NEWER PERSPECTIVES IN LACTATE THRESHOLD ESTIMATION FOR ENDURANCE SPORTS – A MINI-REVIEW

Lactate threshold (LT) estimation in endurance sports continues to be a widely controversial field amongst sports scientists and students despite beyond 50 years of research. With the advent of technology and superior sensors, LT research has ventured into newer fields involving wearables and artificial intelligence. Still, there is a felt need to understand the focused areas of LT research and to guide the students, sports scientists and coaches. The main aim of this mini-review is to identify research categories in a descriptive manner and to synthesize broad themes for future research from latest literature. A comprehensive electronic search in three databases was performed including only original free full text research articles conducted in athletes and heathy subjects, published in English between 2016 and 2020 following PRISMA guidelines. Out of screened 466 articles, 14 articles were finally shortlisted as per inclusion criteria and the findings were summarized. Five research categories were identified and reviewed. for specific sports and the application of valid, reliable noninvasive LT estimation methods in endurance sports. Synthesized broad themes would help guide sports scientists, students and researchers for future research.

Newer Perspectives in Lactate Threshold Estimation for Endurance Sports -A Mini-Review due to reduced lactate clearance. However, lately LT estimation is considered as a point where the individual's physiological system integrates and responds to the stress of the exercise (Poole et al., 2020). Interpretation of the LT estimation provides valuable feedback on the metabolic adaptations that occur with sports training, an important input to the coaches (Kraemer, Fleck, Deschenes, 2011). The estimation of the LT point where the aerobic anaerobic transition happens in endurance sports is nowadays considered a more decisive parameter more than maximal oxygen consumption (VO 2max ) when assessing performance in elite competitive as well as recreational runners (Baron et al., 2008;Etxegarai, Portillo, Irazusta, Arriandiaga, Cabanes, 2018;Meyer, Gabriel, Auracher, Scharhag, Kindermann, 2003). This is generally attributed to the fact that in highly trained endurance athletes, peripheral adaptive changes to training result in improvement in LT percent of VO 2max as compared to VO 2max per se that may become static with training after a certain level in these athletes (McArdle et al., 2017). Approximating the LT point with work rate or speed as well as with time, provides an input to prescribe training intensity to the athlete in an easily understandable and measurable parameter to help improve performance. Assessed individually using other than fixed lactate level LT concepts provides an Individual Anaerobic Threshold (IAT) which generally is used as a benchmark parameter with progression of the training cycle when evaluated longitudinally (Meyer et al., 2000;Poole et al., 2020). The estimation of LT AER and LT ANER have been used widely in prescription of training intensity and to periodize the training microcycle by appropriate load monitoring. LT has been prescribed as ranges of percent of VO 2max , Heart rate reserve (HRR), maximum heart rate (MHR) or rating of perceived exertion (RPE) for practical application of the LT zones in training by coaches and athletes (Etxegarai et al., 2018;Pallarés, Morán-Navarro, Ortega, Fernández-Elías, Mora-Rodriguez, 2016).
However, certain practical difficulties still exist in LT estimation and pose a challenge to the sports scientists and the coaches. Measurement of blood lactate involves an invasive sample collection technique, which is by far the most difficult challenge that generally inhibits athletes to participate whole-heartedly in the evaluation (Onor et al., 2017;Sun, Yi, Li, Li, 2017). Moreover, the sophisticated equipment required for the LT estimation is costly and conduct of the test requires expertise. In addition, with multiple LT concepts and the lack of a standard graded exercise testing (GXT) protocol poses further operational constraints (Faude et al., 2009;Jamnick et al., 2018;Pallarés et al., 2016). In the last few years, with technological advances in lactate analyzers using capillary blood and biomedical sensors with micro-electromechanical systems, the LT estimation research is now focused to develop noninvasive, valid and reliable methods for performance prediction (Amann, Subudhi, Foster, 2006;Bunc, Heller, 1989;Cambri et al., 2016;Candotti et al., 2008;Etxegarai et al., 2018;Onor et al., 2017). There is a felt need to update the sports scientists, researchers, coaches and athletes on the latest area of research in LT concepts and update on the validity and reliability of the commonly used LT concepts in various endurance sports. The main aim of this systematic mini-review is to synthesize latest focus areas in LT concepts research and identify broad research themes for future research studies in endurance sports performance.

Methods
Computerized literature searches following the Preferred Reporting of Items for Systematic Review and Meta-Analyses (PRISMA) guidelines were performed (Moher, Liberati, Tetzlaff, Altman, PRISMA Group, 2009). Search strategy included original research articles only since that was the aim of the study. Free full text articles in English language published between 2016 and 2020 were searched in three scientific databases namely PubMed, Science Direct and Google Scholar. The following keywords were used -'Lactate threshold', 'Anaerobic threshold', 'sports', 'athletes'. The bibliographies of all located articles were screened and a forward citation search was performed.
The search was completed on 20 Nov 2020. Ethical approval was not obtained, as the study essentially was a review of previously published literature.

Study Eligibility
The study eligibility criteria after screening included the following -Free full text, English language, original research article and healthy or actively sporting study population. The exclusion criteria were any type of review articles, systematic reviews and meta-analyses, conference papers, thesis/dissertation works, letters to editors, unpublished data, book chapters and duplicate publications from search databases. Studies not adhering to the inclusion criteria were excluded after assessment for eligibility. Two reviewers did this independently and in case of a difference of opinion, a third reviewer opined on the same.

Data Extraction and Synthesis
A single reviewer did the initial article identification and screening from all three search databases. Two reviewers did screening of 21 free full text articles for study eligibility using the predetermined eligibility criteria. Out of the screened articles, both the reviewers independently without any difference in opinion excluded 07 as per exclusion criteria. Finally, 14 research articles published between 2016 and 2020 were included in this Mini-review. Figure 1 shows the method of study selection as per PRISMA guidelines (Moher et al., 2009). Descriptive summary of the extracted data from these articles were explained with the help of tables and graph.

Results
Two reviewers thoroughly reviewed all fourteen included research articles. The main purpose of the study being to identify key research areas in LT estimation, resulted in classifying the research areas based on the extracted information from the articles into five broad categories. These broad categories of research area and distribution of number articles among these categories are depicted graphically in Figure 2. Analysis of the country of publication of the articles showed that more than 50% i.e. 08 out of 14 research studies were conducted in Europe. With respect to the study design, we observed 11 cross sectional, 01 post analysis of a Randomized control trial, 01 case report and 01 randomized repeated measure design. Summary of Aim and Key findings of the included research articles grouped under the identified broad research categories is as per  The two main confusing and controversial areas of LT research namely LT concepts to be employed and Graded Exercise testing protocols (GXT) to be used were extracted from the included research articles and hence has been tabulated in Table 2. Moreover, the study also aimed at recommending future research themes deriving from the latest broader concepts including the sports type and athletes' type that were studied and hence these findings are tabulated in Table 3 separately.
With respect to LT concepts, we identified as many as 25 LT concepts from these last 5 years of original research after excluding the duplication of concepts. These 25 LT concepts include 22 direct LT concepts with 04 newly studied LT concepts. Three LT concepts were indirect methods to estimate LT using surrogate markers namely Heart rate inflection point (HR_LT), Ventilatory gas Thresholds (VT 1 & VT 2 ) and Electromyogram (EMGth 1 & EMGth 2 ) thresholds. GXT protocols used in these articles have been extracted and synthesized in a structured format namely mode, stage, rest interval, load increment and blood lactate (BLa) sampling technique used for easy comprehension. None of the GXT protocols used in these 14 research articles was similar as shown in Table 2.
In 12 studies out of 14, either competitive or recreational athletes were the study subjects. The sports studied were running (n = 05, 35.7%), cycling (n = 03, 21.4%), swimming (n = 1, 7.1%), ice skating (n = 01, 7.1%), wheelchair basketball players (n = 01, 7.1%) and multiple sports including track & field athletes, basketball and football (n = 01, 7.1%). Two studies included healthy active subjects (Table 3).   To introduce a novel noninvasive individual lactate threshold Heart rate prototype as an alternative for invasive LT tests using a T-shirt integrated with conductive fabric ECG electrodes and LT HR computing algorithm Stage: 1-min, 3-min , 4-min, 7-min and 10-min in 5 GXTs were tested for suitability for LT estimation followed by a Verification Exhaustion bout after cessation of GXT for estimation of VO

Discussion
The main findings of this review showed that five broad research categories (Table 1) have emerged over the last 5 years of LT research with the focus of research skewing more towards noninvasive LT estimation using wearables and artificial intelligence (Figure 2). Most of these research studies were conducted in European countries and developed nations signifying the present research trend and highlighting the need of pursuing focused research on LT in sports in other parts of the world. The common sports in which LT research was conducted are running and cycling, contributing more than 50% of the other endurance sports. A recent study employed LT estimation in performance assessment of wheelchair basketball players also, thereby applying the LT concepts in para-athletes too.
Although in the last decade, many review articles on lactate kinetics and LT training were published, it was seen that most of these review articles focused on either the evolving lactate kinetics or LT concepts (Beneke et al., 2011;Galán-Rioja, González-Mohíno, Poole, González-Ravé, 2020;Hall et al., 2016;Poole et al., 2021;Rogatzki, Ferguson, Goodwin, Gladden, 2015;Sarma, 2018). Hence, our main aim was to identify broad research categories in recent LT research especially in athletes and healthy individuals over the last 05 years for identification of focused areas and to synthesize broad themes for future research in sports. The eligibility criteria was also designed accordingly.

GXT Protocol Design
Graded exercise test (GXT) protocols in terms of stage duration, continuous or intermittent between stages, load increment with each stage as well as the method of lactate measurement have been considered as independent variables that influence the LT estimation irrespective of the concepts that are employed. (Bentley, McNaughton, 2003;Bentley et al., 2007;Jamnick et al., 2018). As shown in Table 2, amongst the 14 research studies included, each study has followed a different protocol design. There has been no clear consensus with respect to appropriate use of GXT for LT estimation in terms of all the protocol components (Jamnick et al., 2018). Jamnick et al. (2018) have studied the validity of almost 16 LT concepts in about five different GXTs with respect to the stage duration using customised load increments based on demographic and Physical activity readiness scale scoring (Jamnick et al., 2018). LT estimation varied with all the GXT duration and the closest LT concepts to MLSS were newer modified Dmax methods employed in this study namely Exponential D max , Log-log Modified D max and Log-log Exponential Mod D max . Log -Poly Mod D max of GXT with 4 min stage duration was the closest to LT MLSS . Moreover, customization of load increment with stages as well as based on the outcome parameters have been recommended by the authors of this study (Jamnick et al., 2018). Faude et al. (2009) have carried out a comprehensive review on validity of various LT concepts (Faude et al., 2009). According to this review, validation of the LT concepts was done either with MLSS or competition performance. LT 4 or OBLA 4mmol/L and IAT (Stegmann, Kindermann, Schnabel, 1981) were by far the most commonly studied LT concepts against MLSS for validation in various endurance sports (Faude et al., 2009). As summarized in Table 1 (S. No. 1), we found four research studies conducted with the aim of evaluating validity, repeatability and predictive value of LT concepts (Fernandes et al., 2015;Heuberger, Gal, Stuurman, Keizer, de Muinck, Miranda, Cohen, 2018;Jamnick et al., 2018;Pallarés et al., 2016). Three studies were conducted in cyclists (Heuberger et al., 2018;Jamnick et al., 2018;Pallarés et al., 2016) and one study in runners (Fernandes et al., 2015). To summarize these findings, LT 4 or OBLA 4mmol/L again fared well in all the studies with respect to validity, repeatability and predictive value. However, use of LT4 underestimated LT in low-trained athletes (Fernandes et al., 2015). Controversy in D max LT concept was observed between these studies (Heuberger et al., 2018;Pallarés et al., 2016). However, Jamnick, Botella, Pyne, Bishop (2018) with newer LT concepts of modified D max found them to be closest to LT MLSS . Apart from these LT concepts, LT + 2.0 mmol/L and Minimum Lactate equivalent (La/ Power) + 1.5 mmol/L have also been studied in these studies with good validity and predictive value in cyclists (Heuberger et al., 2018;Pallarés et al., 2016).

Physiological variables at LT for performance evaluation
Under this research area, we could identify three articles as shown in Table 1. All these three studies were done in different endurance sports like swimming, trail running and wheelchair basketball players. Pelarigo, Greco, Denadai, Fernandes, Vilas-Boas, Pendergast (2016) have studied the relationship of bioenergetics variables and biomechanical variables of female swimmers at various percentages of MLSS. They have found that at 100% MLSS, bioenergetics variables were constant but biomechanical variables namely stroke rate increased and stroke length reduced. This study compared biomechanical variables and their relationship with physiological variables at various MLSS intensities for performance evaluation (Pelarigo, Greco, Denadai, Fernandes, Vilas-Boas, Pendergast, 2016). We identified a similar study relating biomechanical variables of running with energy cost or running economy at LT mainly in runners (Joubert, Guerra, Jones, Knowles, Piper, 2020). Scheer, Vieluf, Janssen, Heitkamp (2019) examined established LT concepts in Trail runners of varying distances for the first time and evaluated LT estimation for performance prediction (Scheer et al., 2019). Otto, Reer, Holtfreter, Riepenhof, Schröder (2019) compared arm crank ergometer with treadmill wheelchair propulsion ergometer using physiological parameters at peak performance and IAT to provide optimal training prescription recommendations in wheelchair basketball players (Otto et al., 2019). All these three research studies provide the latest insight into the utilization of LT concepts and physiological variables at LT in performance evaluation and more so importantly application of LT for training prescription even in para-athletes.

Noninvasive LT estimation (Lactate related)
Because of the major drawback of invasive methodology used for obtaining blood samples, various research studies have focused on noninvasive LT estimation in an attempt to negate this major drawback. The noninvasive LT estimation can be broadly divided into two major research areas as Lactate related i.e. using alternate source of lactate or by using machine learning methods and Indirect i.e. using surrogate markers for lactate itself to identify the LT indirectly. We identified three research studies under this research area. Onor, Gufoni, Lomonaco, Ghimenti, Salvo, Sorrentino, Bramanti (2017) have validated sweat lactate level measurement during cycling exercise with High Performance liquid chromatography in healthy volunteers (Onor et al., 2017). However, LT using sweat lactate method if validated with conventional LT estimation would prove to be of immense value in future LT research. Etxegarai, Portillo, Irazusta, Arriandiaga, Cabanes (2018) from Spain have tried developing machine learning based LT prediction algorithm and validated with blood lactate LT estimation in recreational runners (Etxegarai et al., 2018). In addition, using heuristic approach, the same research group has developed an equation for recreational runners for LT workload estimation based on running speed reserve and initial running speed on treadmill during a GXT (Etxegarai, Portillo, Irazusta, Koefoed, Kasabov, 2019). These methods if further researched and validated would prove very useful for athletes and active individuals where expertise and facilities for LT estimation are not available.

Noninvasive LT estimation (Indirect)
This category of research idea included noninvasive LT estimation using surrogate markers that was popular since the years of Wasserman and Conconi using ventilatory thresholds and heart rate inflection as indirect markers of LT (Conconi, Ferrari, Ziglio, Droghetti, Codeca, 1982;Conconi et al., 1996;Wasserman, McIlroy, 1964). In this review, we located three research articles in this area of LT research, one each from New Zealand, China and Canada. Borges, Driller (2016) in their study had evaluated a wearable device based LT estimation (WLT) using the Near Infrared Spectroscopy principle in runners (Borges, Driller, 2016). The device that was worn over the calf, has an algorithm to predict the LT, and was shown to be valid and reliable in this study. The correlation of WLT was highest with OBLA 4mmol/L and both inter-device as well as intra-device reliability were high (r = 0.97 in both the cases) (Borges, Driller, 2016). Sun, Li, Li (2017) from China had published a case report to introduce a novel noninvasive individual lactate threshold Heart rate prototype as an alternative for invasive LT tests using a T-shirt integrated with conductive fabric ECG electrodes and HR_LT computing algorithm (Sun et al., 2017). In this study, they had devised an indirect HR_LT based algorithm to identify the LT training zones using modified Conconi's method of heart rate inflection point using the ECG electrodes. Despite the inherent accuracy issues with heart rate due to various confounding variables, heart rate based exercise & sports training is popular and commonly used in wearable technology. Hence, this noninvasive indirect LT estimation research category still merits focus among researchers. Piucco, Diefenthaeler, Prosser, Bini (2020) have assessed the EMG th1 and EMG th2 breakpoints from 6 different lower limb muscle sites with VT 1 and VT 2 in Ice skaters (Piucco et al., 2020). This study though did not directly use LT methods to validate was still included to bring out the importance of noninvasive LT research using indirect surrogate markers even for validation, here VT 1 and VT 2 .

Newer Methods
A final research category as Newer methods was framed to include research studies that were not fitting into any of the above four categories. Capellá et al. (2018) from Spain had presented a new concept of inter threshold area between VT 1 and VT 2 among individuals with varying endurance capacities (Capellá, Peinado, Moro, Revenga, Esteves, Montero, 2018). Further application of this new concept in training and performance evaluation of athletes are promising research areas for future research.

Future research themes
One of the objective of this review was also to suggest broad research themes for future research to sports scientists, students, coaches and physical education professionals. Table 3 shows the synthesized broad themes based on the categorized research areas and included original research studies under the respective categories. Despite more than 50+ years of Lactate Research in sports (Poole et al., 2020), consensus in GXT protocol, LT concept for specific sports, validation of common LT sports in endurance sports other than running and cycling, gender difference in LT estimation and LT research in Para-sports, validated noninvasive lactate measurement techniques are lacking. There is certainly immense scope of future research in these broad research themes.
Even though the aim of the review was to identify research categories and provide a roadmap for future research in LT estimation in sports, there were few limitations in terms of the search being restricted to freely available full text articles in the databases. This would have limited the number of research ideas in the field. In addition, since the search was restricted to athletes and healthy active adults, this review lacks research themes done in patient population and clinical research.

Conclusion
Lactate threshold, despite a long research history, is still an actively researched area globally in sports due to its varied applications. The researchers are focused mainly on GXT study protocols, evaluating validity & predictive value of LT concepts and developing noninvasive methods for LT estimation using wearable technology and machine learning arena for performance enhancement in competitive as well as recreational sports. This review has laid the roadmap for future research themes to guide the sports scientists, students and researchers and future research based upon the suggested themes will shed more light upon the conundrum that is LT research.