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HIGHLIGHTED TOPICS
Neural Control of Movement
1Laboratoire des Déterminants Physiologiques de l'Activité Physique, Institut Fédératif de Recherches Etienne-Jules MAREY, Faculté des Sciences du Sport de Marseille-Luminy, Université de la Méditerranée, 13288 Marseille Cedex 09; and 2Laboratoire de Physiopathologie Respiratoire Cellulaire et Intégrée, Institut Fédératif de Recherches Jean ROCHE, Faculté de Médecine Nord, Université de la Méditerranée, 13916 Marseille Cedex 20, France
Submitted 24 July 2003 ; accepted in final form 14 November 2003
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ABSTRACT |
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afferent fibers; fatigue; metabosensitivity; mechanosensitivity; rat; muscle
Treadmill running performed in reinnervating muscle acts on histochemical muscle fiber alterations, contractile properties, enzyme activities, and muscle weight (14, 39, 46, 48). Finally, chronic electrical stimulation of denervated muscle is known to accelerate the recovery of normal function in reinnervated muscle fibers (6, 21, 41). All these strategies focused on the recovery of muscle properties and/or motor pathway and few concerned the mechano- (I and II groups) and metabosensitive (III and IV groups) afferent fibers.
Exercise adaptation is provided by central neuron activity adjustments, which are regulated by activation of group I and II (mechanosensitive) and group III and IV (metabosensitive) afferent fibers. The role played by the latter fibers seems to be crucial for exercise and fatigue tolerance adaptive mechanisms (9). The first role played by group III (thin myelinated) and group IV (unmyelinated) afferent fibers from skeletal muscle is to transmit nociceptive information from muscle to the central nervous system. The second role of these free endings located in the interstitium of the muscle is to induce cardiovascular and respiratory adjustments during muscular exercise. These respiratory and circulatory responses during muscular exercise may be reflexively induced via muscular afferents. The mechanical muscle deformation and the accumulation of metabolites in its intersitium are the cause of raised activity in small nerve fibers, which in turn induces the physiological responses. Furthermore, adjustments of the locomotor activity during exercise are mediated by the thinly myelinated and unmyelinated fibers with endings in the working muscle. These fibers, also called "metaboreceptors" may be responsible for the coupling between ventilation and locomotion, whereas thickly myelinated muscle afferents (i.e., groups I and II) appear to play little role in causing the reflex autonomic responses to contraction (16, 26-31).
The aim of the present study was to focus on muscle properties and the axonal regeneration of mechano- and metabosensitive muscle afferents by evaluating the recovery of their response after nerve injury and repairs by self-anastomosis during 10 wk of treadmill running. In a previous rat study (8), our laboratory reported the responses of tibialis anterior muscle afferents to different test agents [KCl, lactic acid, electrically induced fatigue (EIF), and tendon vibrations] in control (C) conditions, after nerve repair using an epineural suture technique (11), and after chronic electromyostimulation (34).
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METHODS |
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Experiments were conducted in 48 female, 4-mo-old, Sprague-Dawley rats, obtained from Iffa Credo (Les Oncins, France). Housing, surgical procedures, and assessment of analgesia were performed according to the French law on Animal Care Guidelines, and the protocols were approved by the Animal Care Committee of our University. Efforts were made to minimize animal suffering and to only use the number of animals necessary to produce reliable scientific data. Animals were housed in smooth-bottomed plastic cages at 22°C with a 12:12-h light-dark cycle. Food (Purina rat chow) and water were available ad libitum. Animals were randomly placed in four groups.
In the first group (C group, n = 10), animals were left intact. In the others groups [animals with nerve lesion and suture (LS), n = 14; animals with lesion, suture, and chronic muscle rehabilitation by electrostimulation (LSE), n = 12; and animals with 10 wk of treadmill running (LSR), n = 12], the left peroneal nerve was sectioned then immediately self-anastomosed. In the LS group, the left peroneal nerve was sectioned and the animals were housed for the next 10 wk. In the LSE group, the denervated muscle was stimulated daily by a biphasic current, whereas in the LSR group animals ran on the treadmill daily. Electromyostimulation and exercise were performed 5 days/wk for 10 wk. All animals were housed for the next 10 wk in smooth-bottomed plastic cages at 22°C with a 12:12-h light-dark cycle. In this study, we did not include a group with only electrodes placed because we considered that there is no effect of the electrode placement as an isolated variable. Indeed, data reported in the literature indicate that in sham-operated animals (animal with only electrode implanted) no differences were found with the C group (4, 51).
Common Surgical Procedures
As previously described (11, 34), surgery was carried out on animals under deep pentobarbital sodium anesthesia (Nembutal, 60 mg/kg ip, Sanofi, Libourne, France), and atropine (1 mg/kg) was administered intraperitonally to reduce airway secretions. Briefly, with the use of an operating microscope (x40, OPM 11 Zeiss, Oberkochen, Germany), and under aseptic conditions, the common peroneal nerve was exposed in the popliteal fossa on one left side. This procedure was performed by exposing the left peroneal nerve in the posterior thigh, thereby freeing the nerve from the popliteal vessels. Muscles innervated by this nerve are particularly suitable for reinnervation studies, because this nerve lies very superficially at the knee, thereby minimizing the requisite surgery. The distance from the transection nerve entry point to the muscle under study, i.e., the tibialis anterior, was 
10 mm. To obtain best results, the two stumps of the sectioned nerve were immediately (50) stitched without tension (52) by five epineural sutures (Ethilon, 10-0; Omnium, Médical, Neuilly, France). No further surgery was performed until the electrophysiological studies. Muscles and skin were closed with 3-0 suture (Triny, Omnium Médical) and locally disinfected.
Functional Electromyostimulation
In LSE groups, two electrodes were intramuscularly fixed for stimulation (diameter: 500 µm). They were connected to silver cables (PTFE ET 32.07; Draka Fileca, Saint-Geneviève, France) tunneled subcutaneously from the left muscle to the skull. The distal end of cables were attached (Ethilon, 8-0; Omnium Médical) to the muscular epinurium, whereas the other end was connected to a microconnector (Radiospares, Beauvais, France) fixed on the skull with stainless steel screws and dental cement (Ivoclar, Liechtenchtein). Animals were stimulated for 10 consecutive wk (5 h/day, 5 days/wk). The stimulation pattern used was an alternative biphasic stimulation (10 V) modulated in frequency at 4-75-4 Hz and shock duration at 200-150-200 µs. This stimulation pattern lasted 6.5 s and was delivered once a minute by a clinical stimulator (Mulitprocess 16+, Physitech, Electronique Médicale, Marseille, France) used in electrophysiotherapy and physiotherapy. One week after surgery, chronic stimulation was performed 5 h/day, 5 days/wk for 10 consecutive wk.
Exercising Protocol
The running session was performed on the LSE group 1 wk after self-anatomosis on a treadmill (Medical Développement, Saint-Etienne, France). All animals ran in the same condition (0°) at 10 m/min continuously for 1.5 h, twice a day, 5 days/wk. This daily protocol, based on a previous study (53), was chosen to activate the denervated/reinnervated muscle. The exercising and functional electrical stimulation protocols were performed during 10 consecutive wk.
Nerve Recordings
After a 10-wk rehabilitation period, rats were anesthetized by an intraperitoneal injection (1.0 ml solution/100 g body wt) of solution containing pentobarbital sodium and 0.9% sodium chloride in 1:10 volume proportion. The trachea was cannulated for artificial ventilation (Harvard volumetric pump: rate 40-60 breaths/min, tidal volume 2-4 ml; Southmatick, MA). A catheter was also inserted into the right femoral artery and retrogradely advanced as far as the fork of the abdominal aorta to retrogradally inject bolus of KCl and lactate (Lac) solutions to the contralateral studied muscle. The catheter was positioned so that the blood flow to the left tibialis anterior muscle was not interrupted. Animals were paralyzed by an intra-arterial injection of pancuronium bromide (Pavulon, 10 mg/kg; Sanofi, Fresne, France). The left tibialis anterior muscle was dissected and freed from surrounding tissues, but tendons were left intact. Knee and ankle were firmly held by clamps on a horizontal support to avoid limb motion. Dissection at the midthigh level was carried out to expose the common peroneal nerve. Then, it was dissected free from surrounding tissues at a length of 5-6 cm, and its proximal portion was cut.
To record the compound nerve action potential (CNAP), the whole peroneal nerve was placed on bipolar electrodes and was electrically stimulated 1 mm from its insertion point in the muscle with single shocks (0.1-1 ms, long rectangular pulses, supramaximal) delivered by a S8800 stimulator (Grass, Quincy, MA) through an isolation unit. CNAPs were recorded by using bipolar electrodes placed 15 mm from the stimulating electrodes. Raw afferent activity was displayed on storage oscilloscope (DSO 400 Gould, Madison, WI) to average CNAPs evoked by the stimulation of the distal nerve. The conduction velocities, at the peak of the waves, were calculated by using an interelectrode distance of 15 mm.
To record activity from the muscle afferents, the free end of the distal nerve portion was divided into several filament bundles on a nerve support with warm paraffin oil by using an operating microscope (x40, OPM 11 Zeiss). Each bundle was placed sequentially on a monopolar tungsten electrode. The nerve activity was referred to a nearby ground electrode, amplified (50-100 kHz), and filtered (30-10 kHz) by a differential amplifier (MP2 SARL, Marseille, France). The afferent discharge was displayed on a chart recorder (TA 4000 Gould, Balainvilliers, France), and the potentials were fed into pulse window discriminators built in our laboratory, which simultaneously analyzed afferent populations. The output of these discriminators provided noise-free tracings (discriminated units), which were counted by two frequency meters at 1-s intervals (Fimpulses/s) and then displayed on the chart recorder. The discriminated units were counted and recorded on separate tracings. Due to the small sizes of action potentials of the thin afferent fibers in each bundle, the window discriminators allowed us to select two to three units in each afferent population.
Mechanical Muscle Properties
In all groups, muscular responses to nerve and muscle stimulation were studied. Comparison between nerve and muscle stimulation permitted evaluation of the nerve reconnections (end-plate efficiency). To measure the strength of muscle contraction elicited by nerve or muscle stimulation, a steel hook was implanted in the distal tendon and connected to an isometric strain gauge (Microdynamometer S60, Ugo Basile Narco Biosystem, Houston, TX). The output signal of the strain gauge was fed to a chart recorder (model TA 4000 Gould, Madison, WI). The whole cut peroneal nerve was desheathed and placed on bipolar electrodes at 5 mm from its insertion point in the muscle, and two electrodes (interelectrode distance of 4-5 mm) were placed at the midbelly of the tibialis anterior. The muscle or the nerve received single-pulse 0.5-ms stimulation delivered by a stimulator (Grass S8800, Quincy) through an isolation unit. Before pancuronium bromide injection, pulse intensity was determined to be supramaximal after the threshold was determined to elicit a twitch (single contraction). We measured the contraction time (CT) between the stimulus artifact and the peak twitch amplitude (A). The maximal relaxation rate (MRR) was defined as the highest slope measured during twitch relaxation. The MRR-to-A ratio was then calculated.
Response of Muscle Afferent Fibers to Test Agents
The group IV unmyelinated afferents act more than group III myelinated fibers as chemosensory nerve endings because they detect the changes in muscle metabolism (29, 35, 43) and they are selectively activated during and after muscle fatigue (7, 8, 11, 25), Lac (13, 18, 43), H+ (47, 49), and potassium chloride injections (29, 44), and during hypoxemia (1).
For each selected filament bundle, the following tests were performed. 1) The receptive field was determined, i.e., to ensure that the recorded activity was initiated from the muscle, the belly of tibialis anterior muscle was touched with a blunt rod. 2) The compound waves were recorded. The CNAP evoked by electrical stimulation provides a measure of the number and also the properties of the fibers in the nerve; i.e., the calculation of the area under the curve of each peak wave gave an indication of the proportion of the different fibers in each filament bundle. 3) The response of mechanosensitive afferent fibers to mechanical vibrations delivered to the distal tibialis anterior tendon by a vibrator (Ling Dynamic Systems, Baldrock RD, Royston, Herts, UK). Vibrations were applied for 5-s periods, and the vibration frequency was increased from 10 to 90 Hz. Mechanical vibrations were used to identify muscle mechanoreceptors (spindle and Golgi tendon organs) (8, 11). And 4) the response of metabosensitive afferent fibers to intra-arterial bolus injection of KCl (1-20 mM in 0.5 ml of saline) and Lac solution (0.5-3 mM in 0.1 ml of saline) and to a 3-min period of electrically induced muscle fatigue (3-min EIF) was recorded. The duration of each stimulating train was 500 ms, and trains were repeated each second to produce a series of contractions. Pulse durations of 0.1 ms were delivered in each 500-ms train (10 Hz). Muscle fatigue was assessed from the decay of force throughout each 3-min EIF period. The fatigue index, defined conventionally as the percentage of the force lost at the end of the 3-min EIF trial, was then calculated (32, 33).
In all cases, the intensity of muscle stimulation was adjusted to elicit the same maximal increase in muscle force at the beginning of the electrical muscle stimulation trial. Injected chemicals and EIF mimicking the fatigue are used to activate the group III-IV muscle afferents (7, 8, 11).
To avoid interactive effects of the multitude of tests performed on each animal, 30 min of recovery were left between each test. The tests were always conducted randomly.
At the end of the experiment, animals were killed by an overdose of anesthetic, and right and left tibialis anterior muscles were removed, trimmed of excess fat and connective tissues, and weighed.
Statistical Analysis
Statistical analyses were performed with a software program (Instat, GraphPad). Prestimulus (PS) values of frequency of impulses per second of afferent fibers were averaged 2 min before the application of each stimulus. Then, significant changes in the afferent activity induced by each tested agent were determined with respect to the corresponding averaged PS values (100%). Mean values of firing rate (Fimpulses/s) are given as means ± SE, and their variations are expressed in percentage of the corresponding control discharge rate (baseline activity). We used nonparametric one-sample tests (Wilcoxon test) to compare the afferent discharge frequency before and after drug injections, EIF, or tendon vibrations. A one-way ANOVA (with P coefficient being significant at <0.05) was also used to test the intergroup differences. This test was used to compare muscle mass, MRR/A, and the effect of KCl, Lac, tendon vibration, EIF, and fatigue index. We used a Student-Newman-Keuls (with P being significant at <0.05) post hoc test to compare all pairs of groups. The 
2 test was applied to compare the percentage of fibers responding to mechanical vibrations.
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RESULTS |
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In LS and LSR groups, the weight of operated tibialis anterior muscles, expressed as the ratio of the muscle weight to the animal weight, was significantly lower than that of the nonoperated muscle in the C group (61 ± 7%, P < 0.01 and 70 ± 2%, P > 0.05, respectively). By contrast, in the LSE group, the muscle mass did not differ from the C group (93 ± 5%). Results are reported in Table 1.
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Twitch Characteristics
Data were compared with those measured in the C group. Two months after nerve injury and repair, nerve and muscle stimulation in the LS, LSE, and LSR groups always elicited muscle contractions, indicating that muscles were reinnervated by motor axons. Table 2 indicates that A significantly decreased in the LS group but not in the LSE and LSR groups, regardless of whether the stimulation was applied to the nerve or directly to the muscle. MRR/A never varied. Only when twitches were induced by muscle stimulation did CT signifi-cantly (P < 0.01 and P < 0.001) decrease in the LS and LSE groups, but it must be pointed out that a tendency for shortened CT values was also present in the three operated groups in response to nerve stimulation.
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Fatigue Index During EIF
Compared with the C group, rats in the LS group showed an increase in fatigue index, whereas we measured a better tolerance to fatigue in the LSE group. No difference was observed between the LSR and C groups (Fig. 1).
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CNAP
The different peaks of CNAP reflect the proportion of group fibers within the whole nerve. A measure of the surface gives an idea of each proportion. Furthermore, because electrical stimulation was applied at the distal peroneal nerve, the first peak (groups I-II) also contained a contribution of an anti-dromically activated 
-motor efferent fiber, the second one (group III) may also include 
-motor fibers, and the activation of sympathetic fibers may also contaminate the group III and IV waves. Thus we prefer to use the terms of group I/II-like, III-like, or IV-like fibers and discuss the diameter of fibers and quality of each. Table 3 shows an increase in conduction velocities of the slowest fibers between the LSE and C groups. All fiber types (I/II-, III-, and IV-like) were fastest in the LSR group. Relative area measurements of the different CNAP waves show that the distribution of various nervous fibers did not really differ between groups, except an increase in LSR for the I/II-like fibers.
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Afferent Responses to EIF, Lac, and KCl Injections
The baseline activity is the afferent discharge frequency recorded before the battery of test agents is used. This baseline activity was the same in each experimental group.
EIF. In C rats, the discharge rate of the group IV afferent fibers significantly increased after EIF. After 3 min of EIF, the responses were significantly (P < 0.05) lower for the LSE and LS groups (Fig. 2).
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Lac. In C rats, the discharge rate of the group IV afferent fibers significantly increased after Lac solution injection. The response to Lac culminated in the 1 mM concentration (Fig. 3). Responses to Lac injections were similar between the C and LSR groups but differed for 1 and 2 mM concentrations in the LSE and LS groups, respectively.
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KCl. In C rats, the discharge rate of the group IV afferent fibers significantly increased after injections of KCl solutions. The response to KCl plateaued when 10 mM concentration was reached (Fig. 4). Responses to KCl injections were the same between the four groups.
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As previously described (8, 11, 13), the experimental measures returned to PS levels (baseline activity) after each test (Fig. 5).
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Afferent Responses to Tendon Vibrations
In the C group, two populations of muscles mechanosensitive afferents were identified with respect to the frequency of vibrations giving an optimal activation: 40-50 Hz (63%) and 70-80 Hz (37%). The afferent response corresponds to an abrupt increase in the discharge frequency that persisted throughout the vibration period. Compared with the proportion of afferents activated by the different vibration frequencies within the C group, this proportion was not different in the LSE (45 and 55%) group but was significantly different in the LS (28 and 72%) and LSR (33 and 67%) groups. Thus, in the LS and LSR groups, the majority of muscle mechanoreceptors responded to the highest vibration frequencies. Figure 6 represents the changes in afferent discharge (% frequency of impulses per second compared with the baseline activity) elicited by the various frequency tendon vibrations. In LS rats, a significant increase of the nerve response was measured between 60 and 80 Hz compared with C values. In the LSE group, the response increased for 10 and 20 Hz, and from 60 to 90 Hz. Finally, the LSR group only decreased in response to 50 Hz, with a bimodal response such as in the C group.
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DISCUSSION |
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Muscle Properties
Compared with animals without rehabilitation, the postoperative running protocol avoids atrophy. However, the muscle weight recovery is less important than those obtained by chronic electrical stimulation. Several hypotheses could be found to explain these observations. 1) The intensity of direct muscle stimulation, which was adapted daily to induce near-maximal contraction of muscle, was such that it activated a great proportion of muscle fibers. During running exercise, physiological cortical activation never recruited the entire muscle as intensively as the external stimulator did. And 2) with the use of electromyostimulation, muscle fibers were activated before the regenerating nerve fibers completely reached the muscle. Then, in the LSR group, the muscle fibers were reactivated progressively by motor fibers reconnecting the muscle. We could suggest that, in the LSE group, muscles were maintained both for a greater duration and intensity.
Contrary to the LSE and LSR groups, the reduced force-generating capacity (A) was significantly and markedly reduced in the LS group. Reduced force production could be explained by the muscle atrophy, which was marked in LS rats.
The better tolerance to fatigue observed in the LSE group could be explained by the marked reduction of the Lac dehydrogenase enzymatic activity in chronically stimulated muscles and thus the reduced lactic acid production during EIF.
The shorter CT values measured in the LS and LES groups could result from a higher muscular excitability (45) associated with the transformation of the muscle fiber phenotype. Our laboratory already reported that, in a nondenervated rat chronically stimulated with a modulated sine wave current, a marked shortening of the CT and an increase in the proportion of oxidative enzymes concomitant to changes in the contractile properties of muscle fibers toward those of fast twitch ones was present (54). Cole and Gardiner (6) also reported that muscle denervation was effective in attenuating the strength-related but not the speed-related changes in neuromuscular function.
Nerve Recordings
CNAP analysis reveals an increase in proportion and conduction velocity of fast I/II-like nerve fibers in the LSR group indicating that the regenerating nerve contains a greater proportion of large nerve fibers innervating fast motor units. In the LSE and LSR groups, fibers from groups III and IV also present an increase in conduction velocity, indicating that the maintenance of muscle activity act on the regenerating motoneurons. Choi et al. (5) reported an increase in conduction velocity in the regenerating fibers. However, Horch and Lisney (23) reported a reduction in axon diameter (and conduction velocity) that should normalize with recovery with a long period after reinnervation. The fact that the size of the myelinated fibers remains low for a long time after nerve injury, whereas the size of the unmyelinated fibers is less affected, and the fact that the number of regenerated fibers changes over time, with a sustained increase in the number of sprouts supported by myelinated fibers and a decrease of unmyelinated fibers (10), should be taken into account in the interpretation of our results. The maintenance of muscle activity (electromyostimulation and running exercise) over time may affect the proportion of fibers in each group that could be observed several weeks after self-anastomosis in a greater proportion of large-diameter fibers. Furthermore, during reinnervation, trophic factors from muscle and nerve are involved in the communication between growth cones and the target (37). Exercise is known to increase the trophic factors (brain-derived neurotrophic factor, NT-3 and NT-4) release from muscle (17). Then, we can hypothesize, as suggested by Rasband et al. (42), that maintenance of denervated muscle activity increases and modifies the trophic factor release and the potassium channel distribution on regenerating fibers.
The most significant finding in these experiments concerns the better functional recovery of the sensitive afferent fibers from the tibialis anterior muscle after nerve repair and rehabilitation by treadmill running (LSR group). Our observations point out that increasing motoneuron inputs during the regeneration/reinnervation period may influence the functional outcome. Previous studies evidenced that axons of slow motor units appeared to regenerate at a faster rate (12) and that motoneuron sprouting was enhanced in regularly exercising and exercise-trained rats (14, 15). Noah et al. (39), using morphometrical analysis, recently reported a positive effect of exercise on muscle reinnervation. However, Gutmann and Jakoubek (20) indicated a detrimental effect of daily prolonged swimming on regenerated peripheral axon diameter. Herbison et al. (22) suggested that overwork may damage partially denervated muscle that brief contractions may be more beneficial for muscle than a program of exhausting activities.
In humans (2) and cats (36), previous studies have already demonstrated, in noncontracting muscles, that all mechanosensitive endings responded to vibrations, with a wide range of responsiveness attributed to the different type of endings. For example, primary endings of spindles could generally be driven by the highest rates of vibration and the secondary ending by the lowest ones (2). Our laboratory has already suggested (8, 11) that the two populations of mechanosensitive units identified in the rat peroneal nerve, with optimal discharges phase locked to vibration cycles at 70 and 50 Hz, corresponded to primary and secondary spindle endings, respectively. The present experiment also reveals that the pattern of response for the LS and LSR group afferent changed markedly with an increased proportion of afferents having an optimal activation at a vibration frequency of 70-80 Hz. The LS and LSR groups are significantly different from the C and LSE groups, with the LSR group presenting a diminished response at 50 Hz. However, it must be pointed out that, although the proportion of mechanosensitive fiber is inversed in the LS and LSR groups compared with the C and LSE groups, the kinetic of the response curve resembles the C group. In the LSE group, it seems that the fiber discharge rate to vibration is increased for each stimulating frequency. This increase in mechanosensibility response could be enhanced by high and intensive chronic stimulation placed directly on muscle. This confirms previous observations by White and Devor (55), who found that changing the components of physical activity during skeletal muscle reinnervation can alter several attributes of the muscle phenotype. The consensus of several experiments supports the interpretation that proper muscle fiber recruitment and development of force are essential variables. As previously described (34), our results could indicate that after denervation, regeneration, and reconnection with the target muscle, the mechanosensitivity remains partially altered even if a slight beneficial effect is observed when muscles were electrostimulated during the rehabilitation period.
Finally, the present study shows that regenerated thin fibers from all experimental groups are operational to detect the changes in extracellular fluid composition. This signifies that fibers from groups III and IV not only have regenerated toward the muscle target but are able to detect metabolic changes occurring in the interstitium. However, in the LS and LSE groups, even if these metabosensitive fibers were able to respond to muscle metabolic changes, they present an altered response that could disorganize the sensorimotor control in exercising muscle. The absence of adaptive feedback mechanisms (muscle wisdom) and the diminished responses of III and IV afferent fibers after the 3-min EIF could be implicated in the exercise capacity (9). Because our experimental groups' measurements of the fatigue index were obtained after the muscle was disconnected to the central nervous system, the disorganization in the sensorimotor control cannot explain the variability in muscle resistance to fatigue observed in the LS and LSE groups. One explanation could be found by changes in the muscle fiber histological and enzymatic properties. Total absence of significant differences between the C and LSR groups would signify that a running exercise during the rehabilitation period induces more benefits than the chronic electromyostimulation. The chronic electrical muscle stimulation could activate, to a greater extent, the denervated muscle and promote a deleterious effect on muscle fibers (48).
In conclusion, the findings indicate that direct muscle electrostimulation is efficient for maintaining the muscle weight, but other parameters, such as twitch characteristics, fatigue index, mechanosensitivity, and metabosensitivity, are totally restored when an animal performs a running exercise during the rehabilitation period. In the future, it would be interesting to compare different exercise intensities and types and analyze the muscle histochemical properties during the reinnervated process. Clinical applications could be found for patients undergoing nerve injury and repair.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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