When muscles contract and develop force they do so because nerves leading to them become active; the messages result in electrical activity in the muscle which can easily be detected by electromyography (EMG). For much of our lives, however, our muscular capacity is grossly underused. In the course of a normal day, the average sedentary person uses perhaps only 10% of the force-generating potential of the muscles of his limbs. In the past it has been assumed that the passive force in resting muscle could be ignored, but it is now apparent that it may contribute importantly to the stabilization of posture that we take for granted; this is because resting muscle shows thixotropic properties. Thixotropy is a property displayed by many systems, particularly those with complex molecules with long chains, familiar objects being tomato ketchup and thixotropic paint. Tip up a bottle of ketchup and nothing comes out; shake it and it will flow freely. Leave it on a shelf for some time and it stiffens again. Similarly, if there is no movement, muscle stiffens considerably, and much of this change takes place quite rapidly, say within two seconds. This property is one aspect of muscle tone and also contributes to the tone assessed as ‘normal’ for a muscle at rest during a neurological examination. The medical person moves the limb to and fro slowly to estimate resistance. Sometimes further information can be gained by moderate shaking: the limb should not flail excessively.
Muscle properties are influenced by temperature. When the fibres are warm they work faster. People who are cold often complain that their muscles are stiff, and athletes and pianists ‘warm up’ before starting their activities. This may well be due to the well-documented effects that temperature has on muscle thixotropy. When cold it is difficult or impossible to ‘shake out’ the stiffening due to enhanced thixotropy. Some treatments used by physiotherapists may be effective through such mechanisms; deep warmth, as by diathermy, and passive stretching will both have the effect of reducing this resistance.
When a limb is held stationary against gravity, inactive muscle fibres will exist alongside the active ones; the proportion of each depends on the total mass supported, with the active fibres in steady contraction. When sitting, the head is held up by the contraction of muscles at the back of the neck. If someone drops off to sleep in the sitting position, this activity is lost and the head slumps forward. Again, our mouths are normally kept lightly closed by contractions which counteract the action of gravity on the mass of the lower jaw.
Some muscles may be excessively active, resulting in unusual, if not ungainly, postures; in some people when nervous this may be expressed through the facial muscles as a grimace. Sometimes muscles which act against each other contract simultaneously; when severe this can result in spasm or cramp. But even when mild, such spasm can be undesirable, as in writer's cramp or the analogous problems for musicians. Awkward positions, such those inevitable in playing the violin, are particularly likely to generate difficulties.
Most people can relax fully without any special training: if a limb is supported, the muscle action switches off almost at once. ‘Relax’ is used in two senses: mechanical, referring to a reduction of muscular action, and metaphorical, a reference to mental tranquility. The physiological basis of procedures sometimes advocated to achieve such relaxation is often obscure.
Muscle tone, even in healthy people at rest, is thus dependent on quite complex mechanisms. However, it becomes even more difficult to describe under conditions of movement where parts of the body are accelerating and decelerating. For this reason it is usually preferable to speak only of ‘resting muscle tone’.
The word ‘tone’ has the same root as the word ‘tune’, and the tension in the tendon of a muscle can be likened to that in the string of a guitar. Based on these considerations, muscle tone in the resting state can be measured by applying rhythmic forces and observing at which rate of application the motion is the greatest. This is the ‘resonant frequency’. Tone is related to the square of the resonant frequency.
At times the level of tone becomes set incorrectly. The body may be abnormally floppy — a state referred to as ‘hypotonia’ and children may be born with the ‘floppy child syndrome’. But in some relatively common diseases the tone is increased and the limbs are unusually stiff. There are two main types of this hypertonia:
(i) Spasticity may follow injury to a main pathway for messages from the brain downwards. On manipulation of the limb the excessive tone may easily be felt, but the extent of this depends of the rate at which the limb is passively moved; there may be only a little extra resistance if the motion is slow. Stretching a muscle normally causes a reflex contraction; spasticity results when sensors in the muscle responsive to stretch become more than usually effective in causing this reflex contraction. These numerous receptors are important in the regulation of normal posture; it is their unrestrained effects that cause difficulties such as may arise after a stroke, or in children following birth injury (one cause of cerebral palsy).
(ii) Rigidity is distinct from spasticity. In this the resistance is independent of the rate at which the limb is manipulated by the examiner, and indeed, in extreme cases, it leads to a maintained postural abnormality described as dystonia. Such rigidity is one of the signs of Parkinsonism. Occasionally other disturbances are found. Thus someone may have the misfortune of suffering from an involuntary contraction of muscles of the neck so that the head is held in a quite abnormal posture. The medical term for such a ‘wry neck’ is torticollis.
— E. Geoffrey Walsh
See also cerebral palsy; skeletal muscle.
Bibliography
- Walsh, E. G. (1992). Muscles, masses and motion. MacKeith/Cambridge University Press
