The first finding of this study was that HRmax was significantly different for the UMTT compared with the Tlim. One of the different criteria to establish the achievement of maximum effort in a protocol is an HRmax equal to that predicted from a selected formula. In our case, maximum effort was expected in a cohort of well-trained endurance runners during both running tests because of their high tolerance for strenuous exercise. Moreover, in both cases they reported RPEs >19, further signifying substantial effort.
Excluding thermoregulatory factors related to the environment because the athletes ran in a thermoneutral condition (temperature: 22-24 °C; relative air humidity: 60-80%) and because the running exercises were quite short in both cases (<30 minutes), it is reasonable that the greater UMTT duration promoted more corporal temperature elevation and, consequently, more dehydration, with more subsequent cardiovascular stress (34). Indeed, the UMTT promoted full activation of the aerobic processes, whereas not all the subjects in the Tlim test attained oxygen responses equivalent to those previously recorded during incremental tests (22,24) because this test was dependent on each athlete's local muscular endurance (23) and training status (2). From these considerations, the similar levels of acidosis between exercise modes (11.82 vs. 12.01; p > 0.05) was surprising because this represented a similar metabolic activation. With this aspect in mind, we suggest that protocol conditions should be taken into consideration when examining maximal efforts and the attainment of true HRmax in further studies.
The major finding of this investigation was that BHR, HRΔ1, and HRΔ2 were significantly correlated to performance in the UMTT. To our knowledge, this is the first study that has reported such relationships correlating autonomic control and endurance running performance in the field. Indeed, this is the first study that has observed these relationships in a sample of well-trained endurance runners; some studies have reported endurance running performance and HRR relationships for the trained state (18), whereas others have failed to find such relationships in well-trained distance runners (9).
These results support previous evidence that endurance training results in greater performance and greater parasympathetic activity before and after exercise (13,20). The influence of training on the autonomic control of HR is evident in the first weeks (8) and regresses to initial levels after a few weeks of detraining (40). Moreover, a single session of mild exercise performed by sedentary young men has been shown to lead to significant autonomic nervous system improvements (37). Also, endurance training changes in cardiac autonomic nervous system modulation partly contribute to decreases in HR at rest (32). This adaptation of the autonomic nervous system occurs sooner in the immediate postexercise period than at rest (41). From these considerations, it was expected that a relationship between MAS, a specific performance parameter associated with o2max, and BHR and HRR, indirect parameters reflecting greater parasympathetic activation, would be exhibited after a long period of endurance training. We considered that the wide range of running performance in our sample (MAS range of 19-22 km·h−1) would reveal more obviously the relationship between autonomic neural modulation, as reflected in BHR and HRR, and running performance.
On the other hand, other factors that influence running performance, such as improved o2max, anaerobic threshold, and running economy (3), could have influenced our results. Moreover, the relationship between HRV and aerobic performance in subjects with high cardiovascular parasympathetic activity could be controversial because some authors (19), quoted by others (9), have found that HRV increased proportionately with cardiac parasympathetic activity until a critical point, beyond which further increases in parasympathetic activity caused decreases in HRV. Regarding this topic, other authors have suggested that these factors could be based on the anatomic and physiological characteristics of the heart (33). Indeed, others (30) have found a correlation between the increase in the left ventricular internal diameter at end-diastole (LVIDd) and running performance in elite athletes in a longitudinal study. These authors argue that a higher LVIDd results in a much easier recovery for the organism (30). This rationale could contribute to the current exhibited relationship between performance, HRR, and BHR; however, no direct measure of LVIDd was available in our study.
Another interesting result was the correlation between HRΔ1 and HRΔ2 after the UMTT (r = 0.867, p = 0.000). Indeed, the values of HRΔ1 for UMTT and Tlim were also correlated (r = 0.789, p = 0.002). These relationships confirm HRΔ1 as a valid marker of parasympathetic reactivation that is constant across different testing conditions. Further, we suggest that HRΔ1 would be a more valid indictor of the level of parasympathetic reactivation because only HRΔ1 after both running protocols correlated with UMTT performance (HRΔ1UMTT-TUMTT: r = 0.635, p = 0.027; HRΔ1Tlim-TUMTT: r = 0.832, p = 0.001). Others (25), though, have stated that the first 30 seconds of recovery provide a better, more specific index of vagally mediated HRR. It remains to be seen whether HRR during the first 30 seconds or during the first minute provides a superior indication of parasympathetic reactivation, particularly for trained athletes.
An important aspect to consider in conjunction with the current results is HR neural modulation during tapering, because the current study was performed after the last competition of the season (i.e., 7-15 days). Some studies have reported slower HRR in the tapering period after an intensive training period (25,27), whereas others (36) have concluded that BHR did not seem to change during tapering. In our study, the athletes were allowed to maintain a low-intensity running regimen (<80% HRmax) and/or active recovery participation in recreational activities, mixed with rest days ad libitum, after the last competitive event of the summer season. On the basis of prior studies (25,27,36) and the results of the current study, we would suggest that the stage of the season be considered when evaluating BHR and HRR, because the extent of tapering may impact the cardiac autonomic control for athletes.
The significance of the relationships between HRR, BHR, and performance is substantial in terms of monitoring athlete training by coaches who do not have access to laboratory evaluations. From these results, we suggest the use of BHR and HRR in the field to monitor and identify training effects in athletes throughout the training season. Further research is needed to identify ranges of performance associated with ranges of HRR, how HRR and BHR could help to identify training status, and the potential use of HRR for the appropriate training load dosage for increased performance, as suggested by some authors (8).
On the basis of the current results, we suggest that BHR and HRR after maximal running exercises be used as training state parameters during the competitive season. Also, the relationship between HRR (particularly HRΔ1) and performance in the UMTT enhances the utility of this simple field running test for monitoring adaptations in well-trained athletes.
This study was supported in part by a Consejo Superior de Deportes grant (12/UPB31/06). We want to fully recognize the collaboration of all the athletes in this study.
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