By Dr Kenneth Backhouse OBE
For the dancer, balance is far more important than for the average member of the community who is quite happy so long as they don't fall about. A dancer must not only be secure in posture but also in movement. The overall control of the body comes, of course, from the brain that relies upon constantly receiving accurate sensory information. So far as position and movement sense is concerned, this is given the name proprioception but the information comes from the wide variety of sources. For the dancer muscle, tendon and joint sensibility (kinaesthesia) is especially vital but today I want to concentrate on other aspects of importance, namely the eyes, the ears and the soles of the feet.
The eyes are, of course, essential for vision but they also have a very important role in appreciating movement. For full visual acuity the eyes have to point roughly in the direction of gaze but movement awareness covers the whole hemisphere around the front of the head. For instance, if you are sitting in a stationary train or car and something moves even way to the side of your head you are aware of it and for a moment you are likely to think it is you who are moving. If a person stands 'still' there is always some movement, the range of which would increase on closing your eyes. The eyes are constantly recording movement of the body relative to fixed objects around, the information being transmitted to the brain in order to assist in maintaining stability. If your head is not moving too rapidly, the brain is capable of adjusting to this with remarkable efficiency and still maintaining the visual control over posture. If the movement is too great and too rapid, as in a turn, then the facility fails until the movement stops. Then the brain can take control again. This then is the important reason for 'spotting' in turns. The head is turned more rapidly than the body in order to allow a brief period for the eyes to fix on a still object. In order to allow for the ideal carriage of the head, the point for spotting should be something a little above the eye-line.
The Ears, as the eyes, have a double function. They respond to sound for hearing but also transmit to the brain evidence of movements of the head. The inner ear has two components, the cochlear system for hearing and, alongside it, the vestibular system. Although the two systems are connected, they work separately. The vestibular system of each ear is made up of small fluid filled containers to which are attached three 'U' shaped tubes, set at different angles. Each of these has minute sensory hairs projecting from its inner surface into the contained fluid. When movement occurs the fluid in the tube moves at a different rate from the wall. The hairs record this and the information is passed to the brain. The three semicircular canals in each ear being set at right angles to each other and the systems in the two ears also at different angles, there is then, a wide range of tube direction from which the brain can analyse both speed and direction of movement i.e. from six different tube directions. Under normal circumstances these movements are relatively slow and quite limited. Thus rapid, more continuous movement over-stresses the system and produces dizziness. These again are problems that, with training, a dancer has to cope with and to develop means of countering the stresses, which are, in a way, similar to those generated from the eyes. Hence even more, the need for spotting in turns.
From the information that the brain receives from both eyes and ears it can only appreciate that the movement is of the head. Hence, it is important that there is also a highly developed muscle, tendon and joint sense emanating from the dancer's neck in order to complete the sensory picture, relative to the whole body.
The Feet tend to be forgotten from a sensory point of view but, like the hands, they are very finely equipped. They are highly responsive to pressure. As we stand upright there is always a small amount of swaying of the body that is recorded by the pressure sensors in the different parts of the soles of the feet. These add to the information on movement from the eyes and ears that the brain receives. If you stand and have a method of recording the movement of your body, the continuous small, ever-changing movement would surprise you. If then you closed your eyes, that range would increase considerably, even in a trained dancer. It would be much greater in most people. If in our experiment the person were to stand in ice until they lost pressure sensation in the soles of the feet, the position would be far worse. In fact I have seen this experiment carried out and, in some cases with untrained people who then closed their eyes, they lost virtually all sense of control. Now returning to the dancer, on rising to demi-pointe the area of pressure reception is much reduced while when on full pointe there is only pressure on the tips of the toes, which reduces the pressure sensation from the soles of the feet virtually to nil.
In dance, from time to time, all three of these very important sensory controls of equilibrium and movement are subject to wide disruption, which would be unacceptable to an untrained person. Not only must the dancer maintain constant awareness during training of these aspects but even greater emphasis must be directed towards kinaesthesia i.e. muscle tendon and joint sensory awareness. It requires virtually daily activation and reactivation to achieve success
