Increased force endurance and change of the muscle type following anterior cruciate ligament reconstruction
Running title: Stamina increase and fiber plasticity
di Bisciotti Gian Nicola ^(1-5) , Combi Franco ⁽²⁾, Forloni Fabio ⁽³⁾, Petrone Nilton ⁽⁴⁾

Key words: ACL, , stamina, fiber plasticity, rehabilitation.

SUMMARY

In this study we considered 13 subjects whose age, weight and height were respectively 26 +2 years (mean + standard deviation), 72.3 +7.1 kg, 178.6 +4.7 cm; all the subjects regularly performed a sport and had reported an isolated or associated injury of the ACL, that had been surgically treated by means of an arthroscopic reconstruction. Each subject was asked to effect a femoral quadriceps isometric contraction of the two inferior limbs with intensity equal to the 50% of the maximal isometric force, up to the complete exhaustion of the muscle. The times of maintenance of the contraction on the injured limb and on the healthy limb were respectively of 60.00 +14.14 seconds (range 82.55 — 41.99 seconds) and 46.63 +11.85 seconds (range 74.0 — 43.0 seconds). The difference, equal to the 21.84 +8.42%, resulted statistically significant (p‹0.002). The increase in the capacity of the muscular resistance of the pathological limb is probably to be charged to a selective atrophy of the fibres type II and to a conversion of the typology of the fibres themselves from type II o type I. Moreover the values deduced from this study have been proposed as valuation parameter in the protocols for physiotherapy work after a surgical reconstruction of the ACL

INTRODUCTION

One of the peculiar characteristics of the injuries of the ACL is constituted by the loss of maximal force of the extensors of the leg on the thigh, in a period immediately after the operation (post-operation) and after a follow-up period (1, 2, 3, 4), whereas the loss of force of the flexors seems more limited (5). The loss of the capacity of the maximal force after an operation of the ACL is found by means of the mode of isometric contraction (2, 3) and by means of an isokinetic contraction (6, 7,8). This deficit of force in the injured limb is found also in the case in which the ACL has not been surgically treated (6). However, it is necessary to consider that the deficit in force in the injured limb many times is charged, at least in part, to the algic sensation, reported by the patient during a maximal muscular contraction that puts the neo-ligament in tension. This eventuality is particularly recurrent above all in the case in which the dynamometric test is effected in open kinetic chain (OKC), mode during which the anterior translation of the tibia verifies, which may cause an important tensioning of the neo-ligament itself (9). Beside a deficit in contractile force, after the reconstruction of the ACL the injured limb normally shows a more or less marked atrophy of the femoral quadriceps (5, 10) and above all of the vastus medialis obliquus (11). Up to certain levels the maximal force shows a strong correlation with the cross sectional area of the muscle but in the case of ACL reconstruction the loss of muscular trophism shows itself scarcely correlated to the contractile capacity of the femoral quadriceps (6, 12). It is then possible to hypotheses that at least a part of the contractile deficit of the injured limb is due to a change of the metabolic and/or mechanical typology of the muscular fibre (8) and to an altered activation pattern of the motor units caused by the damage of the sensorial receptors of the injured ACL (13, 14). Besides the decrease of the maximal force, an other parameter always relative to the muscular contractility, that may result altered after a reconstruction operation of the ACL, is the muscular stamina: that is the capacity of a determined muscular group to resist to the fatigue induced by a prolonged contraction (15). The few studies found within this field report that the muscular resistance, understood as capacity to resist to a sub-maximum muscular contraction induced by means of an electro-stimulation to the femoral quadriceps, is major in the injured limb after the reconstruction of the ACL than in the healthy counter part (8). Moreover, other studies prove that after a reconstruction of the ACL and the consequent hypokinesia period that follows the first, there is a conversion of the muscular fibres from rapid (FT) to slow (ST) in the femoral quadriceps (16, 17). As a high percentage of ST leads to an increase of the stamina capacities (18, 19, 20), we could hypothesize that an increase of the capacities of the musculature tested to resist to a prolonged sub-maximal contraction may constitute an indirect index of the grade of conversion of the typology of the muscular fibres. The aim of this study is exactly the verification and quantification of the increase of the capacities of the stamina of the femoral quadriceps of the traumatised limb, in patients submitted to surgical reconstruction of the ACL, by means of a voluntary sub-maximal contraction prolonged up to the complete exhaustion of the muscle. The individualisation of this medium value may in fact constitute, in our opinion, an important reference index in the rehabilitative field.

METHODS

Subjects

In this study 13 subjects were taken into account whose age, weight and height were respectively 26 +2 years (mean + standard deviation), 72.3 +7.1 kg and 178.6 +4.7 cm; they all took part in some kind of sports activity (table 1) and had suffered an isolated or associated injury of the ACL, which had been surgically reconstructed by means of arthroscopy (table 2).

During the test period all subjects continued their normal physiotherapy rehabilitation and none showed symptoms of muscular or neuromuscular problems, apart from that described above. When the test was carried out, the subjects were in their 95° +7° post- operative day and had completely recuperated the articular mobility of the injured limb. Moreover, all the subjects had been informed of the aim of the study and of the possible risks involved.

Protocol

After previous warm up, each subject was asked to perform an isometric contraction, with an articular angle standardised at 90°, with the extensors of the leg, whose intensity is equal to 50% of the isometric maximal force preventively measured at the same articular angle. The choice of MIF 50% was dictated by the fact that the maintenance of this percentage of isometric force induced to a total muscular fatiguing in times relatively short of the order of approximately 50’’ (21, 22, 23). The production of force was measured by means of a load cell with a strain gauge (Mod. Ergometer, sample rate 100 Hz, non-linearity histeresis and repeatability 0.002 of RO, temperature compensated 0° to 50°, charge scale 0-300 kg). Each contraction was maintained as long as the value of force expressed did not result minor to the prefixed target for a period superior to 3 second. During the execution of the contraction the subject was supplied with a biofeedback of vision that enabled him to remain within the prefixed target of force production (MIF 50% +5%). The data were read directly on a dedicated software that automatically calculated the time of maintenance of the contraction within the demanded target. The same type of measurement was effected in a randomised way both for the pathological limb and the healthy counter part.

STATISTIC

Ordinary statistical indexes such as average, standard deviation and variance were calculated for each single variable and situation.

The difference between the mean values of maximal isometric force (MIF 100%) and of maintenance of the MIF 50% (with a range of tolerance of +5%) of the injured limb and of the healthy counter part were tested by means of a non parametric Wilkoxon test.

The level of statistical significance was fixed at p‹00.5.

RESULTS

The values of MIF 100% of the healthy limb were equal to 660.61 +162.78 N.

The values of MIF 100% of the pathological limb were equal to 433.66 +132.62 N.

The difference, equal to 33.73 +14.61% was statistically significant (p‹0.001).

The values of maintenance of the MIF 50% in the healthy limb were equal to 46.63 +11.85 seconds (range 74.0-43.0 seconds).

The values of maintenance of the MIF 50% in the pathological limb were equal to 60.00 +14.14 seconds (range 82.55-41.99 seconds).

The difference, equal to 21.84 +8.42%, was statistically significant (p‹0.002).

Table 1: Subjects distribution in function of the practiced sport

Table 2: Subjects distribution in function to the suffered injury.

DISCUSSION

The muscular stamina found in this study, major in the injured limb than in the healthy counter part (21.84± 8.42%, p<0.002), can be compared, although not perfectly superposable to the one reported by Snyders-Mackler and coll. (8), who report that the extensive musculature of the limb that has undergone a surgical reconstruction of the ACL is 11% (p‹0.001) more resistant than the one of the healthy limb. The difference that may be found between these data is undoubtedly to be imputed to the different study protocols, given that in the study mentioned above that required the maintenance of the 20% of the maximal isometric force by means of an electro-induced contraction. There is one datum in particular that must be underlined: independently on the type of contraction considered, voluntary or electro-induced, the extensor musculature of the pathological limb in every case shows a major stamina capacity than the healthy counter part. This difference in the characteristics of muscular stamina may support the hypothesis, already suggested by other Authors, of a selective atrophy of the fibres type II, after the artroscopic reconstruction of ACL (16, 17, 24) and the conversion of the fibres from type II to type I, that is a consequence to the chronic stimulation at low frequency (25, 26, 27, 28), typical during the rehabilitative period after the operation. The conversion of the typologies of the muscular fibres is physiologically justified by the fact that also in adults the same fibres show that they are able to change their molecular composition, altering in such way their gene expression (27). Above all the low frequencies of discharge, typical of the training of muscular stamina and therefore also of the rehabilitative programs, can induce in animals and in men, if repeated chronically and for periods relatively prolonged, a conversion of the light chains (MLC) and of the heavy chains (MHC) of the myosin, from fast to the their slow iso-form (27, 29). The alteration of the nervous activation pattern determines in fact a change of the synthesis of the different contractile proteins (25). In this type of mechanism the main role is mainly played by the nervous activation pattern, but it is also necessary to remind the important role played by other two factors, constituted by the neuromuscular activity and by the mechanical load (28). The possible typological conversion of fibres from type II to type I, consequent to a post-operation event as the ACL reconstruction, is more probable to be imputed to the stimulus at low frequency that the muscle receives during the rehabilitative period, rather to the immobilisation period after the operation. It is in fact known that the musculature of the femoral quadriceps in paraplegic patients shows a predominance of fibres of type II, as a consequence of the loss of the muscular functions (30, 31).

Additionally this change of the fibers typology could explain the fact that the quadriceps femoris loss of force, in the case of ACL reconstruction, is slightly correlated to the cross sectional area of the quadriceps femoris itself (6, 12).

Therefore, considering the major resistance to fatigue of the fibres of type I compared to the fibres of type II (15, 18, 30), the increase of the characteristics of the stamina of the extensor musculature of the injured limb, as found in this study, may be the indirect witness of a selective atrophy of the fibres of type II and of a conversion of the typology of the fibres from type II to type I. An excessive atrophy of the fibres type II united to a massive conversion of the fibres from type II to type I, may prove to be inauspicious especially in sport activities as sprint and jumping but also in team sports as football, where sudden and frequent cutting actions require a rapid and massive recruitment of fibres of type II (20, 32). The value of percentage increase in muscular stamina found in this study (that can be easily recorded by means of an isometric test) may be an important reference parameter in the field of physiotherapy in the rehabilitative protocols of the ACL. As in fact the biological plasticity typical of the muscle permits the reversibility of the structural changes induced in the latter (28, 33), the surmounting of such parameter can suggest the introduction of specific exercises in the work plan of the athletes ,especially of some particular disciplines, in order to induce a preferential recruitment of fibres of type II and balance again the typological situation of the muscles of the two limbs.