What do we know about the development of motor skills in typically developing children?

In this section we consider first what we know about how all human movements are controlled and improved, and then what we know about the developmental progress of motor skills in children. We have provided some detail for the reader because a number of the explanations given for the motor delays of children with Down syndrome can only be discussed if one has some knowledge of the mechanisms involved in movement.

Movement control is complex

The mechanisms of motor control are complex and not yet fully understood.[1] [TODO: 5] [TODO: Add Latash M, L. Levin, M.F. , John P. Scholz,J.P. , Gregor Schöner, G. (2010)] The way in which ordinary movements, such as reaching for a cup or walking, are carried out is clearly immensely complicated. Each time the movement of picking up a cup is performed it is a unique movement pattern, as the distance to be reached, the angle at which the arm extends relative to the body, the size of the cup and the level of liquid in it, will vary. Similarly, when walking, each episode of walking is different as it may be over a different distance, at a different speed, on smooth or rough ground, uphill or against a wind. Research in the field of motor control is very active and during the last few decades increasingly sophisticated mathematical models have been developed to try and explain the way in which the brain carries out everyday movements.[TODO: 1],[TODO: 3],[TODO: 4] The models are tested against actual data on how movements are performed.

All recent theories agree that some actions may be represented in the brain as partly prepared ‘plans of action’ or ‘neuromotor programmes’, but that as each action is carried out it is also controlled ‘on-line’ to ensure effectiveness in that particular situation. The learned neuromotor programmes provide ‘feedforward’ plans that are activated to initiate a movement and information from the senses and body provide continuous ‘feedback’ information. Both ‘feedback’ and ‘feedforward’ systems are used by the brain to carry out each effective movement. Every movement is unique in some of its features, even repeated actions such as reaching to pick up and put down the same cup in the same place several times. In the next section we consider some of the mechanisms involved in ‘on-line’ control and in learning neuromotor programmes.

Systems involved in movement control

Any movement requires the integrated action of the brain, nerves and muscles. As a movement is carried out, there is fast continuous control and adjustment occurring as the brain produces a coordinated action. Feedback of different kinds from the muscles and limbs, from vision and from balance systems is an integral part of all movement control as it is taking place. [Figure 1] illustrates some of the relevant anatomy.

The central nervous system

It is known that all ‘voluntary’ movement is controlled by impulses from nerves that originate in the central nervous system, which consists of the brain and the spinal cord. With the exception of some basic reflexes (spinal reflexes), which may be mediated through the spinal cord, all coordinated movement sequences are determined by the brain. Although there are parts of the brain that are mainly concerned with the control of movement, such as the cerebellum and the motor and pre-motor areas of the cortex, it is important to note that motor control is widely distributed in the brain, with many other areas being involved.

! The coordination of motor control

Figure 1. The coordination of motor control

**Notes to Figure 1:

The muscles

The skeletal muscles are those which move the limbs, trunk, neck and other parts of the body. They are sometimes called ‘voluntary muscles’ because they produce the movements involved in activities such as walking, handling objects and participating in sports. It is important to note that these movements, which involve a very large number of brain processes as well as very many accurate muscle movements, are not really under any form of conscious control at all. When we carry out these functions, we are mainly conscious of the results we wish to achieve rather than the detailed means by which we attain the outcome. For example, we think ‘I will pick up my keys’ - we do not think ‘I need to activate this muscle and then that one to direct my hand to the keys’.

The peripheral nerves

Muscles have two basic forms of nerve supply:

The efferent nerves activate systems which cause the muscle to contract with varying degrees of strength and speed, depending upon the type of message and the type of muscle fibre receiving the impulses. The impulses in the afferent nerves contain feedback information about the movement and position of the muscles and limbs, which the brain uses to ensure that the required movements are correctly carried out.

Feedback systems

In order to maintain and control appropriate body posture, the brain obtains information from a number of sources:

  1. Information about body position from detectors in the muscles and ligaments. This is called proprioceptive feedback.
  2. Signals from the eyes provide visual information about the environment and about body position. This is called visual feedback.
  3. Signals from the semi-circular canals (vestibular system) near to the inner ear provide information about the body in relation to its environment and give it a ‘sense of balance’. This is called vestibular feedback.

All the information provided by these feedback systems is continuously processed in the brain and it enables the brain to send appropriate instructions to the muscles to produce the highly coordinated movement patterns required for normal function.

Ligaments and tendons

Tendons are cable-like structures which connect muscles to the muscle attachments on bones. Ligaments are similar to tendons but attach bones to each other. It is generally agreed that the ligaments in people with Down syndrome are more elastic than in typically developing people.[2] [TODO: 11-12] The effect of having ligaments which are more stretchy than usual is that the joints are capable of a much greater range of movements than is typical. It is likely that this effect has been confused with that of muscle tone.

Change in performance over time

When children or adults begin to learn any new neuromuscular skill, such as walking, drawing or swimming, they initially carry out the task in a clumsy, not very well coordinated fashion. But, with sufficient practice, they will eventually perform the task in a smoother and more efficient manner. The effects of practice on the brain have been demonstrated in a number of studies.[3]

During the period when new types of coordinated movements are being learned, there may be some changes in the muscles involved, such as some increase in strength, but virtually all of the changes related to the development of the new skill take place in the brain. There is little evidence from the available research literature to indicate that factors associated with Down syndrome, such as shortened bone length, stretchy ligaments or altered muscle tone, have any significant effect on basic neuromuscular actions. Presumably this is because brain control systems compensate for these factors during the learning process.

Since muscles ‘do what they are told’, and since the instructions to the muscles all come from the brain, differences in the quality of movement such as slower or less well coordinated movement, can be seen to have their origin in the brain and, where changes in movement patterns occur, they are associated with changes in brain mechanisms.

Neuromotor programmes

Research has demonstrated that practice leads to learning and to the development of ‘neuromotor programmes’ or ‘action plans’ for particular movement sequences in the brain.[3] [TODO: 6] These neuromotor programmes enable movement sequences to be performed more quickly and accurately over time. As practice of the movement continues, the neuromotor programmes become so well learned that they are referred to as being ‘automatised’. It has been suggested that, once neuromotor programmes are automatised, they make less demand on the information processing capacities of the brain.[TODO: 6]

The effects of automatisation can be made clearer by considering a task in which a series of complex movements are learned, such as when learning to drive a car. In this situation the learner has to consciously control the series of movements - i.e. to think what to do next. However, over time, the series of actions may become so well practised and automatised that virtually no conscious control is needed to change gear or to steer the car. Now the driver can give more attention to road conditions and safety, as the conscious information processing demands of controlling the car have been considerably reduced.

Information processing and decision making

Subconscious processing

In all movements, there is a significant information processing requirement as the brain continuously processes feedback and sends control messages to the muscles in order to carry out the activity successfully, but this is at a subconscious level. The individual simply gets up and walks or picks up a cup without any conscious consideration of the controls on the movements needed for the particular situation - any conscious mental activity is simply focused on the goal of the activity.

Conscious processing

In addition, some motor tasks require conscious information processing and decision making before carrying out the movement. The ‘reaction time’ task used in research is one example, as a conscious level of decision making is involved before initiating the movement. The reaction time is the time taken between the signal to start a movement and the movement itself. Here, for example, a person may be instructed to tap the right button when the red light comes on or tap the left button when the green button comes on. He or she has to identify which light is on and then initiate the correct movement. Another example involving conscious processing of information before or during a movement is a physical education lesson, in which the pupil has to follow instructions.

Processing demands and processing abilities may vary

The information processing and the decision making requirements of a motor task influences an individual’s speed and/or precision in carrying out the task. Some individuals may take more time to process information in the central nervous system and some may have more difficulty understanding task requirements or following instructions.

In summary

The production and co-ordination of movement comes from the central nervous system. Movements are controlled by the brain and practice leads to the establishment of learned neuromotor programmes, which increase the speed, accuracy and smoothness of movements. The brain focuses on the endpoint or goal of the activity and it controls the muscles to move the limbs to achieve that goal.[4] The brain gives instructions to the muscles that compensate for the effects of lax ligaments or muscle tone, or arm or finger length, when carrying out a movement. The practice of self-initiated movements is essential for progress.

Motor development from infancy

We have described some of the factors which influence all movements and in this section we consider the way in which motor skills develop during infancy amongst typically developing children.

Motor skills develop in a predictable sequence

There are many studies which have demonstrated that basic gross and fine motor skills usually develop in a specific order and which have documented the ages at which children sit, crawl, walk, jump, run, drink from a cup, use a knife to cut or a pen to write letters, manage buttons and zips. Specific gross and fine motor skills are assessed on many developmental tests, and motor skills also influence the scores of infants on some cognitive (mental) tests as they are expected to demonstrate their understanding by picking up or manipulating objects or toys during these tests.

Individual variation

As all parents know, the age at which healthy, typically developing children reach milestones can vary widely with some walking as early as 10 months and some as late as 24 months. This variation is largely thought to be determined by genetic make-up, but it is also affected by the opportunity to move and explore. For example, one Chinese study demonstrated later walking in those children kept in beds or cots for longer periods than usual because of living circumstances. Other studies show that cultural practices including massage lead to earlier sitting and walking.[5] [TODO: Add Adolph Frank 2016]

Later skills tend to be built on earlier ones

The early gross motor skills of sitting, standing and walking involve increasingly successful control of body posture and balance, and these will be needed for maintaining body stability when bending to reach an object or later when writing and drawing, and when developing sporting skills.

All motor skills improve over time and with practice

This point has already been made, but it is worth emphasising. All children perform movements in a ‘clumsy’ or immature way at first and refine their performance with practice, often over many months or years. For example, for typical children, posture control when walking continues to improve up to at least 7 or 8 years of age.[6] [TODO: 9]

Practice improves the smoothness and accuracy of performance, the speed of performance and leads to automatisation of some of the processes.[7]

A useful and accessible review by Adolph and Frank in 2016 explaining typical motor development and its complexity in more detail is freely available at [TODO: Ref Adolph Frank].

References

1. Todorov, E., & Jordan, M. I. (2002). Optimal feedback control as a theory of motor coordination. Nature Neuroscience, 5(11), 1226–1235. https://doi.org/10.1038/nn963
2. Block, M. E. (1991). Motor Development in Children with Down Syndrome: A Review of the Literature. Adapted Physical Activity Quarterly, 8(3), 179–209. https://doi.org/10.1123/apaq.8.3.179
3. Latash, M. L. (2000). Motor co-ordination in Down syndrome: The role of adaptive changes. In D. J. Weeks, R. Chua, & D. Elliott (Eds.), Perceptual-motor behaviour in Down syndrome. (pp. 199–224). Human Kinetics.
4. Graziano, M. S. A., Taylor, C. S. R., & Moore, T. (2002). Complex movements evoked by microstimulation of precentral cortex. Neuron, 34(5), 841–851. https://doi.org/10.1016/s0896-6273(02)00698-0
5. Campos, J. J., Anderson, D. I., Barbu-Roth, M. A., Hubbard, E. M., Hertenstein, M. J., & Witherington, D. (2000). Travel Broadens the Mind. Infancy: The Official Journal of the International Society on Infant Studies, 1(2), 149–219. https://doi.org/10.1207/S15327078IN0102_1
6. Shumway-Cook, A., & Woollacott, M. H. (1985). Dynamics of postural control in the child with Down syndrome. Physical Therapy, 65(9), 1315–1322. https://doi.org/10.1093/ptj/65.9.1315
7. Dulaney, C. L., & Tomporowski, P. D. (2000). Attention and cognitive skill acquisition. In D. J. Weeks, R. Chua, & D. Elliott (Eds.), Perceptual-motor behaviour in Down syndrome. (pp. 225–248). Human Kinetics.

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