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J Physiol (2003), 553.3, pp. 925-933
© Copyright 2003 The Physiological Society
DOI: 10.1113/jphysiol.2003.048389
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ABSTRACT |
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The aim of this study was to evaluate how a modification in the mechanical conditions under which a muscle is used could induce changes in the characteristics and the spinal drive of its motor units (MU). The distal tendon of the soleus muscle of Wistar rats was transferred to the distal stump of the plantaris muscle tendon. The EMG activity of the soleus was chronically recorded for 8 weeks, every other day, during a 1-min treadmill walk. After spinal ventral root splitting, individual MU contractile properties were measured in control soleus (102 MUs) or in transposed soleus muscles after 4 weeks (41 MUs) or 8 weeks (28 MUs). Muscle/body weight ratio did not vary after transposition, nor did MU tetanic forces. A decrease in MU twitch contraction times and in their half relaxation times was observed at weeks 4 and 8. MU tension-frequency curves varied significantly after tendon transfer, becoming closer to the curves of the fast MUs of the control group. During locomotion, we observed no change in the amplitude of rectified-filtered electromyographic activity, but a significant decrease in mean burst duration and an increase in the median frequency of the power density spectrum. Tendon transposition of the soleus muscle brought about adaptations in MU contractile properties and soleus spinal control.
(Resubmitted 2 June 2003; accepted after revision 5 September 2003; first published online 12 September 2003)
Corresponding author M. Gioux: Laboratoire de Physiologie, Faculté de Médecine, 22 avenue Camille Desmoulins, 29238 Brest Cedex 3, France. Email: maxime.gioux{at}univ-brest.fr
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INTRODUCTION |
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It is well known that two muscles with diverse mechanical functions display many differences in their histological composition, motor unit contractile properties and motor control. In mammalian hindlimb muscles, the soleus muscle whose main function is weight-bearing and stabilisation, is composed primarily of type I fibres (87 ± 4 % of cell amount in the rat hindlimb). In contrast, type II is the predominant fibre type in fast muscles such as the plantaris or the peronei muscular group (Armstrong & Phelps, 1984). Rat soleus muscle contains about 90 % of slow motor units (MU) whereas the extensor digitorum longus (EDL) is composed almost entirely of fast units with contractile properties that differ in contraction time, tetanic force and susceptibility to fatigue (Close, 1967).
Since the motor task requirements differ in duty time, force intensity, movement accuracy, shortening speed and length variation, differences are also observed in the motor drive to muscles. Muscular activation patterns have been widely studied in the locomotion task. In the cat step cycle, the peroneus longus is activated in both stance and swing walking phases while the soleus shows an electromyographic (EMG) activity almost restricted to the stance phase (Loeb, 1993). The percentage of duty time is also significantly different among various muscles of the hindlimb. For example, the soleus is active during longer total times (13.9 %) than the fast EDL (1.9 %) in cat (Kernell et al. 1998). This maximal percentage of EMG activity in the soleus has also been demonstrated in humans (Monster et al. 1978).
The role of neuromuscular activity in controlling muscle phenotype has been extensively investigated. Many studies have reported muscle adaptations induced by altered neural drive : i.e. fibre composition, MU properties. A low frequency (20 Hz) chronic stimulation of the medial gastrocnemius nerve over several weeks induces a conversion towards the slow phenotype with a loss of the 'sag' of unfused tetani for all tested MUs (Gordon et al. 1999). On the other hand, Gorza et al. (1998) have observed a slow to fast MU transformation of denervated rat soleus after high-frequency stimulation (100 Hz): a decrease in isometric twitch duration, and an increase in soleus maximum shortening velocity. Cross-innervation experiments have highlighted the influence of nerves on the myosin composition of the muscle. In fact, when reinnervated by the EDL fast nerve, the soleus exhibits the expression of fast LC3 (light chain 3) isoform (Sreter et al. 1974).
Conversely, very few studies have reported the consequences of a modified mechanical use on the muscular functional properties and motor drive. Such a modification in muscle mechanical output, obtained by tendon transfer, could induce some elements of conversion from pale to red muscle (Bach, 1948). Concerning central motor command, conflicting data have been reported about its adaptation and efficiency after muscular transposition or tendon transfer. In man, after forearm flexion muscles were transposed into an extension location, Illert et al. (1986) observed a new EMG pattern corresponding to their new mechanical wrist extension function. Despite these observations in men, no significant learning has been reported concerning the command of transposed hind limb muscles in animals. In cat, a gastrocnemius muscle sutured to the distal tendon of an antagonistic muscle (tibialis anterior) keeps its previous EMG pattern although it is inadequate for new locomotion requirements (Forssberg & Svartengren, 1983).
Thus, we chose to transfer the tendon between two muscles (the soleus and the plantaris) with different mechanical functions but a close central motor drive, in order to determine whether a qualitative modification in muscle function could induce qualitative and/or quantitative changes in MU mechanical properties and spinal drive
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METHODS |
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Animals
All the experiments were performed on female Wistar rats weighing 290.7 ± 20.2 g (Centre d'élevage Dépré, Saint Doulchard, France). In the first group of animals, we recorded the chronic electromyographic activity of soleus muscles (3 controls and 5 transposed animals) over 8 weeks. All these rats had previously been selected according to their ability to walk on a treadmill at a regular conditioned speed (30.5 cm s-1). This velocity was within the spontaneous range of speeds observed in previous studies (Clarke & Parker, 1986).
In the second group of animals, we studied the MU properties of the soleus muscle (8 controls, 4 rats after a 4 week and 3 rats after an 8 week transposition). At the end of the experimental period, each animal in each group was killed by an overdose of pentobarbital (intraperitoneal injection) and the soleus muscles were excised and weighed.
Surgical procedures for electromyographic recording and tendon transplantation
All the procedures used conformed with our ethical regional committee recommendations.
The experiments were authorised by a departmental agreement no. A29-019-3 and performed according to the recommendations of the European Community directive no. 86/609. The rats were housed in individual cages and had free access to food and water. Each animal in the first group was anaesthetised by administering an intraperitoneal injection of xylazine (Rompun 2 %, 10 mg kg-1) and ketamine (Imalgene, 100 mg kg-1) mixture. Surgery was performed under aseptic conditions. Each animal received either a chronic electrode implantation only (controls) or a soleus tendon transfer and electrode implantation at the same surgical time. Paracetamol (100 mg kg-1 per ora) was systematically administered for postoperative analgesia.
Tendon transfer
For both series of experiments, only one soleus muscle was transposed in each animal. Firstly, the triceps surae muscle was set to a neutral length corresponding to a 90 deg flexion of the ankle. The distal soleus tendon was freed from surrounding tissues and separated from the Achilles' tendon which was entirely resected. Secondly, the distal tendon of the plantaris muscle was cut at the end of its belly. Thus, the plantaris and gastrocnemii muscles were tenotomised. Then, the distal tendon of the soleus was sutured to the distal stump of the plantaris muscle tendon. We used a strong suture protocol so that animals would be able to walk on a treadmill at day 6 after surgery, thus avoiding any leg immobilisation pending tendon healing. Great care was taken not to damage blood vessels and nerve branches. The soleus muscle was thus the only ankle extensor and showed a double function: the extension of the ankle and the plantar flexion of the digits via the flexor digitorum brevis (FDB) (the soleus is linked to the FDB as was the plantaris before tendon transfer). The whole distal tendon of synergistic muscles (gastrocnemii and plantaris) was cut and removed, but the contractile part of these muscles was left untouched.
Surgical procedure for electrode implantation
The electrodes were made up of two Teflon-insulated multistranded stainless steel wires (75 µm diameter), inserted subcutaneously and guided towards the muscle. The Teflon sheath was removed over a length of 1.5 mm and these distal ends were bent back to form a hook and pushed between the soleus muscular fibres. The inter-electrodes distance was set at 5 mm and the wires were stitched to the fascia. Then, the wire ends which emerged at the head were soldered to a connector fixed to the skull by screws and dental cement.
At the end of the experimental period, the location of the implanted electrodes and the free movement of the transposed tendon were verified, in each animal. At the last post-surgery inspection, animals were excluded : (1) when the electrodes had shifted, (2) when too much fibrous tissue could prevent tendon mobility or (3) when the gastrocnemii and plantaris muscles were reattached.
We analysed the EMG of three control muscles and five transposed soleus muscles.
EMG recording sessions and analysis
Recordings began 6 days after surgery. The EMG was recorded every two days, during periods of 1 min steady walking on the treadmill, over 8 weeks. After amplification (Medelec MS6, bandwidth = 8 Hz-32 kHz) and storage on a digital tape recorder (DTR 1803, Biologic France, sampling frequency = 48 kHz), the EMG was analysed off-line.
For each one-minute recording, a sequence of 10-20 bursts was selected. This sequence of raw EMG was rectified and low-pass filtered (50 Hz) using a Biopac Labpro software (MP 100 system). The threshold used to define the limits of a burst of EMG activity was arbitrarily set to twice the amplitude of the background noise of the raw signal and the duration of the bursts was measured. The stride length was estimated from the treadmill speed divided by the average number of bursts per minute. Two analyses were then performed for each sequence. First, the amplitude (average of the amplitude values in the integrated curve) of each burst of sequence was measured using the rectified and filtered EMG (r-f EMG) signal and the average amplitude was calculated for the sequence. Secondly, a power spectral analysis was performed using a fast Fourier transform on the raw EMG signal. For each burst in the sequence, the power density spectrum (PDS) and the median frequency (MF, which divides the PDS into two parts of equal energy content) were calculated (Spike 2 software, Cambridge Electronics Design). The median frequency of the 10-20 steps was then averaged.
Motor units study
At the end of week 4 or week 8, each rat was anaesthetised using an intraperitoneal injection of sodium pentobarbital (35 mg kg-1). Supplementary injections were administered as required to maintain full anaesthesia. The nerve of the soleus muscle was freed and the distal tendon of the muscle was attached to a force transducer (Kulite strain gauge, model BG-300). All other hindlimb muscles were denervated. The skin was stretched over a steel racket to form a pool filled with paraffin oil. The pool was maintained at 37 ± 1 °C with heating elements controlled by thermosensors. The force transducer was mounted on a device allowing the muscle length to be adjusted to give the maximum twitch tension of the whole muscle. This optimal length was kept constant for subsequent measurements.
A laminectomy was performed between L4 and S2 to expose the lumbosacral cord and the skin was elevated to form a pool filled with paraffin oil. The roots were cut near their entry into the spinal cord. The ventral roots were split in oil into filaments and raised onto a silver electrode used as the anode. Another similar electrode was placed on the body mass near the root entry and acted as the cathode. The filaments were stimulated with square pulses (50 µs duration) with constant voltage ranging from 0 to 1 V. Motor axon impulses were detected by electrodes placed under the soleus muscle nerve. Action potentials were amplified by a Grass amplifier (bandwidth : 30 Hz-10 kHz) and displayed on an oscilloscope. A MU was functionally isolated when the stimulation of the filament elicited an all-or-none response in the nerve. This indicated that the filament contained a single motor axon innervating the soleus.
Stimulation sequences and data collection were carried out using a computer with Labview software. Single motor axons to the soleus were isolated and stimulated at rates of 10, 20, 50 and 100 Hz for a duration of 0.7 s, and four twitch responses were recorded and averaged. Peak twitch tension, contraction time, half-relaxation time and maximum isometric tetanic tension at 100 Hz were measured for each isolated MU.
The presence of a 'sag' defined by Burke et al. (1973) as a slight decline in unfused tetani for stimuli trains in which the interpulse intervals are approximately 1.25 times the MU contraction time was assessed for each MU.
When a sag could not be assessed using this frequency of stimulation, we stimulated the motor axon at a frequency of 1/1.5 
contraction time. The fatigue test according to Burke's criteria (trains at 40 Hz lasting 330 ms, repeated every 1 s for a total of 2 min) was performed (Burke et al. 1973).
Immunocytochemistry
The muscles were quickly removed and immediately frozen at constant length in isopentane precooled with liquid nitrogen (-150 °C). They were then stored at -80 °C until use.
For the myosin heavy chain (MHC) analysis, serial cross sections (12 µm) were cut at -20 °C, and then incubated for 45 min in an appropriate blocking solution at 39 °C in a humid chamber. The sections were then incubated for 1 h at 39 °C with different mouse monoclonal antibodies directed against (1) slow type I (Novocastra, Newcastle upon Tyne, UK), (2) all adult fast and developmental isoforms but not slow myosin (MY32, Sigma, St Louis, MO, USA), (3) fast type IIA (clone SC71, DSM Braunschweig, Germany), (4) fast type IIB (clone BFF3, DSM Braunschweig, Germany), (5) embryonic and neonatal MHC isoforms but not adult isoforms (RNMY2/9D2). Staining was revealed by the appropriate peroxidase-conjugated secondary antibody, followed by an avidin-biotin immunohistochemical procedure (kit PK-4002, Vector Laboratories, Burlingame, CA, USA). The plantaris muscle was used to assess the quality of immunostaining. Blank slides (not incubated with primary antibodies) were used to detect any false positive immunostaining. For each muscle, a sample of about 400 fibres was randomly selected from fields equally distributed among the slides.
Statistical analysis
The statistical tests were carried out with Statistica software. Regarding EMG parameters data, a two-way analysis of variance with repeated measurements and Tukey's tests were performed. A one-way ANOVA was used to analyse the contractile properties, the force-frequency relationship and muscle weights. When the data did not fit a Gaussian distribution, a nonparametric Kruskal-Wallis analysis was applied. A 
2 test was undertaken to analyse the distribution of the myosin heavy chain isoforms within the control and transposed muscle groups. Statistical significance was accepted at P < 0.05.
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RESULTS |
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Electromyographic activity
No noticeable difference in posture or locomotion was detected in the transposed animals compared with controls. They moved as freely as the controls and were able to walk on the treadmill from the 6th post-operative day onwards.
Figure 1 shows the EMG activities of a transposed and a control soleus muscle during walking, and their corresponding rectified-filtered signal. The soleus was activated during most of the stance phase of walking.
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Figure 1. Electromyographic activity of a control (A) and a transposed soleus muscle (B) For each recording sequence, a raw EMG sample (lower recording) and the corresponding rectified-filtered EMG sequence (upper recording) are shown. Bursts are delimited by vertical dotted lines. | ||
The soleus burst duration in the transposed group was significantly shorter than that of the controls during the whole experimental period and it was also found to be quite constant. The slight increase observed in the control group during the experimental period was not significant. No statistical difference was observed in the estimated stride length between the transposed and control animals in spite of slightly lower values in transposed animals (Table 1).
Tendon transplantation had no significant effect on the mean r-f EMG amplitude of transposed animals. Within each group, we observed a similar progressive and significant increase in mean absolute r-f EMG amplitude over the 8 week observational period.
The MF was significantly higher in transposed animals than in controls from the first recording session until the end of the experiment. Moreover, MF remained constant within each group during the whole experimental period.
Motor units
We studied eight control rats and we isolated 102 motor units in soleus muscles. Three motor units showed a sag at 40 Hz stimulation frequency and they were thus classified as 'fast' motor units (F) (3 %). The other 99 were classified as 'slow' motor units (97 %). We did not observe any sag among the other motor units analysed either after 4 weeks (41 MUs/4 animals) or after 8 weeks of transposition (28 MUs/3 animals). Moreover, we did not find any fatigue index lower than 0.75 (Burke et al. 1973) whatever the MU analysed in the three groups (controls, 4 week- and 8 week-transposed animals). The different parameters of the contractile properties are expressed as mean ± standard error of the mean (S.E.M.) in Table 2. In control animals, the three fast MUs showed shorter contraction times (CT) and shorter half relaxation times (HRT) than slow MUs. Prior to the statistical analysis, these three MUs were excluded (not represented in Fig. 2).
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Figure 2. Cumulative histogram of the motor unit twitch contraction times in control and transposed animals The slow MU contraction time decreased significantly in transposed soleus after 4 weeks ( | ||
The mean CT of the slow MUs was significantly reduced in muscles transposed after 4 weeks but did not show any further decrease at 8 weeks.
The mean HRT was also reduced in muscles transposed after 4 and 8 weeks.
The mean maximal tetanic tension in transposed soleus after 4 weeks was the same as that in the slow MUs of control muscles but the mean tetanic force slightly decreased at the longer period of transposition (the difference was not significant).
We determined the relationship between the relative tetanic force (the force developed by each motor unit during unfused tetanus and expressed as a proportion of its maximal tetanic force) and the frequency of stimulation (T-f relationship). The fast and slow motor units have different force-frequency relationships. Figure 3 re presents the T-f relationship in several examples of control soleus, and soleus transposed for 4 or 8 weeks. Figure 4 represents this relationship in controls, and in the two groups of transposed muscles (all MUs studied were pooled in each group). The curves obtained in 4 week- or 8 week-transposed soleus were different from those of controls: the midfrequency (the frequency at which the force is 50 % of the maximal tetanic force) increased after transposition. We developed a new criterion in order to classify the slow MUs in transposed soleus muscles because of the changes we had observed in their behaviour. We determined a strength gain for each MU during stimulation: the strength gain is the ratio of the force increase between 10 and 20 Hz of stimulation divided by the force increase between 20 and 50 Hz. In controls, we found 3 'fast' MUs with a ratio 
0.2, thus, the threshold was fixed at 0.2. In the control group (8 rats, 102 MUs), 11 MUs showed a ratio 0.2 (11 %) and 91 MUs had a ratio >0.2 (89 %). We observed a greater number of MUs with ratio 0.2 in the transposed soleus after 4 weeks (26/41 MUs, 63.4 %) and this was still the case after 8 weeks (17/28 MUs, 61 %). Distal tendon transfer of the soleus induced modifications in the T-f curve profiles of the slow motor units and a significant shift towards the right of the T-f curves (P < 0.05).
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Figure 3. Relationship between mean relative force during unfused tetanus and frequency of stimulation of the motor axons in a control rat (A), a 4 week-transposed rat (B) and an 8 week-transposed rat (C) Forces are expressed as a percentage of the maximum tetanic force developed by motor units. In the control animal, fast MU T-f curves are represented by thick dashed lines (n = 2). | ||
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Figure 4. Relationship between mean relative force during unfused tetanus and frequency of stimulation Changes induced by several weeks of distal tendon transposition. The results are expressed as mean relative force ± S.E.M. A , among 102 control MUs, 3 MUs showed a sag (classified as fast MUs (F; the rest are slow, S); represented by thick dashed lines on Fig. 3A). B, the profile of the transposed muscle T-f relationship almost overlapped that of the three control fast MU curves. | ||
Muscle weight
We decided to compare weight ratios between the groups because of the wide range of animal weights. We compared the muscle/body weight ratio of five 4 week-transposed animals, five 8 week-transposed animals to the ratio of six control animals (Table 3). No statistical difference was observed between the transposed muscles and controls (Kruskall-Wallis, Anova).
MHC isoform distribution
About 400 fibres were analysed within each muscle. We verified that MHC distribution was homogeneous in two control soleus muscles, and similar to that reported in previous studies (see Discussion). These controls were then compared with four muscles at the end the 8th week of transposition (Table 4).
In both control and transposed soleus muscles, most fibres contained type I MHC isoform. However, the percentage of fibres expressing the slow MHC type significantly decreased 8 weeks after transposition. A similar decrease in mixed fibres, containing both type I and type IIA MHC, was observed. Simultaneously, the percentage of mixed fibres both expressing type I and type IIX MHC significantly increased (P < 0.001). No detectable immunostaining was observed with the anti-IIB, embryonic or neonatal MHC isoform antibodies, whatever the muscle studied.
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DISCUSSION |
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In these experiments, we observed different changes in MU characteristics and EMG activity of the soleus muscle during gait, which could be due to the fact that this muscle was able to adapt itself to its new mechanical function.
However, no quantitative 'training effect' was observed. Gastrocnemii and plantaris removal did not lead to an overload-induced hypertrophy of the soleus muscle as described by Gardiner et al. (1986, 1991) in the medial gastrocnemius or the plantaris following synergist tenotomy or ablation. The increase in muscle weight and in body mass were equivalent, so that the ratio muscle/body weight in the two transposed soleus groups was not higher after transposition nor was there any increase in the MU tetanic forces.
On visual observation, the animals did not show any apparent change in locomotor behaviour during treadmill walking after tendon transposition, in these well-conditioned animals. The EMG analysis showed a shorter burst duration compensated by a longer inter-burst interval of the soleus muscles of transposed animals while walking on the treadmill at the same speed (see Fig. 1). However, the estimated stride length was slightly shorter in transposed animals compared to controls, indicating a slightly increased gait frequency. Nevertheless, this difference was not statistically significant and confirmed the visual observation.
The analysis of the EMG activity during the locomotor task revealed a slight but significant increase in r-f EMG in both groups. Muscle weight increased as well as body mass during the experimental period. Thus, the electromyographic activity increased because of muscle growth and because animals had to move with an increased weightbearing. This could lead to an increase in the number or in the frequency of recruited MUs, in order to increase the force of the transposed muscle whose MU maximal tetanic forces were not higher than controls at 4 or 8 weeks.
We observed a significant decrease in the mean burst duration in transposed soleus muscles. This duration of EMG activity bursts is similar to that of the control plantaris muscle reported by Gardiner et al. (1986) (306.42 ± 23.55 ms versus about 318 ms at a slightly lower treadmill speed than that used in this experiment). The shorter burst duration could be due to the fact that the soleus was attached to a longer and more compliant tendon, leading to an increase in muscle shortening velocity and/or a modified range of shortening. In fact, the MF was instantly and significantly higher in the transposed muscles compared to controls; the MF may correlate with the MU firing frequency (Fuglsang-Fredericksen & Ronager, 1988).
These decreases in EMG burst duration and the increase in MU firing rate could be the consequence of motor control adaptation. Such adaptations in EMG were observed at the very beginning of the recordings (6th day) and did not change during the next 8 weeks. This is in agreement with an immediate functional adaptation induced by proprioceptive information carried by muscle afferents.
We observed several changes in the soleus MU contractile properties after tendinous transposition. The range of slow MU contraction times in control rats was in agreement with that reported by previous studies in the rat triceps surae (Celichowsky & Grottel, 1997; Carp et al. 1999). After 4 weeks of tendon transfer, mean CT decreased significantly (Fig. 2) as well as mean HRT in the slow MUs. On the basis of these characteristics (faster CT and HRT), slow MUs in the transposed soleus were more similar to fast MUs. This is in agreement with the transfer-induced modifications in their T-f relationship. In controls, where the MUs were classified according to the occurrence of sag, slow MU relative force quickly increased between stimulations of 10 and 20 Hz frequency while the force of the three fast MUs increased at higher frequencies (between 20 and 50 Hz). This feature of the fast MU T-f curves was present to a higher degree in soleus MUs, respectively 63.4 % after 4 weeks, and 61 % after 8 weeks of tendon transfer.
However, no sag was observed in the MUs of transposed soleus, whatever the stimulation frequency used, in spite of a significant decrease in contraction times. Moreover, in the control group, eight more MUs showed this 'fast' behaviour in T-f relationship but no sag was detected. This observation highlights the problem of considering sag as a criterion to distinguish between fast or slow motor units, particularly in the rat. In fact, Carp et al. (1999) have already emphasised that sag assessment in the rat is difficult because the MUs of whole triceps surae display a wide pattern of frequency dependence regarding sag. This is in agreement with Burke (1999) who pointed out the broad ways in which slow and fast types are best recognised. Moreover, when re-innervated by the fast flexor digitorum longus nerve, soleus MUs still exhibit a slow profile in terms of mechanical properties (lack of sag, resistance to fatigue) or fibre histological type (type I), despite a marked decrease in MU twitch contraction time (Dum et al. 1985).
Thus, we looked for another criterion that would allow us to evaluate the trends in functional changes within the MU pool. This criterion was chosen in order to assess the modifications that were observed in the MU T-f relationship behaviour. The T-f curve seems to feature a slow or fast dynamic MU profile. However, this profile is difficult to quantify and subsequently, we propose the 'strength gain' as an 'operating criterion'. Such a right-sided displacement in T-f curves has been reported in the soleus innervated by the fast flexor hallucis longus nerve (Chan et al. 1982).
We found clear changes in MHC expression which showed that tendon transposition significantly affected fibre composition. In both control and transposed muscle groups, most fibres expressed slow MHC type I isoform in agreement with the results reported by Bigard et al. (2000). The absence of embryonic and neonatal MHC staining indicates the absence of tissue damage after the transposition procedure. However, a significant de novo expression of type IIX MHC isoform was detected in mixed fibres (type I/IIX) in the transposed soleus. There was a clear conversion of some type I/IIA MHC and some type I fibres into type I/IIX hybrid fibres. These results confirm a transposition-induced transition of the MHC-isoform component towards a faster profile. This slow to faster transformation has already been described after cross-reinnervation (Sreter et al. 1974; Gauthier et al. 1983).
As regards these three criteria (sag, tension-frequency relationship, MHC phenotype), our data on tendon transfer lead to conclusions similar to those reported after cross-innervation of a slow muscle with a fast muscle nerve. In our experimental protocol, the changes observed could well be due to a modification in the neural muscle drive. Tendon transposition might induce changes in proprioceptive information that could lead to central drive modifications, as pointed out by the increase in MF. Nevertheless, the direct effect of mechanical factors on muscular tissues cannot be excluded.
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Acknowledgements
We wish to thank Danielle Gillet for the immunohistochemical analysis, and Jean-François Clément for helping with the signal analysis software (Unité de Culture Cellulaire, Institut de Synergie des Sciences et de la Santé, CHU Morvan 29609 Brest Cedex). We also want to thank Nathalie Koulmann and Xavier Bigard for their advice and for kindly providing us with SC71 and BFF3 monoclonal antibodies (Département des Facteurs Humains, Centre de Recherche du Service de Santé des Armées, 38 702 La Tronche).
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) compared to control soleus (
) with no greater change after 8 weeks (
).