Interpreting Cardiac Autonomic Responses to Exercise

The day-to-day variations in training load entail large daily variations in cardiac ANS activity (i.e., CV = 10–20% for Ln rMSSD Buchheit et al., 2010b (link), 2013a (link)). In general, intense exercise acutely decreases vagal-related HRV indices for 24–48 h, that may coincide with homeostasis restoration and perceived levels of overall recovery (Stanley et al., 2013a (link)). Following these observations, these day-to-day variations in HRV have been used to guide training contents on a daily basis (i.e., train at high intensity only when vagal-related HRV indices have returned back to normal levels) (Kiviniemi et al., 2007 (link), 2010 (link); Stanley et al., 2013a (link)). Such a HRV-guided training approach has led to greater improvements in endurance performance when compared with “traditional” training programing (Kiviniemi et al., 2007 (link), 2010 (link); Stanley et al., 2013a (link)). However, this training approach may not be as simple as previously thought. Under specific circumstances, such as following heavy training loads in the heat, increased, not decreased, vagal-related HRV indices have been observed within 24 h, despite an acute decrease in perceived wellness (Buchheit et al., 2013a (link)). Additionally, during an intense multi-day desert race (i.e., Marathon des sables 2005, running 253 km in 7 days in extremely hot environment), after the expected initial drop in vagal-related indices during the first 3 days (Brenner et al., 1998 (link)), we observed a clear increase in cardiac parasympathetic activity, which did not match the reported increased perceived fatigue and drop in running performance (Figure 4). This inversed association between vagal-related HRV indices and acute fatigue suggests that in addition to the load of the preceding session(s), data on environmental conditions and hydration status need to be considered to correctly interpret the ANS response to exercise (Buchheit et al., 2013a (link); Stanley et al., 2013a (link)). This is directly related to the fact that increases in plasma volume, that are usual responses to both intense aerobic-oriented exercise (Green et al., 1984 (link)) and heat acclimatization (Ladell, 1951 (link)), tend to increase beat-to-beat HRV, independently of clear changes in fatigue and/or fitness (Spinelli et al., 1999 (link); Buchheit et al., 2009 (link), 2013a (link)). Finally, other factors influencing the recovery time course of cardiac autonomic activity, such as hydrotherapy and sleep should also be considered when interpreting daily changes in HRV (Al Haddad et al., 2012 (link); Stanley et al., 2013b (link)).