Respiratory rate is a fundamental vital sign that is sensitive to different pathological conditions (e.g., adverse cardiac events, pneumonia, and clinical deterioration) and stressors, including emotional stress, cognitive load, heat, cold, physical effort, and exercise-induced fatigue. The sensitivity of respiratory rate to these conditions is superior compared to that of most of the other vital signs, and the abundance of suitable technological solutions measuring respiratory rate has important implications for healthcare, occupational settings, and sport. However, respiratory rate is still too often not routinely monitored in these fields of use. This review presents a multidisciplinary approach to respiratory monitoring, with the aim to improve the development and efficacy of respiratory monitoring services. We have identified thirteen monitoring goals where the use of the respiratory rate is invaluable, and for each of them we have described suitable sensors and techniques to monitor respiratory rate in specific measurement scenarios. We have also provided a physiological rationale corroborating the importance of respiratory rate monitoring and an original multidisciplinary framework for the development of respiratory monitoring services. This review is expected to advance the field of respiratory monitoring and favor synergies between different disciplines to accomplish this goal.

There is an ever-growing demand for measuring respiratory variables during a variety of applications, including monitoring in clinical and occupational settings, and during sporting activities and exercise. Special attention is devoted to the monitoring of respiratory rate because it is a vital sign, which responds to a variety of stressors. There are different methods for measuring respiratory rate, which can be classed as contact-based or contactless. The present paper provides an overview of the currently available contact-based methods for measuring respiratory rate. For these methods, the sensing element (or part of the instrument containing it) is attached to the subject’s body. Methods based upon the recording of respiratory airflow, sounds, air temperature, air humidity, air components, chest wall movements, and modulation of the cardiac activity are presented. Working principles, metrological characteristics, and applications in the respiratory monitoring field are presented to explore potential development and applicability for each method.

The use of wearable sensor technology for athlete training monitoring is growing exponentially, but some important measures and related wearable devices have received little attention so far. Respiratory frequency (f_R), for example, is emerging as a valuable measurement for training monitoring. Despite the availability of unobtrusive wearable devices measuring f_R with relatively good accuracy, f_R is not commonly monitored during training. Yet f_R is currently measured as a vital sign by multiparameter wearable devices in the military field, clinical settings, and occupational activities. When these devices have been used during exercise, f_R was used for limited applications like the estimation of the ventilatory threshold. However, more information can be gained from f_R. Unlike heart rate, $\dot{\text{V}}$O₂, and blood lactate, f_R is strongly associated with perceived exertion during a variety of exercise paradigms, and under several experimental interventions affecting performance like muscle fatigue, glycogen depletion, heat exposure and hypoxia. This suggests that f_R is a strong marker of physical effort. Furthermore, unlike other physiological variables, f_R responds rapidly to variations in workload during high-intensity interval training (HIIT), with potential important implications for many sporting activities. This Perspective article aims to (i) present scientific evidence supporting the relevance of f_R for training monitoring; (ii) critically revise possible methodologies to measure f_R and the accuracy of currently available respiratory wearables; (iii) provide preliminary indication on how to analyze f_R data. This viewpoint is expected to advance the field of training monitoring and stimulate directions for future development of sports wearables.

Recent advances in understanding the importance of respiratory frequency (fR) as a sensitive marker of a variety of physiopathological stressors are fostering growing interest in respiratory monitoring. This interest is further stimulated by the everincreasing efforts that companies are devoting to the development of systems measuring fR. There are a variety of techniques based on different sensors and technologies for fR monitoring. These techniques are commonly classified as contact-based or contactless, depending on whether the system which embeds the sensor is in contact with the body or not. This review is focused on the contactless methods for fR monitoring. We have introduced the main fields of use where contactless respiratory monitoring is important and provided a taxonomy to classify the most popular contactless techniques for fR monitoring. Finally, we have described the performances of the most popular methods, the main open challenges, and the main perspectives.

Systems for cardiorespiratory monitoring hold tremendous promise for many applications in clinical settings, harsh environments, sports science and telemonitoring. However, the translation in the real world is challenging because it is difficult to develop unobtrusive and comfortable systems with high metrological performance. Recent advances in fiber optic technology in the field of sensing and their application to instrument wearable systems can tackle the main issues in this field. In this work, we focused on the recent advances in cardiorespiratory monitoring of fiber Bragg grating (FBGs) sensors which are one of the most popular sensors based on fiber optic technology. We provide an overview of the FBGs working principle and performance. We also describe the most popular and promising configurations based on FBGs used for cardiorespiratory monitoring.

In order to provide further insight into the link between respiratory frequency (f_R) and the rating of perceived exertion (RPE), the present study investigated the effect of exercise duration on perceptual and physiological responses during self-paced exercise. Nine well-trained competitive male cyclists (23 ± 3 years) performed a preliminary incremental ramp test and three randomised self-paced time trials (TTs) differing in exercise duration (10, 20 and 30 min). Both RPE and f_R increased almost linearly over time, with a less-pronounced rate of increase when absolute exercise duration increased. However, when values were expressed against relative exercise duration, no between-trial differences were found in either RPE or f_R. Conversely, between-trial differences were observed for minute ventilation ( _E), O₂ and heart rate (HR), when values were expressed against relative exercise duration. Unlike the relationship between RPE and both _E and HR, the relationship between RPE and f_R was not affected by exercise duration. In conclusion, f_R, but not _E, HR or O₂, shows a strong relationship to RPE and a similar time course, irrespective of exercise duration. These findings indicate that f_R is the best correlate of RPE during self-paced exercise, at least among the parameters and for the range of durations herein investigated.