Computerized Dynamometry in Impairment Evaluations
For those practitioners who regularly perform any type of musculo-skeletal impairment and/or disability evaluations, the recognition that these examinations are performed with less than ideal evaluation tools becomes a hard reality when you perform enough of these tests. This technological limitation contributes to the use of substandard tools that can lead to large amounts of test variance and measurement error. Combined with the lack of formal academic preparation of many practitioners on how to perform a standard impairment assessment, erroneous conclusions may be result regarding the true status of an individual who might be seeking to return to work or sport. This report will focus on the importance of using an objective method to evaluate specific muscle parameters such as strength, muscle endurance, work, power and joint active/passive range of motion, and at different velocities for a more accurate characterization of muscle/joint functional status.
I should begin by defining both disability and impairment and will defer to the AMA’s guide to disability evaluation for these. In the guide, impairment is defined as a deviation from normal in a body part or organ system and it’s functioning. Disability is defined as an individual’s capacity to meet personal, social, or occupational demands or statutory or regulatory requirements, because of an impairment.1 An example of this distinction might be that of a patient who has had a total knee arthroplasty with post-surgical arthrofibrosis development and does not feel capable of ascending stairs. The impairment is the fibrotic knee joint, the disability is the perception of not being able to ascend stairs.
I will use a qualifying statement as I formulate the rationale for this report: none of the aforementioned parameters, in isolation or singularly, correlate well with functional movement as defined by more integrated movement patterns and sequences, including formal work tasks but, in their absence, there is virtual guarantee of impairment and disability. In other words, it does not necessarily follow that because a patient scores poorly on a knee extension test, that he or she cannot climb a ladder or ascend stairs. The biomechanical relationships between individual muscle parameters and function are not that simple for a myriad of reasons including that persons accommodate (compensate or adapt) to physical limitations, both in a healthy and non-healthy manner. The converse is also true. Because the knee extensors of an individual score high on a strength test does not necessarily translate into being able to perform a sit to stand action or reciprocal stepping during stair climbing. In Parkinson’s patients we know that they tend to score very close to the norm in isolated knee extension testing (static or dynamic testing) but many cannot perform a sit to stand action. The problem is not strength—it is power or the application of strength within a certain timeframe (msec) that is limiting the desired activity. Power tasks require the person to be able to generate torque (rotational motion about an axis) at certain velocities and to be able to control for acceleration and deceleration of the limb segments being moved (stabilization). What we do know from sport science research in the area of motor control and orthopedic biomechanics is that lack of knee extensor strength:
- makes it more likely that an elderly person will fall and possibly suffer hip fracture as a result;
- that a female soccer player will suffer an ACL injury that could end her career; or
- that an MS patient will have a compromised ability to ambulate adding further functional decline and morbidity to their already degrading status.
The difference in a work setting (workers compensation case) or other potentially adversarial and/or entitlement scenarios (disability cases) is the confounding “functional overlay” that could be in operation which might include any one or more of the known constellation of psychosocial factors that affect human performance testing. When testing for musculoskeletal human performance, we cannot neglect those factors such as attention, learning, motivation and task understanding that play a role in the final outcome of these tests. It is with this in mind that we focus on, arguably, the most objective form of musculoskeletal performance testing that we have available today—computerized dynamometry (CD), often performed with a Cybex dynamometer.
Isolated Muscle Testing
This form of testing has often been referred to as robotic since it has a passive mode whereby once a safe passive range of motion has been identified in the target joint—such as the knee or shoulder—a numerical range can be inputted (e.g., 0 to 100 degrees) and the device can safely take a patient through the specified arc of motion.
In this report, we will focus on the utility of this form of testing in the impairment evaluation assessment—an area that could use an infusion of objective and meaningful testing so that assessment criteria beyond crude measures of joint active range of motion (AROM) and static strength might be applied in the examination process of a problem muscle/joint. The current AMA format relies heavily on joint AROM which, research has shown, has little correlation to actual impairment.2 In fact, any test that can be managed by the patient to significant levels, by default should be studied for reliability levels and validity. The measurement of AROM is almost solely in the domain of patient control because we are asking the patient to define both the arc of motion and the effort behind it, thus making the test very subjective.
Aside from who controls the movement, an AROM deficit has not in itself been shown in the literature to be highly associated (having specificity) with any condition or pathology other than a component of capsular and non capsular patterns first described by Cyriax.3 Restrictions in mobility (AROM) can be a part of a pathology (e.g., lumbar flexion deficit in a person with spinal radiculopathy) but do not necessarily have to be. Even with full shoulder rotator cuff tears, a highly motivated patient with good substitution/compensatory motor strategies can achieve functional range of motion.