Isokinetic muscle testing for weak patients suffering from neuromuscular disorders: A reliability study
Introduction
Muscle weakness is the major clinical manifestation in hereditary neuromuscular disorders. Precise, sensitive strength testing methods are needed to investigate muscle function in very weak patients. The quantitative measurement of muscle strength can be performed with an isokinetic dynamometer: these machines enable the quantitative assessment of a dynamic muscle contraction in which the velocity, the resistance and the joint position are tightly controlled [1]. Indeed, isokinetic evaluation methods have displayed good reliability and sensitivity for knee and ankle function assessment in people suffering from orthopaedic disorders. To date, isokinetic dynamometers have mostly been used as a reinforcement technique or as a muscle strength assessment method in people with mild or moderate strength impairments. However, the use of isokinetic evaluation has rarely been described in neurological defects in general and in neuromuscular disorders (NMDs) in particular.
A few studies have described the use of isokinetic testing in NMD patients with mild or moderate weakness [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The first studies involving the use of isokinetic dynamometers to evaluate neurological disturbance were reported by Griffin et al. and by Wagner et al. [14], [15]. Both featured the use of a Cybex II® active procedure to investigate muscular dystrophy patients. Wagner et al. studied the efficacy of a reinforcement program on a patient with scapuloperoneal muscular dystrophy. Griffin et al. studied the relation between MMT and isokinetic dynamometry and concluded that the latter method was not suited to subjects with severe muscle weakness. Using a similar approach (isokinetic dynamometry on the Cybex II® with an active procedure), Backman monitored children with Duchenne muscular dystrophy and noted the method’s limitations for assessment of very weak patients [16]. In 1992, Merlini et al. first described the use of a continuous passive motion (CPM) procedure with the Lido-active® dynamometer [11] (it should be noted that the latter apparatus is no longer on the market). These authors reported the isokinetic evaluation (at 30°/s) of patients with Duchenne and Becker muscular dystrophy and spinal muscular atrophy at different disease stages. Comparison of the isokinetic torque with the MMT values for the knee flexors and extensors showed that isokinetic measurement was more sensitive than MMT, even in cases of severe weakness. To date, only one other article has reported the use of this method: Andersen et al. evaluated the strength of ankle flexors and extensors of patients with neuropathy using the Lido-active® dynamometer’s CPM mode and reported that the reproducibility was low for very weak patients [4]. No other study has described the use of the CPM isokinetic method.
Isokinetic strength evaluation in facioscapulohumeral muscular dystrophy (FSHD) has been studied by Kilmer et al. by using the Lido-active® dynamometer [9], [17] to test elbow and knee flexors and extensors with an active procedure that had been described earlier by Lord et al. [9]. Kilmer et al. also evaluated simulated work performance tasks by mean of the Lido Work-SET, a microcomputer-controlled work simulation device which was developed to test functional tasks in patients with neuromuscular disorders [6].
Use of the CPM procedure enables very weak subjects to be tested, which is not possible with active procedures. The method is simple; the patient is instructed to actively accompany the movement (i.e. whilst pushing as hard as possible against the lever arm) and any exerted force is recorded – even if the patient is very weak.
The major criticism of isokinetic methods is that they only allow evaluation at a few joints and, above all, that they lack sensitivity in cases of severe weakness. The standard isokinetic testing method requires the subject to have enough strength to “actively” move the limb and the lever arm. During “active” isokinetic testing, the patient is instructed to push “as hard as possible” on the lever arm in order to move it and reach the predetermined velocity; this is not possible in cases of severe weakness, when patient is not even strong enough to move his limb and the lever arm against gravity.
This disadvantage is circumvented if a continuous passive motion (CPM) programme is chosen; here, the robot moves the limb and the dynamometer lever arm at a preset velocity whilst recording all forces applied to the lever arm. After calculating and compensating for the combined weight of the limb and the lever arm, even slight forces produced by the subject can be detected. Merlini et al. [11] have described this type of procedure for neuromuscular disorders and showed that at low velocity (30°/s), it was possible to test patients scoring less than grade 3 in manual muscle testing (MMT).
Here, we describe an isokinetic testing procedure using the Biodex 3®’s CPM mode. The method includes passive, isokinetic limb mobilisation, which allows one to measure gravity and passive resistance. This measurement is then subtracted from the value recorded during the same mobilisation in which the patient has to actively accompany the motion, i.e. whilst pushing against the lever arm as hard as possible. The torque curve resulting from this subtraction represents the force actually exerted by the patient.
The aim of the present study was to (i) evaluate the reliability of this isokinetic method for knee flexor and extensor testing in NMD sufferers presenting mild to severe muscle weakness and (ii) study the relationship between MMT and isokinetic dynamometry.
Section snippets
Subjects
Fifteen subjects with hereditary neuromuscular disorders (12 women, 3 men) aged from 16 to 67 years (mean age: 40.4) participated in the study. All subjects had a confirmed diagnosis (via molecular biology or muscle biopsy methods) but were exempt from consistent leg muscle or joint pain. All subjects had a knee range of motion (ROM) of about 90°, except for one individual (suffering from limb girdle muscular dystrophy 2A, LGMD2A) with a lower value (50°) for both knees. The population is
Results
All subjects performed the procedure twice for each knee (with the exception of subject 13, who was tested for the left knee only, due to right knee pain on the day of examination).
Pulling the data of both sessions, peak torque values for the knee extensors ranged from 5.6 to 163.8 Nm at 10°/s (see Fig. 2 lower graph) and from 4.6 to 174.4 Nm at 30°/s. Peak torque values for the knee flexors ranged from 1.7 to 67.8 Nm at 10°/s (see Fig. 2 upper graph) and from 2.2 to 72.7 Nm at 30°/s.
Discussion
This study aimed at assessing the reliability of isokinetic measurements of muscle strength in a population of weak NMD patients.
Previous studies about isokinetic dynamometry reliability with normal adult subjects have been conducted with higher velocities (60 to 300°/s) and with different devices: it is admitted that isokinetic dynamometry shows good intrarater and interrater reliability. Nevertheless, reliability of a specific procedure adapted to weak patients with neuromuscular disorders
Conclusions
Our study showed that isokinetic dynamometry is a reliable strength assessment tool, even in weak patients. Isokinetic testing is a possible outcome measure for the follow-up of neuromuscular disorders and could be of value in therapeutic trials. We are now investigating the possibility of applying this CPM methodology to other joints, such as the elbow.
Acknowledgement
This work was supported by the Association Française contre les Myopathies.
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