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1 Section of Sports and Rehabilitation Medicine, Department of Medicine II, University of Ulm, D-89070 Ulm; 2 Institute of Sports Sciences, Department 1: Sport and Movement, Johann-Wolfgang-Goethe University, 60054 Frankfurt am Main; and 3 Eden-Reha, Rehabilitation Clinic, 93093 Donaustauf, Germany
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ABSTRACT |
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Myosin heavy chain (MHC)
isoform expression changes with physical training. This may be
one of the mechanisms for muscular adaptation to exercise. We aimed to
investigate the effects of different strength-training protocols on MHC
isoform expression, bearing in mind that
- MHCslow
(newly identified MHC isoform) mRNA may be upregulated in response to
training. Twelve volunteers performed a 6-wk strength training with
maximum contractions (Max group), and another 12 of similar age
performed combination training of maximum contractions and ballistic
and stretch-shortening movements (Combi group). Muscle samples were
taken from triceps brachii before and after training. MHC isoform
composition was determined by SDS-PAGE silver staining, and mRNA levels
of MHC isoforms were determined by RT-PCR. In Max group, there
was an increase in MHC2A (49.4 to 66.7%, P < 0.01) and a decrease in MHC2X (33.4 to 19.5%, P < 0.01) after training, although there was no
significant change in MHCslow. In Combi group, there was
also an increase in MHC2A (47.7 to 62.7%,
P < 0.05) and a decrease in MHCslow (18.2 to 9.2%, P < 0.05) but no significant change in
MHC2X. An upregulation of
-MHCslow mRNA was,
therefore, found in both groups as a result of training. The strength
training with maximum contractions led to a shift in MHC isoform
composition from 2X to 2A, whereas the combined strength training
produced an MHC isoform composition shift from slow to 2A.
skeletal muscle; exercise;
-myosin heavy chain slow
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INTRODUCTION |
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THE SKELETAL
MUSCLE, ONE OF the most important components responsible for
physical performance and adaptation to exercise, is an extremely
heterogeneous tissue in both structure and function (26).
The alteration of myosin heavy chain
(MHC)1 isoform composition in
the muscle with exercise contributes greatly to this heterogeneity and
serves as an important mechanism for muscle adaptation to exercise. The
multigene families of MHC may be an important substrate to warrant this
heterogeneity (32). For instance, four MHC isoforms, i.e.,
MHCslow, MHC2A, MHC2X, and
MHC2B, can coexist in the hybrid muscle fibers of small
animals (27). The MHC isoform equivalent to the
MHC2B in small animals has not yet been identified in
humans. Much effort has been made to characterize more MHC isoforms. In
particular, the successful characterization of a new slow MHC isoform,
termed
-MHCslow, has attracted considerable
attention recently (12, 28, 30).
-MHCslow, an isoform of slow MHC, was thought to be
expressed abundantly in the myocardium (3, 23) and some
other special muscles like masseter (4-6, 37) and
extraocular muscle (33).
-MHCslow is
functionally distinct from the isoform
-MHCslow (9) and can be considered as an intermediate between
MHC2A and
-MHCslow (28). It has
been reported that
-MHCslow can be transiently expressed
in rabbits during fast-to-slow transition and, therefore, may play an
important role in the muscle adaptation to exercise or other stimuli
(29).
A number of studies have shown that, with an increase in neuromuscular activities, the MHC isoform may shift from fast (2B or 2X) to slow (MHCslow), leading to a fast-to-slow muscle fiber transition (27). It would seem to be apparent that exercise training results in changes in neuromuscular activities, recruitment of different motor units, energy metabolism, and hormonal responses. This may lead to changes in MHC isoform composition, and exercise-induced change of MHC isoform composition seems to be dependent on the kinds of training, muscle groups, and energy metabolism.
Strength training is frequently used to improve the development of force and velocity. Subsequently, various motor units with different recruitment thresholds will be activated (8). It is also well known that strength training has significant impact on MHC isoform expression at the protein level (2). In strength training with maximum contractions, the fast motor units with a high threshold will be activated, leading to a fast-to-slow shift in MHC isoform profile (17). A recent observation demonstrated that a combination of different training methods may improve the effects of strength training on force development (13, 39). The strength training combined with high speed and power, for example, provides a superior stimulus for enhancing intra- and intermuscular coordination (10).
Although there have been a number of studies on the relation between
training and MHC isoform in leg muscles, there are relatively fewer
data available on MHC isoforms derived from arms and, to our knowledge,
no reports on the effects of strength training combined with very fast
ballistic and stretching-shortening movement on MHC isoform expression.
We have, therefore, established a new method for strength training that
combines the traditional strength training with ballistic movement at
lower workload (30%) and stretch-shortening movement. We hypothesized
that this new training approach would have different effects on the
physical performance and MHC isoform compared with the traditional
strength training. Thus, in this study, we aimed to investigate the
effects of the new combined method for strength training on physical
performance and MHC isoform expression at both the protein and mRNA
level, particularly
-MHCslow mRNA in the triceps branchii.
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METHODS |
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Subjects.
Twenty-four male physical education students with experience in
strength training were enrolled in this study. They all had had 3 mo to
5 yr of regular strength training for the arms. They were randomized
into two groups: the Max group received the traditional strength
training with maximum contractions, and the Combi group received the
strength training combined with ballistic and stretch-shortening contractions. Age, height, and body mass were recorded (Table 1). This study was approved by the Ethics
Committee of the Medical Faculty of the University of Ulm (Germany),
and written consent was obtained from all participants.
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Training protocol. The training was performed on Monday, Wednesday, and Friday each week for 6 wk. In the Max group, the same training was carried out on the 3 training days: bench press with maximum contractions [3-repetitions maximum (RM) load, determined by 1 RM] and five series with three repetitions in each series. In the Combi group, the training was the same as that in the Max group on Monday, 10 ballistic movements (concentric-only bench-press throw) with a workload of 30% of 1 RM on Wednesday, and 10 stretch-shortening-type push-ups on Friday. The interval between repetitions and between series was 3-4 s and 6 min, respectively.
Biomechanical measurements. Maximum strength was assessed by means of the 1 RM of the bench-press movement on a multipower station (Technogym, Dreieich, Germany). 1-RM testing was conducted by using the methods described by Kraemer and Fry (18). Briefly, 1 RM represents the highest possible weight that could be successfully lifted until complete extension of the elbow joints.
Maximum movement velocity (m/s) was determined from a pure concentric bench-press movement against a constant load of 16.9 kg on the same multipower station in which the barbell was projected from the hands ("bench-press throw"). The subjects were instructed to apply force as quickly as possible and throw the bar with maximum velocity. Each subject was given three trials, interspersed by a 30-s interval. The velocity measurements were made by means of an optical reflective sensor system (Fichte, University of Frankfurt, Germany) with an accuracy of 4 mm and 1/5,000 s. The highest values were taken for statistical analyses. To ensure sufficient familiarization with the protocol, maximum strength and maximum velocity testing was carried out twice before training. Posttraining testing was performed 3 and 11 days after the completion of the protocol. Again, the highest values were taken for statistical analysis. The correlation coefficients of the two pretraining sessions were 0.98 and 0.94 (both P < 0.01) for 1 RM and maximum movement velocity, respectively. Coefficients of variation were 2.6% for 1 RM and 2.2% for maximum movement velocity.Muscle biopsy.
Muscle biopsy was carried out at rest on a nontraining day, 3 days
before and 7 days after the training protocols. Muscle samples were
taken from the long head of m. triceps brachii of the
dominant side of the subjects by using the fine-needle biopsy technique
(20, 21). After routine disinfection of the skin, a
13-gauge biopsy needle (Peter Pflugbeil Medizinische Instrumente, Zorneding, Germany) was punctured 1 cm into the muscle belly, and
biopsy gauge was shot three times to attain ~3 mg of muscle tissue.
The sample was immediately frozen in liquid nitrogen and then stored at
80°C.
MHC analysis.
The muscle sample was homogenized in 50 µl extraction
buffer (40) containing 100 mmol/l
Na4P2O7 · 10H2O,
5 mmol/l EGTA, 5 mmol/l
MgCl2 · 6H2O, 0.3 mmol/l
KCl, and 1 mmol/l dithiothreitol with an ultrasonic homogenizer
(Bandelin Sonoplus Homogenisator HD2070, Berlin, Germany). The muscle
homogenates were stirred on ice for 20 min and then centrifuged at
4°C and 16,000 g for 10 min. The supernatant was collected
and mixed with an equal volume of glycerol. The total protein
concentration was determined according to Lowry et al.
(22). The protein solution was prepared at the
concentration of 0.25 µg/µl with sample buffer, according to the
method of Laemmli (19). Total protein (1.25 µg) was
loaded for SDS-PAGE, the gel concentration was 7.2%, and the SDS-PAGE was run in the electrophoresis device (Hoefer SE600, Pharmacia, Freiburg, Germany) equipped with a specifically developed cooling system in our laboratory to keep the temperature of running buffer constant (38). Protein detection followed an
ultrasensitive silver staining (25), and the MHC isoforms
were identified (11) (Fig.
1A).
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RT-PCR for mRNA of MHC isoforms.
Total RNA was extracted from the muscle tissue by phenol extraction
(RNA Clean System, AGS, Heidelberg, Germany). The total RNA was
dissolved in 10 µl for each 1-mg muscle tissue. Oligo(dT) primed
synthesis of cDNA was performed by using murine leukemia virus reverse
transcriptase, according to the standard protocol (Perkin Elmer, Roche
Molecular System, Branchburg, NJ). Amplification of cDNA for each of
the MHC isoforms,
-MHCslow,
-MHCslow,
MHC2A, and MHC2X, and
-actin was carried out
by the method reported by Peuker and Pette (31). The
primer sequences used and the corresponding RT-PCR products for each of
the MHC isoforms are summarized in Table
2. The reaction conditions and procedures were described in detail elsewhere (30, 31). In brief, the total reaction volume for each sample was 25 µl and contained 2 µl
cDNA solution, 25 pmol of each primer, 100 µM of each
2-deoxynucleotide 5'-triphosphate, 2 mM MgCl2, and 0.5 unit
of Taq polymerase. Thirty cycles of 45 s at 94°C,
60 s at 56°C, and 30 s at 72°C were performed in an
automatic incubation system (Crocodile III, Appligene, F-67402, Illkirch, France). The RT-PCR products were densitometrically measured
on 3% agarose gel containing ethidium bromide (Fig. 1B).
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Data analysis. Muscle samples were analyzed in duplicate, and the average of the two measurements was taken. The protein bands of each MHC isoform on the silver-stained acrylamide gel, as well as the RT-PCR products for each MHC isoform on the agarose gel, were densitometrically digitalized by using a digital camera (Camedia 2500 L, Olympus, Hamburg, Germany). The densitometric values were derived as an integral of the band density and the band area (21). This procedure was performed by using a software developed in our laboratory specifically for this purpose (MARS98, Ulm, Germany).
For the MHC proteins, the amount of each isoform was expressed as a percentage calculated as
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-actin of the same muscle sample; and
3) these ratios were related to the baseline ratio taken
before the training, i.e., the first biopsy, and expressed as a percentage.
Statistical analyses. All values are expressed as means ± SD, and statistical differences were examined by ANOVA or post hoc Scheffé test. A P value < 0.05 was considered to be statistically significant.
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RESULTS |
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All subjects accomplished the assigned training program without
any adverse complication. 1 RM increased significantly with training in
both groups to a similar degree (6.7 and 6.0% for Max and Combi,
respectively). The increase in maximum movement velocity was
significantly greater in the Combi group than in the Max group (0.1 vs.
0.07 m/s, P < 0.05; Table
3).
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All muscle samples were successfully analyzed. In the Max group, there
was a significant increase in MHC2A (49.4 to 66.7%, P < 0.01) and a decrease in MHC2X (33.4 to
19.5%, P < 0.01), whereas the change in
MHCslow (17.2 to 13.8%) was not statistically significant (Fig. 2A). In the Combi group,
there was also an increase in MHC2A (47.7 to 62.7%,
P < 0.05), no change in MHC2X (34.1 to
28.1%), but a significant decrease in MHCslow (18.2 to
9.2%, P < 0.01; Fig. 2B).
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The mRNA levels of
-MHCslow, MHC2A, and
MHC2X did not change (Fig.
3), whereas
-MHCslow mRNA
increased significantly in both groups. The increase in
-MHCslow mRNA in the Max group (from 100 to 308%,
P < 0.01; Fig. 3A) was significantly
greater than that in the Combi group (from 100 to 160%,
P < 0.01; Fig. 3B).
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DISCUSSION |
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The alteration of MHC isoform composition serves as an important mechanism for skeletal muscle adaptation to exercise. Changes in neuromuscular activities secondary to exercise may lead to such alternations. In the present study, we investigated the MHC isoform expression at protein and mRNA levels in response to strength training and found that, after 6-wk training, MHC2A increased significantly, along with a decrease in MHC2X in maximum contraction training and a decrease in MHCslow at the protein level in combination training. On the other hand, both training strategies improved the maximum strength (1 RM) significantly (Table 3). However, the improvement in the maximum movement velocity was statistically significant only in the Combi group.
At first sight, our results are similar to those of previous studies
(7, 13, 35). Harris et al. (13) demonstrated that a combined strength training was superior to the strength training
with high force or high power alone with regard to the improvement of a
wide variety of athletic performance variables requiring strength,
power, and speed. Newton et al. (24) showed that the
ballistic training, in addition to resistance training, in well-trained
athletes could improve the explosive force (vertical jump). The effects
of training on muscles have been documented at muscle fiber-type
transition and MHC isoform at the protein level. In general, strength
training leads to fast-to-slow muscle fiber transition (i.e., IIb
IId/x
IIa
I) and to distribution shift of MHC isoform
composition at the protein level from 2B
2X
2A
slow
(27). However, the effects on MHC isoform composition may
vary with different training methods and muscles studied. In this
study, the training with maximum contractions led to an increase in
MHC2A and a decrease in MHC2X at the protein
level, indicating an MHC isoform composition shift: 2X
2A. This is indeed similar to the findings by Allemeier et al. (1) and Jürimäe et al. (17). The combined strength
training in this study produced an increase in MHC2A and a
decrease in MHCslow at the protein level, suggesting an MHC
isoform composition shift: slow
2A. This is of novelty, as there
has not been similar findings showing a exercise-induced slow-to-fast
shift in MHC isoform composition at the protein level in the literature
to date, although Jansson et al. (15) demonstrated a 4- to
6-wk sprint training-induced slow-to-fast muscle fiber transition (slow
IIa) in vastus lateralis using myofibrillar ATPase stain.
The changes in MHC isoform composition observed in the present study
shed some light on the mechanism of improvement in performance as a
result of training. The strength training-induced increase in
MHC2A was probably responsible for the improvement in
maximum strength, which was documented in both groups. In the Max
group, the lack of a significant increase in maximum velocity may be attributed to the reduced MHC2X composition, which then
attenuated the effect of the slow
2A shift in MHC isoform
composition. However, the maximum movement velocity in the Max group
did not decrease as a result of the 2X
2A shift. This was probably
a compensatory response as the decrease in MHCslow was not
significant. In the Combi group, MHC2X was better
preserved, which, together with the slow
2A shift in isoform
composition, has possibly led to a significant improvement in maximum
movement velocity.
We are not absolutely certain why the two training protocols produced different MHC isoform compositions. One, however, should consider the following. First, it is known that the neural adaptation plays an important role in the strength training (34). The neural impact on muscle response to training may exert at the level of motor units with different recruitment thresholds and at the level of firing rates (8). The two training protocols may differ in the activation of neural control, the recruitment of motor units, and the firing models. Second, mechanical factors may contribute to the differences between the two groups. It is apparent that the ballistic and stretch-shortening movements in the Combi group have different mechanical impacts on muscle compared with the maximum contractions.
The mRNA of all MHC isoforms except
-MHCslow did not
change after the 6-wk strength training, which differed from the
results of MHC isoform composition at the protein level. This
discrepancy between protein and mRNA level may be mainly attributed to
their different kinetics (16). Jashinski and colleagues
(16) determined the time-dependent decay of
MHC2B in low-frequency-stimulated rat extensor digitorum
longus muscle at both the protein and mRNA level. The decay of
MHC2B mRNA was much faster than that of MHC2B protein (half-time = 62 h vs. 11 days). This discrepancy, on
the other hand, suggests that the posttranscriptional mechanisms play an important role in muscular adaptation to exercise.
The changes in
-MHCslow mRNA arouse much interest.
-MHCslow is considered to be an intermediate between
-MHCslow and MHC2A in functional
characteristics (9, 14). The upregulated expression of
- MHCslow thus suggests an active transition procedure
in MHC isoforms (28, 29). However, there have not been
similar observations on
-MHCslow mRNA in humans. In our
present study, we have investigated the expression of
-MHCslow at the mRNA level, and the results show that
the level of
-MHCslow mRNA was significantly elevated in
both groups, whereas the mRNA level of the other MHC isoforms remained
unchanged. This elevated level of
-MHCslow mRNA may
suggest that the alteration in MHC isoform composition induced by
strength training was not accomplished over the study period. Another
possibility is that satellite cells were activated through exercise
(36). It has been reported that, in animals with
compensatory muscular hypertrophy, satellite cells are activated with
increased muscle activities (36). Because
- MHCslow is considered to be an embryonic form of
MHC, the activation of satellite cells may well be responsible for an
upregulation in
-MHCslow mRNA observed in the present
study. This would, therefore, explain why there was a persistent high
level of
- MHCslow mRNA. Unfortunately, the separation
of the two isoforms of MHCslow at the protein level, i.e.,
-MHCslow and
-MHCslow, was technically by
no means simple and thus not performed in this study. It, therefore,
remains unknown whether the expression of
-MHCslow has
also changed at the protein level. In the present study,
MHCslow at the protein level consisted of
-MHCslow and
-MHCslow. If
-MHCslow were a small part of MHCslow, the
change in MHCslow at the protein level would be mainly that
of
-MHCslow. This is supported by our present results, a significant decrease in MHCslow at the protein
level, along with a significant increase in
-MHCslow
mRNA in the Combi group, but not in the Max group, although the
increase in
- MHCslow mRNA posttraining was even
greater than that in the Max group (Figs. 2 and 3). It has been
demonstrated in animal study that
-MHCslow mRNA was
upregulated during a fast-to-slow muscle fiber transition shown by
single-fiber analysis (30), but it is not clear whether
-MHCslow mRNA can also be upregulated during a
slow-to-fast muscle fiber transition. Because
-MHCslow
is regarded as an intermediate between
-MHCslow and
MHC2A, it is possible that
- MHCslow mRNA can be upregulated by the events of slow-to-fast muscle fiber or MHC
isoform transition. In our present study, there is a shift in MHC
isoform composition at the protein level from slow to 2A (i.e., slow to
fast) in both groups, although it was not statistically significant in
the Max group. This was accompanied by a clear upregulation of
-MHCslow mRNA. It is, therefore, possible that
-MHCslow mRNA can be upregulated during a shift in MHC
isoform composition from slow to 2A.
In conclusion, a 6-wk strength training produces significant changes in
isoform composition in the muscle of triceps brachii. Strength training
with maximum contractions leads to an increase in MHC2A and
a decrease in MHC2X, indicating a 2X
2A shift in MHC
isoform composition at the protein level, whereas strength training
combined with maximum contraction and ballistic and stretch-shortening movement induces an increase in MHC2A and a decrease in
MHCslow, indicating slow
2A shift of MHC isoform
composition. The strength training can produce a distinct upregulation
of
-MHCslow mRNA, the physiological significance of
which deserves further investigations.
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ACKNOWLEDGEMENTS |
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This study was supported by the Bundesinstitut für Sportwissenschaft, Cologne, VF0407/05/08/2000 and VF0407/01/22/2000-2003.
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FOOTNOTES |
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Address for reprint requests and other correspondence: Y. Liu, Sport- und Rehabilitationsmedizin, Universität Ulm, Steinhövelstr. 9, D-89070 Ulm, Germany (E-mail: yuefei.liu{at}medizin.uni-ulm.de).
1 By definition, MHC denotes the myosin heavy chain at the protein level, whereas MHC isoforms at the mRNA level are especially denoted by mRNA.
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.
10.1152/japplphysiol.00830.2002
Received 11 September 2002; accepted in final form 28 January 2003.
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