Cardiac Acceleration at the Onset of Exercise: A Potential Parameter for Monitoring Progress During Physical Training in Sports and Rehabilitation

There is a need for easy-to-use methods to assess training progress in sports and rehabilitation research. The present review investigated whether cardiac acceleration at the onset of physical exercise (HR<inf>onset</inf>) can be used as a monitoring variable. The digital databases of Scopus and PubMed were searched to retrieve studies investigating HR<inf>onset</inf>. In total 652 studies were retrieved. These articles were then classified as having emphasis on HR<inf>onset</inf> in a sports or rehabilitation setting, which resulted in 8 of 112 studies with a sports application and 6 of 68 studies with a rehabilitation application that met inclusion criteria. Two co-existing mechanisms underlie HR<inf>onset</inf>: feedforward (central command) and feedback (mechanoreflex, metaboreflex, baroreflex) control. A number of studies investigated HR<inf>onset</inf> during the first few seconds of exercise (HR<inf>onsetshort</inf>), in which central command and the mechanoreflex determine vagal withdrawal, the major mechanism by which heart rate (HR) increases. In subsequent sports and rehabilitation studies, interest focused on HR<inf>onset</inf> during dynamic exercise over a longer period of time (HR <inf>onsetlong</inf>). Central command, mechanoreflexes, baroreflexes, and possibly metaboreflexes contribute to HR<inf>onset</inf> during the first seconds and minutes of exercise, which in turn leads to further vagal withdrawal and an increase in sympathetic activity. HR<inf>onset</inf> has been described as the increase in HR compared with resting state (delta HR) or by exponential modeling, with measurement intervals ranging from 0-4 s up to 2 min. Delta HR was used to evaluate HR<inf>onsetshort</inf> over the first 4 s of exercise, as well as for analyzing HR<inf>onsetlong</inf>. In exponential modeling, the HR response to dynamic exercise is biphasic, consisting of fast (parasympathetic, 0-10 s) and slow (sympathetic, 1-4 min) components. Although available studies differed largely in measurement protocols, crosssectional and longitudinal training studies showed that studies analyzing HR<inf>onset</inf> in relation to physical training primarily incorporated HR<inf>onsetlong</inf>. HR <inf>onsetlong</inf> slowed in athletes as well as in patients with a coronary disease, who have a relatively fast HR<inf>onsetlong</inf>. It is advised to include both HR<inf>onsetlong</inf> and HR<inf>onsetshort</inf> in further studies. The findings of this review suggest that HR<inf>onset</inf> is a potential tool for monitoring and titrating training in sports as well as in rehabilitation settings, particularly in patients with ventricular fibrillation. Monitoring HR<inf>onset</inf> in the early phase of training can help optimize the effectiveness of training and therapy. More research is needed to gain a better understanding of the mechanisms underlying HR<inf>onset</inf> in relation to their application in sports and rehabilitation settings. © 2014 Springer International Publishing Switzerland.