Reliability & Validity of Isokinetic FCE:
Objectivity. Objectivity refers to the indication that the test measurements are relatively unbiased by either the participant or the evaluator. Objectivity in testing can be increased when the testing procedures and practices for observation and scoring are clearly defined and documented.
In isokinetic testing this is assured as the tester is independent of the either the referrer or the participant. The provider of the isokinetic evaluation gets the same remuneration regardless of the result. Employees are referred simply to the nearest centre geographically.
Reliability. Reliability refers to consistency in measurement and scoring. The test measures and scores must be consistent across evaluators, participants, and the date or time of test administration. Inter-rater and Intra-rater reliability have been noted to be the two most important forms of reliability in FCE test administration and evaluation.
Isokinetic tests have been shown to offer excellent intra and inter tester reliability. As they are simply very accurate measurement devices they have very good consistency in the results. In fact isokinetic evaluation has been assessed for reliability on every major area of the body. Correlation coefficients are normally as high as 0.98 to 0.99
Inter-rater Reliability. Inter-rater reliability refers to the ability to achieve similar test results on an evaluation when administered by separate evaluators. It ensures that the FCE scores were not due to differences in the evaluator's skills, test administration, and scoring techniques. Inter-rater reliability can be measured by having two or more different evaluators give a test independently to the same test participant or group of subjects. The test scores are then compared against the others by calculating the coefficient of variation. Ideally, the coefficient of variation should be less than 5%.
Isokinetic evaluations should only be repeated by the same tester to ensure inter-rater reliability. Providers have dedicated members of staff for this purpose.
Intra-rater Reliability. Intra-rater reliability refers to the stability of a test score derived from the implementation of one test to another when the same evaluator administers the FCE test. Factors that potentially affect the consistency of test-taking with the same evaluator include: the duration of time between the two tests, the treatments received by the participants, the medical stability of the participants' conditions, and the cooperation of each of the test participants.
This is only applicable if the employee has had interventions to correct a problem. The reliability of repeated measures has been shown to be excellent as long as the tester is the same in the same facility.
Safety. Safety refers to the level of risk in performing the test. The test must be safe to administer. The FCE must not be expected to lead to injury.
This is established in a pre-evaluation questionnaire and pre-test during subjective examination. One of the primary benefits of isokinetic testing is its safety. The machines inherently only give the amount of resistance the subject can safely move. This resistance is changed 5000 times per second making it seamless and interactive beyond the level at which human muscles can change.
Utility. Utility refers to the test being useful. The evaluation procedure must meet the needs of, and provide useful information to, the participant, the evaluator, the referral source, and the payor. The FCE must include useful information about specific return-to-work questions.
Isokinetic tests can focus on single joint actions through to complex tasks. Often a complex task will be affected but the reason for this deficit can be elusive. Isokinetics machines can not only test the complex task but they can also break down the task into it's elements allowing each action within the chain to be analysed.
Validity. Validity measures how real the test results are. A score is considered valid if it accurately measures the factor it intends to measure, and if it can be used to estimate or reflect performance on a target task. With regard to FCE testing, the functional test scores and measures can estimate and predict real-world function and the ability to perform work-specific tasks and activities. Without validity testing, there is no way of knowing whether the results are truly accurate and meaningful. Validity is an essential requirement of all FCE measurements.2
Isokinetics tests have been performed since the late 1960's. In this time extensive research has been performed which has demonstrated all of the types of validity that are appropriate to FCE testing they are listed and explained below:
Is what you measure relevant to performance?
Isokinetic dynamometry has demonstrated content validity with respect to muscle performance.
For instance, maximum isokinetic derived power (during plantar flexion) is reached at the functional velocity equal to that of toe off during walking.
Or as velocity increases force generation decreases but average power increases (Fugl-Meyer et al. 1982).
Fabiś J (2007) found isokinetic training increased muscular function following ACL reconstruction
Sandrey MA, Kent TE. (2008) and Fox A et al. (2008) found the ankle became more unstable as the muscles became weaker
Think of this as looking for a series of relationships with factors that are well established. The main ones are covered below.
Gender differences. Many isokinetic studies have shown that men are significantly and consistently stronger than women (this is often documented as an obvious observation concerning gender differences).
Effect of Age. Backman and Oberg (1989) demonstrated consistent strength increases in the dorsiflexors through the ages of 6, 9 and 12 in girls and boys. (Up to this stage strength was equal). Girls then plateau and boys continued to get stronger.
Strength normally reaches its peak in the third decade and there after declines moderately with age until the seventh decade where there is a steeper decline. Timm (lifting capacity 1988) and Borges (knee strength 1989) found these variations using isokinetic knee and lumbar spine testing.
Silder A, Heiderscheit B, Thelen DG (2008) found hip strength became more vital as you aged and could compensate for poor ankle strength
Activity Level. Fugl-Meyer (1981) found isokinetic ankle plantar and dorsiflexor strength differences between trained versus sedentary subjects.
Body weight. In human studies muscle mass rises proportionately with body weight. Heavier subjects do produce higher isokinetic moments.
Muscle characteristics. As shown by Backman and Oberg (1989) the slope of moment angular velocity curve changes with age in that children cannot utilise stretch shortening cycles as adults can, possibly because of softer muscles (in other words they are more flexible). This is seen in isokinetics especially in adolescents who generate more moment at slower speeds.
Nordgren et al. (1983) demonstrated this best when they found that local anesthetic injected into the knee joint resulted in normalisation of an otherwise depressed moment curve in patients with osteo-arthritis. Or in other words subjects with osteo-arthritis demonstrate altered shapes in their torque curves which subjects without the symptoms do not.
Pienimäki TT, Siira PT, Vanharanta H. (2002) showed isokinetic strength directly related to grip and elbow dysfunction. Whilst isokinetic treatment could reduce grip and elbow tendon problems.
Criterion Referenced Validity:
This is split into 2 parts - Concurrent or whether what you find isokinetically can be related to another activity, and predictive, or is the isokinetic measure able to forecast a future event.
Gerdl et al. (1986) found a strong correlation between isokinetic endurance testing during repeated plantarflexion contractions and the maximal functional walking capacity in patients with peripheral arterial insufficiency and intermittent claudication. That means if a patient has a low score on this endurance test their walking capacity will be correspondingly limited (this test may be more suitable as it involves less risk to the patient).
Pain sensation scores during knee testing have a high correlation with patello femoral pain syndrome, particularly at lower test angular velocities (Dvir 1995).
The examples of concurrent validity are extensive for every major joint of the body including the spine
The power to predict dysfunction from testing.
Prediction of injury. This has probably been best studied by Gilliam et al (2002). They looked at two different companies' isokinetically screening prospective employees. One was an airline and the other was a large parcel delivery company. They were able to reduce injuries by 75% using isokinetic screening.
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Pienimäki T, Tarvainen T, Siira P, Malmivaara A, Vanharanta H. Associations between pain, grip strength, and manual tests in the treatment evaluation of chronic tennis elbow. Clin J Pain. 2002 May-Jun;18(3):164-70.
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