How is human movement constructed? How does the brain organise human movement? How can movement be effected/changed? Decomposing, understanding and improving human movement is a complex task to review. You only have to read one research or review article (Gurfinkel et al 1988, Massion 1992, Pearson 1993; Duysens et al 1998) or look up a neuroscience text (Cordo & Harnad 1994, Gazzaniga 1997, Stein et al 1997, Berthoz 2000, Latash 1998 & 2002) to discover just how complex it gets.

However amongst this complexity some simplifying concepts have been proposed. Fortunately, what is difficult for us to understand of the complexities of human movement is also difficult for the nervous system to solve to enact these movements. Hence, the concept of "synergies" was proposed by Bernstein and his group (1967) as a mechanism for how the nervous system can simplify the control of movement . To determine and control for every component and every degree of freedom in motion of an entire vertebrate structure would be impossible. Hence the nervous system has a repertoire of simple and complex movements which involve groups of muscles acting in synergies to produce functional actions. "Motor synergies are the basis of movement" (Berthoz 2000 p154).

Instead of one or groups of neurons per muscle firing from the brain in a coordinated manner to produce movement, one or a group of neurons fire to activate a group of muscles in a functional pattern of motion. A synergy can be whole body movement or a component of a movement. When these groups or ensembles of neurons activate rhythmic motor functions like vertebrates and invertebrate flying (locust, bird), walking (cat, rat, locust), swimming (stingray, turtle, leech), escape (crayfish), jumping (fly), singing (cricket) they are called central pattern generators or CPG's (Pearson 1993; Grillner et al 1997). CPG's have also been found to exist distally in the spinal cord as well as more centrally in the mid brain (see Grillner and Wallen 1985; Grillner et al 1995 for review).

Synergies are flexible that is they can be arranged to produce several different functions- for example in the cat scratching and locomotion have been shown to have shared circuitry (see Saltiel et al 2000 for a review).

It is important to note from the beginning that though one area of the brain may be particularly responsible for a motor act many parts of the brain participate actively in what is known as distributed parallel processing. Nine regions of the brain showed electrical activity and blood flow during extension of the right index finger (Jahanshahi et al 1995).

Any vertebrate movement is thought to be organised using synergies.

The place to begin a description of synergies is with simpler nervous systems- the lamprey and tetrapod locomotor trunk and limb action have been extensively researched and provide simple mechanisms for understanding how movement can be controlled by the brain. The primary role of nervous sytems is movement control (Cordo and Harnad 1994) and the number one movement is locomotion hence we have to return to these old favourites in more detail from the previous presentation of evolution of human locomotion.

Evidence for synergies began with the observation that decorticate cats (ie the cerebral cortex was removed) could be made to locomote in a regular coordinated leg-trunk pattern by electrical stimulation of the mid brain (MLR mesencephalic locomotor region) (Shik and Orlovsky 1966). It was also found that higher strengths of electrical stimulation produced trotting and galloping. Hence a very simple mechanism for controlling for variability of speed for the locomotion synergy with the same spinal circuitry.

the lamprey

The lamprey ("Petromyzon marinus" an aquatic lower vertebrate with a nervous system similar to higher vertebrates, see figure below) has been studied extensively to reveal its neural circuitry for locomotion and postural control: see Grillner and Wallen 1985; Grillner et al 1995, Orlovsky et al 2002). The lamprey body is divided into segments each controlled by neural circuitry (CPG) at each level of the spinal cord and controlled from above by the MLR. Muscle activation along the spine is produced by bursts of neural activity between left and right sides of each spinal segment in a wave of contraction that passes from segment to segment in a coordinated manner that is achieved by a phase lag of excitation of successive segmental spinal circuitry (CPG's) (Griller and Wallen 1985; Grillner et al 1997 review).

This coordination can be controlled from the front to the back (rostral to caudal) producing forward swimming or back top front (caudal to rostral) producing backward swimming (Griller and Wallen 1985).

the lamprey

Coordination of individual segment output can be varied considerably by the ability of any one segment of the spinal cord to generate a phase lag in either direction (rostral and caudal) allowing not only a smooth regular pattern of spinal motion but also a very varied one (Grillner 1996).

the salamander

The transition from legless to legged locomotion can be observed in the one same animal- the salamander swims and locomotes on land with the same trunk pattern that can be modelled using a simple interaction between trunk and limb action.

The salamander belongs to the group of tetrapods ie four legged vertebrates (Tetrapods include amphibians, reptiles, birds, and mammals) and represents the most primitive of tetrapod locomotion. Tetrapod mechanics have common features of trunk and limb coordination that have been well described (Gray 1943, Strickberger 1990; Wolf 1999; Ijspeert 2002). The legs act as both weight bearing struts and levers. In lower tetrapods like the salamander these are short and placed wide of the body (see figure below).

Look at the animation below of salamander locomotion. You will see that all four legs and the trunk move in a regular repeatable pattern of motion. Very simply we could define just two motor synergies for the reptilian locomotion: 1. Alternate leg extension and flexion (or protraction and retraction) and 2. alternate trunk side bending.

Watch the animal locomote. I have added a solid line to schematically define the action of the trunk which alternately bends side to side. Watch the regular repeatable trunk side bending symmetrically side to side. Focus on one leg only for a while. See that its motion can be defined as alternate extension to reach ground ahead and then flexing as this ground is covered (Note there is an important rotation of the femur in this action that is not presented at this time). There can be three limbs on the ground (tripod) at all times and one advancing limb as in walking or two limbs weight bearing and two advancing as in trotting. Now think that the nervous system has the ability to combine these two synergies in a meaningful, economic way to produce locomotion. So what is very complex can be at one level viewed as simple. This is the concept of synergies.

In a hypothetical case for purposes of understanding movement we could design a system/animal with only trunk action (rigid legs) or a system with only leg action (rigid trunk)- see respective animations below:

You may agree that these two options do not appear to be as functionally useful or as economic as when they are combined.

These images were created from the work of Gray (1944) and Ijspeert (2002). In fact to create the second two images I actually used the first image to illustrate locomotion and then I retained leg action but stiffened the trunk and in the second I did the reverse. Now it is possible and likely that nervous systems can "shift" in this way. Combining and not combining synergies is a mechanism for constructing motion (Bizzi 1995; Saltiel et al 2000). Berthoz (2000) describes the selection and sequencing of synergies as a "strategy".

It is more and more realised now that the nervous system is not hard wired and is in fact flexible and manipulable or "plastic". Neuromuscular plasticity is yet to be fully utilized in current rehabilitation routines. It is however one basic premise for the Feldenkrais Method.

What I am proposing or asking here is that can the concept synergies be used in the clinic to assess and improve function? Is it possible in human movement at the very simplest level to determine a trunk synergy and a limb synergy to describe the motor behavior? How many basic synergies are there for the trunk and limbs? For Reptilian locomotion there is one synergy for the trunk and one for the limbs (legs, arms- all the same thing).

One reason for using the reptile is because it is very easy to represent and view trunk action as a whole and consider it as one synergy. Presenting the human trunk would immediately invoke preconceived notions about trunk movement that would likely interfere with the ability to see whole actions. However with the reptilian motion in mind it is an easier task to proceed to human movement with some shared and agreed clarity and definition of trunk synergies.

Before proceeding with human movement lets take a look another animal locomotion and trunk synergy- the cheetah (The reptile and cheetah have been presented on another page with respect the evolution of human locomotion. These images are used here again to define synergies and trunk synergies especially as the basis for all human movement).

Observe the cheetah sprint- what is the trunk synergy? It is flexion and extension- see the red lines illustrating the flexing and extending of the trunk. Similarly the legs flex and extend. Trunk action lengthens the stride and contributes to the power of the legs (Gray 1944). Again just two synergies for locomotion and possibly at least 5 CPG's- one for the trunk and one each for the limbs (Grillner and Wallen 1985). The shift from alternate trunk side bending to alternate trunk flexion-extension allowed for higher speeds to be attained by animals (Gracovetsky 1985). Note the transition in cheetah to the longer limbs, the vertical alignment of the limbs (to bring them under the body (instead of outside the body limits) and sagittal plane flexion-extension of the limbs (instead of lateral).

Remember this is a process for viewing movement- "seeing synergies"-it is a tool to decompose movement in meaningful ways and it is how the nervous system controls for simple and complex movements.

The cheetah is known to run at speeds of 71 miles per hour (114 kilometers per hour). How much does the trunk contribute to the power and speed of this action? From the animations above imagine how fast this animal could run with a stiff trunk and what would that look like. If any of you have very old pets (dogs or cats) they frequently become stiffened in the lower half of the trunk and walk quite differently to the regular pattern.

Lower vertebrates tend to use whole limb synergies while in more evolved nervous systems the whole limb synergy can be fractionated into smaller components allowing for more precise control like prehension (grip) (see Grillner and Wallen 1985).

Alexander (1989) maintained that all terrestrial vertebrate gait patterns are determined by optimisation of energy costs. The shift in gait pattern of the horse from walk to trot to gallop is accompanied by changes in oxygen consumption and energy efficiency such that it is more economical to trot than walk faster. Similarly it becomes more energy efficient to gallop than to trot faster (see referenced in Alexander 1989). Hence it is important to keep in mind that one determinant in trunk and limb-trunk synergies is energy efficiency. In animals inefficient movement patterns increases the time necessary for foraging for food and decreases the time for reproduction. It probably also increases the risk of predation. In humans it contributes to increased joint strain and possible pain and damage (as proposed by Feldenkrais 1972 p89-90).

Look at the animation of the man ascending steps. Can you see a trunk and limb synergy? Sure the leg flexes and extends (with rotation) and the trunk basically side bends (with rotation and flexion and extension). Have you ever observed human locomotion/function is this way?

Now lets deconstruct components of this synergy or strategy- look below to see the trunk synergy only, the leg synergy only and then a combined leg-trunk synergy:

	In standing, hitch the pelvis upwards on the left. Feel, observe trunk side bending and pelvic tilting (in the frontal plane).
Raise your left leg only and deliberately resist any trunk action. Only hip and knee flexion. Try ascending and descending stairs with only leg action and no trunk action.
	Now combine a pelvic hitch with the leg raise.

I have over simplified this action and the animation is not yet fully accurate however the concept of "seeing synergies" and simple (trunk side bend, leg flexion-extension) and more complex synergies (coordination action in all three planes) I propose to be a potent way to begin an assessment of human locomotor behavior. This is a good tool to consider in your repertoire for studying human movement.

Why is seeing synergies so important? Imagine the effect on the hip and knee without trunk action when ascending stairs. Imagine the effect on the knee if the femur does not rotate during extension of the leg (ie action in one plane only). What would be the result on the action and on the structure if the thorax did not accommodate the powerful side bending of the pelvis and lumbar spine? Ascending stairs exaggerates the same trunk synergies for gait on a flat surface (Krebs et al 1992). It provides an easy way to view the combined trunk-leg synergy of human locomotion. These are features of what I see in human motion and provide clues to solving motor abilities and clues for rehabilitation.

Whilst motion occurs in all three planes it is the trunk side bending and the leg flexion and extension (with rotation of the femur) that provide the basic components of the action and I propose they are the most powerful components of the action. Now go back to the reptile and imagine the salamander and the crocodile ascending stairs. The similarities are scary. Whether the basic leg-trunk synergy of the reptile remains in human motor behavior or whether the brain actually controls human motion with such synergies is not the question here, what is significant for the clinician is to draw upon the story; ie the concept of simple and complex synergies, the similarities in action and the possible similarities in neuronal control to provide a tool for assessing and rehabilitating human movement.

These simplified components of trunk and leg action can be easily viewed and practiced both actively and passively. For me this combined trunk side bending and leg action/synergy for ascending stairs is a basic human function and can be found in many activities other than stairs (locomotion on flat surface, reaching overhead, standing on one leg and hence balance control). It has clear evolutionary roots (both from the early tetrapod locomotion to the evolution to standing up from the ape posture), links to neuronal control of motion (See Grillner) and represents an important function to be assessed as one of the major motor behaviors of human function.

If this action/synergy is well organised in an individual then you have a strong well controlled base for skilful powerful action for standing on one leg, climbing, balance, throwing and reaching, arising from a chair and locomotion.