It is generally accepted that testing muscle performance isokinetically is not directly related to function. It would seem obvious that movements performed at constant angular velocity are completely unrelated to those seen in most activities particularly sports where rapid periods of acceleration are followed by high speed limb movements which can not be reproduced by today's dynamometers! Further to this most movements in the real world occur due to the interactions of multiple joint and muscle systems unlike most isokinetic tests which occur at single joints it's that simple. 

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When we set a speed for the Isokinetic ROM we presume this is the speed the machine will force the lever arm to turn at (or in other words the speed at which the exercise/test will take place at). In reality this is almost true but if we want to be really technically correct then we have to consider the preset angular velocity margins. 

Think of it like this:-

When the subject pushes on the load cell (whatever attachment is on it) the machine allows acceleration to the preset speed (shown below as the green line), once this is reached (shown as the green star) the machine applies more force to slow the movement to the correct speed, which as you can imagine, takes a certain (albeit small) amount of time.

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Whenever a movement is performed isokinetically we set a pre determined speed for the actuator to turn around (we presume the joint turns at a speed relative to this).

However when the subject is waiting to begin the movement the machine is obviously not moving.

So if we set an exercise to be performed at 60 degrees per second there would have to be a period at the beginning where the machine accelerated (or allowed acceleration) during which time minimal resistance would have to occur (otherwise no acceleration). 

This is demonstrated in this MAP curve.

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As we have already seen acceleration and deceleration affects the isokinetic range of motion significantly (see here for a full description). It has been shown that including the acceleration and deceleration portions of the range of motion during a test can seriously affect the results once the speed is increased (Taylor et al. 1991).When any test begins (isokinetic concentric or eccentric) there is a brief moment where the speed is equal to zero or in other words there is no movement. The tested body part must then be accelerated to the pre-set speed, as the speed approaches that which has been pre-selected the dynamometer slows the movement to the pre-set speed. It has been show by many authors (Perrine and Edgerton, 1978, Sapega et al., 1982, Taylor et al., 1991 and Wilson, Walshe and Fisher, 1997)  that the speed during acceleration and deceleration can be up to double the pre-set velocity. This means that the machine has to have a braking effect on the movement which can cause a spike as the movement is slowed to the pre-set velocity. At slower speeds this spike is small (Brown, 2000) but increases in magnitude with increasing speed.

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Isokinetics machines are measurement devices. They provide us with information about the moving mechanical performance of muscle groups. They assume (very incorrectly) that the muscles they are testing move at a constant angular velocity (or in other words that the muscle move isokinetically). Unfortunately, most biological joints do not possess a fixed axis of rotation and hence the machine will make errors. The extent of the error will depend on the joint tested and the position of the subject.It is possible to measure multi joint motion and moment using the machine. It is generally understood that isokinetic refers exclusively to the motion of the mechanical elements external to the body (not the body itself).

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