Isokinetic Knee Extensor and Flexor Testing

Posted on 10 April, 2013

• Preamble on Isokinetic Knee Testing

• Positioning for Testing the Knee Extensors

• Positioning for Testing the Knee Flexors

• Anatomical Referencing

• Gravity Correction

• Range of Movement

• Warm-up

• Pre-loading the Isokinetic Movement

• Selecting Maximal Isokinetic Efforts

• Which and How Many Velocities?

• Quantitative Measures of Strength

Preamble

The KIN-COM dynamometer is often used to conduct isokinetic strength assessments of muscle groups constrained to work about a single (isolated) joint. The isokinetic exercise mode of the Kin-Com offers significant advantages, particularly when conducting the assessment using the Evaluation approach. In this mode and approach, individual or discrete muscle actions may be performed in a movement event. Each movement event involves a single type of muscle action through a defined range of motion at a prescribed angular velocity of movement. A discrete muscle action is defined as movement events separated by a recovery phase of specified minimum time. Movement events may be repeated until either a certain number are completed or until reproducibility of the torque output is achieved.

The discrete testing approach used in the Exercise Science Laboratory (408.4503) typically consists of three to five (or more) maximal concentric and three to five (or more) maximal eccentric muscle actions per velocity, with each muscle action separated by a thirty second recovery time. A two minute recovery time is interspersed between each angular velocity of movement. Angular velocities of 60, 120 and 180 deg/s are typically used. The default isokinetic evaluation exercise protocol defines many parameters to control the movement. The default program 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.

Positioning for Testing the Knee Extensors

Since testing began on our first Kin-Com in 1984, the protocol has incorporated both concentric and eccentric muscle actions. In testing both the knee flexors and knee extensors, a neutral hip position is chosen. This requires the subject to be positioned in supine lying for the knee extensor tests, as shown below. The knee extensor group would typically be tested first.

<img src=”images/kesetup.jpg” width=”496″ height=”349″ alt=”Overview of the subject positioning for a knee extensor strength assessment on the Kin-Com 500H” title=”Overview of the subject positioning for a knee extensor strength assessment on the Kin-Com 500H” Position the subject in supine lying so that the knee joint is protruding over 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 egde 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. This space is illustrated in the picture below.

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 not allowed 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. The resistance pad is located distally on the leg (with the furthest edge of the pad about two centimetres, or two finger widths, 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.

Check the alignment of the knee joint centre by passively moving the leg through the full range of motion. If correctly aligned, the resistance pad should not tilt or slide along the leg.

Positioning for Testing the Knee Flexors

Since testing began on our first Kin-Com in 1984, the protocol has incorporated both concentric and eccentric muscle actions. In testing both the knee flexors and knee extensors, a neutral hip position is chosen. This requires the subject to be positioned in supine lying for the knee extensor tests, as shown above. The knee extensor group would typically be tested first.

Position the subject in prone lying so that the knee joint is protruding over the end of the end of the Kin-Com bench. The thigh needs to be well supported, but the bench should not impinge on free movement of the patella.

To stabilise the pelvis in the prone lying position we have recently contructed a V-split strap that directs pressure onto the PSIS and ischial tuberosities of the pelvis (previously we have have used the two straps shown below). The subject either places their hands under their chin or on the bench surface above their head. They are instructed not to grip the sides of the bench.

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The knee joint centre (lateral femoral condyle) is aligned with the axis of rotation of the dynamometer by moving the Kin-Com bench. The resistance pad is located distally on the leg (at a distance where the furthest edge of the pad would be 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. The lever arm length for the knee flexor and knee extensor tests should be the same!

Check the alignment of the knee joint centre by passively moving the leg through the full range of motion. If correctly aligned, the resistance pad should not tilt or slide along the leg.

Anatomical Referencing

<a href=”definitions.html#anat ref”>Anatomical referencing</a> is 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 tester 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.

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Gravity Correction

The <a href=”definitions.html#grav cor”>Gravity correction</a> or gravity compensation procedure requires a horizontal position to be defined. Gravity correction is then performed by then placing the lever arm to 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”. That position would stretch the knee flexors and result in greater resistance than that due to limb segment weights 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.

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Range of Movement

For a concentric/eccentric sequence of movement events (muscle actions) for the knee extensors, the stop angle is entered by moving the lever arm, resistance pad and limb to an angle of zero degrees (active full knee extension). The start angle is entered by moving the lever arm, resistance pad and limb to an angle of ninety degrees of knee flexion.

Warm-up

I typically choose to use submaximal efforts to familiarise the subject with the required discrete muscle actions and ensure that the muscle is sufficiently warm to perform maximal efforts. I therefore prefer to use the Start button (which allows discrete efforts) rather than the Warm-up button which allows continuous concentric/eccentric cycles. The usual progression is to ask the subject to produce about 50% of maximal effort in the first muscle actions (concentric and eccentric), then 70 %, then 90%, then 95%, and finally 100%. The eccentric efforts may fall below the requested intensity as they seem more difficult to familiarise the subject with (although the first effort is often strong). Accept the best effort from the screen display until near maximal efforts are produced. Near maximal efforts are required for accurate selection of preload forces.

Pre-loading the Isokinetic Movement

An individualised, not constant, <a href=”definitions.html#preload”>preload</a> is used in all of our strength assessments. This preload may not be optimal, but it should be sufficient to activate the muscle to better than 75% of maximal activation. Achieving an appropriate preload relies on the subject being warm and producing 90 to 95% (of their maximum) effort in their final warm-up efforts. The reason why the preload does not have to be optimal is because we routinely remove the <a href=”definitions.html#”></a>non-isokinetic phases of movement when calculating our measures of strength.

In setting a preload, the aim is to achieve a force (or torque) at the starting angle which is equal to the force (or torque) which would have been achieved had the muscle been maximally activated and moving through the start angle at a constant angular velocity. What this force (or torque) should be may be estimated by extrapolation from the force-angle (or torque-angle) curve produced when the movement is isokinetic (see Figures 3 and 4). For concentric muscle actions, the preload will be less than that force or torque produced in a maximal isometric muscle action at the specified angle, and will decrease with an increase in the velocity of shortening. For eccentric muscle actions, the preload cannot be greater than that force or torque produced in a maximal isometric muscle action at the specified angle, and so can only be close to that value.

Selecting Maximal Isokinetic Efforts

The overlay screen displayed when using the Evaluation approach to testing shows either the last accepted force-angle curve or an average force-angle curve of all accepted muscle actions. The aim of the testing procedures at each velocity is to obtain the greatest overall force-angle (and hence torque-angle) curve. An example of some representative data is provided.

Figure 5 shows an example of the individual curves which were produced in five concentric muscle actions of the knee flexor muscle group at a velocity of 60 deg/s.

Figure 6 shows the individual curves of five eccentric muscle actions of the knee flexor muscle group at the same velocity.

Figure 7 and Figure 8 show the minimum, average and maximum curves derived from the individual curves of Figures 5 and 6 respectively. These are the curves that would be generated in the Report section of the software. Some astute curve selection during the assessment may have resulted in only one curve being stored for this velocity.

The single best curves for the concentric and eccentric muscle actions (with greatest torque values) are shown Figure 9.

Which and How Many Velocities?

A suggestion for setting the slowest angular velocity of movement may be provided by taking the range of motion and dividing the value by two. Thus, for a ninety degree knee flexion range of movement, the slowest velocity would be 45 degrees per second. This speed setting would be justified from an energy systems viewpoint with the argument that each discrete muscle action would last approximately 2 seconds and use energy derived from ATP stores and the immediate energy system (McArdle, Katch and Katch, 1991).

The value of multiple testing velocities may be in determining that the data acquired is valid data (by comparison with the torque-velocity relationship). Different torque-velocity profiles may be evident for particular muscle groups.

Quantitative Measures of Strength

Data reduction facilities are available after curves have been accepted during the testing phase and the datafile stored. The Numeric and other reports will allow a number of measures of isokinetic strength to be determined. Typically these will include the full range average torque (<a href=”definitions.html#frat”>FRAT</a>), truncated range average torque (<a href=”definitions.html#trat”>TRAT</a>), peak torque (<a href=”definitions.html#peak torque”>PT</a>), angle specific torque (<a href=”definitions.html#ast”>AST</a>), and work (<a href=”definitions.html#work”>W</a>) (Figure 6). An average power (<a href=”definitions.html#average power”>AP</a>) may also be derived, but this is typically done by using <a href=”/home/staff/strauss”>Geoff Strauss</a>’s Kin-Com Analysis Package (<a href=”definitions.html#KCAP”>KCAP</a>) or by importing an ASCII text copy of the data into a spreadsheet and using a formula to calculate average power. Data can be generated from all torque-angle curves and averaged for each velocity and muscle action, or the best data (highest torque values) may be utilised. In subsequent analyses, ratios such as the torque (peak or average) to body weight ratio, peak torque to average torque ratio, non-dominant to dominant limb ratio, injured to uninjured limb ratio, and/or concentric to eccentric ratio may be derived.

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Isokinetic Knee Extensor and Flexor Testing

Contents

Preamble

Positioning for Testing the Knee Extensors

Positioning for Testing the Knee Flexors

Anatomical Referencing

Gravity Correction

Range of Movement

Warm-up

Pre-loading the Isokinetic Movement

Selecting Maximal Isokinetic Efforts

Which and How Many Velocities?

Quantitative Measures of Strength