Studies that have investigated the relationship between performance and muscle fiber type composition (i.e., determined by either myosin ATPase histochemistry or myosin heavy chain [MHC]) analyses) have reported mixed results. Some of the most commonly examined performance variables include maximum isometric strength (23); rate of force development during an electrically stimulated isometric twitch (12); isokinetic peak torque (8,21,22); percent decline in isokinetic peak torque during repeated maximal muscle actions (29); and measures of aerobic performance, including the speed corresponding to the onset of blood lactate accumulation during treadmill running, the average speed achieved during a marathon race (13), and cycling efficiency during a submaximal cycle ergometer test (17). Generally speaking, these investigations (8,13,17,21-23,29) have reported moderate to high (i.e., r = 0.5-0.9) correlations between muscle fiber type composition and athletic performance.
Several studies, however, have also examined the potential influence of differences in muscle fiber type composition on the amplitude and/or frequency characteristics of surface electromyographic (EMG) and/or mechanomyographic (MMG) signals (2,3,10,11,14,20). For example, our laboratory (3) recently investigated the patterns of responses for EMG and MMG amplitude and mean power frequency (MPF) for the vastus lateralis during a sustained 30-second isometric muscle action of the knee extensors at 50% of the maximum voluntary contraction (MVC) in resistance-trained and aerobically trained athletes. Muscle biopsies from the vastus lateralis showed that the resistance-trained athletes expressed primarily the MHC Type II isoform, whereas the aerobically trained athletes expressed mostly the MHC Type I isoform (3). It is interesting to note that both the resistance-trained and aerobically trained athletes demonstrated significant increases in EMG amplitude and decreases in EMG MPF during the sustained muscle action, and there were no differences between the 2 groups of athletes for mean absolute EMG amplitude and MPF values at all time points during the sustained muscle action. The resistance-trained athletes, however, demonstrated significantly greater mean absolute MMG amplitude and MPF values than the aerobically trained athletes at all time points during the sustained muscle action (3). The differences between the resistance-trained and aerobically trained athletes for MMG amplitude and MPF responses (3) suggested that 1 or more of the parameters from the MMG signal could be useful for examining differences in muscle fiber type composition. Thus, the purpose of the present study was to reanalyze the data from our previous investigation (3) to determine if the combination of isometric knee extension strength and MMG median frequency (MDF) could be used to explain a significant proportion of the variance in % MHC Type II isoform content of the vastus lateralis in resistance-trained and aerobically trained athletes.
Experimental Approach to the Problem
Two independent groups of subjects (i.e., a resistance-trained group and an aerobically trained group) were tested in this study to determine if the combination of isometric knee extension strength and MMG MDF could be used to predict the % MHC Type II isoform content of the vastus lateralis. Each subject performed isometric muscle actions of the knee extensors at progressively higher torque levels as MMG signals were detected from the vastus lateralis. Immediately after recording the MMG signals, a muscle biopsy was taken from the vastus lateralis and analyzed for MHC isoform content. Absolute MMG MDF and isometric knee extension strength were then entered individually, and in combination, into linear regression models to predict the % MHC Type II isoform content.
Five resistance-trained (mean ± SD age = 23.2 ± 3.7 years; body mass = 101.9 ± 37.9 kg; height = 176.3 ± 8.2 cm) and 5 aerobically trained (mean ± SD age = 32.6 ± 5.2 years; body mass = 67.5 ± 4.2 kg; height = 176.0 ± 2.5 cm) men volunteered to participate in this study. All resistance-trained subjects could perform a 1 repetition maximum (1RM) back squat with at least twice their body mass, and the mean ± SD 10-km running time for the aerobically trained subjects was 33.9 ± 0.9 minutes. The investigation was approved by the University Institutional Review Board for Human Subjects, and all subjects completed a health history questionnaire and signed a written informed consent document before testing.
Each subject performed isometric muscle actions of the dominant knee extensors (based on self-reported kicking preference) on a modified knee extension machine (York Barbell Company, York, Pennsylvania, U.S.A.). The cable length of the machine was adjusted to allow a knee joint angle of 90 degrees between the thigh and leg. Prior to the isometric testing, the subjects performed a warm-up of five 6-second isometric muscle actions at approximately 50% of their estimated MVC. Following the warm-up trials and 2 minutes of rest, the subjects performed 2 maximal, 6-second isometric muscle actions of the knee extensors. The highest torque output from the 2 maximal muscle actions was selected as the MVC value.
A tension-compression load cell (MLP-500, Transducer Techniques, Temecula, California, U.S.A.) attached in-line with the cable and frame of the knee extension machine was used to detect the knee extension force generated by each subject. The analog force signal was amplified with a signal conditioner (TMO-2, Transducer Techniques) and sent to a BNC connector board (BNC 2080, National Instruments, Austin, Texas, U.S.A.) interfaced with a 12-bit analog-to-digital converter (AT-MIO-16E-10; National Instruments). The force signal was then digitized at 1,000 Hz and stored on a personal computer.
The MMG signal from the vastus lateralis was detected with an accelerometer (Entran EGAS FT 10, bandwidth 0-200 Hz, dimensions: 1.0 × 1.0 × 0.5 cm, mass 1.0 g, sensitivity 10 mV/g) placed over the lateral portion of the muscle at approximately the midpoint between the greater trochanter and lateral condyle of the femur (6). The accelerometer was fixed to the skin using double-sided adhesive tape, and an in-line amplifier (gain: ×200) was used to amplify the MMG signal before digitization.
The raw MMG signal was digitized at 1,000 Hz and stored in a personal computer for subsequent analysis. All signal processing was performed using custom programs written with LabVIEW programming software (version 7.1, National Instruments). The MMG signal was bandpass filtered (fourth-order Butterworth) at 5 to 100 Hz and selected for a 2-second time period corresponding to the middle 33% of the 6-second MVC. The median frequency (MDF) of the MMG data segment was calculated by processing the signal with a Hamming window and the discrete Fourier transform (DFT) algorithm.
A muscle biopsy (80-160 mg) was taken from the vastus lateralis using the percutaneous needle biopsy methods of Bergström (4). The muscle sample was taken from the exact location where the accelerometer was placed. After careful cleaning of the sample site, a local anesthetic (2% lidocaine) was injected cutaneously, and a small (approximately 1 cm) incision was made through the skin and deep fascia with a No. 11 scalpel. The biopsy sample was taken with U.C.H. needles (Popper and Sons, New Hyde Park, New York, U.S.A.) using the double-chop method (24,27) and suction (7). After the biopsy was taken, the incision was closed with sterile strips, and a pressure bandage was placed over the incision site. The subjects were required to return to the laboratory 24 to 48 hours after the biopsy procedures to ensure that the incision was healing properly.
All biopsy samples were oriented in tragacanth gum so that the fibers were parallel and on end. The samples were then flash frozen in isopentane cooled to −160°C with liquid nitrogen immersion and stored at −80°C. The MHC isoform content was analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) (5,19) with modifications for single human muscle fibers (25). Then, 10 to 12 serial sections 12-μm thick were taken from each biopsy sample in a cryostat at −20°C. The sections were then lysed by placing them in 0.5 mL of a buffer containing 10% (wt/vol) glycerol, 5% (vol/vol) β-mercaptoethanol, and 2.3% (wt/vol) sodium dodecyl sulfate (SDS) in 62.5 mmol·L−1 Tris hydrochloride buffer (pH 6.8) and heated in a dry bath at 60°C for 10 minutes. To determine MHC expression, small amounts of the extracts (3-5 μL) were loaded on 4 to 8% gradient SDS polyacrylamide gels with 4% stacking gels (1), run overnight (19-21 hours) at 120 V, and stained with Coomassie blue. The MHC isoforms (Type I, IIa, and IIx [also known as IIb]) were identified according to their molecular masses as compared with those of marker proteins and migration patterns from single fiber analyses (25,26).
Absolute MMG MDF (Hz) and isometric knee extension strength (N) values were calculated for each subject during the MVC. Knee extension strength was calculated as the average force value during the middle 33% (i.e., 2.0 seconds) of the 6-second MVC. This portion of the force signal was selected to correspond with that from the MMG signal. Absolute MMG MDF and isometric knee extension strength were then entered individually, and in combination, into linear regression models to examine the relationships with the % MHC Type II isoform content. An alpha of 0.05 was used to determine statistical significance for each regression model. Previous test-retest reliability from our laboratory for 8 male subjects measured 48 hours apart indicated the range of intraclass correlation coefficients (R) for MMG MDF to be from 0.72 to 0.82. In addition, Fry et al. (9) reported that the SDS-PAGE technique for determining MHC isoform composition (i.e., the same procedures used in the present study) provided results that were highly correlated (r = 0.80-0.93) with the percentage fiber type area.
Table 1 shows the mean ± standard error of the mean (SEM) values for % MHC Type I, IIa, and IIx isoform contents and the isometric knee extension strength and MMG MDF values for the resistance-trained and aerobically trained subjects. The % MHC Type IIx isoform was nearly 0% for both the resistance-trained and aerobically trained subjects. Thus, the % MHC Type IIx and IIa isoforms were summed to calculate the % MHC Type II isoform content. Table 2 shows the results from the linear regression analyses used to examine the relationships among % MHC Type II isoform content, isometric knee extension strength, and MMG MDF. Regression models 1 and 2 examined the associations among isometric knee extension strength or MMG MDF and % MHC Type II isoform content, respectively. Regression model 3 shows the relationship between % MHC Type II isoform content and the combination of isometric knee extension strength and MMG MDF. The results showed that regression models 1 and 2 were not significant (p > 0.05), but regression model 3 was significant (p < 0.05). The zero-order correlation between isometric knee extension strength and MMG MDF was not significant (p > 0.05), with an r-value of 0.079. The equation that describes the association between the % MHC Type II isoform content and the combination of isometric knee extension strength and MMG MDF follows:
% MHC Type II isoform content = 0.023(isometric knee extension strength in Newtons) + 0.887(MMG MDF in Hz) - 9.512
This equation had a multiple correlation of R = 0.773 and a SEE of 15.4% of MHC Type II isoform content. Figure 1 shows the relationship between the actual and estimated (i.e., from the equation shown above) % MHC Type II isoform contents.
The primary finding from the present study was that neither isometric knee extension strength nor MMG MDF alone was significantly correlated with the % MHC Type II isoform content of the vastus lateralis. In combination, however, these 2 variables explained a significant proportion of the variance in % MHC Type II isoform content. Furthermore, the standardized regression coefficients for regression model 3 in Table 2 indicated that isometric knee extension strength and MMG MDF contributed approximately equally to explaining the variance in % MHC Type II isoform expression.
Previous investigations have reported mixed results regarding the association between measures of isokinetic performance and muscle fiber type composition. For example, Thorstensson and Karlsson (29) found that during 50 consecutive maximal concentric isokinetic muscle actions of the knee extensors at a velocity of 180 degrees/s−1, the percent decline in peak torque was positively correlated (r = 0.86) with the percentage of fast-twitch muscle fibers in the vastus lateralis. An additional study (28) showed that the percentage of fast-twitch muscle fibers in the vastus lateralis was positively correlated with maximal isokinetic knee extension peak torque at 180 degrees/s−1 (r = 0.50) and the maximal velocity achieved during an unloaded knee extension (r = 0.50). Similar results were reported by Ryushi and Fukunaga (21), who found that there were positive correlations between the percentage of fast-twitch oxidative glycolytic muscle fibers in the vastus lateralis and peak torque during maximal isokinetic knee extensions at 30, 60, 120, and 180 degrees/s−1. In contrast, Froese and Houston (8) reported that there were no relationships between the percentage of fast-twitch muscle fibers in the vastus lateralis and peak torque during maximal isokinetic knee extensions at 45, 90, 135, 180, 225, and 270 degrees/s−1. These results were similar to those from the present study because there was no significant relationship between isometric knee extension strength and the % MHC Type II isoform content in the vastus lateralis (Table 2). Table 2 also shows, however, that there was no significant relationship between MMG MDF and the % MHC Type II isoform content in the vastus lateralis. Thus, the findings from the present study indicated that measurement of isometric strength or MMG MDF alone was not sufficient to identify differences in MHC isoform expression.
Although no previous investigations have correlated the percentage of Type II muscle fibers with the frequency of the MMG signal, several studies have examined the MMG frequency parameters of muscles that demonstrate differences in fiber type composition (15,16,18). For example, Mealing et al. (15) reported that during submaximal isometric muscle actions of the plantar flexors and forearm flexors at 50% MVC, the MMG power spectrum for the biceps brachii was bimodal, with 2 distinct spikes in power at approximately 6 and 13 Hz. The MMG power spectrum for the soleus muscle, however, was unimodal, with a single spike in power at approximately 6 Hz (15). Mealing and McCarthy (16) reported similar results during isometric muscle actions of the soleus and orbicularis oris muscles. Specifically, the average peak frequency of the MMG signal for the soleus was 10.8 Hz, whereas that for the orbicularis oris was 22 Hz (16). Thus, it was suggested (16) that the shape of the MMG power spectrum may be related to muscle fiber type composition and/or average motor unit firing rates. In addition, Orizio and Veicsteinas (18) found that during a sustained isometric MVC of the knee extensors, MMG MPF for the vastus lateralis decreased rapidly for sprinters from 0 to 65% of the total contraction time, followed by a slower reduction from 65% to 100% of the total contraction time. For long distance runners, however, MMG MPF decreased rapidly from 0 to 35% of the total contraction time, followed by a slower reduction from 35 to 100% of the total contraction time (18). In addition, at the same time point during the sustained muscle action, the mean MMG MPF value for the sprinters was always higher than that for the long distance runners (18). Thus, it was suggested (18) that the unique patterns of responses for MMG MPF between the sprinters and long distance runners likely reflected differences in muscle fiber type composition.
Unlike these previous studies (15,16,18), the present investigation showed that there was no significant univariate relationship between the % MHC Type II isoform content and MMG MDF during an isometric MVC. It is important to note, however, that the combination of isometric knee extension strength and MMG MDF was significantly related to the % MHC Type II isoform content. Furthermore, the standardized regression coefficients shown in Table 2 indicated that isometric knee extension strength and MMG MDF contributed approximately equally to the multiple regression model. Thus, these findings indicated that the simultaneous measurement of isometric knee extension strength and MMG MDF could provide a simple, time-efficient, and noninvasive method for estimating the % MHC Type II isoform content.
In summary, the results from the present study showed that there was no significant relationship between the % MHC Type II isoform content of the vastus lateralis and isometric knee extension strength or MMG MDF. In combination, however, isometric knee extension strength and MMG MDF explained a significant proportion (i.e., 59.8%) of the variance in the % MHC Type II isoform content. Thus, it is possible that a simple, time-efficient, and noninvasive test that simultaneously measures isometric strength and MMG MDF could be used to estimate the % MHC Type II isoform content. Future investigations need to be performed, however, to determine if this test can be used with other muscles.
The results from the present study suggested that the combination of isometric strength and a frequency parameter from the MMG signal could be useful for predicting the % MHC Type II isoform content of the vastus lateralis. These findings were similar to those from our previous investigation (3) that found different MMG amplitude and MPF responses for resistance-trained and aerobically trained subjects during a fatiguing isometric muscle action. It is important to interpret these results with caution, however, because the data were from trained individuals and only 1 muscle. They may be useful, however, for coaches and/or physiatrists who are interested in noninvasive techniques for estimating muscle fiber type composition. Future investigations are needed to determine if the combination of isometric strength and a frequency parameter from the MMG signal can be used to predict the % MHC Type II isoform content in trained individuals and other muscles.
This study was funded by a research grant from The University of Memphis-FedEx Institute of Technology.
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Keywords:© 2009 National Strength and Conditioning Association
muscle fiber type; mechanomyography; force