Isokinetic muscle testing for weak patients suffering from neuromuscular disorders: A reliability study

Abstract

Precise, sensitive muscle strength testing methods are needed to investigate muscle function in patients with neuromuscular disorders (NMD). Here, we describe an isokinetic knee flexor and extensor testing procedure using the Biodex 3^®’s continuous passive motion (CPM) mode. The torque values recorded during passive isokinetic motion were subtracted from the torque values obtained for the same movement with maximal, concentric effort. The aims of the present study were to (i) evaluate the method’s reliability in NMD patients presenting mild to severe muscle weakness and (ii) study the relationship between manual muscle testing (MMT) and isokinetic dynamometry. The fifteen participating patients were tested twice; the respective intraclass correlation coefficients (ICCs) for the two sessions ranged from 0.91 to 0.99 for the peak torque, work and power and from 0.50 to 0.90 for the angle at peak torque. The Spearman rho correlation coefficients comparing isokinetic values and MMT values ranged from 0.67 to 0.74 (p < 0.01). This reliable, dynamic method appears to be of great value in NMD evaluation when sensitive strength measurement at the knee is required.

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.