The overall aim of this thesis was to investigate HRV (parasympathetic indices) as a tool for monitoring recovery from exercise. The specific aims were to (i) understand physiological mechanisms and kinetics underlying cardiac parasympathetic reactivation following exercise, (ii) determine the effects of hydrotherapy on post-exercise cardiac parasympathetic reactivation, subsequent exercise performance, perceptions of recovery and other physiological variables, and (iii) assess how training load induced changes in HRV can be interpreted to assess positive and
The objective of exercise training is to initiate desirable physiological adaptations that ultimately enhance physical work capacity. Optimal training prescription requires an individualised approach, with an appropriate balance of training stimulus and recovery. Fundamental to achieving an optimal training-recovery balance is the ability to assess the fatigue or recovery state of an individual. HRV has emerged as a simple, non-invasive tool to assess the homeostatic effect of both acute and chronic exercise on the cardiac autonomic nervous system. The cardiac autonomic nervous system operates purposefully in response to external stimuli such as exercise training. As a marker of recovery, cardiac parasympathetic reactivation following a training session is highly individualised.
A first literature review demonstrated that cardiac parasympathetic reactivation after exercise likely depends on metaboreflex stimulation in the short-term (i.e., 0−90 min post-exercise) and baroreflex stimulation in the intermediate term (i.e., 1−48 h post-exercise). Complete autonomic recovery requires up to 24 h for low-intensity exercise, 24−48 h for threshold-intensity exercise, and at least 48 h following high-intensity exercise. Based on limited data, exercise duration is unlikely the greatest determinant of cardiac parasympathetic reactivation. Cardiac autonomic recovery occurs more rapidly in individuals with greater aerobic fitness.
Accelerating the recovery process may potentially allow athletes to complete more quality training thereby enhancing the training effect. Therefore, whether manipulation of cardiac parasympathetic reactivation can alter recovery state and subsequent exercise performance is of interest to athletes and coaches. The three experimental studies contained in this thesis investigated whether acceleration of cardiac parasympathetic reactivation following exercise via hydrotherapy wasbeneficial for recovery and subsequent exercise performance.
In the first experimental study we investigated how changes in cardiac parasympathetic reactivation during recovery are associated with perceptions of recovery and subsequent exercise performance. Well-trained cyclists completed a high-intensity training session (HIIT), followed 20 min later by either passive rest (PAS), cold water immersion (CWI), or contrast water immersion (CWT). Thereafter they rested quietly for 160 min, after which they completed a work-based time trial. In the second experimental study we investigated performance and HRV over consecutive days of cycling with post-exercise CWI or PAS. Well-trained cyclists completed two separate 3-d training blocks (120 min cycling per day), followed by 2 d of recovery-based training. Following each training session the cyclists stood in cold water (10°C) or at room temperature (27°C) for 5 min. In the third experimental study we investigated how acceleration of post-exercise cardiacparasympathetic reactivation via CWI affects physiological adjustments during a standardised HIIT session. Well-trained cyclists completed two HIIT sessions separated by 30 min during which, they stood in cold water (10°C) or at room temperature (27°C) for 5 min.
Finally, successful implementation of HRV as a tool for individualising training prescription and monitoring recovery state requires accurate interpretation of observed changes in cardiac parasympathetic activity. A second literature review demonstrated that various approaches are available to interpret how changes in parasympathetic HRV reflect adaptations to endurance training regimes in elite athletes. These include: (i) monitoring individual adaptive responses using weekly and 7-day rolling averages, rather than single-day values of parasympathetic-derived indices of HRV, and (ii) assessing the HRV to R−R interval ratio.
The data in this thesis suggest that post-exercise cardiac parasympathetic reactivation kinetics may reflect the overall recovery process and therefore have application for individualising training prescription and monitoring recovery status of athletes (via the novel method for interpreting how changes in HRV reflect adaptations to training we have proposed). Acceleration of cardiac parasympathetic reactivation through hydrotherapy benefits perceptions of recovery during the acute period post-exercise, and may allow athletes to train harder without compromising autonomic nervous system status. However, hydrotherapy during short-term recovery between exercise bouts alters physiological responses during subsequent high-intensity exercise, possibly to the detriment of performance.