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10 Articles in Volume 9, Issue #9
Neuroethics at the Close of the Decade of Pain Control and Research
Cumulative Response from Cranial Electrotherapy Stimulation (CES) for Chronic Pain
Dextrose Prolotherapy for Unresolved Wrist Pain
Adult Growth Hormone Deficiency in Fibromyalgia
Middle Ear, Eustachian Tube, and Otomandibular/Craniofacial Pain
Computerized Dynamometry in Impairment Evaluations
Co-Morbid States Are the Rule—Not the Exception—in Pain Practice
Nutritional Supplements in Pain Practice
Testosterone Replacement in Female Chronic Pain Patients
A Practical Guide for the Use of Opioids in Chronic Pain

Computerized Dynamometry in Impairment Evaluations

Isokinetic computerized dynamometry provides objective, reproducible, and valid measurements in clinical decision making regarding return to sport, return to work,

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.

In isolation, AROM has very little diagnostic value and must be interpreted as part of the larger diagnostic profile. The point is that if our intent is to characterize the state of health of a particular muscle or joint, we must take into account the more global capabilities or attributes of the target muscle including mobility, strength, muscular endurance, power and total work capabilities—not simply AROM. All of these rely on the intrinsic force-generating capacity of the muscle. As well, AROM measurements taken with goniometry and/or inclinometry have not been shown to be consistently reliable between examiners or on the basis of inter rater reliability.4,5 The most consistent, responsive and valid measure of these parameters is generated from computerized dynamometry. Any other form of testing—including hand held dynamometry, manual muscle testing, dynamic screening and functional testing—cannot provide the same data at similar precision levels. These secondary measures introduce considerably greater error into the individual estimates for those attributes of interest. Furthermore, there is no reliable way to comment on patient effort levels.

Data Collection

Figure 1 displays the Cybex system in isolation including the chair, monitor/CPU and torque generating apparatus. Testing and evaluation involves setting the patient up on the Cybex chair and connecting the patient to the machine using the corresponding attachments. For example, Figure 2 illustrates the configuration for a a knee joint evaluation. This form of evaluation is ideal in isolating a specific anatomical functional unit—specifically, a target joint with all its relevant muscular attachments. Figure 3 illustrates testing of the shoulder internal and external rotators or rotator cuff unit, while Figure 4 illustrates an ankle joint assessment of plantar/dorsiflexion.

The dynamometer axis of rotation is aligned with the patient’s natural axis of rotation. For the purists, we understand that some joints such as the knee have a polycentric axis of rotation (several rotation points based on joint angle) but, for the purposes of this test, a specific “best fit” point is chosen. The patient is seated comfortably and a series of practice trials (flexion/extension cycles) at a preselected speed is performed. This warms up and stretches the joint while at the same time minimizes the learning effect inherent in test procedures. When we test a target joint, we ensure that there is an adequate rest period between sets of repetitions so muscles can properly recover during rest. The Cybex device can perform 23 different joint/muscle tests including continuous passive motion (CPM) mode and functional patterns mode when the device is being used for training, as opposed to testing. The versatility of a such a system is that it can perform most all training functions that a full service line of exercise equipment could perform—all using a much smaller footprint than an entire line of selectorized or plate loaded equipment. We will focus on the evaluation function of this device since this is the feature that is most useful to impairment assessment practitioners.

Figure 1. Cybex system including the chair, monitor/CPU and torque generating apparatus. Photo courtesy of Cybex and CSMI. Figure 2. Demonstration of patient performing a knee joint evaluation. Photo courtesy of Cybex and CSMI. Figure 3. Testing of the shoulder internal and external rotators or rotator cuff unit. Photo courtesy of Cybex and CSMI.

What has been impressive to those who use computerized dynamometry (CD) in performing impairment/disability or return to work/sport assessments—including independent medical exams (IMEs) and Functional Capacity Evaluations (FCEs) —is the level of measurement precision that is unmatched by any other technology. Muscle outputs such as inherent muscle strength (torque), endurance and power are calculated with a high degree of accuracy. When isokinetic mode is selected as the test preference, the user also selects the speed—e.g., 60 degrees per second (slow) or 240 degrees per second (fast)—depending on the parameter being tested. The device will test up to 500 degrees per second and makes it ideal to test muscle/joint integrity at ultra-high speeds in mimicking athletic tasks such as world class sprinting and/or soccer kicking motions. It is recommended that CD testing actually be performed prior to disability testing that involves more functional movement patterns versus the isolated profile that CD will provide us. The CD test should really precede the more global FCEs that focus on job simulation skills (movement patterns) if it is ultimately to be used in this type of setting.

Figure 4. Assessment of plantar/dorsiflexion of the ankle joint. Photo courtesy of Cybex and CSMI.

The unique nature of this testing method certainly lends itself to evaluation protocols that require reliable and precise measurements of specific joints and muscles. The added advantage is being able to measure consistency of repeated measures and generate a measure of repetition variation (variance between repetition) using a coefficient of variation (CV) which allows the practitioner to at least speculate intelligently on the probability of non-valid sub-maximal effort by a patient when compliance calls for full or maximal efforts. This aspect of testing can be useful when patient effort levels are in question. In the sports milieu, the problem is usually quite the opposite—with athletes attempting to return to play too soon after injury—and so accurate assessment of joint/muscle integrity is essential. Radiologic assessments combined with orthopedic examination, although important, do not always accurately characterize the functionality of the injured area. When in doubt, computerized dynamometry is used to accurately quantify the level of muscle/joint function and is often the missing link between hypothesized and true patient status.

Muscle Functions

The use of isokinetic testing in CD is especially useful because the speed is held constant so we factor out injurious forces created by acceleration/deceleration of a load—especially in those moments when we really want to test, not train, the muscle. The speed options are variable and numerous which means we can test slow, during which the muscle generates high internal forces, or at higher speeds which allow muscles to contract faster but generate lower internal forces, consistent with length/tension relationships. Testing a joint at different speeds provides good data on the ability of the muscles to generate forces at various speeds. The myriad of human tasks calls for the muscular system to be flexible in its ability to adapt to various loads and speeds of movement. Muscles must be able to function optimally in concentric (tension while shortening), eccentric (tension while lengthening) and isometric (static tension) modes. A comprehensive muscle/joint testing tool such as the Cybex can test all these muscle actions. We also get data on where in the path of motion or torque curve the tested muscle/joint achieves peak forces. The ability to generate a certain amount of force is important, but the timing of that force generation is just as critical. The neuromuscular events such as time to peak force and joint angle of peak force have implications for functional movements and help explain why some persons can perform a task, while others cannot.

Another very useful feature of isokinetic testing using CD is that when there is joint fatigue, the device automatically accommodates to the change in force generation allowing a true representation of total work performed by a person. In any other test format, fatigue leads to test termination or a reduction in resistance or load when in isotonic (same load) mode—such as when a person is performing a test using a defined load on a weight machine. In this case, if the load is set to 50 lbs and knee extension is being tested and the person can no longer lift or move the 50 lbs, the test stops. With isokinetic testing, the test actually continues as the muscle fatigues and torque-generating capacity decreases. The device accommodates lowered force production and allows muscle action to continue until momentary muscular failure occurs at a later point. The end result is a more accurate representation of muscle endurance and total work performed.

From a safety standpoint, a painful twinge occurring during an isokinetic CD scenario assessment would be instantaneously sensed by the device as a force reduction—unlike a conventional strength assessment test that uses a fixed load where the same occurrence would cause immediate reflex inhibition and muscle weakness causing the load to drop. In so many other test scenarios, the weight continues to exert a potentially injurious force until the person undergoing the test or an outside force—such as the weight stack, spotter, or built in range of motion—stops it.

Conclusion

The hallmark features of isokinetic computerized dynamometry are objective, reproducible, and valid measurements. There are instances in clinical decision making where only high quality, meaningful data should be used to formulate an opinion leading to important recommendations such as return to sport, return to work, and impairment and disability assessments. Such a decision impacts the lives of many in significant ways and human performance data-based decisions should not be made without consideration of measurement error and the impact of that error on the final clinical decisions. It is recommended that the computerized dynamometry form of testing be used to better assess the status of muscles and joints prior to disability-type evaluations involving more complex movement patterns and/or specific performance tasks.

The use of isokinetic CD is safe, efficient, and a highly reliable form of testing that controls, to a large degree, the patient-device interaction. Such a system can perform in isometric, isotonic, and isokinetic modes, as well as having passive motion capability. The versatility and utility of this device (with 23 movement patterns) would make it an important measurement tool in any type of muscle/joint performance assessment. In summary, isokinetic computerized dynamometry brings a more sophisticated level of muscle analysis with a protocol that allows not only normative data comparison, but also side to side comparison data, when appropriate. Superimposition of strength curve data between involved versus uninvolved limbs allows easy comparison of torque tracings and curve amplitudes makes strength, endurance, total work performed and power comparisons simple and accurate.

Acknowledgement

I would like to thank Rob Rindos (Cybex) distributor and Robert Potash of CSMI for supplying the photos used in this report.

Last updated on: January 28, 2012
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