Changes

Muscles of the same shape can behave very differently  

Muscles of the same shape can behave very differently  

Figure 5.7 compares three muscles of the same mass, and the same general shape.  Each of these muscles could fit into the same space in the body.  If the fibres are parallel to the force vector of the whole muscle (Fig. 5.7 a), the number of sarcomeres in series is maximal, and the number of sarcomeres in parallel is minimal, for a muscle of this shape.  The fibrous architecture of such a strap muscle gives maximal range of movement, and minimal strength.

Figure 5.7 compares three muscles of the same mass, and the same general shape.  Each of these muscles could fit into the same space in the body.  If the fibres are parallel to the force vector of the whole muscle (Fig. 5.7 a), the number of sarcomeres in series is maximal, and the number of sarcomeres in parallel is minimal, for a muscle of this shape.  The fibrous architecture of such a strap muscle gives maximal range of movement, and minimal strength.

If the fibres are aligned at an angle to the force vector of the whole muscle (Fig. 5.7 b, c), the effective force and range of movement of each fibre is reduced since it is proportional to the cosine of this angle.  Compared with Fig. 5.7 a, the number of sarcomeres in series has been reduced, and the number of sarcomeres in parallel has been increased.  Thus the force has been increased in spite of the angulation of the fibres, but the range of contraction has been decreased.  In the direction of the fibres, the work done during contraction is similar for the three muscles, since their mass is the same.

If the fibres are aligned at an angle to the force vector of the whole muscle (Fig. 5.7 b, c), the effective force and range of movement of each fibre is reduced since it is proportional to the cosine of this angle.  Compared with Fig. 5.7 a, the number of sarcomeres in series has been reduced, and the number of sarcomeres in parallel has been increased.  Thus the force has been increased in spite of the angulation of the fibres, but the range of contraction has been decreased.  In the direction of the fibres, the work done during contraction is similar for the three muscles, since their mass is the same.

:::::Diagrammatic representations of three muscles of similar mass but different shape but of widely varying fibrous architecture. Approximate values for range of contraction, force and work are given for the pennate muscles b and c, relative to those of the strap muscle a = 100, when the angle of pennation  = 25°.  The "functional transverse area" is indicated by the dotted lines.  Note that the effect of pennation has been to reduce the range of contraction and the work effective in the direction of contraction, but to increase the force.  Note also that the more sarcomeres that are in parallel within the muscle, the more tendinous apparatus must be in series with it.

:::::Diagrammatic representations of three muscles of similar mass but different shape but of widely varying fibrous architecture. Approximate values for range of contraction, force and work are given for the pennate muscles b and c, relative to those of the strap muscle a = 100, when the angle of pennation  = 25°.  The "functional transverse area" is indicated by the dotted lines.  Note that the effect of pennation has been to reduce the range of contraction and the work effective in the direction of contraction, but to increase the force.  Note also that the more sarcomeres that are in parallel within the muscle, the more tendinous apparatus must be in series with it.

:::::'''Fig 5.8 Extremes of pennation'''  

 

:::::'''Fig 5.8 Extremes of pennation'''  

:::::The properties of a muscle vary with the proportion of collagen built into its architecture, even though its external appearance, as judged by its shape and size, remain much the same.

:::::The properties of a muscle vary with the proportion of collagen built into its architecture, even though its external appearance, as judged by its shape and size, remain much the same.