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KINETIC CHAINS

of the other joints to restore balance. For example, if the knee were flexed in isolation, whilst the foot was planted on the ground, the mass centre would be displaced backwards and may fall outside the base of support. To counteract this, the hip and ankle joints have to flex to bring about an anterior displacement of the mass centre.

At face value this distinction may appear to be useful, but it doesn’t really stand up to scrutiny. For example, in so called ‘open’ chain movements, although the distal segment may be free to move in space this is not without constraint. There are often either extrinsic or intrinsic factors which limit the range of viable solutions. So although there may be a theoretical free- dom of movement, what is actually seen is a very stable and predictable pattern. In the example above, the leg motion in swing is constrained by the need to minimise the energy cost and so the degree of variability possible is in fact small.

There are also anatomical constraints limit- ing the independence of the joints within the chain. In both upper and lower limbs there are biarticular muscles which play an important role in the co-ordination and control of movement (8, 9). These biartic- ular muscles are affected by displacement at either of the joints they cross and although they are not necessarily responsi- ble for the generation of a large proportion of the active joint moment, they do serve a key role in regulating the tension in the uniarticular muscles. This co-ordinative role acts regardless of whether the limb is thought to be in either an ‘open’ or ‘closed’ chain.

Further to this, we have the difficulty that even in what appear to be obvious ‘closed’ chains, where the distal segment is in contact with the ground or other surface, we should not forget that the pelvis and upper body are usually free to move in space. Why should we only consider one end of the chain when defining its status as ‘open’ or ‘closed’? The need to keep the centre of mass above the base of support is often important and so might limit the degree of freedom in the chain but none the less the chain is free at one end.

So we have a classification system which is rather arbitrary in its definitions and does not really differentiate the critical biome-

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chanical characteristics it is supposed to. It is a system which has only limited application and is seldom used in the biomechanical analysis of movement. But despite these concerns this is not the principle reason why I suggest that the literature concerning ‘open’ and ‘closed’ chain exercises does not link up. The ‘open’ versus ‘closed’ chain literature does, for the most, act as a smokescreen for the real issues, with authors assigning differences in outcome measures to the nature of the chain rather that to the real differences between the exercises examined. As stated previously, the analysis methods employed are sound and I do not wish to cast doubt on the quality or importance of the findings of these studies.

If the literature is reviewed one will find that the vast majority has considered the question ‘open’ and ‘closed’ chain exercises involving knee extension (eg.3,5,7,10). There are some studies considering the upper limb (eg.11,12) but similar points can be made about these as will be made concerning knee exercises.

In most cases the comparison has been made between isometric or isokinetic knee extension exercises and movements such as squats or lunges (2). Others have used isolated knee extension movements in comparison to leg-press type movements (5). Well let’s start by considering the use of isometric exercises. These must require that the segments are fixed in location at both ends. Therefore applying the definition presented previously, any isometric contraction should be thought of as happening in a ‘closed’ chain. Alternatively it could be said that they are not in a chain at all, if we interpret the joints (links) as being fixed, with no movement possible, rendering the limb in effect, a single, semi-rigid segment.

Further to this, a chain must consist of at least two segments and one link, otherwise it is just a single segment! So by definition, isolated knee extension exercis- es, where a single segment rotates about a single joint, are not kinetic chain exercises of any form and their classification as ‘open’ or ‘closed’ is meaningless.

So what do these various studies tell us about the appropriate use of exercises during the rehabilitation process? To answer this question it is necessary to

revisit the activities used, to see if there is a more useful classification which can be used to differentiate them. One obvious and striking difference between the exercises studied is the number of joints employed in the movement. A great many of the studies cited so far (and in the wider literature) have in fact compared single and multi-joint movements and it is this, rather than the notional ‘open’ or ‘closed’ chain nature, which is responsible for the differences reported.

Exercises involving only a single joint are attractive to the therapist in so far as they permit greater control over the range of motion and easier identification of the involved muscle activation. It is thus possible to regulate both the range and intensity of movement which have obvious attractions from a clinical perspective. It is also possible to use such movements in a diagnostic sense to associate particular patterns of joint torque with specific muscle or joint disfunction.

A number of studies have however suggested that there may be problems with the use of single joint movements, especially during the early stages of rehabilitation. Greater shear force across the knee/tibiofemoral joint have been reported for single than in multi joint knee extensions (6,13), although this finding is not universal (2). The reason most commonly cited for this difference is the greater co-activation (sometimes referred to as co-contraction), specifically of the hamstrings, during multi-joint motions (9, 10). Where the hamstrings are co-active they are able to contribute a posterior directed shear force to counteract the anterior shear of the quadriceps. This has obvious implications when considering the loading of tissues such as the healing ACL in post injury or post surgical rehabilita- tion.

Development of muscular strength is a common goal within rehabilitation exercise programmes. To achieve gains in strength it is important that the target muscles are recruited and loaded at the desired level. During single joint exercises the movement is generally isolated and controlled/ constrained such that they require activa- tion of only the prime movers for the action, with only limited co-activation of stabiliser and antagonistic muscles (10). It is therefore possible to regulate the

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