# The Role of Cholinesterases in Post-Exercise HRV Recovery in University Volleyball Players
## 4 Discussion  
The main contribution of our study is that ChE shows a behavior during postexercise recovery that is similar to the RMSSD indexes, SS, and S:PS ratio of HRV; therefore, we can consider these internal load markers. It has been described in the literature that during a postexercise recovery, the interaction of the parasympathetic and sympathetic systems is complex [5; 12] and mediated mainly by parasympathetic reactivation [36; 37]. This includes multiple regulations such as the descent of circulating catecholamines, blood pressure, baroreflexes, and metabolome-flexes that cause a drop in sympathetic stimulation [4; 5; 17] due to an efferent reflex effect of vagal stimulation [6]. This effect is regulated by cholinergic signaling in the sinoatrial node, and in a certain way, ChE are mediators of the vagal impulse by hydrolyzing ACh [6; 20; 21]; thus, an increase of ChE in blood indirectly reflects an increase in the rate of ACh hydrolysis leading to ACh descent [27; 38; 39]. Our results support this idea since. in postexercise measures, the increase of ChE widely correlated with an increase in sympathetic activity (SS and S:PS ratio) and a decrease in RMSSD (Table 3). Afterward, in the 24H and 48H measures, ChE normalized, and RMSSD returned to her initial values, as well as the SS and the S:PS ratio. These changes may be because AChE facilitates the signaling of the onset and termination of the cardioinhibitory effect by hydrolyzing ACh in the synaptic cleft in a very short time (<1 s) [4; 40]. Subsequently, the parasympathetic nerve will be stimulated in the phasic mode, where released acetylcholine will hyperpolarize the sinoatrial node by binding to its muscarinic receptors decreasing the depolarization slope and thereby delaying the next sinoatrial node impulse [18; 19; 41] and, thus, favoring postexercise HRV recovery. This seems to indicate that these variables evaluate the internal load regardless of the type of exercise performed since, although some authors relate the recovery with duration [4; 42] or with intensity [5; 43], in our results, it can be seen that the workload was very similar for all subjects (Table 1), demonstrating that the changes in response show the individual assimilation of that load. Therefore, the changes in these variables would fit into the concept of internal load. The indexes, RMSSD, SS, and S:PS ratio have been used jointly in various studies to see the behavior of the sympathetic and parasympathetic systems, both in training control and in competition, concluding that these indicators are valid and reliable for training and competence monitoring [13; 44; 45; 46]. On the other hand, cholinesterase values (Table 2) are within the range of acceptable clinical values for the technique used (1278 \(\pm\) 338 pg/mL), so the changes observed in this variable are not at any time abnormal or pathological values. Thus, we observed that the postexercise ChE behavior coincides with the increase reported in the study by [PERSON] et al. [29]. However, this does not seem to be the case with that reported by [PERSON] et al. [30], as the behavior of ChE in women is very similar to our results but not so in men. This could be because the impact of the session was not sufficiently demanding to provoke high rates of ACh hydrolysis. The values of RMSSD, SS, and the S:PS ratio coincide with those presented by other authors [13; 44; 47], and they would be within the 25-50 percentile of the reference tables reported by [PERSON] et al. [47]. The behavior of HRV indices following such an exercise is similar to what has already been well-documented in the literature [13; 48], where it has been reported that HRV is a reliable indicator to measure fatigue and postexercise recovery with acceptable relations [3; 4; 5; 8; 9; 49], establishing the average values for a good state of the athlete (recovered) of RMSSD above 30-40 ms, SS below 10 AU and S:PS ratio below 0.5 AU [11; 44; 50].