Standardized Protocol For Monitoring Muscle Fatigue and Biomechanics In Amateur Cyclists Using Infrared Thermography

Summary

Cycling biomechanics relies on efficient muscle activation and core stabilization. Infrared thermography (IRT) offers non-invasive monitoring of muscle fatigue. This protocol establishes a standardized methodology for integrating IRT into biomechanical analysis, aiming to optimize performance and prevent injury in amateur cyclists through precise physiological monitoring.

Abstract

The use of stationary bikes has increased significantly, making physical activity more accessible to the general population. However, the lack of precise, non-invasive monitoring during training sessions can mask muscle imbalances and inefficient biomechanical patterns. These factors, subclinical in their initial stages, can lead to a high incidence of musculoskeletal injuries. Currently, exercise evaluation relies on the subjective perception of effort or conventional performance metrics, creating a critical gap in the early detection of physiological issues before they progress to chronic conditions. Given the crucial role of exercise in public health, it is imperative to implement sustainable evaluation strategies applicable to various fitness levels. In this context, infrared thermography (IRT) emerges as a promising alternative due to its ability to quantify skin surface temperature, a direct indicator of metabolic activity, thermoregulation, and blood flow. This project establishes a standardized methodological protocol using IRT to monitor the physiological response during cycling sessions in a non-athletic population. The application of this protocol revealed that standardized IRT can effectively detect subclinical thermal asymmetries and localized "hotspots" in joints (e.g., the knee at 32.8 °C) that are otherwise undetectable through subjective effort perception (Borg scale) or pedaling cadence monitoring. By systematically controlling environmental and subject-related variables, this research analyzes surface temperature distribution in the lower body to evaluate biomechanics and muscle fatigue. Specifically, the protocol facilitates the correlation between thermal patterns, fatigue levels, and pedaling cadence. This approach not only supports technological innovation in preventive medicine and public health but also lays the groundwork for an early detection tool capable of mitigating injury risks and optimizing the well-being of the population engaged in this activity.

Introduction

The biomechanics of cycling is a multifaceted discipline that integrates principles of human movement, muscle physiology, and energy transfer to optimize performance and prevent injury. At its core, cycling involves a cyclical alternation between the power phase—where the quadriceps, gluteal muscles, and gastrocnemius generate the majority of the propulsive force—and the recovery phase, during which the hamstrings and hip flexors facilitate the return of the leg to its initial position. Efficient energy transfer to the pedals is not only a function of lower limb muscle activation but also depends critically on core stabilization, which ensures biomechanical efficiency and minimizes compensatory movements that could predispose the cyclist to injury^1,2,3.

Currently, researchers are focusing on the assessment of muscle activation patterns, fatigue, and thermoregulatory responses during cycling^3,4,5. This interest is due to the recognition that muscle fatigue is a complex phenomenon, impairing performance and increasing injury risk, and that its early detection and management are essential for both elite and recreational cyclists^2,6. Among the array of available assessment tools, infrared thermography (IRT) has been established as a promising, non-invasive technique for monitoring muscle activity and fatigue⁵. IRT enables the visualization of skin temperature changes, which may reflect underlying muscle activation and thermoregulatory processes during exercise that have a direct effect on athletes' metabolism^5,7.

Despite its potential, the application of IRT in cycling biomechanics research and practice faces several methodological challenges. While some standard operating procedures (SOPs) have been proposed in general sports and clinical contexts^8,9, there is a recognized need for further refinement of existing protocols to address the unique physiological and convective cooling demands of amateur cycling^7,10. The variability in environmental conditions, inconsistencies in subject preparation, and the lack of consensus regarding the selection of anatomical regions of interest have limited the reproducibility and comparability of findings across studies^1,5,7,9. These limitations underscore the need for rigorous methodological frameworks to harness the full potential of IRT in the context of cycling^7,10,11,12.

The literature reveals that the main thematic axes in this field include exercise physiology, IRT, skin temperature monitoring, exercise performance, and thermoregulation^2,5,7. Studies have demonstrated significant correlations between skin temperature and muscle activation parameters, such as those obtained via surface electromyography (sEMG), suggesting that IRT can serve as a surrogate marker for muscle fatigue during dynamic exercise⁵. However, external factors such as perspiration, ambient temperature, and individual differences in thermoregulatory capacity could influence thermographic results, necessitating careful control and reporting of experimental conditions⁷.

Likewise, advances in non-invasive muscle fatigue monitoring have expanded beyond IRT to include techniques such as sEMG, mechanomyography (MMG), near-infrared spectroscopy (NIRS), and ultrasound imaging^{13,14,15,16,17}. Each modality offers unique insights into muscle function: sEMG quantifies electrical activity during contraction, MMG captures mechanical vibrations, NIRS assesses muscle oxygenation, and ultrasound visualizes muscle deformation. The integration of IRT into this multi-modal landscape holds particular promise for cycling, where continuous, non-contact monitoring is advantageous. IRT can complement other modalities by providing spatially resolved, real-time data on skin temperature distribution, which may reflect localized muscle activation, asymmetries, and potential sites of overuse or injury^14,18.

To overcome the methodological inconsistencies identified in previous research, the protocol proposed in this study is strictly aligned with international standards for clinical and sports thermography. Specifically, the parameters for subject preparation and environmental control—such as the restriction of stimulants and physical activity, and the use of standardized acclimatization periods—are based on the guidelines established by the International Academy of Clinical Thermology (IACT), the American Academy of Thermology (AAT), and the Glamorgan Protocol⁸. Furthermore, the environmental and technical settings, including emissivity and thermal strain assessment, follow the ISO 9886 standards and the recommendations of the European Association of Thermology (EAT). By integrating these established regulations, this study ensures that IRT measurements are a reliable reflection of physiological responses, providing a robust framework for monitoring muscle fatigue and biomechanics in recreational cyclists.

Nevertheless, the successful application of IRT in cycling biomechanics requires the establishment of standardized methodologies. This includes the definition of pre-exercise acclimatization periods, control of ambient temperature and humidity, consistent subject preparation, and the precise delineation of anatomical regions of interest for thermal analysis^5,7. Furthermore, the development of robust data analysis protocols will enhance the interpretability and utility of IRT data⁵.

The present study addresses these gaps by proposing and validating a comprehensive methodological framework for the integration of IRT into biomechanical analysis in recreational cyclists. By systematically controlling for environmental and subject-related variables, and by correlating thermal data with established markers of muscle activation and fatigue, this approach aims to optimize the monitoring of muscle fatigue, contribute to injury prevention, and enhance performance outcomes^2,5,7,14.

Protocol

The research protocol was conducted in accordance with the ethical principles of the Declaration of Helsinki. Due to the non-invasive and non-contact nature of infrared thermography, the safety of the participants was guaranteed throughout the monitoring process. All subjects provided written informed consent before any procedures were initiated, agreeing to the use of their physiological data for research purposes. The study followed the internal research guidelines of the Faculty of Engineering at the Universidad Autónoma de Querétaro, ensuring participant anonymity and data protection.

NOTE: To overcome the methodological inconsistencies identified in previous research, the protocol proposed in this study is strictly aligned with international standards for clinical and sports thermography. Specifically, the parameters for subject preparation and environmental control follow the guidelines established by the International Academy of Clinical Thermology (IACT), the American Academy of Thermology (AAT), and the Glamorgan Protocol. Furthermore, technical settings such as emissivity and thermal strain assessment follow the ISO 9886 standards and the recommendations of the European Association of Thermology (EAT). The feasibility of this protocol was validated with 60 biomedical engineering students from the Universidad Autónoma de Querétaro, aged between 20 and 25 years, to ensure protocol adherence and baseline homogeneity. This specific population ensured high compliance with pre-test restrictions and provided a homogeneous physiological baseline to verify the consistency of the thermographic captures.

1. Subject preparation and environmental control (Replicability and demographic impact)

1. Inclusion criteria and subject factors
   1. Record the age and sex of each subject for analysis of age differences and sex effects.
   2. Record the height and mass of the subject for use in anatomical parameters.
   3. Administer the Borg Rating of Perceived Exertion (RPE) scale (6–20). Instruct the subject to point to the number that represents their current exertion level before and after the test to correlate physiological data with subjective fatigue.
2. Consumption restriction and skin preparation
   1. Prohibit the subject from consuming alcohol and nicotine for 24 h prior to measurement.
   2. Prevent the subject from consuming caffeine and from engaging in intense exercise during the 3 h prior to the test.
   3. Instruct the subject not to apply creams, cosmetics, or ointments to the lower extremities during the 12 h prior to the test.
   4. Confirm that the subject is adequately hydrated and record this on the pre-test data sheet.
   5. Use a clinical scale to record body mass before the test. Ask the subject to consume 300 mL of water 30 min before the session to ensure a standardized baseline hydration state.
3. Standardization of clothing and posture
   1. Instruct the subject to completely undress the lower extremities of the body.
   2. Provide standardized cycling shorts (black Lycra with no visible seams).
4. Standardization of environmental and resting conditions
   1. Place a digital thermo-hygrometer 1 m from the subject at a height of 1 m. Verify that the ambient temperature is between 18–25 °C and the relative humidity is between 40% and 55%. Record these values every 5 min in a spreadsheet.
   2. Ensure that there are no direct drafts, exposure to sunlight, or sources of heat/cold that could influence thermoregulation.
   3. Perform a 15 min static, seated rest period for acclimatization to environmental conditions.
   4. Place the subject on the stationary bicycle in the required posture.

2. Configuration and acquisition of thermographic images

1. Open the thermal camera's internal settings menu. Navigate to Object Parameters and manually enter 0.98 for emissivity (ε). Set the Reflected Apparent Temperature to match the ambient temperature measured in step 1.4.1.
   1. Use the infrared thermograph (IRF) thermal camera with a minimum resolution of 320 x 240 pixels.
   2. Set the emissivity (ε) to 0.98 for the surface of human skin.
   3. Use a laser distance meter or a physical measuring tape to place the tripod exactly 100 cm from the lateral plane of the bicycle's crankset. Mark the tripod leg positions on the floor with adhesive tape to ensure zero displacement between captures.
   4. Enter the relative humidity of the environment in the instrument settings.
   5. Set the atmospheric temperature of the laboratory and enter the external temperature.
2. Anatomical positioning and acquisition.
   1. Place the thermograph on a tripod and set the camera height to 30 cm above the floor.
   2. Position the lens at a perpendicular angle (≈90°) to the lower end of the subject.
   3. In the thermal analysis software, select the Polygon/Rectangle ROI tool. Click and drag to delineate a 10 x 5 cm box over the Rectus Femoris (midway between the anterior superior iliac spine and the patella). Click Calculate to extract the Mean Temperature (T_mean).
3. Measurement point protocol
   1. Capture the thermographic image immediately after completing the 15 min rest period.
   2. To ensure capture within 5 s, have the researcher stand behind the camera with the focus pre-set. At the signal "Stop pedaling", extend the dominant leg to a 6 o'clock position; immediately press the Capture/Freeze button on the camera trigger.
   3. Post-exercise recovery: Capture the third thermographic image 10 min after the exercise is completed.

3. Exercise protocol execution

1. Set the stationary bicycle to Ergometer Mode with a fixed workload of 150 W (adjusting for the subject's weight if normalized power is required).
2. Place a digital metronome or use the bicycle's integrated console to enforce a constant cadence of 80 RPM. Instruct the subject to maintain this pace within a ±5 RPM margin to ensure a steady-state thermoregulatory response.
3. Verify that the thermal camera is set to a Standard recording mode with a temperature range of 20 °C to 45 °C and an emissivity of 0.98.

4. Data analysis and standardization

1. Calculate the thermal shift (ΔT) using the formula: ΔT = T_post - T_pre. Perform this calculation for both the dominant and non-dominant limbs to determine thermal asymmetry (ΔT_asymmetry = |T_right - T_left|)
2. Avoid manipulating or cropping the original thermographic image.
3. Preserve the initial image resolution and complete metadata for analysis.

Results

Cycling biomechanics:
Cycling biomechanics operate in the pedaling cycle. The Push Phase (Downward) generates the main force: Quadriceps (Rectus Femoris, Vastus Lateralis, Medialis, and Intermedius) are the primary motors, extending the knee and driving the pedal. The Glutes assist with hip extension, providing power and stability. The Calves (Gastrocnemius and Soleus) complete the transfer of force. The Pull and Recovery Phase (Upward) prepares the leg: Hamstrings flex the knee and pull the pedal up...

Discussion

The research is based on two pillars: the effectiveness of Infrared Thermography (IRT) as a non-invasive tool for measuring physiological stress and the urgent need to standardize its methodological protocol. The justification for using IRT to assess fatigue in cycling is confirmed by a network analysis (Figure 2), which establishes Skin Temperature and Exercise Monitoring as central themes in the literature^1,2,4.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Borg Scale	Borg Perception	RPE Scale 6-20	6-20 RPE scale used to correlate subjective fatigue with objective thermal asymmetries and subclinical hotspots.
Controlled Chamber	Internal Facilities	Physiology Lab	Environment maintained at 18 °C to 25 °C and 40–55% humidity to ensure stable skin temperature and minimize convective interference.
Data Log Sheets	Google	Google Forms (Digital Entry)	Standardized forms for recording anthropometric data (age, sex, BMI) and hydration status to control for individual thermoregulatory variables.
Informed Consent	Local Ethics Committee	Institutional Format	Documentation approved by the local Ethics Committee, ensuring adherence to the Declaration of Helsinki for non-invasive monitoring.
Infrared Camera	FLIR Systems	49001-2001	High-resolution (320 x 240 px) thermal imager set to ε = 0.98 to measure localized metabolic heat and thermal shifts (ΔT).
Marking Tools	BIC / Body Med	Skin Marker / Floor Tape	Skin-safe markers and floor tape used to fix camera-to-subject distance at exactly 100 cm and ensure consistent anatomical ROI alignment.
Stadiometer & Scale	SECA	763 (Digital Station)	Tools for recording height and body mass; necessary for calculating normalized power and documenting subject-specific physiological baselines.
Standardized Shorts	Nike	HV0411-010 / CZ9831-010	Black Lycra garments used to standardize skin emissivity and provide unobstructed access to the lower limb Regions of Interest (ROIs).
Stationary Bicycle	Monark	828E Ergomedic	Electromagnetic ergometer set to a fixed workload (e.g., 150 W) and constant cadence (80 RPM) to ensure steady-state metabolic heat production.
Stopwatch	Casio	HS-3V-1RET	Critical for timing the 15-min acclimatization period and ensuring IR capture occurs within 5 s post-exercise to avoid rapid thermal decay.
Thermometer-hygrometer	Control Company/Traceable	4040	Real-time monitoring device; used to input ambient temperature and humidity into the IR camera's object parameters for atmospheric correction.
Tripod	FLIR Systems	T197731	Stable support set at a 30 cm height and perpendicular angle (≈90°) to the lower extremities for repeatable image acquisition.