Tipos de medidas de protección

Control of movement is dependent upon the interaction between the nervous system and the musculoskeletal system. While the central nervous system generates patterns of movement or motor programmes, their effectiveness is dependent upon the viscoelastic properties of muscles and the anatomical alignment of bones and joints. This section discusses the nerve- muscle interaction which may be affected by neurological impairment.

Neural components

The motor unit. The combination of the motoneurone, its axon and the muscle fibres that it innervates is known as the motor unit or 'final common pathway'. It is activated according to the sum of excitatory and inhibitory inputs and it is through this route that all processing in the CNS is finally transformed into movement (Rothwell 1994, Vrbova et al 1995).

Each muscle fibre is supplied by only one motoneurone but each motoneurone supplies many muscle fibres. The number of fibres sup- plied by a single motoneurone is referred to as the innervation ratio. Smaller muscles such as the intrinsic hand muscles have a lower innervation ratio - each motoneurone supplying fewer muscle fibres - and it is suggested that there is at least a moderate correlation between innervation ratio and the ability to finely grade muscle force (Enoka 1995).

Muscle fibre type. Skeletal muscles are com- posed of a variety of functionally diverse fibre types. They are dynamic structures capable of changing their phenotypic profile according to the functional demands placed upon them. This adaptive responsiveness is the basis of fibre type transitions (Pette & Staron 1997).

There are three basic fibre types:

• Slow-twitch oxidative (SO) or type I. These are innervated by small motoneurones and have a small fibre diameter. They are slow to contract, generate a small force and are very resistant to fatigue.

• Fast-twitch oxidative glycolytic (FOG), or type IIA. These have a medium-small fibre diame- ter, are fast contracting, generate an intermedi- ate force and have some resistance to fatigue. • Fast-twitch glycolytic (FG) or type IIB. These

have motoneurones with large cell bodies and have large-diameter axons. They are fast con- tracting, generate a large force and fatigue readily (Rothwell 1994).

The characteristics of the three main fibre types are summarised in Table 3.1.

However, other fibre types have been reported in both animal and human studies. There are reports of an intermediate type IID/X in skeletal muscle (Vrbova et al 1995, Pette & Staron 1997) and it is suggested that a considerable percentage of hybrid fibres are present in normal adult muscles (Pette & Staron 1997).

It may be that there is some misclassification of fibre types in human muscles in that type IIB fibres are more similar to the rat IIX than to the rat type IIB (Ennion et al 1995, Pette & Staron 1997, Goldspink 1999a). Hill (1950 as cited by Ennion et al 1995) hypothesised that because the muscles in larger animals are longer, with more sarcomeres in series, their intrinsic velocities of shortening would have to be lower than those of small animals; otherwise, the rate of force devel- opment would be unsustainable. It is suggested that the type IIB fibre type expressed in rodents is associated with too high an intrinsic velocity for humans and is therefore not expressed. This difference between type IIB fibre type in rats as compared to humans was described most succinctly by Goldspink (1999b). If humans moved at the same speed as rodents, the ballistic movements of the limbs would be so extreme as to cause them to become detached from the torso.

A muscle does not consist of a homogenous population of muscle fibres; there are several different fibre types within a muscle, each of which has different mechanical properties. The range of operation of the whole muscle is extended beyond that of any one fibre type alone. A predominance of one fibre type gives the muscle its characteristic properties (Rothwell 1994).

Muscles with predominantly SO fibres partici- pate in longer-lasting but relatively weak con- tractions, such as in postural control, whereas those with predominantly FG fibres generate large forces but are more readily fatigued. A sporting analogy is to compare the marathon runner with the sprinter. The marathon runner has endurance, due to a predominance of SO fibres, whereas the sprinter, with predominantly fast-twitch muscles, demonstrates explosive power but which is of short duration and there- fore cannot be sustained.

Gene expression/fibre type determination. Fibre phenotype is dependent not only on neural activ- ity but also to a large extent on mechanical factors, specifically a combination of stretch and muscle activity (Lin et al 1993, Goldspink 1999a). All muscles stay phenotypically fast unless they are subjected to stretch and force generation. For example, in rats, soleus, with predominantly slow oxidative fibres, if immobilised in a shortened position, subjected to hypogravity or denervated by spinalisation, reverts to expressing the fast gene (Goldspink et al 1992). Furthermore, experiments have shown that the number of SO fibres in the rat soleus muscle increases after birth as the animal uses this muscle for support. If this supporting function is negated by hind limb suspension, the increase in slow-twitch fibres is arrested or fails to occur. A similar finding results following tenotomy of soleus preventing it from being stretched. The contractile speed of the tenotomised muscle becomes fast although the innervation is unchanged (Vrbova et al 1995, Pette & Staron 1997). The main regulation in the expression of the slow phenotype seems to depend on the repres- sion of the fast-type as much as activation of the slow-type genes (Goldspink et al 1992).

Development, innervation, increased and decreased neurological muscular activity, over-

Figure 3.3 Schematic representation of major factors affecting sequential fibre-type transitions. (From Pette & Staron 1997 with permission.)

loading and under-loading, hormones and ageing have all been shown to be influential in determining the phenotypic expression of skele- tal muscle fibres. In general, activity and loading result in fast-to-slow fibre type transition and decreased neuromuscular activity or load cause transitions from slow-to-fast (Pette & Staron 1997, Pette 1998) (Fig. 3.3).

Certainly in animal muscle, on which most experiments have been conducted, this fibre type transition is a graded event (Vrbova et al 1995)

The nerve has a powerful influence on the properties of muscle fibres. Nerve transfer in animals shows fast muscles supplied by a slow nerve become slow contracting, and slow muscles supplied by a fast nerve become fast acting, although the cross-innervated muscles are not as slow or as fast as predicted and this nerve trans- fer does not produce a complete change in phe- notype (Vrbova et al 1995).

Muscles may alter their characteristics depend- ing on their usage (Dietz 1992). FG muscles will readily hypertrophy when they are recruited and overloaded during sustained exercise training, as under normal conditions, compared with the SO muscles, they are recruited relatively infre- quently. Conversely, although the SO muscles may also hypertrophy, this will be to a lesser extent as they already participate to a more

maximal degree in postural control (Rothwell 1994). However, exercise alone cannot radically change fibre type composition or contractile characteristics. During exercise, the motor units are still recruited in an orderly fashion although activated more often and the sequence of recruit- ment of motor units remains the same, there being no evidence that the motoneuronal proper- ties which influence the pattern of firing are changed by increased use (Vrbova et al 1995).

Recruitment order. The recruitment order of a motor unit depends on the size of its motoneu- rone. Motoneurones with the smallest axons are excited before those with larger axons: i.e. in the order SO > FOG > FG (Henneman et al 1965 as cited by Rothwell 1994). In this way skeletal muscles are well designed and matched for highly specific functions in the orchestration of any movement by the CNS (Rothwell 1994, Vrbova et al 1995). Postural muscles, which are made up predominantly of SO muscle fibres and participate in long-lasting but relatively weak contractions, are recruited before those which are predominantly fast-twitch and generate larger forces but are more readily fatigued.

For example, it has been shown with electro- myographic (EMG) studies that postural muscle fibres such as those of soleus, are activated almost continuously during standing and walking where- as fibres in other skeletal muscles are activated only 5% of the time (Goldspink 1999a). With a common input to the medial gastrocnemius and soleus motoneuronal pools, soleus, which has a predominance of SO muscle fibres, is always recruited before gastrocnemius, which has a greater percentage of fast-twitch fibres. It is not only active during tonic contractions: even in jumping, soleus is maximally active but, in this case, the gastrocnemius produces the most force (Rothwell 1994).

Non-neural components

Contractile properties of muscle. Skeletal muscle fibres are gathered together into bundles called fascicles which are surrounded by a connec- tive tissue sheath. The internal structure of the muscle fibre is complex, with the main elements

being the myofibrils which constitute the contrac- tile machinery of the muscle. Each myofibril con- sists of longitudinally orientated filaments called myofilaments, the bulk of which are composed of two proteins, actin and myosin. Actin and myosin filaments overlap in the sarcomeres when a muscle contracts. This is referred to as the 'sliding fila- ment' hypothesis. The critical stage of force gener- ation is rotation of the myosin head. This causes the filaments to slide over one another if they are free to move, producing an isotonic contraction, or otherwise an isometric force is generated.

At rest, the concentration of calcium in the sarcoplasmic reticulum is very low, so the major- ity of actin-myosin bonds remain unformed. The release of neurotransmitter at the neuromuscular junction initiates an action potential along the cell membrane of the muscle fibre, causing a release of calcium from the sarcoplasmic reticu- lum into the sarcoplasm. This process allows for the formation of actin-myosin bonds. Calcium is then actively pumped back into the sarcoplasmic reticulum and the contraction ceases.

Mechanical properties of muscle. There are three elements in the mechanical behaviour of muscle. The contractile element has its own mechanical properties, viscosity and stiffness, which change with the level of muscle contraction. The contrac- tile mechanism resides in the interaction between the actin and myosin filaments. The series elastic element lies in the tendinous insertions of the muscle and in the actin and myosin cross-bridges themselves, and transmits the force of contrac- tion. The parallel elastic element resides in the sarcolemma of the muscle cells and in the sur- rounding connective tissue. This both distributes the forces associated with passive stretch and maintains the relative position of fibres (Goldspink & Williams 1990, Rothwell 1994). The contractile element actively generates muscle force, whereas the series and parallel elastic ele- ments are passive components, acting as mechanical springs.

There are two important relationships which influence the control of muscle contraction. The first is the length-tension relationship whereby, as the muscle length is increased, the force exerted rises. The tension is due not only to the activity of

the contractile element but also to passive stretch of the non-contractile elements. The force pro- duced therefore differs depending on the muscle length, maximum work being performed by a muscle shortening at intermediate muscle lengths. The example cited by Rothwell (1994) is the extended position of the wrist while the fingers flex powerfully on to an object in the palm of the hand. The power of the grip is substantially weak- ened with a more flexed wrist position. This may be significant following stroke, where flexor hypertonia is often predominant in the upper limb. Many people are unable to extend the wrist, with a resultant decrease in grip strength and impairment of manipulation skills.

The second is the force-velocity relationship in that the rate at which a muscle can shorten depends upon the force exerted: the greater the resistance to movement, the more the velocity of movement is decreased.

The length-tension and force-velocity con- tribute to compensation for unexpected disturb- ances. A sudden increase in load can produce increased muscle tension not only through reflex pathways but also by the nature of the length- tension characteristics of the muscle. Even in the deafferented man, the force output adjusts although not to the same extent as normal (Rothwell 1994).

The elastic compliance of muscle is dependent upon the concentration of collagen, a major com- ponent of intramuscular connective tissue. This concentration is higher in slow-twitch as opposed to fast-twitch muscle and is reflected in the passive length-tension curves which show that fast-twitch muscle has a higher compliance. Experiments with rats have shown that slow-twitch postural muscles are particularly sensitive to immobilisation. The intramuscular connective tissue increased more rapidly in the immobilised soleus than in gastroc- nemius, which is composed primarily of fast- twitch fibres, and this was more prevalent when the muscles were held in a shortened position (Given et al 1995).

The force generated by a muscle is dependent upon the number of cross-bridges which can be engaged between actin and myosin filaments and this in turn depends on the overlap of these

filaments in the sarcomere. The number of sarcomeres in series determines the distance through which a muscle can shorten and regula- tion of the sarcomere number is considered to be an adaptation to changes in the functional length of muscle. If a muscle is immobilised in a short- ened position, there is a reduction in the number of sarcomeres and conversely, if a muscle is immobilised in a lengthened position, there is an increase in the number of sarcomeres (Herbert 1988, Goldspink & Williams 1990). However, in animals, the changes in sarcomeres have been shown to be age dependent, the young adapting to a fixed lengthened state by lengthening the tendon as opposed to adults adapting with sarcomeres growing longer under the same conditions (Whitlock 1990).

The interaction between the actin and myosin filaments gives the muscle a certain resting tension and short-range dynamic stiffness. This plastic behaviour or stiffness within the muscle fibre itself is known as thixotropy (Hagbarth 1994); this is an engineering term which is used to describe the dynamic viscosity of fluids. When applied to muscle, thixotropy has been attributed to abnormal cross-bridges between actin and myosin filaments, producing an inherent muscle stiffness (Sheean 1998).

Clinical implications

Neural activity is a major factor in influencing the characteristic properties of skeletal muscles (Dietz 1992). Changes in function imposed by neurological impairment may produce muscle fibre type transformation and/or change in muscle fibre type distribution that is dependent upon the amount and the pattern of neuronal input (Jones & Round 1990, Cameron & Calancie 1995). Within the upper motor neurone syn- drome both fibre type transitions have been reported (see Chapter 5).

Prevention or reversal of denervation atrophy depends on the capacity of the nerves of surviving motoneurones to sprout and reinnervate as many denervated fibres as possible. As a result of sprout- ing, each motoneurone supplies an increased number of muscle fibres. The innervation ratio is

therefore increased, each motoneurone supplying more muscle fibres, leading to a decrease in selec- tive movement (Gordon & Mao 1994).

Although exercise to improve muscle strength is recommended, strenuous activity of partially denervated muscle can lead to an irreversible increase in weakness (Bennett & Knowlton 1958). It is suggested that, in order to avoid overwork weakness, there must be:

• a balance between rest and activity

• an emphasis on submaximal intensities of exercise

• the development of preventative muscular strengthening programmes specific to the patterns of weakness

• the creation of endurance training pro- grammes for normal cardiovascular responses to exercise (Curtis & Weir 1996).

In patients with neurological disability, muscles may be constrained in a shortened position due to the prevalence of abnormal tone. There is loss of sarcomeres and a reduction in muscle fibre length with an increased resistance to passive stretch. This alters both the length-tension and force-velocity relationships of muscle contraction, and optimal force production is impaired. Sustained stretch of shortened structures may go some way to improv- ing the biomechanical advantage and enabling more effective activation of muscle, a factor noted in clinical practice. Many patients automatically elongate shortened muscles and report greater ease of movement following sustained stretch. Splinting may also be used as a means of imposing sustained stretch. The advantages and dis- advantages of this intervention are described in Chapter 10.

AN APPROACH TO THE ANALYSIS