Summary

• Wearable technology has provided new avenues for maximising athletes' performance, health, and safety more than ever in the sport's history and this technology is constantly evolving.

• Wearables are frequently used to monitor training, recovery, fatigue and performance among athletes.

• Though fatigue is not measured directly by wearable devices, they use various internal (Electrocardiogram and photoplethysmography) and external sensors (accelerometer, magnetometer, gyroscope, and GPS) to make predictions about the overall health of athletes from different angles, ultimately offering information on fatigue.

• Advanced algorithms within wearable technology allow us to understand the meaning of thousands of real-time data both off the field and on the field.

• By measuring variables such as heart rate variability, training load, sleeping, body temperature, energy expenditure and psychological variables with wearables, now coaches, trainers, and scientists can better understand physical demands in real-time and their effect on fatigue.

• Wearable technology is still developing and has not reached its full potential, meaning there are some issues regarding its reliability and validity in monitoring fatigue. However, when this technology reaches its full potential, this would be the ultimate tool for monitoring fatigue.

What are wearables?

Wearable technology has revolutionised the modern sports industry, where special attention is being given by researchers and engineers to develop this technology to a whole new level. Also, being sophisticated and embedding the latest technology into small devices have made wearable devices more complex to understand. However, advanced algorithms and programmes allow us to process thousands of real-time data into more meaningful information, where we can simply understand what all those numbers mean. 

In general, wearable technology can be defined as a type of electronic device which can be used as accessories and can be attached or placed on clothing or on user’s body to collect highly sensitive biometric data in real-time . These data predict specific parameters such as an individual’s health, energy expenditure and sleeping quality.

How do wearables work?

Different types of internal and external sensors are being used to understand the complex interactions between training and its influence on athletes’ body systems. The internal sensors tend to measure physiological changes (heart rate, breathing rate, muscle oxygen level etc.) that change due to both training and competition stress. In contrast, external sensors can measure variables outside the body, such as total distance covered, acceleration, velocity, etc.

Internal sensors

The internal sensors provide a glimpse of what is happening physiologically within our bodies and often track quantitative data. Internal physiological variables such as heart rate, pulse oximetry, breathing rates and skeletal muscle oxygenation are frequently used to measure individuals' relative intensities. Different types of internal sensors are being used to track these physiological variables. For instance, heart rate is one of the main physiological variables that is used in sports and the majority of heart rate monitoring devices are equipped with the technology of electrocardiography and photoplethysmography.

External sensors

The most frequent external sensors that can be found on wearable devices are the Inertial Measurement Unit (IMU) and Global Positioning System (GPS). The IMU is made up of a combination of accelerometers; which measures acceleration, gyroscopes; which measures angular velocity, and magnetometers; which measures changes in magnetic vector. Also, the integration of GPS technology with IMU has enabled us to measure accurate real-time data on velocity, distance and precise location. However, these external sensors do not provide perspectives about our physiological variables, such as heart and breathing rates.

What is fatigue?

Fatigue is identified as an inevitable common problem in athletes' daily training routine. If it is not managed correctly, it could lead to many short- and long-term detrimental issues. Even though fatigue is a complex phenomenon with different perspective definitions, generally, it can be stated that an overall feeling of tiredness or lack of energy due to stress occurs in terms of physical, psychological, and social aspects. In sports, it can be described as an exercise-induced diminishment of performance during events.

What causes fatigue?

There are several factors that are associated with fatigue. Let's look at a few of them briefly.

• Excessive training load –Training load is one of the significant predictors of fatigue monitoring. Increasing the training frequency, intensity, and duration beyond an athlete's threshold can lead to adverse effects on performance.

• Lack of sleep – Adequate sleep is essential to physiological and psychological recovery. Sleep deprivation hinders our ability to think and react quickly. Also, it develops irritability, risk of anxiety and depression.

• Energy depletion or low energy availability- Our body needs enough energy to execute powerful movements. During sports events, the stored energy sources (ATP, ATP-Pcr, muscle and liver glycogen) within our body decrease gradually and athletes cannot perform desired movements unless replenished adequately.

• Metabolic acidosis – As a result of muscles converting stored chemical compounds into mechanical energy, athletes can perform high-power outputs. However, the lower availability of stored energy within the body makes performing high-intensity powerful activities challenging for athletes. Often, waste products such as lactic acid are formed within the muscle, hindering muscle performance.

• Psychological factors- Psychology plays a vital role in higher performance among athletes. Alteration of factors such as stress, confidence, motivation, well-being and sleeping can influence either negatively or positively on performance and could portray the level of fatigue and readiness for exercises.

• Not adequate time for recovery – Adequate recovery is an important factor for proper training adaptations. Also, it allows the body to restore energy levels, rebuild and repair muscle tissues, and eliminate mental stress. Not having enough recovery leads to a decrease athletes' performance while risk of developing overtraining syndrome over time

Why it is important to monitor fatigue?

Training is a continuous process and athletes must tolerate a higher level of physiological and psychological stressors throughout the year to strive to improve performance. Also, coaches and sports scientists make regular adjustments on training load during a training cycle to ensure that athletes are performing within their optimal range. These adjustments cause either to increase or decrease the level of fatigue among athletes.

However, athletes are different from each other, and with the complexity of integrating different training variables, monitoring fatigue has become more challenging.  Also, measuring various metrics which affect fatigue on a regular basis is a daunting and time-consuming task, as athletes cannot be monitored or supervised for continuous 24 hours. Therefore, monitoring fatigue is a paramount factor for acquiring optimal performance and it will provide valuable information about the athletes’ sensitivity to training and their ability to perform. One of the common ways of understanding the athletes' sensitivity to training is to understand the dose-response relationship, defined as the physiological and physical changes that occur due to the training load.

Implementing strategies to monitor fatigue is critical to evaluate the effectiveness of any given training programme. Also, having those data will help coaches and sports scientists make accurate decisions and adjustments on training if things are not happening according to the annual training programme, but also they can identify other complications such as overtraining syndrome, illness, and injuries.

Monitoring fatigue with wearables

Wearable devices can collect real-time data for a long time, and with the help of advanced algorithms, these data can be translated into more meaningful information. Most of the available wearable devices are not directly measuring the level of fatigue, but these devices can monitor the factors that are associated with fatigue. Internal and external sensors often monitor factors such as training load, sleep, heart rate, sweat, and body temperature. These collected data are compared with either normative values or set-up goals and will give you interactive information on your activity level, the current status of sleeping, energy level, training loads etc.
There are a few ways wearable devices are helping athletes track the factors associated with fatigue.

1. Measuring Heart Rate Variability

2. Monitoring Training Load with GPS devices

3. Tracking Sleep

4. Other (Body temperature, energy expenditure, psychological changes
   
   

Heart Rate Variability

Heart rate variability (HRV) has become the dominant form of measuring athlete's stress in the 21^st century. In general, HRV can be defined as the variation of time intervals between each successful heartbeat and assessing the patterns of variability over. These wearable devices assess patterns and variability of the heartbeat intervals, showcasing the activities of the autonomous nervous system, a combination of sympathetic (increasing heart rate with fight or flight responses) and parasympathetic (decreasing heart rate with rest and digest response) nervous systems.

HRV typically measures in milliseconds (ms) and can be obtained with two main approaches in wearable settings. In the first method, HRV is measured with the traditional Electrocardiograph (ECG), which is mainly found in laboratories. In the second method, photoplethysmography (PPG) is used to detect HRV, a low-cost and non-invasive method. The ECG captures the heart's electrical activity and is considered a valid method of measuring the heart rate. However, these devices must be in adequate contact with the skin and athletes are sometimes reluctant to wear these devices during dynamic exercises due to the irritability caused by the devices. On the other hand, the PPG collects the mechanical signal of the peripheral pulse waves much simpler than ECG devices.

As athletes develop fatigue, an imbalance can be seen in the autonomic nervous system, where sympathetic influence is improved while decreasing the parasympathetic impact. Identification of these variables in short and over time, has been suggested as a way of indicating fatigue and preventing overtraining syndrome.

Most wrist-worn devices/watches are built based on PPG technology, and athletes fondly use these wearable watches to track their heart rates. Watches such as Apple, Fitbit and Garmin can be identified as the most popular wearables nowadays as these watches provide information on heart rate and stay connected with mobile devices. All these three brands measure the heart rate within ±3% error which is only a small margin of error. Also the validity and reliability of these three brands are satisfactory and slight differences in heart rate variability can be found across different populations. However, further improvement is needed to reduce the error and increase the validity and reliability of measuring heart rate among athletes.

Examples of measuring HRV

• Apple

• Fitbit

• Garmin

Training load and GPS

Using GPS devices to monitor external training load has become popular in team and individual sports. GPS devices from manufacturers like Catapult Sports and GPSports are widely popularised in football, hockey and Australian Football. These devices can generate different variables that are either directly or indirectly associated with performance, recovery, fatigue, and risk of injuries. Moreover, these devices can collect around 1000 data points per second on player load, offering more than 262 measuring parameters.

GPS devices can accurately record the distance covered by an athlete. Coaches, athletes and sports scientists can now understand about accurate distance covered within training or competitions, whereas necessary recovery and training strategies can be taken to mitigate higher stress. This type of information is not only helpful for endurance-based sports like running, swimming, and cycling but also for other high-intensity sports such as football, hockey and rugby.

GPS devices can record speed and pace in real-time and allow athletes to stay within desired training zones, while coaches can have instant feedback training doses. Also, GPS can map out a player's exact locations during a match; this data helps to identify patterns, evaluate training strategies and plan future training programmes. For instance, heat maps in football are commonly used to evaluate players' whereabouts during a game and will be used over time to map the playing patterns. Also, these devices can measure acute to chronic workload ratio which is a good indicator of athlete’s readiness and level of fatigue.

For more information
Catapult 
Sonda

Tracking Sleeping

To have a physical balance and a high quality of life, all human beings must have adequate sleep, a primary biological function. Not having sufficient sleep hinders athletes' cognitive and physical performance, ultimately reducing their overall athletic performance. Using wearable devices to track sleeping has gained popularity among athletes to uncover many unforeseen variables. Usually, sleeping wearables come with either watches or rings. Most devices use the PPG to collect variables such as sleep latency, duration, and efficiency.

One of the main variables that wearable devices are tracking is sleeping duration. It detects the periods of inactivity and compares them with usual sleeping patterns. It offers insights into your time to fall asleep and the number of times or duration of any wakeful period. Also, some of the devices which have heart rate monitors and accelerometers are able to measure sleeping quality and different sleeping stages, such as deep sleep, light sleep and awaking periods Moreover, wearable devices can monitor how many times you woke up during sleeping, either intentionally or unintentionally. By collecting these types of data points over days, weeks, and months, athletes can have insights into their sleeping patterns and improve their sleep quality.

Even though wearables provide both quantitative and qualitative data on sleeping, there is a debate on whether these data are 100% accurate within the athlete population. Some devices have been validated with polysomnography, the golden standard of measuring sleeping among the general population, but others have not. Therefore, there is a significant possibility of overestimation or underestimation of sleeping quality and quantity among athletes. However, this technology is still at its grass root level and constantly evolving with the corporation with artificial intelligence.

Other

As wearable technology is constantly evolving, there are other areas that have been paid attention to monitoring internal and external responses to understanding athletes’ performance. Some wearable devices are attached to text tiles and shoes, while others can be stuck to the skin. Also, several other devices predict energy availability and psychological variables, such as mood, by measuring the variability of both internal and external stressors. However, these wearable devices are still being tested to ensure their validity and reliability in athlete settings.

Body Temperature
During exercises, the homeostatic nature, which is the ability of the body to maintain a stable internal environment, is disrupted. By increasing or decreasing body temperature, the human body tries to get it back to normal. Also, exposure because of continuous contraction and relaxation of muscles heat is produced. Increasing the core body temperature leads to a negative impact on athletes’ performance and can be led to heat-related illness. Therefore, monitoring either core or surface body temperature could be valuable in understanding the interaction of an athlete’s body for training and environmental stressors, ultimately in insights about fatigue levels.

Energy Expenditure
Athletes need to have a higher energy intake to meet the extremely high demands on their training and performance. However, depending on the intensity, duration, type of sport, specific goals, and individual factors, energy expenditure (EE) can vary between athletes. To measure the EE, methods such as direct calorimetry and indirect calorimetry are used, but with their cost and practical restriction, most of the time, it is not applicable within the field. By incorporating physiological responses such as heart rate, body temperature and individual characteristics: height, weight, sex, and body composition, wearable devices can estimate the daily energy expenditure of athletes. To capture different physiological variables, wearables are included with accelerometers, gyroscopes, electrocardiograms, photoplethysmography, skin temperature and near-body temperature sensors. However, due to the higher variability of intensity, training, and competition, sometimes measuring EE can be over or underestimated. Therefore, special attention should be given to the validity and reliability of these devices among athletes.

Psychological variables
Mental fatigue is developed among athletes due to prolonged exposure to mental exertion and physical fatigue. If mental exhaustion is not appropriately managed, sports performance will decrease and negatively impact on individuals’ health. Even though monitoring mental fatigue is highly complex by the individual psychological variation of athletes, wearable technology has started to investigate monitoring mental fatigue among athletes. By detecting the changes in eye movements, physiological changes, speech and brain activities, wearable devices could provide insights into mental fatigue. However, the validity and reliability of these devices are significantly lower than those detecting physical fatigue.

Reference

• Ahmad, N., Ghazilla, R. A. R., Khairi, N. M., & Kasi, V. (2013). Reviews on various inertial measurement unit (IMU) sensor applications. International Journal of Signal Processing Systems, 1(2), 256–262.
• Aughey, R. J. (2011). Applications of GPS technologies to field sports. International Journal of Sports Physiology and Performance, 6(3), 295–310.
• Beattie, Z., Oyang, Y., Statan, A., Ghoreyshi, A., Pantelopoulos, A., Russell, A., & Heneghan, C. (2017). Estimation of sleep stages in a healthy adult population from optical plethysmography and accelerometer signals. Physiological Measurement, 38(11), 1968.
• Bongers, C. C. W. G., Hopman, M. T. E., & Eijsvogels, T. M. H. (2017). Cooling interventions for athletes: an overview of effectiveness, physiological mechanisms, and practical considerations. Temperature, 4(1), 60–78.
• Coutts, A. J., Crowcroft, S., & Kempton, T. (2021). Developing athlete monitoring systems: theoretical basis and practical applications. In Recovery and Well-being in Sport and Exercise (pp. 17–31). Routledge.
• Driller, M. W., Dunican, I. C., Omond, S. E. T., Boukhris, O., Stevenson, S., Lambing, K., & Bender, A. M. (2023). Pyjamas, Polysomnography and Professional Athletes: The Role of Sleep Tracking Technology in Sport. Sports, 11(1), 14.
• Fuller, D., Colwell, E., Low, J., Orychock, K., Tobin, M. A., Simango, B., Buote, R., Van Heerden, D., Luan, H., & Cullen, K. (2020). Reliability and validity of commercially available wearable devices for measuring steps, energy expenditure, and heart rate: systematic review. JMIR MHealth and UHealth, 8(9), e18694.
• Grandner, M. A., Hall, C., Jaszewski, A., Alfonso-Miller, P., Gehrels, J.-A., Killgore, W. D. S., & Athey, A. (2021). Mental health in student athletes: associations with sleep duration, sleep quality, insomnia, fatigue, and sleep apnea symptoms. Athletic Training & Sports Health Care, 13(4), e159–e167.
• Halson, S. L. (2014). Monitoring training load to understand fatigue in athletes. Sports Medicine, 44(Suppl 2), 139–147.
• Hargreaves, M. (2005). Metabolic factors in fatigue. Sports Science, 18(3), 98.
• Iizuka, T., Ohiwa, N., Atomi, T., Shimizu, M., & Atomi, Y. (2020). Morning heart rate variability as an indication of fatigue status in badminton players during a training camp. Sports, 8(11), 147.
• Knicker, A. J., Renshaw, I., Oldham, A. R. H., & Cairns, S. P. (2011). Interactive processes link the multiple symptoms of fatigue in sport competition. Sports Medicine, 41, 307–328.
• Kodithuwakku Arachchige, S. N. K., Burch V, R. F., Chander, H., Turner, A. J., & Knight, A. C. (2022). The use of wearable devices in cognitive fatigue: current trends and future intentions. TheoreTical Issues in Ergonomics Science, 23(3), 374–386.
• Kranjec, J., Beguš, S., Geršak, G., & Drnovšek, J. (2014). Non-contact heart rate and heart rate variability measurements: A review. Biomedical Signal Processing and Control, 13, 102–112.
• Luo, H., Lee, P.-A., Clay, I., Jaggi, M., & De Luca, V. (2020). Assessment of fatigue using wearable sensors: a pilot study. Digital Biomarkers, 4(1), 59–72.
• Makivić, B., Nikić Djordjević, M., & Willis, M. S. (2013). Heart Rate Variability (HRV) as a tool for diagnostic and monitoring performance in sport and physical activities. Journal of Exercise Physiology Online, 16(3).
• Mielgo Ayuso, J., Maroto Sanchez, B., Luzardo Socorro, R., Palacios Le Blé, G., Palacios Gil Antuñano, N., & Gonzalez Gross, M. M. (2015). Evaluation of nutritional status and energy expenditure in athletes. Nutricion Hospitalaria, 31(Supl. 3), 227–236.
• Moeller, J. L. (2004). The athlete with fatigue. Current Sports Medicine Reports, 3(6), 304–309.
• Mourot, L., Bouhaddi, M., Perrey, S., Cappelle, S., Henriet, M., Wolf, J., Rouillon, J., & Regnard, J. (2004). Decrease in heart rate variability with overtraining: assessment by the Poincare plot analysis. Clinical Physiology and Functional Imaging, 24(1), 10–18.
• O’Driscoll, R., Turicchi, J., Hopkins, M., Horgan, G. W., Finlayson, G., & Stubbs, J. R. (2020). Improving energy expenditure estimates from wearable devices: A machine learning approach. Journal of Sports Sciences, 38(13), 1496–1505.
• Purvis, D., Gonsalves, S., & Deuster, P. A. (2010). Physiological and psychological fatigue in extreme conditions: overtraining and elite athletes. Pm&r, 2(5), 442–450.
• Sadek, I., Demarasse, A., & Mokhtari, M. (2020). Internet of things for sleep tracking: wearables vs. nonwearables. Health and Technology, 10(1), 333–340.
• Sargent, C., Lastella, M., Romyn, G., Versey, N., Miller, D. J., & Roach, G. D. (2018). How well does a commercially available wearable device measure sleep in young athletes? Chronobiology International, 35(6), 754–758.
• Seshadri, D. R., Drummond, C., Craker, J., Rowbottom, J. R., & Voos, J. E. (2017). Wearable devices for sports: new integrated technologies allow coaches, physicians, and trainers to better understand the physical demands of athletes in real time. IEEE Pulse, 8(1), 38–43.
• Seshadri, D. R., Li, R. T., Voos, J. E., Rowbottom, J. R., Alfes, C. M., Zorman, C. A., & Drummond, C. K. (2019). Wearable sensors for monitoring the internal and external workload of the athlete. NPJ Digital Medicine, 2(1), 71.
• Shaffer, F., & Ginsberg, J. P. (2017). An overview of heart rate variability metrics and norms. Frontiers in Public Health, 258.
• Sweeting, A. J., Cormack, S. J., Morgan, S., & Aughey, R. J. (2017). When is a sprint a sprint? A review of the analysis of team-sport athlete activity profile. Frontiers in Physiology, 8, 432.
• Taoum, A., Bisiaux, A., Tilquin, F., Le Guillou, Y., & Carrault, G. (2022). Validity of Ultra-Short-Term HRV Analysis Using PPG—A Preliminary Study. Sensors, 22(20), 7995.
• Theodoropoulos, J. S., Bettle, J., & Kosy, J. D. (2020). The use of GPS and inertial devices for player monitoring in team sports: A review of current and future applications. Orthopedic Reviews, 12(1).
• Wells, G. D., Selvadurai, H., & Tein, I. (2009). Bioenergetic provision of energy for muscular activity. Paediatric Respiratory Reviews, 10(3), 83–90.
• Yasar, K. (2022). What is wearable technology? - Definition from WhatIs.com. [online] SearchMobileComputing. Available at: https://www.techtarget.com/searchmobilecomputing/definition/wearable-technology.