Exercise training is recommended for cardiovascular disease (CVD) patients aiming to restore their maximum level of activity and promote their cardiovascular adaptation [1, 2]. Usually, the exercise intensity is prescribed based on parameters from the cardiopulmonary exercise test (CPX), which is recommended on a regular basis for evaluation of adults with chronic diseases [3]. The CPX should be applied whenever there is clinical indication and availability [4]. However, some more accessible tools, at lower cost, have been used for prescription and monitorization of the exercise training such as the Borg Scale [5], and the Talk Test [6]. These alternative tools are important especially when the CPX is not available, potentially providing useful parameters to exercise prescription, facilitating the applicability and dissemination of the aerobic exercise intervention. They can also be useful for aerobic exercise prescription to cardiopulmonary patients in a home-based setting or when social distancing is necessary, as it has been experienced in the COVID-19 pandemic. Besides that, these prescription resources may be applied for the large part of the global population that is sedentary and presents a low level of physical activity.

The Talk Test (TT) is a non-invasive procedure that assesses metabolic stress through speech provocation, indicating the ideal intensity of aerobic exercise [6, 7]. Methodologically, the TT is divided into progressive stages with speech provocation at the end of each stage. The subject is asked to recite a paragraph and to answer the question: “Are you able to speak comfortably?”. There are three answer options: “Yes”, for positive stages; “More or less”, for equivocal or uncertain stages; and “No”, which corresponds to the negative stage, a condition used to finish the test [8, 9]. Another strategy for speech-provoking, which is less commonly used, is counting as a continuous measure of ventilatory stress. The subject is required to count out loud at their normal rhythm, with the number of counts attained in one breath used as a measure [7, 10].

There is a conflict between metabolic and phonetic functions, caused by increased gas exchange and reduced expiratory time during exercise [11]. Studies have observed that the first ventilatory threshold identified during CPX (e.g., the moment when pulmonary ventilation starts to increase disproportionally in relation to oxygen consumption), may be associated with the moment when this conflict is insurmountable. This usually happens during the uncertain or negative stage of the TT [12, 13]. Furthermore, the relationship between exercise intensity and physiological thresholds in the TT seems to be maintained even with different strategies for speech provocation [12].

The application of the TT has been explored since 2002 in different populations, such as athletes [13, 14, 15], men with prostate cancer [16], overweight and obese patients [17], and patients with CVD [18]. It is presently a recommended test in the current guidelines for CVD patients even without a clear definition of the most appropriate protocol [19, 20]. In addition, its applicability to patients with pulmonary disease is still unknown. This increased interest and usefulness of the TT as an evaluation and prescription tool for patients with cardiopulmonary diseases calls for further knowledge about its applicability, protocols, and properties. Therefore, the present study aims to synthesize the application methods and measurement properties of the TT in individuals with cardiopulmonary diseases.

This systematic review was registered in the Prospective Register of Systematic Reviews (PROSPERO) under code CRD42018106893 and was reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA statement) [21].

To our knowledge, this is the first review to systematically synthesize the application methods and the measurement properties of the TT in individuals with chronic cardiopulmonary diseases. The present review included ten studies that showed the feasibility of the TT application and its usefulness in individuals with chronic cardiopulmonary diseases, mainly in coronary artery disease patients. Most of the studies included a small sample size and replicated the maximal exercise test to the TT protocol. The TT+ was related to the intensity of aerobic exercise prescription corresponding to the VT${}_1$. However, the studies present a divergence between the protocols of the TT application and the findings regarding the validity and reliability of the test.

There are several studies in the literature that support the relationship between the TT stages and ventilatory thresholds [18, 36, 37]. However, this relationship may change depending on the population. When applied to healthy individuals, the TT$\pm$ stage has been correlated with the VT${}_1$ [37]. Quinn & Coons found that when exercising at the TT+ level, individuals were within the recommended training zone based on %VO${}_2$max, % of maximum heart rate and subjective rating of perceived exertion [38].

For both CVD patients and healthy individuals, there are discrepancies in the responses due to the different forms of the TT application. The method of speech provocation, mainly the paragraph length, can influence the response [12]. Schroeder et al. [39] reported that long paragraphs may result in lower VT${}_1$, for instance, but with lower prediction errors. This is aligned with the studies included in this review, which did not show much variation in this respect as most studies used a 30-word paragraph. The duration of each TT stage, when the load is increased and the subject answers the speech provocation, is also a component of the TT protocol that can affect subjects’ response. Stages with a longer duration produce less prediction error. The load reached by the individuals in the points equivalent to the ventilatory thresholds significantly decreases when the stages duration increase [40].

Despite the diversity in the TT protocols and the resulting difficulty in arriving at conclusions, the reliability of the TT was satisfactory [29], both for the assessment by the individual and the physiotherapist [31]. Good results regarding the TT reliability in the context of cardiovascular rehabilitation (CR) were found [41, 42]. However, Petersen et al. [33] found poor reliability when the test was administered by more than one evaluator, even with acceptable ICC values.

Nevertheless, when assessed by the COSMIN scale [22], only one study achieved the “adequate” rating for its reliability assessment design [33]. It is noteworthy that the reliability can be increased by standardizing the TT protocols (e.g.,: ergometer, load increase pattern, speak provocation method, stage duration,…), which would reduce the variation of the test parameters. Among the measurement properties, it is noted that the validity and reliability of a test are critical to ensure that the same can be replicated and used to relate to clinical outcomes [43].

Although systematic reviews are not available on the present topic, literature and narrative reviews shed light on the gaps and strengths around the TT. In a narrative review of major contributions to the literature, Foster et al. [12] presented the TT applicability for exercise assessment and prescription. The review showed strong evidence regarding the heterogeneity of TT protocols, also presented here in individuals with cardiopulmonary diseases. In agreement with other review articles [6, 44, 42], Foster et al. [12] demonstrated the physiological mechanisms involved in the relationship between the TT stages and ventilatory thresholds, reinforcing that TT can allow aerobic exercise practice at optimal intensities when the intensity is adjusted to the highest level of comfortable speech.

The TT was well tolerated by individuals with ischemic heart disease, and its application is safe and effective [35] with minimal measurement errors [31]. Its good clinical applicability is related to its easy administration and low cost [6]. This can potentially impact the CR context, in which the exercise component is very beneficial to patients with CVD [1, 20]. Although there are still barriers to CR implementation, its importance has been well consolidated in the literature, showing a positive impact on several risk factors and on reducing the risk of adverse health events [1, 19, 20]. Among the parameters used for exercise prescription (frequency, intensity, time, type), the intensity seems to be the one that most requires attention so that the benefits of the exercise component of CR may be acquired. The TT can be an alternative tool that has the potential to facilitate exercise prescription and self-monitoring in the recommended target intensity [6, 18, 42]. As highlighted by Saini et al. [42], the TT can be useful especially in contexts like India, where low-cost and more accessible tools are necessary to overcome some causes of the CR underutilization.

Investigation of the TT in future research may include establishing standards for the application protocol in individuals with cardiovascular and pulmonary disease, as well as assessing responsiveness after rehabilitation programs. Because of the exploratory nature of this study, future research is warranted. Furthermore, it is necessary to evaluate the tolerance and validity of the test when applied exclusively to individuals with pulmonary diseases, since these individuals may present some peculiarities related to dyspnea resulting from dynamic hyperinflation [45].

The present study has some limitations. Although it was not part of the study planning and purpose, it was not possible to meta-analyze the results due to the lack of similarity in the analysis and the presentation of the findings by the included studies. Besides, the tool used to evaluate the quality of studies may not have been sensitive enough, underestimating the classification of most studies concerning their methodological quality (low and moderate). Although the population included by the studies covered in this review is very representative of cardiovascular rehabilitation patients, the present study could not address the impact of functional limitations and various disease stages, affecting the validity of the TT. Evidence is missing regarding the applicability of the talk test in a more heterogeneous population, or as cited previously, pulmonary disease patients that present different physiological responses to exercise. As mentioned previously, future research is warranted, especially to explore the validation of an individualized talk test protocol.

Our findings reveal important implications for clinical practice and research. Until now, there is no well-defined and in-depth explored TT protocol. The most common protocol is the graded talk test, replicating the maximal exercise test. This protocol is applied on a cycle ergometer, with one-minute stages and asking the subject to read a standardized paragraph to evaluate the speech comfort level. Because of the heterogeneity in protocols and findings, caution is required when applying and interpreting it. The evidence indicates that the TT is suitable, safe, and feasible for assessment and prescription of exercise in individuals with CVD, showing no case of electrocardiographic evidence of myocardial ischemia. The TT+ and TT$\pm$ stages can be indicated to prescribe exercise when targeting the VT${}_1$. The TT is a valid and low-cost tool that has the potential to be applied in clinical practice.

AMV and MK designed the research study. AMV, EMM, AA, DAR, and GSR performed the research. DLM and MK provided help and advice on the initial research, the results, and the final writing. AMV, EMM, AA, DAR, GSR and MK analyzed the data. AMV, EMM and AA wrote the manuscript. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.

Not applicable.

We would like to acknowledge the support of the GEPCardio research group and the collaborating professor who made this research feasible.

This research was funded by Higher Education Personnel Improvement Coordination (CAPES, grant number 001), and by Institutional Program for Scientific Initiation Scholarships at the Santa Catarina State University (PROBIC/UDESC, PIC2018030000146).

The authors declare no conflict of interest.