Different approaches were used to extract signal segments for analysis of the signals in time and frequency domains. In the time domain, the average amplitude of the sEMG signal (ARV) was calculated separately for each stroke from the signal values contained within the active phase. The active phase for ARV estimation was defined as the time interval containing the local maximum of the power envelope corresponding to this particular stroke, in which the power of the signal was >1% of the maximum power for this stroke (Figure 1A). In the frequency domain, the standard periodogram method based on the short time Fourier transform (STFT) was used to estimate the power spectrum of sEMG for each stroke, and the median frequency of the power spectrum (MDF) was then calculated. The STFT is best used as spectral estimation method for stationary conditions (achieved, e.g., over short periods of sustained static muscle contractions). However, it has been shown that it can also be used successfully to assess spectral changes in dynamic conditions (25). To avoid the most nonstationary parts of the sEMG signal corresponding to low activity at the end and at the beginning of the active phase of each stroke, the authors decided to use a narrower part of each active phase than in the case of ARV estimation. Our own testing has shown that the differences between MDF values obtained using either 10, 30, or 50% of the local maximum power as the cut-off values for extraction of the active phases for MDF estimation were not significant. The correlations between MDF values obtained using these 3 different cut-off values were high (R2 > 0.80) for all muscles and all tested subjects. The active phase for MDF estimation was therefore defined as the time interval containing the local maximum of the power envelope corresponding to this particular stroke, in which the power of the signal was >50% of the maximum power for this stroke (Figure 1B). By using this relatively high cut-off value (as opposed to the 1% cut-off level used for ARV estimation), only the most intensive part of the sEMG signal from each stroke was used in the spectral analysis.
Measurement of Oxygen Consumption
Oxygen consumption during the tests was measured using a Cosmed transportable measurement system (model K4b2, Rome, Italy). Besides the maximum relative (relV̇O2max) and absolute (absV̇O2max) volume of oxygen consumption per minute and the total volume of oxygen consumption (totV̇O2), obtained during each 6-minute rowing test, the coefficient of gross economy of rowing (kECON = totV̇O2·d−1 ; where d is the rowing distance) was also calculated.
Measurement of Mechanical Parameters of Rowing
Using the software that was enclosed with the Concept IIc ergometer, the total rowing distance (Dist) and the average rowing power (Pmean) were measured, along with the average stroke frequency (Frmean) and the stroke frequency at time point 10, 300, and 360 seconds (Fr10, Fr300, Fr360, respectively) in both tests. At each time point, the stroke frequency was averaged the same way as the ARVs and MDFs were (see Experimental Approach to the Problem).
The data were processed by the SPSS 13.0 for Windows statistical package (SPSS Inc., Chicago, IL, USA). Standard statistical methods were used for the calculation of means, SDs, and to test normality of the distribution of variables. The general linear model analysis of variance with repeated measures (RM ANOVA) was used to test the changes in variables over time (time points 10, 300, and 360 seconds) within the same rowing test. If statistically significant differences were found, additional post hoc analysis was performed for RM ANOVA using the Bonferroni test. A paired samples t-test was used for testing the differences between variables of the submax and all-out rowing tests. If variable distribution deviated from normal distribution, a Wilcoxon signed rank sum test for 2 paired samples was used for testing the differences. The p ≤ 0.05 criterion (2-tailed test) was used for establishing statistical significance.
Biomechanical, Biochemical, and Respiratory Parameters Before, During, and After a 6-minute Rowing Exercise
Table 2 shows the basic statistical parameters of biomechanical, biochemical, and respiratory variables before, during, and after both rowing tests and the statistical significance of the differences between the tests. All variables, but not the maximum relative oxygen consumption (relV̇O2max) during the submax test (p = 0.011), and the stroke frequency at the time point 360 seconds (Fr360) during the all-out test (p = 0.003) were normally distributed. Significant differences between the tests (p < 0.05) occurred in all variables, except for gross economy of rowing (kECON).
During the all-out test, the subjects covered a longer distance (p = 0.001), developed higher average power output (p < 0.001), and achieved higher average stroke frequency (p < 0.001) compared to the submax test. Lactate concentration at rest (LArest) before the all-out test was significantly higher than before the submax test (p = 0.005). The maximum lactate concentration (LAmax) and the change in the lactate concentration (dLA) after the all-out test were significantly higher compared to LAmax and dLA after the submax test (p < 0.001 for both parameters). The relative and absolute V̇O2max and the total oxygen consumption (totV̇O2) were also significantly higher during the all-out test (p = 0.047, p = 0.023, and p = 0.017, respectively), whereas the kECON did not differ significantly (p = 0.459) between the tests.
The stroke frequencies were significantly higher at each time point (Fr10, Fr300, and Fr360) during the all-out test (p = 0.003, p < 0.001, and p = 0.018, respectively). However, there was a significant decrease in the stroke frequency between Fr10 and Fr360 (p = 0.033) during the submax test and a significant increase between Fr10 and Fr360 (p = 0.025) and Fr300 and Fr360 (p = 0.005*) during the all-out test (Figure 3).
Activation and Fatiguing Patterns During the Submax and All-Out Tests
Figure 1 shows the course of changes of normalized amplitudes (nARV) of the sEMG signal of the analyzed muscles with respect to their initial value for the submax and all-out tests. During the submax test, the nARV values of the GC, LD_up, BR, and BB muscles remained relatively steady, whereas nARVs of the RF, VL, BF, GM, ES, and LD_lo were increasing during the entire observation period. However, during the all-out test, there was no change in the normalized sEMG signal amplitude of the ES, BF, and BB muscles, a decrease in nARV of the GC, LD_lo, LD_up, and BR muscles and an increase in nARV of the RF, VL, and GM muscles. Additional increase in the nARV was also observed in all muscles during the finishing action in the last minute of the all-out test.
Only in the RF muscle was the ARV at the time point 360 seconds (ARV_360) of the submax test significantly higher than the ARV at the time point 10 seconds (ARV_10; p = 0.041), whereas in the RF and LD_lo, the ARVs were significantly higher at the time point 300 seconds (ARV_300; p = 0.023 and p = 0.004, respectively) compared to ARV_10 (Figure 4). There were no statistically significant differences between ARV_300 and ARV_360 in any of the muscles.
During the submax test, the average MDF did not differ significantly (p > 0.050) between individual time points (10, 300, and 360 seconds) in any of the analyzed muscles.
During the all-out test (Figure 5), the ARV_360 values were significantly higher than the ARV_10 values in the RF (p = 0.038), VL (p = 0.009), and GM (p = 0.044) muscles, and the ARV_300 values in the RF (p = 0.045) and LD_up (p = 0.007) muscles. Only in the GC muscle were the ARV_300 (p = 0.002) and ARV_360 (p = 0.048) significantly lower than the ARV_10. There were no statistically significant differences (p > 0.050) in other muscles.
On the other hand, the average MDF (Figure 6) at the time point 360 seconds (MDF_360) was significantly lower compared to the time point 10 seconds (MDF_10) of the RF (p = 0.022), ES (p = 0.037), and LD_lo (p = 0.027) muscles. No statistically significant differences (p > 0.050) were established between other time points of these and other muscles.
Surface Electromyographic Signal Differences Between the Tests
The greatest statistically significant differences in the ARV between the tests were observed in the VL muscle, because the ARV was constantly higher during the all-out test than during the submax test, and the difference increased with the duration of rowing (Figure 7). A different structure of the differences between the tests was seen in the GC, RF, LD_lo, and LD_up muscles, where the amplitude of the sEMG signal during the all-out test was significantly higher (p < 0.05) at the time point 10 and 360 seconds but not at the time point 300 seconds. In the BB muscle, the ARV_10 did not significantly differ between the tests, whereas at the time point 300 and 360 seconds, it did (p = 0.021 and p = 0.015, respectively). In the GM and BR muscles, only the ARV_360 was significantly higher during the all-out test compared to the submax test (p = 0.040 and p = 0.016, respectively). In other 2 muscles (BF and ES), no statistically significant differences were found between the tests.
The MDF significantly differed between the tests only in the RF muscle at time points of 300 seconds (−7.15 ± 4.60%, p = 0.004) and 360 seconds (−10.96 ± 6.22%, p = 0.001) but not at the time point 10 seconds (Figure 8). There were no statistically significant differences in the MDF in other muscles.
As had been expected, during the all-out test the subjects covered a longer distance and achieved higher average power output, higher average stroke frequency, higher LA concentrations, higher maximum relative and absolute, and total oxygen consumption compared to the submax test; however, no substantial differences were observed in gross economy of rowing. The changes in the EMG parameters during the submax test were minimal. The amplitude of the sEMG signal systematically increased only in the RF and LD_lo muscles, whereas no decrease in the MDF was observed in any of the muscles. Slightly larger changes in the sEMG signal occurred during the all-out test, as the ARV of the VL, RF, and GM muscles increased, while at the same time the MDF of the RF, ES, and LD_lo muscles decreased. Compared to the submax test, during the all-out test, the highest increase in the ARV was observed in the VL muscle and also in the ARV of the RF, LD_lo, LD_up, BB, and GC muscles, whereas the MDF decreased only in the RF muscle.
Compared to other studies, the highest LA concentration during the all-out test in this study (12.47 ± 1.94 mmol·L−1) was slightly lower than the one measured by So et al. (36) (13-14 mmol·L−1) during a maximum 6-minute effort and was within the range of the values reported by Fiskersrand and Seiler (11) for competitive and supramaximal intensity (8-14 mmol·L−1) and by Shephard (35) in his 1998 review article (11-19 mmol·L−1). The maximum relative (62.88 ± 8.67 ml·kg−1·min−1) and absolute (5.49 ± 0.51 L·min−1) oxygen consumption values were also slightly lower when compared with other all-out tests on rowing ergometers (Hagerman : 6.1 ± 0.6 L·min−1; Hagerman et al. : 5.95 L·min−1 or 67.6 ml·kg−1·min−1; Lakomy and Lakomy : 4.71 ± 0.39 L·min−1; Mäestu et al. : 67.4 ± 7.4 ml·kg−1·min−1). However, the average power output during the all-out test in this study (371.67 ± 44.69 W) was similar compared to that reported by Mäestu et al. (27) for the national-class rowers (329.44 ± 41.37 W), whereas the average time over 500 m (1:38.6 ± 4.1 seconds) was lower compared to a similar study conducted by So et al. (36) (1:42.1 ± 0.5 seconds) on 2 lightweight rowers. These comparisons lead to the conclusion that the subjects performed the test according to the best of their ability and that the results are relevant for highly trained rowers.
Rowing with the submaximal intensity was indicated by the low blood LA concentration at the end of rowing (4.64 ± 0.77 mmol·L−1). The submax test loading was prescribed with a predefined intensity (4 mmol·L−1), and it was performed by almost complete absence of substantial changes in the sEMG signal parameters. Of all analyzed muscles, the ARV increased significantly only in the RF and LD_lo muscles (20 and 13%, respectively), whereas the MDF values did not change significantly throughout the test. In the study involving professional cyclists who cycled with an intensity of 80% of V̇O2max (comparable to the submaximal intensity in this study) the RMS of the VL muscle increased by about 10, 15, 30, and 40% after 10, 20, 30, and 40 minutes, respectively (33), whereas no statistically significant changes were observed in the power spectrum of the sEMG signal, even though the oxygen consumption was increasing all the time (24,33). It can therefore be concluded that the fatigue of the RF and LD_lo muscles was similar to that reported by Petrofsky (33) for the VL muscle during submaximal intensity cycling (80% of V̇O2max). The increase in the amplitude of the sEMG signal during rowing indicates that new motor units were recruited (31) and that the form of the action potential changed (7) because of fatigue. A higher exposure of the RF muscle in means of higher activation, represented by significant increase in ARV during rowing, appears to be the consequence of the muscle's participation in both knee extension and hip flexion. Therefore, the submax test represented an almost perfect steady state at high-intensity rowing that can serve as a base for comparison with the all-out test.
It was expected that the muscles that showed the signs of fatigue at the submaximal intensity would fatigue even more during the all-out test. As the amplitude of the sEMG signal of the RF muscle increased already during the submax test, it increased significantly also during the all-out test. However, during the all-out test, the amplitude increased not only in the RF but also in the VL and GM muscles. These muscles engage in leg extension. Despite the above, the changes in the ARV in the first 300 seconds were not particularly large, because in some muscles they increased additionally and substantially over the last minute, which indicates the existence of some reserve before the last 60 seconds of the all-out test. The increase in the ARV of the RF, VL and GM muscles until the 5th minute (13-18%) was similar to the increase in the amplitude observed by Mäestu et al. (28) for the VL muscle during 2000-m simulated race on the rowing ergometer (about 20%) and Petrofsky (33), also for the VL during cycling at 100% of VO2max until failure (about 15%). As well as for the first 5 minutes, the increase in the ARV of the abovementioned muscles in the last minute of the present study was similar to that reported by Mäestu et al. (28) and Vesterinen et al. (40) (about 25%) during the last 50 m of each of the four 850-m series of cross-country skiing on roller skies at racing velocity (a combined (summed) amplitude of triceps brachii and VL muscles). However, the overall increase in the amplitude during the all-out test in this study (by 30-44%) was perceptibly (10-14%) greater (depending on the muscle) than the overall change in the amplitude reported by Petrofsky (33), but still similar to those (about 47%) determined by Mäestu et al. (28) for well trained rowers. On the other hand, changes in the power spectrum were not that high (no more than 11%). In a similar study in rowers, So et al. (36) established that the MPF of RF and ES can decrease by >25% during maximum rowing before reaching the lowest value (i.e., endurance level). However, Petrofsky (33) reported a 20% decline in the MPF of VL during cycling at 100% of V̇O2max. The above indicates that the increase in the ARV in the last minute of rowing was mainly because of the finishing action (and not fatigue), when the stroke frequency increased significantly as well. It seems obvious that the subjects rowed with some reserve until the last minute during the all-out test.
A change in the sEMG signal can also stem from the structure of the muscle fibers. In muscles with a higher proportion of slow-twitch and fatigue-resistant fibers, as typically observed in rowers (37), less fatigue and thus smaller changes in the EMG parameters at submaximal intensities can be expected compared to those with a higher proportion of fast-twitch fibers (22,23). Additionally, a temperature compensation for the decline in the MPF (and MDF) can be expected (26). However, given the preliminary 10-minute standardized warm-up, this effect does not appear very significant because it has been established that the MPF changes by 2.82 ± 0.27 Hz°C−1 during high-intensity dynamic exercise (26) and MDF by 3.48% ° C−1 at 80% of the maximum voluntary contraction (30), whereas with higher intensity of loading from rest to 100% of V̇O2max the temperature can rise by only about 3.5° C (33).
The last minute of rowing during the all-out test differs considerably from the earlier pattern of muscle activity (Figure 2). It seems less likely that the change could be related only to fatigue. Most probably, the bulk of the increase in the ARV stems from the tactics related to finishing, which is common at the end of a rowing race. Even though the ARV increased in the last minute in most of the muscles, only the changes in the RF and LD_up muscles were statistically significant. One of the reasons lies in the fact that the subjects performed the finish in different ways or with a different intensity, while on the other hand the reason could be in greater fatigue or technical exposure of the 2 muscles. The increase in the amplitude of the sEMG signal because of fatigue is probable only with the RF muscle but not the LD_up muscle, because the ARV of the latter did not change substantially with respect to the initial value until the 300th second, and no significant change occurred in the MDF. Therefore, the increase in the amplitude of LD_up muscle in the last minute of rowing was mainly attributed to tactical reasons.
The GC occupies a special place among the analyzed muscles, because during rowing, its EMG amplitude decreased below the initial value. It is assumed that the plantar flexion was used more prominently in the initial phase of rowing than in the continuation. The GC muscle is a 2-joint muscle enabling a transfer of energy between the segments (42). Moreover, it is a muscle, which is characterized by a high share of fast-twitch fibers (3) and fatigues quickly. However, fatigue is probably not the reason for the lower amplitude of the sEMG signal, because it would also cause a decrease in the MDF, which did not happen. It is therefore more likely that in the later rowing phases a programed decrease in plantar flexion occurred.
The majority of differences in muscle activation between the submax and all-out tests at the start were related to the leg (GC, RF, and VL) and shoulder girdle (LD_lo, LD_up) extensors. It is interesting to note that the hip and trunk extensors and the arm flexors did not engage. In the middle, steady-state phase of rowing, the activation of most of the analyzed muscles did not differ considerably between the 2 tests. Higher activation was only observed in the VL and BB muscles (Figure 7). It therefore appears that these 2 muscles were the most responsible for the maintenance of the pace in that rowing phase, particularly the VL muscle, because the changes in its activation were the most systematic of all the muscles. In the final phase of rowing, the major differences could be seen in muscle activation (and less in fatigue), mainly on account of the finish. Here, the same muscles as at the beginning of rowing were engaged and, additionally, the arm muscles (BR and BB) were also activated. It thus seems that the temporary increase in the tempo (at start and finish) required a complex response of the majority of muscles participating in the execution of a stroke, whereas the maintenance of the pace was because of engagement of only VL and BB muscles.
The increase in the ARV of the GM muscle was the largest in all investigated rowing phases, yet the changes were not statistically significant, with the exception of those at the finish. This shows that rowers might apply different strategies in using this muscle. Particularly interesting was the absence of an increase in the activity of the BF and especially ES muscle, which was, along with RF the most stressed muscle in the study of So et al. (36). This might be related to the specific rowing technique used by the subjects in this study. Slovenian rowers are taller (in this study: 188.73 ± 5.78 cm), and because of this, they do not have to lean too much forward with the trunk at the catch. Thus, they are probably used to rowing with an extended and fixed trunk. However, the Chinese rowers are on average smaller (175.1 ± 3.2 cm ) and supposedly, they have to lean forward more. In this way, they have to activate and stress the trunk extensors more. It may thus be concluded that in this study, trunk stabilization was more important than its extension. On the other hand, this may also be associated with pain in the lower back, which is common among rowers (34).
It can be concluded that the majority of changes in the sEMG signals of the analyzed muscles during rowing at a competitive level were related to the increase in the amplitude of the sEMG signal, whereas the changes in the frequency content of the signal were relatively small. The above shows that, at a competitive level, the fatigue processes could be compensated for by additional recruitment of motor units, which is probably owing to rowers' higher proportion of slow-twitch muscle fibers. During the all-out test, the most activated were the leg extensor muscles in general, yet, surprisingly, no substantial increase in ARV was observed in the trunk extensors. At the same time, neuro-muscular fatigue occurred in the RF, ES, and LD_up muscles, whereas no substantial fatigue was seen during the submax test.
A comparison of the submax and all-out tests showed that at the start, the leg and shoulder girdle extensors were the most important muscles. In the middle part of the simulated race, the knee extensor and elbow flexor muscles were mainly responsible for maintaining a constant rowing pace. However, during the last minute, when the subjects performed the final strain (finishing action), the leg and shoulder girdle extensor muscles were involved again, additionally accompanied by the elbow flexors. Interestingly, trunk extensor muscles do not seem to be critical, which might be related to the specific rowing technique used by the subjects.
This study was funded by grants from the Slovenian Research Agency (grant no. L5-6570 and grant no. L5-7112), the Foundation for Financing Sports Organizations in Slovenia and the Government of the Republic of Slovenia, Ministry of Higher Education, Science and Technology. Special thanks go to the Rowing Federation of Slovenia, to the selector Miloš Janša, and all other coaches who supported this study. The researchers would also like to thank Dr. Radoje Milić and his assistants from the Laboratory for Sports Physiology, Faculty of Sport, University of Ljubljana, and Miloš Kalc for their help with the measurements.
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