To assess radial muscle belly displacement in each muscle, a digital displacement transducer (GK 40, Panoptik d.o.o., Ljubljana, Slovenia), which incorporates a constant spring of 0.17 N·mm−1 (7), was set perpendicular to the thickest part of the muscle belly. The sensor's position was determined individually for each muscle because of the anatomic differences observed in the athletes, a characteristic already pointed out (32,34). The thickest part of the muscle belly was established visually and through palpation of the muscle during a voluntary contraction, after the anatomic indications (9). Once the exact point was found, it was marked with a dermatological pen. The self-adhesive electrodes (5 × 5 cm, 2 mm·h−1. Conlin Medical Supply Co., Ltd., China) were placed symmetrically approximately 5 cm away from the sensor, the positive electrode in the proximal area of the muscle above the measurement point and the negative electrode in the distal area below the measurement point. The maximum muscle belly displacement point was checked in all measurements through the calculation of each muscle's characteristic optimal curve and using some low-intensity measurements, where the sensor is moved slightly within a determined area to ascertain exactly the maximum muscle belly displacement point. The electrostimulator used was TMG-S2 (EMF-FURLAN & Co. d.o.o., Ljubljana, Slovenia).
Each cyclist participated in 2 measurements. The first one was carried out in the first week of December, within the Season's preparation period (PP) of training when the athletes had already cycled an average of 1,150 ± 279 km (range of 700–1,600 km) and had not yet entered any competition. The training was focused on the development of strength resistance through moderate intensity and the development of general aerobic capacity through running, mountain biking, and swimming. The volume of kilometers per week in the training cycle before the assessment was about 550 km with an intensity higher than VT1 and below VT2. The second one, 7 months later, was carried out in the month of July, 5 weeks before the beginning of the “Vuelta a España” competition, within the Season's competition period (CP), when the average cycled distance had been 20,600 ± 2,319 km (range of 18,000–24,000 km). The volume of kilometers per week was similar to the previous assessment period, around 600 km, but the intensity was much higher, with several days of weekly training over VT2 or near V[Combining Dot Above]O2.
Both measurements took place during a recovery microcycle (5 days), specifically in the active rest day that each cyclist had planned, with a separation of at least 60 hours between the assessment and the last training load that they had made in the previous microcycle. All the cyclists were in a healthy state.
In both measurements, the TMG test was carried out by using an electrical stimulus whose duration was 1 millisecond and whose intensity was raised in increments of 10 from 30 mA until total intensity was reached at 110 mA (maximal stimulator output). For each cyclist, out of the total set of measurements obtained from each muscle, only the one that represented the highest DM (maximal radial response) was selected for the analysis. Between consecutive stimuli, a resting time of 10 seconds was allowed to avoid fatigue effects in the muscle (19). In each measurement, 2 parameters were observed: maximum radial muscle belly displacement (DM) expressed in millimeters and contraction time (TC) established between 10 and 90% of maximum response of DM in the ascending curve, expressed in milliseconds. These parameters were preferred over others obtained through TMG (Delay time, as the time from the onset of electrical stimulus to 10% of DM; Sustain Time, as the time between 50% of DM on both sides of the curve; and Relaxation Time, as the time from 90 to 50% of DM on the descending curve) because of their excellent short-term repeatability and reproducibility with an ICC of 0.98 for DM and 0.97 for TC, with normalized standard error for all parameters <2% (19) and their interrater reliability an intra-class correlation coefficient (ICC) (95% confidence interval [CI]) of 0.97 for DM and 0.92 for TC (31), and ICC (95% CI) of 0.92 for DM and 0.83 for TC (4).
All data are expressed as mean ± SD and % differences between “preparation period” and “competition period.” Statistical significance was assessed using Student's t contrast for paired groups p < 0.05 (*) and p < 0.01 (**). Cohen's d effect sizes for identified statistical differences were determinate. Effect sizes (ES) with values of 0.2, 0.5, and 0.8 were considered to represent small, medium, and large differences, respectively (5). Lastly, to study the possible influence of the assessed muscle, the side of the body where the muscle is located, the individual characteristics of each cyclist or the possible interactions with the Season's period on the variables TC and DM, a repeated measures analysis of variance (ANOVA) was applied p < 0.05 (*) and p < 0.001 (**). Each one of those factors was used as between-subject variable and the Season's 2 studied periods as within-subject variable. Data were analyzed using the statistical package SPSS (Statistical Package for the Social Sciences) for Windows version 17.0.
In Table 1, a significant increment can be observed between the TC values of the PP and CP in the VM (p < 0.05), VL (p < 0.05), and left RF (p < 0.01) muscles, these increments, in the CP, range between 30.6 and 55% with a large effect size (d range between 0.79 and 1.27). Nevertheless, the TC of the right BF (p < 0.05) presents a significant decrease in the CP (32.2%) with a large effect size (d = 1.22).
In Table 2, although DM values are lower during the CP in all muscles, except left VM, a difference between the 2 studied moments can only be observed in the left VM (p < 0.01) with the only increase found of 34.3% and a large effect size (d = 1.17). The effect size is small in the VL and RF, ES is medium in the right BF, and even there is no ES in the right VM and left BF.
In Table 3, the repeated measures ANOVA confirmed that there are significant differences between the TC of the 2 evaluated periods (f = 20.225; p = 0.001), among the TC of the 4 evaluated muscles (f = 3.474; p = 0.020), and among the cyclists themselves (f = 8.257; p = 0.001). Significant differences were also found in the interaction “period × muscle” (f = 10.613; p = 0.001). These differences, in practical meaningfulness, are specified in that the VM and VL in both legs present the lowest value in TC during the PP, with differences of >7 milliseconds with respect to the other muscles, whereas the BF muscle is the shortest TC during the CP, with differences of >12 milliseconds with respect to the other muscles. The knee extensors (VM, VL, and RF) offer a significantly longer TC during the CP than during the PP. However, the knee flexor (BF) has a significantly lower TC during the CP than during the PP (Table 1).
However, the analysis did not reveal significant differences in TC values between the cyclists' left and right lower limbs (f = 0.015; p = 0.902) nor in the interaction “period × side” (f = 1.059; p = 0.307) or “period × cyclist” (f = 1.155; p = 0.337).
On the other hand, it is confirmed that no significant differences were shown by the ANOVA analysis either between the DM of the 2 evaluated periods (f = 1.681; p = 0.199) or between the cyclists' left and right lower limbs (f = 0.066; p = 0.798) or in the interaction “period × side” (f = 2.280; p = 0.135).
However, significant differences do exist among the DM of the 4 evaluated muscles (f = 8.469; p = 0.001) and among the cyclists (f = 4.725; p = 0.001), and in the interactions “period × cyclist” (f = 4.398; p = 0.001) and “period × muscle” (f = 2.721; p = 0.05). In fact, in practical meaningfulness, the muscles that show the highest DM value are the RF (8 ± 2.4 mm) and VM (7.8 ± 1.7 mm). By contrast, the VL and BF show the lowest DM during both periods with differences of >2 mm with respect to the RF and VM (Table 2).
Finally, a significant increase with a large effect size has also been found in the CP of the intensity of stimulation (milliamperes) to achieve the maximum response in all evaluated muscles: BF (59.4 ± 7.4 vs. 78.8 ± 15.2 mA; p = 0.003; d = 1.62); RF (61 ± 7 vs. 79.7 ± 12.9 mA; p = 0.001; d = 1.80); VL (57.5 ± 5.1 vs. 78.3 ± 13.9 mA; p = 0.005; d = 1.98); and VM (57 ± 8 vs. 72.7 ± 10.6 mA; p = 0.006; d = 1.67).
The main findings of this study have been reference values of TMG that show significant differences between TC values in all muscles for both periods with a large effect size (d range between 0.8 and 1.53), a significant increase with a large effect size (d range between 1.62 and 1.98) in the CP of the intensity of stimulation (milliamperes) to achieve the maximum response, and there are also significant differences among the muscles, and among cyclists, in both TC and DM. Nevertheless, we have found neither significant differences between DM values for both periods, nor between left and right lower limbs in TC and DM.
The TC is considered as an important parameter that describes the biomechanical characteristic of the skeletal muscles (32), and it has been suggested that the information on skeletal muscle structure is very important, to observe changes in muscles and improve the training process in athletes (7). The TC of the cyclists' VM during the PP (28.7 ± 5.5 milliseconds) is slightly higher than that obtained by Pišot et al. (24) in 10 healthy young subjects (25.2 ± 2 milliseconds) and much higher during the CP (40.6 ± 14.4 milliseconds). However, the TC of the cyclists' BF shows a higher value during the PP (35.9 ± 9.9 milliseconds) and a slightly lower value during the CP (28.2 ± 5.2 milliseconds), compared with the young subjects (31.0 ± 7.6 milliseconds). The cyclists' TC during the PP is similar to that of healthy young individuals, with a lower TC in the knee extensor than in the knee flexor. Nevertheless, this relationship is reversed during the CP. This fact could be an indicator of the process of adaptation that is produced during the Season in those muscles involved in pedaling, because the training, in the long term, causes differences in the structure and composition of specific sets of muscles depending on the mechanical performance model of each sport. Also, the TC of the cyclists' RF (35.9 ± 6.9-millisecond PP; 45.9 ± 16.2-millisecond CP) is higher, in both periods, than that obtained by García-Manso et al. (13) in 19 ultraendurance triathletes (63.5 ± 13.1 milliseconds, summed left and right leg mean values), and the TC of the cyclists' BF is higher in the PP (35.9 ± 9.9 milliseconds) compared with the triathletes (65.1 ± 22.1 milliseconds, summed left and right leg mean values), but it is lower in the CP (28.2 ± 5.2 milliseconds).
The behavior shown by the knee extensors and the knee flexor between the 2 periods could be accounted for by the high intermuscular coordination, which is necessary in these cyclists to obtain optimal mechanic efficiency during the pedaling movement. Specifically, the BF is activated along with the knee extensors to facilitate pedaling between 45 and 180° (16). This activation of the flexors propitiates the conditions for the necessary elevation when the pedal gets close to 180°. Thus, the BF is involved both in the hip's extension and in the subsequent flexion of the knee because of its condition of biarticularity muscle. This double function of excitation-relaxation during the pedaling process requires high intermuscular coordination. This biarticularity muscles performs the functions mentioned in the studies compiled by Hug and Dorel (16): contribute to joint stability, or in the transfer of energy between joints at critical times in the pedaling cycle and in the control of the direction of force production on the pedal.
The results showed a significantly large increase between the TC values of the PP and CP in the muscles VM (28.7 ± 5.5 vs. 40.6 ± 14.4 milliseconds; 41.4% p < 0.05, d = 1.1), VL (28.3 ± 4.9 vs. 40.6 ± 10.2 milliseconds; 43.4% p < 0.05, d = 1.53), and RF (35.9 ± 6.9 vs. 45.9 ± 16.2 milliseconds; 27.8% p < 0.05, d = 0.8). Nevertheless, TC of the BF presents a significantly large decrease in the CP (35.9 ± 9.9 vs. 28.2 ± 5.2 milliseconds; −21.4% p < 0.05, d = 0.97). Furthermore, a significant large increase (d range between 1.62 and 1.98) has also been found in the CP of the intensity of stimulation (milliamperes) to achieve the maximum response in all evaluated muscles. In this regard, using TMG, the effect that 10 days of endurance training had on the VL of 10 athletes has been detected, observing an increase in TC and DM (18); also, it has been suggested that the effect of muscle fatigue on the RF was different in TC and DM from the BF, in a long distance race (13), and an enhanced recruitment of motor units in VL, assessment by surface electromyography (root mean square voltage), has been observed between rest, precompetition and CPs of the Season in professional cyclists (20). This change in the knee extensors in the competitive period could be because of the optimization in the use of type I fibers that take place throughout the Season, as a result of the long mileage and the intensity of training carried out within 7 months of each the 2 measurements. This suggestion would be in keeping with the fact that the necessary efficacy in pedaling during a prolonged effort seems to be possible because of the higher percentage of type I fibers that are found in the VL muscle (27).
It is also supported in that the TC has been significantly related to the spatial distribution of the different types of muscle fibers determined by the histochemical method (6,7), and more recently, the TC of VL has correlated with the proportion of myosin heavy chain I in a biopsy obtained from the same muscle (29). The TC value for the BF (36 ± 9 milliseconds) obtained by Dahmane et al. (7) showing a significant correlation with the percentage of type I fibers (43 ± 10%; r = 0.93, p < 0.05), and the TC values for the BF (between 29.7 ± 10.9 and 34.8 ± 8.6 milliseconds) obtained by Dahmane et al. (6) showed a significant correlation with the percentage of type I fibers in the superficial part of the muscle (44.1 ± 1.8%; r = 0.76, p < 0.05) and in the deep part of the muscle (50.9 ± 2.3%; r = 0.90, p < 0.05). Therefore, TMG serves as an indirect noninvasive and selective estimation method of skeletal muscle fiber type.
The DM of the cyclists' VM (7.8 ± 1.7 mm) is slightly lower to that determined by Pišot et al. (24) in healthy young individuals (8.7 ± 1.6 mm) and in the case of BF (5.3 ± 1.0 mm) it is similar to the cyclists' (5.7 ± 1.9 mm). Also, the DM of the cyclists' BF (5.7 ± 1.9 mm) and RF (8 ± 2.4 mm) is in line with the triathletes (10.8 ± 3.5 mm, summed left and right leg mean values) obtained by García-Manso et al. (13). This indicator is associated with muscle stiffness and diameter changes (24). Muscle stiffness is the ratio of the change in force applied to a muscle to the resultant change in length (21). Nevertheless, the DM could also be affected by the tendon mechanical properties, because an increase in the DM has been attributed to a decrease in muscle, and tendon stiffness (24). A negative correlation has been established between the decrease in muscle thickness and the increase of DM, which is indicative of a lower muscle resting tension because of muscle atrophy and, possibly, to changes in the viscoelastic properties of intramuscular and tendon connective tissue (24). The DM of the VM and BF in their research increased by 24% (p < 0.01) and by 26% (p < 0.01), respectively, after a 35-day bed rest.
This fact is especially relevant in the case of professional cyclists because research has shown that their bone mineral density is lower than that of healthy and highly active young men. The femur neck is the most affected part (−18%) because, despite the activity's inherent muscle contractions, an excess of endurance training and a lack of resistance training, which are typical of this sport, are 2 factors associated with lower bone mineral density (3).
The DM does not change significantly during the season, although a slight decrease with small size (d ranges between 0 and 0.64) is noticeable in all the studied muscles in the cyclists, except for the left VM. It is probably because they maintain muscular and tendon stiffness during the Season.
The ANOVA confirmed that these differences in both periods depend on the evaluated muscles (TC p = 0.02; DM p = 0.001) and on the cyclists (TC p = 0.001; DM p = 0.001) and does not depend on the side of the body.
The results have shown no significant differences between right and left lower limbs, as far as TC and DM are concerned. This fact seems to be in line with a cyclical sport where some kind of symmetry can be expected as a result of sharing the effort between both legs. Nevertheless, previous research has found bilateral asymmetries in other athletes, such as professional football players, regarding both concentric and eccentric contractions of the knee flexors (8,25). It is considered that an asymmetry exists when the difference between either side of the body is >10% (25).
However, it was found that the TC of VM and VL is lower than the others in the PP, and BF presents the lowest TC in the CP (p < 0.02). The RF and VM show the largest radial displacement in both periods (p < 0.001). These results are consistent with fact that the measurement of the muscle belly responses to electrical stimuli revealed differences among muscles mainly because of their respective structures (11,12,28,33). The TC of the VL (20 milliseconds) has been shown to be significantly lower than the BF (60 milliseconds) in healthy subjects (32). The differences among the erector spinae, biceps brachii, VL, and BF in 9-year-old children have also been verified (23). Also, significant differences have been suggested between trunk flexors and extensors in young women (12), between knee flexors and extensors in professional cyclists (11), and even in men and women volleyball players, a higher normalized response speed score in the VL and VM compared with the RF and BF (28).
Also, each cyclist seems to show a different contractile properties pattern for the parameters TC and DM (p < 0.001), which is also represented, in a very graphic way, by the high SD, which characterizes this study's sample.
It is necessary to obtain more knowledge about the adaptations of the TC of the BF during the Season. An analysis of the interday reproducibility of the TMG measurements would also be interesting to ascertain the necessary duration of the stimuli during the training required to cause significant changes in the TMG parameters and subsequent adaptations in the muscle fibers. It could also be important to control, in the assessment of TMG, the skinfold thickness, because it could have a significant variation during the Season.
In conclusion, our findings suggest a reference values of TC and DM in the professional cyclists, a behavior of TC values during the Season, which show marked differences between the knee extensors (large increase of TC) and the knee flexor (large decrease of TC), and a large increase in the CP of the intensity of stimulation to achieve the maximum response. This is probably a consequence of the effect of training and competition loads have on type I fibers during the Season (adaptation process) in knee extensors. However, the DM has not changed significantly during the Season, that is, the muscular and tendon stiffness have a small increase in the CP. Furthermore, these differences in both periods depend on the evaluated muscles and on the cyclists, and do not depend on the side of the body. Thus, the TMG could be a suitable, selective, and noninvasive tool for the individual assessment, during the Season, of the most important superficial muscles involved in pedaling, along with other methods of evaluation.
The assessment of TMG, close to the main competition, has made possible to establish reference values of TC and DM of the cyclist's muscle to a 3-week stage race. The high performance obtained by these cyclists in their main competition (5 weeks after the second measurement) they were winners of the “Vuelta a España” in the General Teams Classification and one of them finished in 5° place in General Individual Classification suggests that the TC and DM values, and the level of stimulation required to achieve them, shown by cyclists in the competitive period could be used a reference for the coaching of other cyclists in the same period.
Through the TMG, the coach could control the cyclist's muscle during the Season, because the TC values of the competitive period could respond to an optimization in the use of type I fiber product of accumulation of kilometers, the increase of the intensity of stimulation (milliamperes) to achieve the maximum response in the CP could respond to an increase of the cyclist's neuromuscular fitness in this period, and the DM values could be useful to control the cyclists' muscle and tendon stiffness during the Season.
The significant differences among the cyclists and among the muscles confirm the need, for the coach, of individualization and control of the training loads, because every cyclist has different objectives at the beginning of the season, which accounts for the high interindividual variability of the training status in the PP of this sample, and because the knee extensors and flexors seem to change in opposite directions during the Season. The information obtained through noninvasive tool TMG could help to provide suitable advice for the design and control of individualized training loads, and to obtain information about the training effect, side-to-side asymmetries and muscle group imbalances, or neuromuscular fatigue.
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Keywords:Copyright © 2013 by the National Strength & Conditioning Association.
tensiomyography; cyclist; muscle symmetry; periodization; training load