*Descriptive*

A global descriptive analysis of the cohort (n = 63) including the different cardiovascular parameters at the different measurement times is shown in Table 1.

During the isokinetic protocol, the maximum SBP value was 207 mmHg (at the fourth measurement), maximum DBP was 103 mmHg (at the third measurement), and maximum HR was 148 bpm (at the final measurement). The minimum SBP, DBP, and HR values were 102 mmHg, 44 mmHg, and 39 bpm. Intra-subject differences in DBP with respect to the measurement at rest never exceeded 13 mmHg.

Because all the parameters described followed normal distribution in the cohort (n = 63), repeated-measures ANOVA was performed to compare each parameter among the different measurement points and to assess the effect of time on the obtained means (Figure 2).

ANOVA revealed an effect of measurement time on SBP (F5.363 = 52.91; *p* < 0.001; ɳp2 = 0.5). When comparing pairs of measurements, a significant effect was produced at rest and after 10 min warm-up exercise compared with that at the other measurements (*p* < 0.001). There were no significant differences between the third measurement (post RLE 60◦/s) and the other measurements or between any of them (*p* > 0.05). DBP was also affected by different measurement times (F = 9.30; *p* < 0.001; ɳp2 = 0.149). The pairwise comparison showed statistically significant differences between the measurement

at baseline and those at rest except for the sixth (*p* = 0.313) and final (*p* > 0.5) measurements. There were also significant differences between the third and fourth measurements (post RLE 60◦/s and post RLE 180◦/s) with respect to the sixth measurement (*p* = 0.045 and *p* = 0.016, respectively) and between the fourth and final measurements (*p* = 0.03). HR was also affected by different measurement times (F3.521 = 188.37; *p* < 0.001; <sup>ɳ</sup>p2 = 0.780). HR resulted in highly significant differences (*p* < 0.001) among all the measurements except between the fifth (post RLE 240◦/s) and seventh (post LLE 180◦/s) measurements.

**Table 1.** Description of the cardiovascular parameters during the isokinetic protocol at different measurements. Means and standard deviations obtained for the global sample (n = 63) and according to the field position and competition level of the soccer players are shown.


Data presented as mean +/− SD. Max, maximum value obtained by a sample subject; Min, minimum value obtained by a subject in the sample. Abbreviations: RLE, right lower extremity; LLE, left lower extremity; HR, heart rate; SBP, systolic blood pressure; DBP, diastolic blood pressure; MAP, mean arterial pressure; RPP, rate pressure product. \* significantly higher than the resting measurement *p* < 0.001; † significantly higher than post-bike measurement *p* < 0.001; ‡ significantly higher than the measurement at rest *p* = 0.001. significantly higher than the 6th measurement (LLE 60◦/s) *<sup>p</sup>* < 0.05. significantly higher than the 8th measurement (LLE 60◦/s) *p* < 0.05. ђ significantly higher than all previous measurements *p* < 0.001. significantly higher than all previous measurements *p* < 0.001, except the 5th. Ꮘ significantly higher than previous measurements *p* < 0.001, except for the 5th and 6th. Ω significantly higher than Forwards in the same measurement *p* < 0.05.

Finally, the effect of the different measurement times on MAP (F5.538 = 29.47; *p* < 0.001; <sup>ɳ</sup>p2 = 0.357) and RPP (F4.577 = 168.55; *p* < 0.001; <sup>ɳ</sup>p2 = 0.761) were observed. There were significant differences in MAP between the baseline measurement with respect to the rest measurement (*p* < 0.001) and between the second and fourth measurements (*p* < 0.01). There were differences in RPP among all measurements except among the fifth, sixth, and seventh measurements (*p* > 0.05) and between the fourth and sixth measurements (*p* = 0.086).

**Figure 2.** Heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP) response of the global sample (n = 63) during the proposed isokinetic testing protocol (error bars show the SD of the mean in each measurement). Abbreviations: RLE, right lower extremity; LLE, left lower extremity.

Repeated-measures ANOVA was performed to assess the effect of time on cardiovascular parameters in the participants according to their field position (defenders, midfielders, and forwards) as well as the interaction of this effect with field position.

The changes in SBP (F10.745 = 1.02; *p* = 0.463) and RPP (F9.252 = 1.66; *p* = 0.096) with respect to the measurement points followed the same profile as that in the global sample. There was no interaction of the effect "time" and the factor "field position" when comparing the means of the different measurement points. An interaction of the effect of time with respect to field position was obtained for DBP (F = 2.1; *p* = 0.012; ɳp2 = 0.076), with a significant difference at the fourth measurement (180◦/s RLE) between the defenders and forwards (*p* = 0.043). Although there were no significant differences among the field positions in the other measurements, the DBP response at the fifth measurement differed among the groups. In turn, an interaction effect of "time–field position" was found for HR (F7.086 = 1.76; *p* = 0.042; <sup>ɳ</sup>p2 = 0.065) during the isokinetic protocol measurements. The mean HR values of the defenders were higher than those of the forwards and midfielders, with significant differences at the sixth measurement (*p* = 0.037); the other measurements displayed homogeneous HR response in all groups. Finally, there was an interaction effect of "time–field position" in MAP (F14 = 1.96; *p* = 0.02; <sup>ɳ</sup>p2 = 0.072), although when performing pairwise comparison, no significant differences were found among the groups at any of the measurements. MAP in the forwards changed with respect to that in the defenders and midfielders, without significant differences.

When the participants were categorized according to competitive level (first- and second-division teams), the cardiovascular parameters during the isokinetic protocol followed a normal distribution. No interaction of "time effect" and "category factor" in the cardiovascular parameters was recorded in the comparison of the means at the different measurement points. SBP (F5.361 = 0.31; *p* = 0.914), DBP (F = 0.48; *p* = 0.84), HR (F3.559 = 0.78; *p* = 0.520), MAP (F5.507 = 0.34; *p* = 0.9), and RPP (F4.601 = 0.67; *p* = 0.62) showed high homogeneity in their response in both categories.

Table 2 shows the mean values of the cardiovascular parameters analyzed at the end of both tests and the significance of the comparisons.


**Table 2.** Comparison of the cardiovascular values achieved by the global sample (n = 63) at the end of the isokinetic protocol and treadmill ergospirometry.

Data presented as mean +/− SD. HR, heart rate; SBP, systolic blood pressure; DBP, diastolic blood pressure; MAP, mean arterial pressure; RPP, rate pressure product. \* Signification *p* < 0.05.

There were significant differences in the final SBP, MAP, HR, and RPP values of both assessment tests, with all parameters significantly higher at the end of the treadmill ergospirometry.

#### **4. Discussion**

Notably, none of the participants presented complications or abnormal BP or HR responses to the exercises [53]. The cardiovascular reference values used to determine abnormal responses were those used for dynamic incremental exercise (treadmill test) [38,54,55] because no study in the literature has established non-physiological BP and HR responses for isokinetic exercises. Therefore, because "normal" cardiovascular response values during an isokinetic testing protocol have not been established, it is advisable to use those established for stress tests as a fundamental reference when studying cardiovascular responses during maximum exercise.

The novelty of this study lies in describing the behavior of BP and HR in the group of professional soccer players not because they are just another population group but because they will be subjected to this assessment test on a regular basis during the seasons in which they compete. This means that there is a high prevalence of this test in this specific population group. Furthermore, this test could be used in the future to obtain more global and detailed information on the AT and HR of a soccer player to these physical demands and prevent possible future undesirable or pathological clinical events.

There were no players in whom SBP decreased in relation to that at rest, which is recommended [56]. This is considered a normotensive response to the effort because maximum SBP and DBP values of up to 240 and 115 mmHg, respectively, have been established in highly trained participants [57,58], with an increase in DBP of up to 15 mmHg considered normal during maximum-intensity exercise [54,59]. The maximum HR recorded was clearly below the cardiovascular safety limits as expected from intense exercise with limited duration.

#### *4.1. Heart Rate*

The mean HR of all participants showed a practically linear significant increase throughout the assessment protocol except between the fifth and sixth measurements, with differences between each consecutive recording time of approximately 7–13 bpm, and the mean HRmax was 112.9 ± 18.9 bpm at completion of the protocol. It is evident that the interruption in the linear increase in HR at the sixth measurement (LLE 60◦/s) is related to the change in the LE made by muscular effort, subsequently continuing the progressive increase in HR with the same profile as before the change of LE. In fact, it is probable that if the six series of isokinetic testing were performed with the same LE, higher maximum values than those obtained in this study would be achieved.

On analyzing the variations in HR between each proposed angular velocity, we observed that the greatest increases occurred after warming up on an exercise bike (13.4 bpm) and in the series at high angular velocity (240◦/s), where it increased by 13.1 bpm (RLE 240◦/s) and 11.6 bpm (LLE 240◦/s) compared with that in the immediately previous series measurement. In these series at 240◦/s, 25 repetitions of knee flexion/extension were performed, leading to a longer effort time than that in the series at 60◦/s (5 repetitions) and 180◦/s (10 repetitions). This finding corroborates the results of other studies, in which the increase in HR depends more on the duration than on the intensity of the exercise [31,60–62]. Because isokinetic testing protocol is designed with consecutive series and little recovery time [30,60], it results in higher HR responses than those to isolated series at a given angular velocity or at rest intervals greater than 90 s between each series [35,63]. In fact, in a continuous isokinetic testing protocol, the mean HR increased to values close to those obtained in maximum stress tests until the participant reaches exhaustion [64].

However, in the usual isokinetic tests of the knee musculature in soccer players [27,65,66], no more than three or four series are performed at different velocities, so the effort of each series does not exceed 1 min; therefore, HRmax values are not attained. Studies that report greater increases in HR in adults after a series of isokinetic exercises generally have longer durations of the said series [30,67].

On comparison of the HR response of our sample during the isokinetic testing protocol with that in the studies, it is evident that HR in soccer players is much lower than that in untrained adults [30,31,34,62,68]. This suggests that cardiovascular adaptations to soccer players' training trigger a smaller increase in HR during isokinetic exercises despite our sample having a lower mean age than that of samples in other studies. Given that higher HR values are obtained during isokinetic exercises in young individuals [30,35,68], the adaptations to training by this population group are more significant than the participants' age.

This reflection seems to be confirmed by the absence of differences in the HR response during isokinetic exercises between the first-division team (24.5 years) and second-division team (19.9 years) players. In fact, the behavior of HR between both groups was very similar at all measurements, indicating the limited influence of age and competitive level in the HR response.

#### *4.2. Blood Pressure*

The SBP values of the global sample increased progressively until the fourth measurement (RLE 180◦/s), when, after reaching a mean value of 155.2 ± 15.7 mmHg, it remained practically unchanged at the subsequent measurements. It even decreased slightly at the seventh and eighth measurements until completion of the isokinetic testing protocol, with a mean value of 154.3 ± 15.3 mmHg. After the third and fourth measurements, there were no significant differences in the increase in SBP, resulting in an incremental curve that reached a plateau (fourth measurement), and it remained stable until completion of the protocol. This SBP response is like that described in healthy adults performing compared to incremental dynamic exercises [69,70] although the mean values in our study were lower. Similarly, no decrease in SBP was observed after changing LE between the fifth and sixth measurements as in HR, so this change did not influence the overall response to the protocol.

We did not identify an influence of angular velocity on the BP response because the increases occurred during the first two series, and SBP subsequently remained unchanged. Some researchers who assessed the SBP response in isokinetic exercise series at different velocities recorded higher SBP at low angular velocities [32,34,67,71], whereas others did not report significant differences between knee flexion–extension series at different angular velocities, as in our case [60,62]. Therefore, it appears logical that in consecutive series of exercise protocols with limited recovery time, angular velocity is not a relevant element in the SBP response.

The SBP values in this study are hardly comparable with those in other studies because no similar designs were found that measured cardiovascular parameters during an isokinetic testing protocol, and no studies assessed professional soccer players [34,72]. Thus, the lower SBP values in our participants are likely related to better cardiovascular adaptations to exercise by soccer players; the BP response profile may follow a similar pattern in healthy adults. It is evident that an isokinetic protocol with a series of contractions established at different velocities cannot generate excessive increases in SBP as reported during protocols performed to exhaustion [32] or with heavy-resistance exercises involving large muscle groups [73,74].

The position of the soccer players was not associated with differences in the SBP response although the midfielders had higher baseline values (130.8 mmHg) and maintained them during essentially all measurements compared with the other players, with a non-significant increase in final SBP of 5 and 7 mmHg compared with that in forwards and defenders, respectively. However, these small differences do not follow a stable pattern that justifies an influence of the field position on the SBP response. These differences are even smaller when comparing players based on their competitive level, in which both the SBP response and the mean values obtained by the two groups are very similar and not significantly different.

The DBP value increased by 8 mmHg after warming up and remained almost unchanged until the fourth measurement (RLE 180◦/s), after which it gradually decreased except for a slight increase in the seventh measurement, reaching a mean value of 74.9 ± 9.9 mmHg at protocol completion. This slight increase in DBP is lower than that reported in other isokinetic (non-exhausting) exercise designs in untrained participants [75,76] and clearly lower than that in studies with isometric exercise protocols for HR or percentage of VO2 max [77–79]. Thus, the adaptation to exercise by individuals with a high level of training seems to trigger lower values of DBP response during isokinetic exercises. Notably, the expected DBP response to non-exhausting isokinetic exercise protocols is a slight increase of ≤15 mmHg in highly trained participants. This behavior differs from that to dynamic incremental exercises [50] but is very similar to that to non-incremental exercises [80]; therefore, the duration and particularly the progressive intensity of the exercise seem to be key elements in the behavior of this parameter.

The greatest increase in DBP occurs after warming up, and it remains largely unchanged thereafter. Therefore, it is evident that the possible hypertensive effect related to isokinetic exercise would only be associated with exhausting isokinetic exercise designs [32,64] as in the case of isometric exercise [79,81].

The DBP response patter was very similar in both groups of soccer players, with no differences greater than 3 mmHg in the mean values. No influence of age or competitive level was noted in the observed response although some studies involving untrained healthy individuals reported a slight dependence of age on this response during isokinetic exercise [30,68]. This effect is decreased in highly trained individuals; thus, the DBP response is determined by the adaptations to training by soccer players.

However, certain variations in the DBP response according to field position were observed. In general, defenders had DBP throughout the protocol, with the forwards reporting lower values until the sixth measurement, after which they exceeded the mean DBP of the midfielders. These results differ from those of SBP; i.e., midfielders had higher SBP than forwards and defenders; at protocol completion, the differential BP of the midfielders (86 mmHg) was higher than that of the forwards (79.3 mmHg) and defenders (74 mmHg). These findings agree with those of previous studies that evaluated other types of exercises [82,83]; there is a linear relationship in which as the subject's training level, maximum TAS, and differential BP increase. Thus, due to the physical demands of their position, midfielders may have better BP adaptation to an intermittent protocol of isokinetic exercises at various velocities. However, this aspect is not recorded in continuous incremental aerobic exercises, as reflected in our ergospirometry results or those of Ramos [50], resulting in greater differences according to field position when faced with high-intensity

intermittent efforts (isokinetic testing) than when performing a continuous incremental aerobic effort. This may all be influenced by an increasing interest in improving the aerobic capacity of field soccer players [84–87] regardless of their position, whereas adaptation to aerobic–anaerobic efforts are determined to a greater extent due to the demands of the footballer's position during competition.

Finally, MAP and RPP were determined because TAM has been previously used to assess BP response during isokinetic exercise [31,68,76,88], and RPP allows us to clinically objectify myocardial O2 consumption during the test [89,90].

MAP progressively increased until the fourth measurement, with the cycling warmup clearly resulting in a more marked increase in TAM (10.4 mmHg) and with values clearly lower than those recorded in other designs of isokinetic exercise both in young adults [68,76,88] and older, untrained subjects [31,68] No differences were observed in MAP according to the mean age of the participants in a previous study [68] similar to our results. There was no influence on the type of contraction selected because in general, a series of concentric contractions are considered more "hypertensive" than eccentric contractions at the same angular velocities [31,68,91] However, this influence on the MAP response according to the type of contraction selected appeared to be related only to samples from untrained and mainly older subjects, in which concentric-type isokinetic exercises trigger higher MAP, SBP, and DBP [31,68].

Regarding field positions, a slightly different MAP response was observed in the forwards, who had increased MAP in the final two measurements compared with those in the previous measurements, which, although not significant, showed increased BP near the end of the effort. This may be related to the type of physiological effort they usually perform, such as short and intense efforts but with longer recovery times; thus, an intermittent isokinetic testing protocol would reflect more differences in BP response according to the field position than commonly used treadmill tests.

However, this phenomenon was not reported for RPP, in which the behavior of the players was very similar in all groups. The clinical estimate of myocardial O2 consumption that results from this parameter [89,92] would show a very homogeneous behavior among the groups of soccer players. The RPP values in our study are similar to those of other researchers who assessed this parameter in untrained participants [31,68,93] and with designs of isolated isokinetic exercises or with rest intervals greater than 90 s between each series, which results in higher BP and lower HR. Findings similar to the maximum RPP values in our participants during the isokinetic protocol were 15.000–17.000 units lower than those obtained by professional soccer players in an ergospirometry in previous studies [14,50].

This study shows at least one limitation. The between-groups and between-categories results are not strong. Some commonly utilized physiological parameters (e.g., HR or VO2 max.) may not be sensitive enough to detect specific physiological adaptations occurring in response to fatigue/training [94–96]. One possible explanation for this may stem from the fact that these parameters provide little information on the specific nonlinear dynamic interactions between organic subsystems involved in exercise physiology [95]. Therefore, it would be interesting to evaluate the effects of isokinetic/treadmill protocols utilizing variables able to quantify how respiratory, cardiovascular systems, and neuromuscular systems coordinate during exercise in future studies.

#### **5. Conclusions**

The findings indicate that the performance of an intermittent isokinetic testing protocol of the knee triggers normal and safe BP and HR responses in healthy professional soccer players, with no values exceeding the recommended cardiovascular stability limits.

The angular velocity is not a determining element in the SBP and DBP response.

HR increased linearly during the isokinetic testing protocol until reaching submaximal values, and its increase depends to a great extent on the duration of the isokinetic effort than on its intensity.

The HR of the defenders was higher than those of the forwards and midfielders but was independent of the competitive level.

The SBP and HR values achieved at completion of the treadmill test were significantly higher than those during the isokinetic testing protocol. The final DBP in the isokinetic protocol was higher than that measured at completion of the treadmill test, but the results were not significantly different.

**Author Contributions:** Conceptualization, L.F.-R. and F.G.-M.S.J.; methodology, L.F.-R., O.C.-O. and F.G.-M.S.J.; formal analysis, A.P.-G. and T.P.-F.; investigation, A.P.-G. and F.G.-M.S.J.; writing original draft preparation, A.P.-G. and O.C.-O.; writing—review and editing, F.G.-M.S.J. and T.P.-F.; supervision, L.F.-R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki, and this study was approved by the Ethics Committee of the CEU San Pablo University in Madrid. No. 238/17/18; approval date: 30 July 2019. The study conforms with The Code of Ethics of the World Medical Association.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical considerations.

**Acknowledgments:** We wish to acknowledge John Jairo Aguilera-Correa for his writing assistance.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

