Changes

:::::A muscle contracts a distance d and lifts a weight W against gravity (a).  When two such muscles are in series, the distance lifted is 2d (b). Each muscle is effectively loaded as in (a). When the muscles are in parallel, as in (c), the load is shared and the muscles can lift twice the weight 2W through their contraction distance d.  Note that the work done (2d x W, or d x 2W) in lifting the weight by pairs of muscles is the same regardless of their configuration.  Similarly the work done by a muscle depends on the number of contractile units (sarcomeres) within it (hence its weight), and not on the geometrical arrangement of the contractile units (or, in the arrangement shown in the diagrams, its shape).

:::::A muscle contracts a distance d and lifts a weight W against gravity (a).  When two such muscles are in series, the distance lifted is 2d (b). Each muscle is effectively loaded as in (a). When the muscles are in parallel, as in (c), the load is shared and the muscles can lift twice the weight 2W through their contraction distance d.  Note that the work done (2d x W, or d x 2W) in lifting the weight by pairs of muscles is the same regardless of their configuration.  Similarly the work done by a muscle depends on the number of contractile units (sarcomeres) within it (hence its weight), and not on the geometrical arrangement of the contractile units (or, in the arrangement shown in the diagrams, its shape).

==='''Movement strength and work'''===

==='''Movement, strength and work'''===

Range of contraction is therefore proportional to the number of sarcomeres in series, or the length of the muscle fibre.  The force produced (strength) is proportional to the number of sarcomeres in parallel, or, since the contents of a fibre are predominantly myofibrils, to the transverse sectional area of the muscle fibre.  The strength of muscle is approximately 0.3 MN.m–2 of transverse sectional area.  Note that the work done by a muscle (force x distance) is the same in both cases (Fig 5.1).  The possible work that a muscle can do, or mechanical energy that it can generate, is proportional to the total number of sarcomeres or, in other words, to the mass of the whole muscle.

Range of contraction is therefore proportional to the number of sarcomeres in series, or the length of the muscle fibre.  The force produced (strength) is proportional to the number of sarcomeres in parallel, or, since the contents of a fibre are predominantly myofibrils, to the transverse sectional area of the muscle fibre.  The strength of muscle is approximately 0.3 MN.m–2 of transverse sectional area.  Note that the work done by a muscle (force x distance) is the same in both cases (Fig 5.1).  The possible work that a muscle can do, or mechanical energy that it can generate, is proportional to the total number of sarcomeres or, in other words, to the mass of the whole muscle.

Consider two activities of a cat.  A large force is required to accelerate the mass of the body from the position of crouching, ready to spring.  The more sarcomeres that are in parallel, i.e. the greater the transverse area of the muscles brought into use, the greater the acceleration. A cat in this stance arranges its hind limbs and back to recruit as many as possible sarcomeres in parallel (Fig 5.2 a).  It also positions each sarcomere at the optimum length for the development of a contractile force (Fig 4.3).  You can easily observe this by watching a kitten at play, pretending to stalk and spring.   

Consider two activities of a cat.  A large force is required to accelerate the mass of the body from the position of crouching, ready to spring.  The more sarcomeres that are in parallel, i.e. the greater the transverse area of the muscles brought into use, the greater the acceleration. A cat in this stance arranges its hind limbs and back to recruit as many as possible sarcomeres in parallel (Fig 5.2 a).  It also positions each sarcomere at the optimum length for the development of a contractile force (Fig 4.3).  You can easily observe this by watching a kitten at play, pretending to stalk and spring.   

A galloping cat protracts the forelimbs to lengthen the stride as much as possible (Fig 5.3).  This movement demands of the muscles protracting the forelimbs a large range of contraction.  The muscle involved must have as many as possible sarcomeres in series.

A galloping cat protracts the forelimbs to lengthen the stride as much as possible (Fig 5.3).  This movement demands of the muscles protracting the forelimbs a large range of contraction.  The muscle involved must have as many as possible sarcomeres in series.

:::::'''Fig 5.2 Sarcomeres in parallel'''

:::::'''Fig 5.2 Sarcomeres in parallel'''

:::::In (a), a cat is crouching ready to spring. The propulsive force will be provided by the extensors of the vertebral column x, and the extensors of the hip y. These are the most massive muscles in the body, and a high number of their sarcomeres are therefore in parallel. The result of the contraction is seen in (b).   

:::::In (a), a cat is crouching ready to spring. The propulsive force will be provided by the extensors of the vertebral column x, and the extensors of the hip y. These are the most massive muscles in the body, and a high number of their sarcomeres are therefore in parallel. The result of the contraction is seen in (b).   

:::::In the stage of the gallop of the cat in which the forelimb is retracted, the brachiocephalic muscle (dotted line) is fully stretched (a). When the forelimb is protracted, (b) this muscle is fully contracted.  During contraction of the muscle, the forelimb is not in contact with the ground; the muscle accelerates only the limb and not the whole cat. The emphasis is therefore on range of movement rather than strength; the sarcomeres in this muscle are therefore predominantly in series. 

:::::'''5.3 Sarcomeres in series'''

==='''Each muscle is a unique organ delivering unique torques'''===   

==='''Each muscle is a unique organ delivering unique torques'''===