All systems when used isokinetically work on the same principle. The lever arm (the bit you attach the patient on to see above) moves at a pre-set angular velocity (PAV) see below.
Remember you set this velocity.
Now the user can push as hard or as little as they like and the machine will only move within small boundaries of the speed you set. If the user pushes harder the speed does not increase but the resistance does and the movement is maintained within very narrow margins about the PAV. If the user pushes with a force unable generate the speed required for the lower margin of the PAV then the system will interpret this as the user wishes to stop (this will often be seen as a jerky movement on the machine).
The width of the PAV margins are a performance characteristic of each individual system which are often unchangeable (Kin-Com, Isocom, Isomed) but can be altered on some systems (Cybex Norm (sensitivity)).
Remember, the moment (force) generated does not have to be maximal but it has to be enough to generate the PAV otherwise the machine will not run (or in other words, if the machine stutters and jerks then the speed needs to be turned down (or the PAV altered if available) as the movement is actually isotonic whilst this happens).
Each of the systems available has its own features but basically they are all the same in that they have a rotating lever arm which moves in a single plane (take note - this does not mean that the motion of the joints has to be in a single plane e.g. diagonal movements at the shoulder are reproducible).
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 also 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).
When the machine determines how much resistance to offer the subject to keep the PAV it has to determine the resistance given by the subject. This piece of information that the machine needs to create the movement is vital to the tester to get the results we all know and love. All machines are said to have a sample rate (this is how many measurements of strength are required to maintain isokinetic motion e.g. 100Hz or equal to 100 measurements per second). In reality it is quite plausible to take the base signal (analogue) and record this at any speed for research purposes. Older designs still use the 100hz sample rate whereas the latest generation of machine use speeds up to the current fastest of 5000hz achieved by the Isocom.
Fortunately, the machine will take these basic measurements, record them and then display them (usually in a graph (individual machine characteristics can be seen here)) as muscle performance parameters.
Muscle performance testing usually consists of a minimal number of maximal contractions which gives the machine a representative moment angular position (MAP) curve. The number of repetitions required is different depending on the protocol used.
There are many parameters which must be set in advance to control the test/exercise. These are usually categorized as two groups (well Dvir 1995 separates them) - subject orientated and machine orientated inputs.
Subject orientated Control Parameters
These vary somewhat between individual joints, however, there are some general rules.
ROM
This is measured in degrees and describes the total allowable angular displacement of the lever arm. It should not be confused with the motion taking place at the joint (which is often even further removed by the misalignment of the instantaneous axis of rotation).
The Isokinetic ROM (IROM)
This is always smaller than the ROM as each contraction cycle starts and terminates at a static position i.e. if we set a test speed of 60 degrees/second at the end of each movement the segment has to be accelerated or decelerated from 0 degrees per/second to 60 degrees per/second. This takes time (see acceleration/deceleration curve below).
Magnitude of ROM
There is evidence to suggest that an increase in peak moment will be experienced with increasing ROM i.e. if you test the knee over a 90 degree ROM and then over a 120 degree ROM you should expect an increase of approx. 9% in peak moment. This may be due to tension developments or greater neural activation within a muscle.
Angular Velocity
Usually measured in degrees/second but you will often see it stated as radian/second (one rad is approx. 57.3 degrees). Remember that pre-set velocity cannot be related to muscle linear contraction velocity (as described by Hinson et al 1979). It should be noted that the higher the angular velocity the less movement will be performed isokinetically.
Machine Orientated Control Parameters
Damp Setting
Before the movement reaches isokinetic values there will often be a spike at the beginning of the contraction. This has been referred to as the 'Impact Artefact' (or torque overshoot, impact torque, moment overshoot or overshoot). This spike does not represent muscle performance at the pre-set velocity. They occur in the acceleration and deceleration phases (often called moment signal transience). These sectors are usually equal to the ROM and speed respectively (e.g. over 90 degrees at 30 degrees/second the transient phase will be equal to 1 degree at either end of the ROM). Dampening the signal reduces this (see below).
More modern dynamometers allow for this problem by using ramping or computer controlled acceleration. This is one reason why strength results cannot be compared from one make to another.
Although the overshoot is not meaningful with regards to peak moment it is very important as a reflection of the capacity to recruit the neuromuscular apparatus and generate a moment. For this reason most good dynamometers will allow you to alter the damp settings on a manual basis so both aspects can be investigated.
For an in depth review of moment artifact please click here.
Isometric Pre-activation.
This refers to the static tension generated in the muscles before motion (often called pre-loading). This was first described by Gransberg and Knutsson (1983) who said it had a restraining effect on the initial moment oscillations.
Lower Isometric Bias.
This is often referred to as the minimal start and back forces. It describes the amount of moment that has to be maintained in order to ensure smooth progression of isokinetic motion and it can be used to compensate or compliment isometric pre-activation.
Upper Moment Limit.
This is the maximal moment that may be exerted by the machine against the subject. It was incorporated to ensure the safety of vulnerable structures e.g. ACL after reconstruction, but it may also be used for the purpose of fine motor performance analysis.
Feedback.
Can be either auditory or visual (the visual feedback varies wildly between machines). Both can have profound effects on performance (up to 30%).
The performance characteristics of each individual machine can be seen here.
