Musculoskeletal pain is widespread.
In Norway, 50% of longterm sick leaves and one third of new disability pensions are based on musculoskeletal diagnoses. The fairy tale of Cinderella illustrates one possible injury mechanism. Some muscle fibres, known as the Cinderella fibres, are continuously activated even when the total muscle load is low. This Cinderella hypothesis may partly explain why systematic relaxation obtained by methods like Acem Meditation may reduce musculoskeletal pain in everyday life. During meditation, even the Cinderella fibres are allowed to rest.
Typical modern jobs include few or no physically strenuous tasks, but many employees spend a large proportion of the working day performing functions that involve low-level muscle loads, e.g. when they operate advanced production equipment or use a keyboard or a computer-mouse. Muscle tension due to stress and psychological strain may add to the physical load and possibly increase the overall level of muscle activity. There may also be insufficient time for recreation and stress-reducing activities. Does a low-level muscle load over long periods represent a risk for overload and muscle pain?
Why do musculoskeletal disorders put such a burden on national health insurance schemes, now that heavy workloads are largely a thing of the past, both at home and at work? The Cinderella fairy tale illustrates a popular explanation (Hägg 1991). The life of Cinderella before she meets the prince exemplifies how psychological stressors and low-level physical loads may increase the risk of developing muscle pain. Physical activity in itself is not a bad thing; in fact, muscles need to be used in order to be strong and fit. But we need a good balance between activity and rest. In heavy physical work, this balance may be jeopardised, at least for the elderly or the untrained. The Cinderella hypothesis suggests that this may also be the case in physically light work without enough variation.
What characterised the distribution of work in Cinderella’s household? She lived with three other women (her two stepsisters and her stepmother), but she alone did all the housework. Instead of helping her, the other women made Cinderella work even harder by their demanding attitude, frequent complaints and criticisms, and ever-growing lists of household chores. Rather than four women cooperating and sharing the domestic labour equally, which would have left all of them with time to rest and recuperate, Cinderella had to work steadily and under considerable pressure from early morning until late in the evening.
Can Cinderella’s miserable life with her stepfamily illustrate how our muscles are organised? Hägg (1991) and several other researchers maintain that even a very low physical load may become a high load for some parts of a muscle if it is very unevenly distributed. In order to understand this point, we have to look into the muscle and see how it is put together and how it is controlled during work.
A muscle consists of a number of so-called motor units. A motor unit is a group of muscle fibres controlled by one motor neuron in the medulla of the spinal cord, sending a long branch called an axon all the way out to the muscle. The axon has direct contact with all the muscle fibres in the motor unit. A nerve signal in the form of an electrical impulse sent through this axon makes all the fibres shorten and thereby adds to the total muscle force. Muscle force is mainly regulated by the number of motor units that are involved at a given time. A muscle has from several hundred up to several thousand motor units, each consisting of several hundred muscle fibres.
One might think that the optimal situation would be for the motor units to be activated in a cyclical order when the need for muscle force is moderate or low. If that were the case, light work would represent a low load for the muscle fibres, even if the muscle as a whole were working for a long time with no pauses. But this is not how human musculature functions.
Instead, the internal organisation of the muscle resembles the situation in Cinderella’s house before the entrance of the prince. The motor neurons of the medulla, having command of their respective troops of muscle fibres, differ with respect to how easily they get involved – they have different thresholds for involvement. If the need for muscle force is low, only the low-threshold motor units are recruited. Motor units with higher recruitment thresholds are totally inactive. This pattern is not substantially changed, even if the low-force work is long-lasting or very repetitive. As a rule, the same motor units are activated all the time. The analogy to Cinderella and her stepsisters is obvious.
A motor unit is a group of muscle fibres controlled by one motor neuron in the medulla of the spinal cord.
Why has the evolution of the species resulted in the situation described above? Several explanations may be suggested.
Some muscle fibres are specially adapted to work steadily for a long time, and low-threshold motor units contain such fibres. Other muscle fibres produce powerful force and react swiftly, but are easily exhausted. This is the type of fibres found in high-threshold motor units.
The way the muscles are organised is also an advantage for the motor control centre in the brain. Since the motor neurons differ with respect to their recruitment thresholds, the complexity of motor control is reduced. Instead of having detailed control of each motor unit, the central motor command may just change the signal strength to all motor neurons. We do not have to decide which motor units should be activated, as they are recruited in an orderly manner following the recruitment thresholds of their motor neurons.
Whatever the explanation might be, light physical work only activates the low-threshold fibres, the so-called Cinderella fibres. Thus, the same muscle fibres are used whenever a need for muscle force arises. These fibres continue to be active even when the need for muscle force increases, then acting together with muscle fibres with higher recruitment thresholds. When the demand for muscle force is reduced, the Cinderella fibres are the last fibres to stop being active.
Threshold limits for physical load? In work contexts, threshold limits have been established for harmful substances, e.g. gases, solvents or dust components. If the threshold limits are exceeded, the employer has to intervene with countermeasures. The idea of having a similar threshold limit for muscle load has been addressed, but in vain. Even with low muscle force, the risk of incurring musculoskeletal pain increases when this muscle force is needed for major portions of the working day.
The Cinderella hypothesis indicates a possible explanation. Even if the muscle as a whole produces a very moderate force (e.g. only 1% of the maximal force of this muscle), the Cinderella fibres are continuously active at a level giving optimal force production for this type of muscle fibres.
The muscle as a whole may produce a force of 1%, 5%, 15% or 30% of maximal force, but the situation for the Cinderella fibres is not substantially changed. When working at 30%, we quickly become tired and need to pause for a rest. With the very low loads, however, we are able to continue working for hours. The Cinderella fibres are, after all, specialised for work of long duration. But even these fibres can be overloaded.
Human beings are not adapted for persistent, monotonous loads in the neck, shoulders and arms, like those experienced in many computer-based jobs. Our genetic code was selected in the eras of our primate ancestors and huntergatherer forebears, when physical challenges were completely different.
Because it is impossible to establish a safe threshold limit for muscle load, and because any continuous muscle load probably is a risk factor when of sufficient (still unknown) duration, several researchers have begun to look beyond physical work loads for explanations of muscle pain.
We all know that mental and psychological factors can produce muscle activity. This is reflected, for instance, in the colloquial expression that one is ‘tense’. If the stress level is reduced, e.g. through meditation, the muscles commonly relax.
As a rule, however, this ‘psychogenic’ muscle activity is very low and is therefore not generally taken into account in evaluations of the muscle load of work situations. Instead, the only focus is on the physical force needed to perform the work tasks.
Psychogenic muscle activity may be present both at work and in leisure time. What characterises this muscle activity? The answer may be illustrated by a series of experiments (Wærsted 1997) recording the activity of the trapezius muscle, the muscle of the shoulder-neck that typically feels stiff and tender during stress and tension, and is responsive to massage.
In these experiments, subjects were asked to solve simple computerised tasks while being seated at well-adjusted computer workstations. The trapezius muscles did not have to be active for the subject to perform the physical response at the keyboard. In the majority of the subjects, however, the recordings revealed periods of continuous, low-level muscle activity in the trapezius muscles (typically around 1% of maximal muscle force) while the computer tasks were being solved. The presence of this psychogenic muscle activity was influenced by emotional and psychological factors, such as the introduction of an extra money incentive for good performance.
Stress and worries can be present both at work and in leisure time, and may contribute to an increased activity level in the Cinderella fibres, even when there is no obvious need for muscle force. Recent research has documented that the trapezius muscle may show continuous, low-level activity even during sleep, and indicates that this phenomenon is more common in subjects with muscle pain (Mork and Westgaard 2004).
Psychogenic muscle activity probably adds to the muscle activity already present as a result of physical demands. Also, psychogenic muscle activity probably prevents pauses in physical work from providing effective rest for the Cinderella fibres. To what extent this is the case, and how far this mechanism may contribute to the development and maintenance of muscle pain states, has not yet been established. We know from experience, however, that it is beneficial to vary muscle activity and to have a good balance between muscle activity and rest.
Using a systematic method for relaxation, such as Acem Meditation, seems to be very effective in preventing prolonged psychogenic tension. In this way, we all have the opportunity to let our Cinderella fibres find their princes, enjoy their well-deserved rest periods and live ‘happily ever after’.