) and so a greater pennation angle reflects an energy saving. Consequently, the magnitude of force a skeletal muscle can generate is determined by the number, length and arrangement of fibres it is composed of. These parameters can be summarised by the physiological cross-sectional area (PCSA) of a muscle (Powell et al. 1984). PCSA is the cross-sectional area of the muscle fibres in a muscle and has been found to correspond proportionally to the maximum force generation potential of a muscle (Lieber and Fride´n, 2000). The formulation of PCSA is often derived with the formula (Alexander and Vernon 1975);
PCSA (mm²)= muscle mass (g)
muscle density (g/mm³) · fibre length (mm)
1.9.3 Microscopic anatomy of skeletal muscle
As with bone, skeletal muscle is a hierarchical tissue (Fig.1.13). At the macroscopic level skeletal muscle appears striated. Each striation is a muscle fascicle, which is a bundle of muscle fibres and is surrounded by a layer of connective tissue, the
endomysium (Purslow 2010). Skeletal muscle fibres are among the largest and most complex cells found in vertebrates (Resnicow et al. 2010) and are themselves composed of smaller units termed myofibrils. Myofibrils are long cylindrical
multinucleate structures composed of an arrangement of end-to-end contractile units called sarcomeres. Sarcomeres are composed of thin actin filaments and thicker myosin filaments. Thick and thin filaments are arranged in inter-digitating rows, which interact with each other to generate force. The number of sarcomeres present in a myofibril is dependent upon the length and thickness of the fibre, which is why these parameters play such a crucial role in force production (Lieber 2002).
45 a motor nerve, and a number of muscle fibres. Nerves communicate with fibres via motor end-plates, or neuromuscular junctions (NMJ) (Schiaffino and Reggiani 2011). This functional grouping of fibres allows for selective recruitment of motor units, allowing a muscle a wide range of contractile states, not simply on or off.
Figure 1.13 Diagrammatic representation of the hierarchical structure of
skeletal muscle. Skeletal muscle is composed of a vast number of muscle fascicles bounded by connective tissue. Each fascicle is made up of a number of muscle fibres, which themselves are composed of myofibrils. Each myofibril is composed of the proteins actin and myosin. Figure modified from Whiting and Rugg (2006).
1.10 Force production and transmission