1.1

Introduction

Stroke is the first cause of adults’ acquired handicap in Western countries . Vascular cerebral accidents lead to cognitive and/or sensorimotor sequels in more than one third of the patients. Upper limb deficiency is the most frequently encountered sensorimotor sequel. While close to four fifths of stroke survivors learn to walk once again , only one third recover their ability to functionally use their upper limb (from 18% in severe strokes to 79% in minor ones) . The restoration of motor capacities in this population relies on interlinked mechanisms of neurological recovery, adaptation and compensation . The concepts underlying our rehabilitation practices dramatically improved over the past 20 years , as knowledge on brain neurophysiology and on the mechanisms of brain flexibility, now known as “neuroplasticity”, improved. Today’s rehabilitation programs carry out intense and highly repetitive exercises over a time that often exceeds three months subsequent to a stroke.

The interest of “task-oriented” rehabilitation has been demonstrated and its basic principle has been implemented in numerous programs, including: constraint induced therapy , body-weight support training, robotized rehabilitation , functional electrostimulation (FES) … In a similar way, new concepts based on experimental neurosciences have led to innovative programs based, for instance, on the theory of mirror neurons (rehabilitation with “mental imagery”, “mirror-based therapy”…) or on the notion of “inter-hemispheric balance” (bimanual rehabilitation).

As part of these new rehabilitation techniques, and while muscle strengthening techniques in stroke rehabilitation used to be largely decried, methods consisting in strength and endurance training are now part of the programs regularly offered to patients after damage of the central nervous system . Indeed, it is widely known that the strength deficit following stroke significantly contributes to functional limitations, both with regard to the upper and the lower limb . Morris et al. and Ramas et al. recently published two interesting literature reviews pertaining to the interest of muscle reinforcement in rehabilitation of stroke patients, whatever the strengthening technique employed. In the articles mentioned by these authors, the methods employed include both manual and instrument-assisted muscle strengthening techniques. The protocols generally involved only a limited number of repetitions (lower than 9), at approximately 60% of the theoretical muscle strength, over a period ranging from four to 12 weeks, with two to five sessions a week. Reported results are encouraging in terms of both strength level of the trained muscles and function, but heterogeneity of the studies precludes elaboration of precise recommendations on how to concretely implement such programs.

Isokinetic muscle strengthening (IMS) has been widely used for many years in rehabilitation after musculoskeletal lesions and consequently represents a potentially interesting technique of rehabilitation with regard to central nervous system damage.

The objective of this work is to review existing literature on the use of IMS in rehabilitation of hemiparetic patients, in order to directly apply it in current clinical use, particularly with regard to the upper limb.

1.2

Materials and methods

Our research was conducted with MEDLINE, using the following English-language keywords in combination with each other: “stroke”, “rehabilitation”, “isokinetic”, “upper limb” and “training”. Sixty-eight articles were found. Only the studies mentioning use of IMS for stroke rehabilitation were selected (17 studies), and no distinction was made between its application to the lower or the upper limbs. We did not keep works simply proposing isokinetism as a tool for assessing muscle strength of paretic limbs (nine articles), nor did we keep studies mentioning IMS as one among other techniques of rehabilitation (three articles), since they did not address the question of the interest of IMS as such.

Finally, we selected only six studies dealing with isokinetic rehabilitation IMS in hemiparetic patients. Three readers were involved in this review and applied the reading method drawn from the reading template of the French national health authority (Haute Autorité de santé).

1.3

General considerations on isokinetism

Isokinetic dynamometers may be used as instrumental muscle strength assessment devices or as muscle strengthening tools. Indeed, constant technological advances over the past 15 years facilitated the development of highly effective dynamometers usable for assessment and/or rehabilitation of most joints in upper and lower limb. These devices are regularly used in osteo-articular disorders or in the framework of sports training programs.

In the lower limb, muscular groups most often involved in assessment and/or isokinetic strengthening are knee muscles and, to a lesser extent, peri-articular muscles of the ankle. In the upper limb, rotators and abductors of the shoulder are generally involved, as are, to a markedly lesser extent, peri-articular muscles of the elbow.

Most of the isokinetic machines used in pathology are rotating dynamometers. Results of an isokinetic assessment are generally expressed as the peak torque value developed by the tested muscle group. It is also possible to evaluate the work provided by the same muscle group and the power developed at the joint.

Assessment and isokinetic strengthening protocols are generally described in terms of mode of muscle contraction (concentric or eccentric), value of imposed angular velocity, and number of repetitions. The choice of the working protocol depends on the subject and on the required objectives, as well for the upper and for the lower limb.

The concentric mode is the most widely employed because it is easy to use and well tolerated. The eccentric mode has been less regularly proposed, because this mode of contraction is difficult for patients, and may lead to musculo-tendinous micro-injuries (especially aches or delayed onset muscle soreness: [DOMS]), which are more frequent than in concentric work. Moreover, interpretation of values obtained in eccentric mode is largely discussed . On the other side, some consider eccentric muscular contraction to be closer than the concentric mode to normal muscle physiology: indeed, eccentric contraction may be observed in many activities of daily life (fight against gravity or against inertia), often in combination with the concentric mode (“plyometric” contraction).

The choice of the working speed determines the level of strength that the muscle is supposed to produce. In the coupling that is characteristic of the concentric mode, strength and speed are inversely related: the higher the speed, the less the muscle develops strength. In the eccentric mode, this relation is reversed. Consequently, training that aims at increasing strength is carried out at low speed, while exercises at high speed demand more muscle power. It should also be noted that slow speeds engender more joint stress but are closer to normal muscle physiology. More generally speaking, the choice of speed depends on both the objectives of the training program and on the function of the to-be-trained muscle group. Isokinetic speeds usually proposed for the concentric mode range from 0 to 300°/s. Distinctions are made between low speeds (from 0° to 100°/s), medium speeds (from 100 to 200°/s) and high speeds (from 200 to 300°/s). The speeds suggested for the eccentric mode are generally lower, practically never exceeding 150°/s (only for some apparatus models).

Due to the fatigue induced in trained muscles, the number of repetitions for low speeds generally does not exceed five. For higher speeds, the number of repetitions varies from five to ten. Stress tests, also called endurance tests, are frequently carried out at high speed (180°/s minimum), and the number of repetitions may reach 50 . Once again, however, exercise speed in stress tests depends both on the muscle group being tested and on the kind of patients concerned by the exercise . In sport training programs, the number of repetitions may rise or fall, depending on the goal of the session .

1.4

Results

1.5

Discussion

The few studies we found on the topic do not present sufficient methodological qualities, which would allow us to claim positive effects of IMS when this form of strengthening is used in rehabilitation of stroke patients. In order to confirm its interest and, more particularly, to demonstrate its superiority in comparison with other already validated rehabilitation methods, randomized controlled trials would be necessary.

Anyway, in a general manner, the interest of muscle strengthening in rehabilitation after stroke is likely to exist . Since IMS is just a different way to conduct muscle strengthening, one may consider this kind of rehabilitation to be effective like other strengthening programs.

There is no scientific evidence that IMS is of more interest than other muscle strengthening methods, especially in neurological patients. Concerning athletes, Croisier et al. suggest that the development of a maximal moment of force over the whole range of amplitude of movement may be of interest in muscle physiology, sustaining the superiority of isokinetic reinforcement compared to conventional one. Moreover, most of the authors consider that the main interest of IMS, when compared with other muscle strengthening methods, consists in the following: reproducibility of exercises, accuracy in scheduling and building contents of work sessions, adherence to the program facilitated by the visual and/or auditory feedback provided by the machine in real time, easy monitoring of patient performance. This argumentation can be applied to patients.

When elaborating an IMS protocol for a stroke patient, the choice of a concentric or eccentric operating mode should be discussed in light of the previous published works on the topic. While the concentric mode has been more frequently used, some points suggest the interest of the eccentric mode, notwithstanding the above-mentioned difficulty in implementation:

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  eccentric muscular contraction exercise is known to be highly efficacious with regard to muscle strengthening not only in athletes , but also in patients after a stroke ;

  
  
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  for Rouleaud et al. , eccentric exercises appear to be well-tolerated in hemiplegic patients since they require a lesser degree of energy expenditure;

  
  
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  rapid concentric movements stretch the antagonist muscles and for some patients, this stretching could theoretically enhance spasticity. Eccentric exercises could avoid this phenomenon .

None of the identified studies mention application of the “continuous passive mobilization” (CPM) mode in stroke patients. CPM is a variant of isokinetic exercise. In the CPM mode, movement is carried out at constant speed by the machine, whatever the patient is able to develop as muscle couple of force. The patient may produce either concentric contraction (in the same direction as the movement induced by the dynamometer) or eccentric contraction (braking force of the movement induced by the dynamometer) . CPM is suspected to be an interesting way to assess muscles’ strength in weak patients suffering from neuromuscular diseases , for example when severity of the neurological deficit does not allow them to move the limb against gravity.

Its potential interest could be summarized as follow:

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  whatever the patient’s degree of fatigue, the addition of CPM ensures that the movement will be carried out through a full range of motion, even when the torque produced by the person is quite weak;

  
  
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  the “guiding” of the movement by the machine may facilitate the achievement of the task in a context where sensory or cognitive disorders could render isokinetic training difficult.

The theoretical risk of enhanced spasticity during muscle strengthening after central nervous system damage has been widely discussed in the literature. Most authors mentioned in this review took this point into consideration . On the basis of these preliminary results, it is possible to claim that, when applied following central neurological injury, IMS does not aggravate spasticity. On the other hand, the existence of high spastic hypertonia with an Ashworth score greater than 3/5 may limit the use of the technique because installation of patients may be compromised. To our knowledge, this point has not been evaluated previously in the literature.

The paucity of literature focusing on IMS use in the treatment of the paretic upper limb following a stroke may be explained in several ways:

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  frequency of complete upper limb motor deficiency rendering use of the tool impossible ;

  
  
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  frequency of pain syndrome, particularly at the shoulder, rendering installation and strengthening work particularly difficult in this population ;

  
  
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  absence of consensual data on the muscle groups to be strengthened, including the literature dealing with IMS use in osteo-articular pathology ;

  
  
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  connection between strength gain and functional improvement still quite contestable at the level of the lower limb , but also at the level of the upper limb. While this question has been raised by several authors, the answer is not clear ;

  
  
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  lastly, “task-oriented rehabilitation techniques” have gradually assumed greater and greater importance in rehabilitation programs of the upper limb after vascular cerebral accident. These methods include not only conventional occupational therapy techniques , but also “constraint-induced therapy” . Other emerging rehabilitation methods may be considered as “task-oriented”: “robotic rehabilitation” or “electro-mechanical assisted” rehabilitation , functional electro-stimulation , virtual reality or interactive video gaming rehabilitation . The role of upper limb muscle strengthening in therapeutic strategies remains unclear: this method may usefully complement task-oriented rehabilitation techniques , but will not replace them .

1.6

Conclusion

Numerous arguments from the literature suggest the interest of IMS in rehabilitation of hemiplegic patients, particularly with regard to the lower limb. Indeed, the beneficial effects of IMS in rehabilitation of the hemiparetic lower limb need to be confirmed in further studies and its place in rehabilitation of the upper limb has to be clarified. The choice of IMS protocols needs to be carried out in light of the works cited above, and special attention should be paid to the CPM mode. Many questions remain with regard to the interest and feasibility of eccentric exercises, especially for the upper limb. Lastly, the role of IMS to complement other rehabilitation methods for patients following a stroke should be taken into consideration.

Disclosure of interest

The authors declare that they have no conflicts of interest concerning this article.