Assessing Resistance to Passive Movement about the Knee
Contents
• Preamble on Assessing Resistance to Passive Movement about the Knee
• An Isokinetic Protocol for Resistance to Passive Movement
• Positioning
• Anatomical Referencing
• Gravity Correction
• Range of Movement
• Selecting Repetitions for Analysis
• Quantitative Measures of Resistance to Passive Movement
Preamble on Assessing Resistance to Passive Movement about the Knee
The KIN-COM dynamometer is often used to conduct isokinetic strength assessments of muscle groups constrained to work about a single (isolated) joint. A variation on the isokinetic exercise mode is the passive isokinetic mode. In this mode, the Kin-Com is programmed to move independently of the force exerted against the load cell. That is, the Kin-Com lever arm will move the limb segment(s) through the pre-defined range of motion without account of whether the individual is assisting or resisting the movement. This is potentially dangerous if the movement occurs contrary to the individual’s intentions. Therefore, a low maximum force limit is incorporated in the protocol as a safety consideration.
The protocol we have used since 1984 involves a variation on the standard isokinetic protocol instituted under the Exercise Option. Minimum and maximum force limits are changed to reflect limb weight and safety considerations. The minimum force is set about 20 N less than the maximum negative force encountered during the movement (when the limb weight downwards on the load cell when the lever arm is in a position close to the horizontal, a large negative force will be registered by the load cell). The maximum force is set approximately 50 N above the largest positive force encountered during the movement.
A pause time of three seconds is interspersed between the extension/flexion and flexion/extension phases (ie at both ends of the range of movement). Four cycles (repetitions) are programmed at a speed of 40 degrees per second. Slower and faster speeds of 6, 12, 20, 60, 80, 120 and 160 have also been used. Resistance to passive movement in individuals with neurologic dysfunction may be velocity dependent, so multiple velocities may be useful.
A thirty second ‘recovery’ time is interspersed between each angular speed of movement. The passive variation on the isokinetic exercise protocol with the parameters to control the movement is shown below. This may be modified and stored as a different test protocol if desired. It must then be accessed via the Protocol selection button.
An Isokinetic Protocol for Resistance to Passive Movement
The KIN-COM dynamometer is often used to conduct isokinetic strength assessments of muscle groups constrained to work about a single (isolated) joint. A variation on the isokinetic exercise mode is the passive isokinetic mode. In this mode, the Kin-Com is programmed to move independently of the force exerted against the load cell. That is, the Kin-Com lever arm will move the limb segment(s) through the pre-defined range of motion without account of whether the individual is assisting or resisting the movement. This is potentially dangerous if the movement occurs contrary to the individual’s intentions. Therefore, a low maximum force limit is incorporated in the protocol as a safety consideration.
The protocol we have used since 1984 involves a variation on the standard isokinetic protocol instituted under the Exercise Option. Minimum and maximum force limits are changed to reflect limb weight and safety considerations. The minimum force is set about 20N less than the maximum negative force encountered during the movement (typically when the limb weight acts downwards on the load cell and the lever arm is in a position close to the horizontal, a large negative force will be registered by the load cell). The maximum force is set approximately 50N above the largest positive force encountered during the movement.
A pause time of three seconds is interspersed between the extension/flexion and flexion/extension phases (i.e. at both ends of the range of movement). Four cycles (repetitions) are programmed at a speed of 40 degrees per second. Slower and faster speeds of 6, 12, 20, 60, 80, 120 and 160 have also been used. Resistance to passive movement in individuals with neurologic dysfunction may be velocity dependent, so multiple velocities are useful.
The parameters for the Resistance to Passive Movement Test are listed below:
Positioning
In assessing the resistance to passive movement about the knee, either a neutral hip position or 80 degree hip flexion position is chosen. The neutral hip position requires the subject to be positioned in supine lying. The 80 degree hip flexion position uses the normal position of the Kin-Com back support.
The subject is positioned so that the knee joint is protruding over the end of the end of the Kin-Com bench. While the thigh needs to be well supported, the leg also must be able to achieve at least 90 degrees of knee flexion without the posterior aspect of the leg contacting the front edge of the bench. Check this position ensuring a space of two to three finger widths between the freely hanging leg and the front of the Kin-Com bench.
To stabilise the pelvis in the supine lying position, two straps are placed over the ASIS and slightly more distal. The subject either places their hands under their head or folds their arms across their chest. They are instructed not to grip the sides of the bench. The knee joint centre (lateral femoral condyle) is aligned with the axis of rotation of the dynamometer by moving the Kin-Com bench and the dynamometer head. The resistance pad is located distally on the leg (with the furthest edge of the pad about two centimetres proximal to the anterior ankle crease). It is important that the centre of the resistance pad is aligned with a whole centimetre value on the lever arm ruler – this number is then the lever arm length.
Anatomical Referencing
Anatomical referencing may be performed by having the subject actively extend their knee to full extension. The resistance pad is placed in firm contact with the limb while the testor stabilises the distal thigh (a thigh stabilisation strap may be used for this purpose). This active knee extension position is entered a zero degrees. The computer instruction to move the lever arm in a positive direction requires the lever arm and resistance pad, and the subject’s limb, to be moved into flexion. In cases of increased antagonist restraint or tight antagonist musculature either passive knee extension or a knee flexed position may be used for anatomical referencing. Ninety degrees of knee flexion may be set with a goniometer.
Gravity Correction
Gravity correction or gravity compensation is performed by then positioning the lever arm at an angle corresponding to 45 degrees of knee flexion. This angle could be an angle between 30 and 45 degrees of knee flexion, but should not be ‘as close to the horizontal as possible’ because of the antagonist restraint. That position would stretch the knee flexors and result in greater resistance than that due to limb weight alone. Check that the subject has relaxed the leg and foot by gently moving the foot. Accept the limb weight reading once it is steady. Gravity corrected data is not necessary for the measure of resistance to passive movement which we have devised.
Range of Movement
For a concentric/eccentric sequence of movement events (muscle actions), the stop angle is entered by moving the lever arm, resistance pad and limb to an angle of twenty degrees of flexion (twenty degrees from full knee extension). The start angle is entered by moving the lever arm, resistance pad and limb to an angle of ninety degrees.
Selecting Repetitions for Analysis
Usually the last three or all four of the repetitions are repeatable, i.e. the curves for all repetitions are superimposed. On some occasions the first curve is different from the next three and is omitted from the analysis.
Quantitative Measures of Resistance to Passive Movement
Unfortunately our measure of choice is not available in the software provided with the dynamometer. We therefore take the .CHT datafile and use our KCAP software to import the binary file, identify individual repetitions (which we term cycles) and graph them. Either the full range of movement from 20 to 80 degrees of knee flexion, or a truncated range from 25 to 75 degrees, or from 30 to 70 degrees (depending on the speed of movement), is selected for analysis. Each curve is then analysed by calculating the area between the passive extension and flexion curves – termed a hysteresis loop.. This area reflects the work done (in Joules).
An alternate approach would be to import an ASCII text copy of the data (an ASCII dump of the data is available in the KinCom software) into a spreadsheet and use a formula to calculate the area between the passive extension and flexion curves.
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