Changes

==='''Resistance involves work although no external work is done'''===

==='''Resistance involves work although no external work is done'''===

Some muscles have a high proportion of slow twitch fibres, using energy at a low rate.  They are especially useful where a force slows down or movement prevention is required.  This decelerating action requires relatively little energy, which is released as heat.  The term "isometric", usually used to describe these fibres, is misleading, since this type of muscle action can take place while a muscle is being stretched, if the muscle is resisting stretch.  This "braking" action of muscles occurs, for instance, in certain forelimb muscles when a horse lands from a jump (Fig. 6.2).  Slow twitch muscle fibres usually predominate at sites where they are able to oppose the force of gravity, both in standing and moving animals, as in the example just given.  In limbs, such postural muscles act over joints that would otherwise flex and cause the limb to collapse (Fig. 6.3).

Some muscles have a high proportion of slow twitch fibres, using energy at a low rate.  They are especially useful where a force slows down or movement prevention is required.  This decelerating action requires relatively little energy, which is released as heat.  The term "isometric", usually used to describe these fibres, is misleading, since this type of muscle action can take place while a muscle is being stretched, if the muscle is resisting stretch.  This "braking" action of muscles occurs, for instance, in certain forelimb muscles when a horse lands from a jump (Fig. 6.2).  Slow twitch muscle fibres usually predominate at sites where they are able to oppose the force of gravity, both in standing and moving animals, as in the example just given.  In limbs, such postural muscles act over joints that would otherwise flex and cause the limb to collapse (Fig. 6.3).

Fast twitch fibres differ from slow twitch fibres in several ways.  Fast contraction demands fast control of the stimulating mechanisms with in the fast twitch fibre: thus there is more sarcoplasmic reticulum in these fibres.  There is also an enzymatic difference in the rate at which myosin adenosine triphosphate (ATP) is split. Fast twitch fibres have a high activity, and slow twitch fibres have a low activity of the enzyme myosin adenosine triphosphatase (myosin ATPase).  Fortunately for the easy study of the contraction speeds of different muscles, a histochemical method exists that differentiates between the two fibre types.  This is because each fibre type has a different myosin isoenzyme, the difference in the enzyme activity of which is exaggerated at a high pH.  By this means, fast and slow twitch fibres can be visualised in histological sections (Fig. 6.4).  

Fast twitch fibres differ from slow twitch fibres in several ways.  Fast contraction demands fast control of the stimulating mechanisms with in the fast twitch fibre: thus there is more sarcoplasmic reticulum in these fibres.  There is also an enzymatic difference in the rate at which myosin adenosine triphosphate (ATP) is split. Fast twitch fibres have a high activity, and slow twitch fibres have a low activity of the enzyme myosin adenosine triphosphatase (myosin ATPase).  Fortunately for the easy study of the contraction speeds of different muscles, a histochemical method exists that differentiates between the two fibre types.  This is because each fibre type has a different myosin isoenzyme, the difference in the enzyme activity of which is exaggerated at a high pH.  By this means, fast and slow twitch fibres can be visualised in histological sections (Fig. 6.4).  

Muscles may have both propulsive and postural roles.  Several muscles with broad attachments possess a high population density of slow twitch fibres nearest the pivot over which the muscle acts, and a high density of fast twitch fibres towards the periphery (Fig. 6.5).  During standing or quiet walking, only the area of the semitendinosus muscle in which slow twitch fibres are dense is active (Fig. 6.6).  The area dense in fast twitch fibres is recruited only during more violent activity.  We have already discussed the mechanics of different areas of such a muscle (Fig. 5.4).  The heterogeneity of fibre type distribution within a muscle may arise because of the necessity for fast twitch fibres to have a greater torque.  Another advantage of this arrangement will be explained shortly.   

Muscles may have both propulsive and postural roles.  Several muscles with broad attachments possess a high population density of slow twitch fibres nearest the pivot over which the muscle acts, and a high density of fast twitch fibres towards the periphery (Fig. 6.5).  During standing or quiet walking, only the area of the semitendinosus muscle in which slow twitch fibres are dense is active (Fig. 6.6).  The area dense in fast twitch fibres is recruited only during more violent activity.  We have already discussed the mechanics of different areas of such a muscle (Fig. 5.4).  The heterogeneity of fibre type distribution within a muscle may arise because of the necessity for fast twitch fibres to have a greater torque.  Another advantage of this arrangement will be explained shortly.   

   

   

:::::'''Fig 6.3 Limb posture'''  

:::::'''Fig 6.3 Limb posture'''