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Medicine & Science in Sports & Exercise:
doi: 10.1249/01.mss.0000232022.21361.47
BASIC SCIENCES: Original Investigations

Calcium Sensitivity of Human Single Muscle Fibers following Plyometric Training

MALISOUX, LAURENT1; FRANCAUX, MARC1; NIELENS, HENRI1; RENARD, PATRICIA2; LEBACQ, JEAN3; THEISEN, DANIEL1

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Author Information

1Department of Physical Education and Rehabilitation, Faculty of Medicine, Catholic University of Louvain, Louvain-la-Neuve, BELGIUM; 2Laboratory of Biochemistry and Cellular Biology, University of Namur, Namur, BELGIUM; and 3Department of Physiology, Faculty of Medicine, Catholic University of Louvain, Brussels, BELGIUM

Address for correspondence: Daniel Theisen, Ph.D., 1, place P. de Coubertin,B-1348 Louvain-La-Neuve, Belgium; E-mail: daniel.theisen@ucl.ouvain.be.

Submitted for publication January 2006.

Accepted for publication May 2006.

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Abstract

Purpose: To study the effect of plyometric training on Ca2+ sensitivity and the influence of troponin T (TnT) isoforms on Ca2+-activation properties in skinned human muscle fibers.

Methods: Biopsies were obtained from the vastus lateralis of eight men before and after the training period. Chemically skinned fibers were evaluated regarding their Ca2+-activation properties and were classified according to their myosin heavy chain (MHC) contents and analyzed regarding their slow and fast TnT isoforms.

Results: After training, significant improvements (P < 0.05) were found for static jump, countermovement jump, 6 × 5-m shuttle-run test, and leg-press performances. An 8% increase in the proportion of type IIa fibers (P < 0.05) was observed. Single-fiber diameters increased by 11% in type I (P < 0.01), 10% in type IIa (P < 0.001), and 15% in type IIa/IIx fibers (P < 0.001). Peak fiber force increased by 35% in type I (P < 0.001), 25% in type IIa (P < 0.001), and 57% in type IIa/IIx fibers (P < 0.01). The Ca2+-activation threshold was not altered by training, but the Ca2+ concentration required to elicit half-maximal activation showed a decreasing trend, with significant changes in type I fibers (P < 0.001). Cooperativity at low Ca2+ concentrations was increased in type I and type IIa/IIx fibers (P < 0.05). Type I fibers exclusively expressed slow TnT isoforms, and type II fibers were always associated with fast TnT isoforms, independent of training status. Therefore, changes in Ca2+ sensitivity after training could not be explained by differential fast or slow TnT isoform expression.

Conclusion: Plyometric training increased single-fiber Ca2+ sensitivity, especially in type I fibers. These changes could not be explained by a modified TnT isoform expression pattern.

Stretch-shortening cycle movements occur during functional activities such as walking, running, or jumping. They are characterized by an eccentric muscle contraction followed immediately by a concentric contraction. This combination of negative and positive muscle work potentiates concentric force generation and greatly increases mechanical efficiency (14) via the storage and subsequent restoration of elastic energy in the muscle fascia and tendinous tissue (11). Plyometric training is based on the principle of repetitive, high-load, stretch-shortening cycle exercises. This type of training has gained popularity in recent years and has been used successfully in different contexts. Most evidently, lower-limb plyometric exercises improve leg-extension force, contraction velocity, and jumping performance, especially when high stretch velocities are used (22). Others have found that plyometric training can improve running performance, probably through increased stiffness of the muscle-tendon unit (21). Plyometric training has also been suggested to be an effective intervention to reduce knee injuries by improved muscle-activation strategies and functional stability at the knee joint (3).

The benefits of plyometric training on lower-limb performance are believed to be essentially attributable to altered patterns of muscle activation. Hakkinen et al. (10) found that power-type strength training based on exercises involving explosive force production provoked faster neural activation and force development of the trained muscles. Only nonsignificant changes were described concerning muscle hypertrophy. Similarly, Chimera et al. (3) reported that their participants displayed an earlier and greater muscle activation and an increased coactivation of agonist and antagonist muscles during the landing phase following plyometric training. Kyröläinen et al. (13) demonstrated that 15 wk of maximal-effort power training did not induce significant changes in muscle-fiber type or size and argued that enhancements in jumping performance could be the consequence of altered joint-control strategy and rate of force development at the knee joint.

Based on these results, it would seem that plyometric training is not appropriate to induce significant improvements at the level of the muscle structure or contractile function. However, these aspects have not been investigated to any large extent. We have recently shown that short-term (8 wk) plyometric training could induce considerable changes in the contractile function of chemically skinned muscle fibers obtained from the vastus lateralis muscle (15). Single-fiber cross-sectional area, peak force, and maximal shortening velocity were significantly increased, giving rise to some 25-50% increases in single-fiber peak-power generation. These positive effects were found on fibers expressing type I, type IIa, and hybrid type IIa/IIx myosin heavy chain (MHC) isoforms and were associated with increased explosive force development. Therefore, enhanced functional performance following plyometric training could be partly explained by changes in the contractile apparatus of the muscle fibers, independent of fiber-type expression.

Previous studies have investigated the sensitivity of myofilaments to Ca2+ in different animal and human models, most of which focused on muscle unloading leading to decreases in Ca2+ sensitivity (2,5,16,23). Investigations on the effect of training interventions on myofilament Ca2+ sensitivity are scarce. Godard et al. (9) observed an increase of type I fibers Ca2+ sensitivity in older women after resistance training. On the other hand, no differences were found between highly trained swimmers and control subjects (6) or between master runners and age-matched controls (25). The effects of plyometric training on the Ca2+ sensitivity of myofilaments have not yet been investigated. Therefore, the purpose of the present study was to analyze the influence of an 8-wk plyometric training program on the Ca2+ sensitivity of chemically skinned single-muscle fibers. Based on our previous observations (15), we hypothesized that plyometric training could also enhance the Ca2+ sensitivity of fibers expressing type I and type II MHC isoforms. Because troponin (Tn) T isoform expression has been shown to modulate Ca2+-activation properties (18), our secondary hypothesis was that changes in Ca2+ sensitivity of single muscle fibers are influenced by differential slow or fast TnT isoform expression within the fiber. The data of this study are an extension of the results presented in a previous investigation (15).

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MATERIALS AND METHODS

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Subjects.

Eight healthy young men participated in this study after having given their written informed consent. Their main characteristics at the beginning of the study were as follows (mean ± standard error, SE): age = 23 ± 1 yr, height = 177 ± 2 cm, body mass = 68 ± 4 kg. The participants were involved in regular practice of physical activities (hockey, gymnastics, cycling, soccer, judo, and swimming) at a recreational level for 3.1 ± 0.4 h·wk−1 (mean ± SE). They were instructed not to change their usual activity pattern during the study period. None were engaged in physical activities involving repetitive jumps. The protocol of this study was approved by the faculty ethical review committee and complied with the principles of the Declaration of Helsinki.

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Plyometric training program.

The training program consisted of different stretch shortening cycle exercises and was scheduled for a period of 8 wk, with three sessions per week for a total of 5228 jumps. Each session was preceded by a warm-up consisting of a 5-min run, free-weight squats, and rope jumping. The sessions lasted 20 min in the beginning to approximately 45 min at the end of the training period. Training consisted of different types of jumps performed without additional weight to promote high-velocity contractions. The program included the following exercises (Table 1): static jump (SJ), countermovement jump (CMJ), drop jump, double-leg and single-leg triple jump, and double-leg and single-leg hurdle jump. The one-leg exercises were performed alternatively on each side to provide a similar workout for both legs. The daily program was composed of approximately two to five series of three to five different exercises. The exercise series were composed of 6-12 repetitions performed in sequence and interspersed by 2 min of recovery. The participants were instructed to perform all jumps at maximal intensity. They were also encouraged to amplify their knee flexion during the landing phase to reach an approximate angle of 90° and to maximize the eccentric component imposed on the knee extensors. To minimize the risk of injury, the number of jumps was progressively increased during the first 4 wk, and the heavier exercises were introduced progressively (Table 1). All training sessions were supervised by one of the investigators or by an experienced coworker.

Table 1
Table 1
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Evaluation of leg strength and power.

A series of functional tests was performed before and after the plyometric training program to evaluate the changes in knee-extension force, velocity, and jumping performances. One-repetition maximal force (1RM) of leg extensors (two-leg press), vertical jump height (SJ and CMJ), and time to perform a 6 × 5-m shuttle run test were measured 1 wk before the first training session and 4 d after the last training session.

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Muscle biopsies.

The needle biopsy technique with suction was used to obtain muscle samples from the vastus lateralis muscle of the right leg. A pretraining sample was obtained 3 d after the pretraining functional test session to minimize the risk of studying damaged fibers. Similarly, a posttraining muscle sample was obtained 3 d after the functional test session following the training period. Muscle samples were immediately bathed in a skinning solution (see below) at 0°C and divided into small bundles of fibers on ice. The bundles were stored in skinning solution at 4°C for 1 h and then transferred to fresh skinning solution and stored at −20°C for at least 5 d before the first experiment.

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Solutions.

The skinning solution contained (mM): 125 propionic acid, 2.0 EGTA, 1 MgCl2, 4.0 ATP, 20 imidazol (pH 7.0), 50% (v/v) glycerol, and protease inhibitors: 0.5 mM PMSF and 20 μg·mL−1 leupeptin. The composition of the relaxing and the activating solutions were based on calculations using an iterative computer program described by Fabiato and Fabiato (4) with apparent stability constants adjusted for temperature, pH, and ionic strength. Both solutions contained (mM): 7.0 EGTA, 20 imidazol, 14.5 creatine phosphate, 1.0 free Mg2+, and 4.0 MgATP. The free Ca2+ concentration in the relaxing and the maximally activating solutions was pCa 9.0 and pCa 4.5 (pCa = −log [Ca2+]), respectively. In both solutions, pH was adjusted to 7.0 with KOH and total ionic strength to 180 mM with KCl. Submaximal activating solutions were prepared by mixing appropriate volumes of activating and relaxing solutions to obtain a series of different free Ca2+ concentrations ranging from pCa 5.0 to pCa 6.6 (25).

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Single-fiber preparation.

Fiber bundles were stored at −20°C in the skinning solution for a maximum of 6 wk after the biopsy. On the day of an experiment, a bundle was transferred into relaxing solution on ice, and a single-fiber segment (approximate length of 3.5 mm) was carefully pulled free from the end of the bundle using a stereomicroscope with a maximal magnification of 90× (1626-Z60, Euromex, Germany). The fiber preparation was attached to an isometric force-measuring set-up via two aluminum T-clips folded over its extremities. The output signal from the force transducer (sensitivity 29.41 mV·mN−1) was amplified and digitized at a rate of 200 Hz using a Lab-PC + 12-bit DAQ board (National Instrument, Austin, TX). Collected data were stored on a personal computer and analyzed offline using custom-made software (LabView; National Instruments, Austin, TX). Once mounted on the set-up, the fiber was stretched to a sarcomere length of 2.5 μm, as evaluated by laser diffraction. This length corresponds to the beginning of the plateau of the length-tension curve in human muscle fibers (19). During the entire experiment, fiber and solutions were maintained at a constant temperature of 15°C by a Peltier element fitted underneath the experimental wells.

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Protocol for fiber-force measurements.

The isometric force-measuring set-up was arranged in such a way that the fiber segment could be rapidly transferred between the wells of a small Teflon plate containing either relaxing, maximally activating, or submaximally activating Ca2+ solutions. First, baseline force was recorded for at least 1 min while the fiber was in a relaxing solution of pCa 9.0. The fiber was then successively activated in a Ca2+ solution and subsequently transferred back into the relaxing solution. The fiber was maintained in the activating solution until the developed force showed a plateau. The first three contractions were performed in a pCa 4.5 solution to measure the maximal Ca2+-activated force, P0. Throughout the experiment, the fibers were maximally activated with the Ca2+-saturated solution (Fig. 1A). The force developed at submaximal activating levels (Pr) was expressed relative to the preceding and following P0 recorded at pCa 4.5. Sequential fiber activations were administered every 1.5 min.

FIGURE 1-Representat...
FIGURE 1-Representat...
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Measurement of single-fiber calcium sensitivity.

Using an iterative nonlinear curve-fitting procedure (Marquardt-Levenberg algorithm), the force-calcium relationship was evaluated for each fiber based on the Hill equation: Pr = pCan/(pCa50% n + pCan), where pCa50% is the Ca2+ concentration at which half-maximal activation occurs, and n is the Hill coefficient, an indicator of the slope of the relationship (Fig. 1B). Separate Hill plots were constructed by plotting log[Pr/(1 − Pr)] against pCa, and the Hill plot coefficients, n1 and n2, were calculated as the slope of the least-square regression lines fitted to points above and below half-maximal activation, respectively (Fig. 1C). The Ca2+-activation threshold was calculated as the pCa for which log[Pr/(1 − Pr)] = 2.5 using data points below half-maximal activation (25).

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Single-fiber diameter.

A separate pool of fibers than those tested for Ca2+ sensitivity were used to evaluate single-fiber diameter, because this measurement could not be performed on the isometric force-measuring set-up. The fiber was fixed between two connectors placed over the stage of an inverted microscope (Axiovert 25C, Zeiss, Germany) and viewed with a magnification of 400× while resting in relaxing solution. Sarcomere length was adjusted to 2.5 μm by use of a calibrated eyepiece micrometer. Subsequently, the fiber was photographed with a digital camera (Camedia C3020 Z, Olympus) while being suspended in air for about 5 s. Fiber diameter was defined as the average fiber width determined from the calibrated picture on three locations along the fiber segment.

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Fiber MHC isoform determination.

Single muscle fibers were classified according to their MHC isoform contents, as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) (1). After completion of the Ca2+ sensitivity test, the fiber was removed from the ergometer, dissolved in 25 μL of SDS sample buffer [160 mM Tris, pH 6.8, 4% SDS, 2 mg·mL−1 bromophenol blue, 20% glycerol, leupeptin 30 μg·mL−1, and 1% β-mercaptoethanol] and stored at −20°C before further processing. Later, the aliquot was heated at 95°C for 3 min, and 4-μL samples of this extract were run on a polyacrylamide gel using a Mini-protean 3 cell system (Bio-Rad, Hercules, CA). Total acrylamide concentrations were 4 and 8% in the stacking and the separating gel, respectively (acrylamide: bis-acrylamide, 50:1). Gels were run at a constant voltage of 140 V for 17 h at 4°C and subsequently silver stained (Silver Stain Plus kit, Bio-Rad, Hercules, CA). A sample of homogenized human skeletal muscle was used as the standard for the identification of MHC isoform bands (Fig. 2). The MHC expression was determined on each fiber segment used for the mechanical tests as well as on about 140 fibers per biopsy to determine the MHC profile in each subject.

FIGURE 2-Example of ...
FIGURE 2-Example of ...
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Fiber TnT isoform determination.

After MHC isoform identification of single fibers, 15 μL of each extract was used to determine TnT isoforms using SDS-PAGE, as described above, followed by Western blotting (Fig. 3). Total acrylamide concentrations were 5 and 15% in the stacking and the separating gel, respectively (acrylamide: bis-acrylamide, 37.5:1). Gels were run at 40 mA for 2 h at 4°C. Next, the proteins were electrotransferred to a PVDF membrane for 2 h at 80 V. The membranes were blocked for 3 h with a phosphate-buffered saline solution (PBS, pH 8) containing 5% dry milk, then incubated overnight in PBS containing 1% dry milk and a polyclonal antibody directed against slow TnT isoforms (sTnT) of human skeletal muscle (C19, Santa Cruz Biotechnology, Santa Cruz, CA) diluted 1:600. After several washes with PBS, the membranes were incubated in the presence of a peroxidase-conjugated secondary antibody (anti-goat IgG, Calbiochem, San Diego, CA), and the marked proteins were detected by enhanced chemiluminescence (ECL + Plus kit, Amersham Biosciences, Uppsala, Sweden) using hyperfilms. Subsequently the membranes were incubated at 50°C for 30 min in a stripping buffer solution (1 M Tris, 10% SDS, 0.8% 2-mercaptoethanol, pH 6.8) to remove all antibodies and washed twice for 2 h with PBS. The identification of the fast TnT isoform (fTnT) was then performed as described above using a polyclonal antibody directed against fast human skeletal muscle TnT isoforms (C18, Santa Cruz Biotechnology, Santa Cruz, CA) diluted 1:600.

FIGURE 3-TnT isoform...
FIGURE 3-TnT isoform...
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Statistical analyses.

Results are presented as means ± SE. One-way analyses of variance (ANOVA) for repeated measures were used to determine the effect of plyometric training on functional test results and MHC profiles. Two-way ANOVA, with "subject" and "fiber type" as main factors, were used to evaluate differences between fiber types (according to MHC isoforms) in pretraining samples. Pre- and posttraining results were compared using a two-way ANOVA, with "subject" and "training status" as the main factors. That is, single fibers were considered as individual observations while taking into consideration the intersubject variability. Tukey's post hoc tests were used to identify individual differences. Because of lack of data for type I/IIa and IIx fibers, no statistical analysis was performed to evaluate the training effect in these fibers. Statistical significance was accepted at P < 0.05.

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RESULTS

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Functional performance.

All participants engaged in the experiment completed the full set of plyometric exercise sessions of the training program. Individual and mean performances during the pre- and posttraining functional tests are presented in Figure 4. Average vertical jump performances were increased for the SJ by 0.036 ± 0.010 m (P < 0.01) and for the CMJ by 0.055 ± 0.007 m (P < 0.001). The time during the 6 × 5-m shuttle run test was decreased by 0.36 ± 0.15 s (P < 0.05), and 1RM strength developed at the leg press was increased by 25 ± 6 kg (P < 0.01).

FIGURE 4-Individual ...
FIGURE 4-Individual ...
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MHC isoform composition of pre- and posttraining fibers.

MHC isoform profiles of pre- and posttraining fibers are shown in Figure 5. Fiber type was determined on 1162 pre- and 1050 posttraining fibers from the vastus lateralis muscle of the eight participants. The vast majority of muscle fibers analyzed contained type I, IIa, or IIa/IIx MHC in pre- and posttraining samples (90 and 93% of all fibers analyzed, respectively). After plyometric training, the percentage of type IIa fibers was increased from 33 ± 5 to 42 ± 4% (P = 0.038), whereas there was a trend for a decreased percentage of type IIx fibers (from 7 ± 3 to 2 ± 2%, P = 0.054). There was also a tendency for an increased percentage of hybrid type I/IIa fibers from 2 ± 1 to 5 ± 1% (P = 0.050).

FIGURE 5-Effect of a...
FIGURE 5-Effect of a...
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Single muscle-fiber diameter and P0.

Fiber diameter was determined on a total of 207 pre- and 163 posttraining fibers, of which 111 expressed type I, 152 expressed type IIa, and 77 expressed IIa/IIx MHC. Fiber diameter was significantly bigger in pretraining type IIa fibers compared with type I and type IIa/IIx fibers, the latter two not being significantly different from each other. Accordingly, P0 of type IIa fibers was significantly higher than P0 of type I and type IIa/IIx fibers, which had similar values themselves. After plyometric exercise training, the mean diameters of type I, IIa, and IIa/IIx fibers were significantly increased from 76.9 ± 1.7, 90.4 ± 1.2, and 82.7 ± 1.9 μm to 85.3 ± 2.0, 99.8 ± 1.3, and 95.1 ± 1.8 μm, respectively (Fig. 6A). At the same time, P0 rose from 0.80 ± 0.04, 0.95 ± 0.03, and 0.75 ± 0.05 mN to 1.08 ± 0.05, 1.19 ± 0.03, and 1.18 ± 0.07 for type I, IIa, and IIa/IIx fibers, respectively (Fig. 6B). The total number of fibers analyzed is indicated in Table 2.

FIGURE 6-Single-fibe...
FIGURE 6-Single-fibe...
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Table 2
Table 2
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Force/pCa relationships.

The results obtained for pCa50%, activation threshold, and Hill plot coefficients n1 and n2 are given in Table 2. Before training, type I fibers needed significantly lower free Ca2+ concentrations than type IIa and type IIa/IIx fibers to develop 50% of P0. Type I fibers also required a significantly smaller free Ca2+ concentration than type IIa fibers to reach the activation threshold. The force/pCa relationship was affected by plyometric training in type I, IIa, and IIa/IIx fibers, but the responses were slightly different. Ca2+ sensitivity of type I fibers was enhanced, as reflected by the increased pCa50%. Type IIa and hybrid IIa/IIx fibers showed a similar tendency but without reaching the threshold of statistical significance. No changes were observed in the Ca2+-activation threshold for any fiber type. Although the Hill coefficient n1 decreased in type IIa fibers, the coefficient n2 showed a general trend to increase, with significant changes in type I and IIa/IIx fibers after plyometric training.

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TnT isoform composition.

TnT isoform compositions of single fibers expressing the different types of MHC were determined on a total of 92 pre- and 143 posttraining samples. Table 3 presents the pooled data from pre- and posttraining fibers. Perfect correspondence between TnT and MHC isoform expression is observed within the same fiber segment. sTnT was detected only in fibers containing also type I MHC isoforms, that is, type I and hybrid type I/IIa fibers, independent of the subjects' training status. Similarly, plyometric training did not alter the systematic association of fTnT and type II MHC isoform expression within the same fiber.

Table 3
Table 3
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DISCUSSION

Previous results strongly suggest that performance enhancements following plyometric training can be largely attributed to altered muscle-activation patterns (3,10,13). However, these studies have not always provided in-depth analyses of the modifications induced at the level of the contractile apparatus of the trained muscles. The data presented here suggest that plyometric training can have a considerable influence on the contractile performance of the muscle via fiber-type expression, as well as changes in fiber diameter, force, and force/pCa relationships of the myofilaments. After plyometric training, the proportion of type IIa fibers was increased, as previously observed after resistance training (24). Fiber force was improved for all fiber types, thus confirming previous data (15), probably as a result of increased fiber diameter (24). Our main hypothesis, that plyometric training increases the Ca2+ sensitivity of single muscle fibers, was partly confirmed. The Ca2+ concentration needed to elicit half-maximal activation showed a general decreasing trend, with significant changes found only in type I fibers. The cooperativity-activation process at low Ca2+ concentrations was improved in type I and IIa/IIx fibers, as reflected in the increase of the Hill coefficient n2, with a similar tendency in type IIa fibers. However, the activation threshold for force development was not changed in any fiber type.

Plyometric training as performed in the present study is characterized by high-velocity eccentric muscle contractions, followed immediately by rapid and powerful concentric contractions. Considering these characteristics, it was assumed that fast-twitch fibers expressing type II MHC would be reactive to such training. This, however, was not observed, because most of the effects were found in type I fibers. Similar results were found by Godard et al. (9), who studied resistance training in elderly women. They observed significant increases in pCa50%, Ca2+-activation threshold, and Hill coefficient n2, but only in type I fibers. It should be noted, however, that their sample size of type IIa fibers was small, which may explain their nonsignificant results. From these results, it seems possible that fibers expressing type I MHC are more responsive to training stimuli. This feature is also illustrated in the results obtained from unloading models where Ca2+ sensitivity of slow-twitch fibers was largely affected, whereas no significant changes were observed in fast-twitch fibers (5,7). It can be speculated that type I fibers have a greater functional plasticity than type II fibers in response to training or unloading.

As already reported by others (25), the Ca2+ sensitivity of fibers expressing type I MHC was higher in our pretraining samples than those expressing type II MHC. Ca2+ sensitivity during skeletal muscle activation is determined by the complex formed by three Tn subunits: inhibitory TnI, Ca2+-binding TnC, and tropomyosin-binding TnT. Ca2+ binding to TnC triggers a series of interactions between the Tn subunits, tropomyosin and actin, resulting in actomyosin interaction and contraction. The sensitivity of a fiber to Ca2+ has been shown to depend on the type of TnC isoform present in the fiber segment (17). The fast TnC isoform has two Ca2+ binding sites and confers to the fiber a lower sensitivity to Ca2+, as opposed to the slow isoform, which has only one binding site. Ca2+ sensitivity of a given fiber could thus be affected by the specific TnC isoform expressed within that cell (8). However, TnC isoforms do not fully account for the differences in the Ca2+-activation properties of single fibers (8). Potter et al. (18) have suggested a direct regulatory role for TnT, which could (via different isoform expression) be involved in the regulation of Ca2+-activated force development (20). When Ca2+ binds to the Ca2+-specific regulatory site of TnC, TnI dissociates from the actin filament, thus neutralizing its inhibitory activity. However, Ca2+-dependent activation of the actomyosin ATPase was found to be possible only in the presence of TnT. Therefore, they (18) concluded that TnT plays a direct role in Ca2+ regulation of the actomyosin ATPase activity.

Bastide et al. (2) studied single chemically skinned fibers from rat soleus and tibialis anterior muscles and classified their samples according to TnT isoforms (slow, fast, or both). They demonstrated that fibers expressing only the fast TnT had a lower sensitivity to Ca2+ than those expressing only the slow protein, with intermediate values for hybrid fibers. Furthermore, they showed that 2 wk of hindlimb unloading induced a rapid shift in the expression of TnT isoform expression. Based on the aforementioned observations, we speculated that changes in Ca2+ sensitivity following plyometric training could originate from differential TnT isoform expression. However, our results revealed that for both pre- and posttraining samples, coexpression of sTnT and fTnT was never detected in any fibers expressing exclusively type I or type II MHC. The slow form of TnT was always associated with MHC I expression, and, similarly, the fast TnT was only present in type II fibers, with coexpression being found in hybrid type I/IIa fibers (Table 3). Conversely, Kischel et al. (12) showed that slow TnT isoforms (low molecular weight) were expressed in fast rat muscles (tibialis anterior and extensor digitorum longus) known to be composed almost exclusively of type II MHC. Considering the findings of Bastide et al. (2) and Kischel et al. (12), it would seem that coexpression of slow and fast TnT isoforms is likely in rat muscle fibers, which may not be the case for human fibers. Similar results to ours have been reported previously on fibers from rat diaphragm (8). Therefore, although differences between fiber types in terms of Ca2+ sensitivity could indeed be attributed to TnT isoform expression, the changes in Ca2+ sensitivity observed following plyometric training within a given fiber type cannot be explained on the basis of differential fast or slow TnT expression. Deeper analysis of the changes of the different slow or fast TnT isoforms might reveal plausible explanations to the shift in Ca2+ sensitivity after training. For example, it is possible that a shift in the sTnT isoforms could have caused the change in pCa/force relationship of type I fibers. Moreover, other proteins of the contractile apparatus may play an important role in determining Ca2+-activation properties, such as TnC, TnI, and tropomyosin (8). A combined analysis of these protein isoforms following changes in muscle loading is necessary to provide a better picture of the determinants of fiber-activation characteristics.

The present data demonstrate that short-term plyometric training can have a considerable impact on the mechanical functioning of single muscle fibers, thus complementing earlier work, which illustrates the neurological origin of performance enhancements. Of special consideration is the training protocol used in this investigation. Because the participants had never followed such a program, training intensity and volume were increased progressively to avoid injury (Table 1). Special focus was placed on the knee-extension velocity during the executions, using only maximal effort jumps with no overweight. Although the training period was limited to 8 wk, the fact that we used three weekly sessions may have been more favorable to induce specific adaptations of the muscle fibers, even though our subjects maintained their usual physical activities (3.1 ± 0.4 h). Prior studies have not analyzed the impact of such training at the single-fiber level and have, at best, investigated the changes induced in the muscle structure. Furthermore, comparison with earlier work is difficult because of the different purposes pursued leading to various execution modalities. Kyröläinen et al. (13) tested a plyometric training protocol of 15 wk, loading mainly the triceps surae muscle group and finding that maximal voluntary contraction was enhanced in plantar flexors, but not in the knee extensors. The distribution and size of fibers from the gastrocnemius muscle were not modified. Although their results may seem contradictory to the findings of the present study, it should be noted that their protocol included only two weekly sessions with a lower exercise volume (80-180 actions per training vs 75-350 here) and involving participants who were already more active (±6 h·wk−1 in their study vs ± 3 h·wk−1 here). Using a 6-wk training protocol similar to ours, Spurrs et al. (21) found that running economy in trained long-distance runners was significantly improved. They also reported performance enhancements in the CMJ test similar to our results, as well as increases in the maximal isometric force of the triceps surae muscle. Unfortunately, no force analyses were performed on knee-extensor muscles.

Plyometric training has the potential of improving the efficiency and performance of human movements in many aspects. This investigation provides new explanations on the mechanisms involved in the training enhancements. Our results suggest adaptations in fiber-type expression, as well as structural and functional enhancements at the single-fiber level, thus confirming our previous observations (15). Specifically, plyometric training changed the force/pCa relationship of single muscle fibers, but increased Ca2+ sensitivity was not associated with differential slow or fast TnT isoform expression.

This study was supported by a grant (to D. Theisen) from the Fonds Spéciaux de Recherche, Université catholique de Louvain, and the Fonds de la Recherche Scientifique Médicale, Belgique (Convention No 3.4547.04).

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Keywords:

STRETCH-SHORTENING CYCLE; SKINNED FIBERS; FIBER HYPERTROPHY; TROPONIN T ISOFORMS

©2006The American College of Sports Medicine

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