How does the muscle respond to different intensities of strength training?

A study analyzed two key variables — skin temperature and muscle oxygen saturation — to better understand what happens in the lower limbs during and after localized strength exercise.

In this article, we explore its findings and how they can be applied in the field of sports performance.

What did they measure, and why is it relevant?

The authors evaluated seventeen trained men while performing leg extensions at two different intensities: high (70% of their one-repetition maximum) and low (35%). Over the course of three sets of 15 repetitions, they measured:

• Skin temperature using infrared thermography.
• Oxygen saturation in the vastus lateralis muscle using near-infrared spectroscopy (NIRS).
• Subjective perception of effort after each set.
• The evolution of these variables during exercise and recovery (7 and 15 minutes after).

Key Findings

Skin Temperature (Thermography)

• Thigh: Temperature increased progressively, especially with high intensity.
• Knee: Also increased, but to a lesser extent.
• Lower leg (non-active area): Temperature decreased during exercise, reflecting the redistribution of blood flow toward the active muscles.

This thermal behavior shows that the muscle generates more heat as effort increases, while other areas "give up" blood flow to support the metabolic demand.

Muscle Oxygen Saturation

• Oxygen saturation significantly decreased during the sets, more so under the high-intensity condition.
• The oxygen extraction capacity (∇%SmO₂) and the hyperemic response during recovery (Δ%SmO₂) were also greater with a heavier load.
• During recovery, oxygen saturation rose above baseline levels in those who trained with high intensity, reflecting a more pronounced hyperemic response.

This pattern indicates greater metabolic activation and a stronger need for muscle reoxygenation after more demanding exercises.

Perceived Effort

• Participants reported significantly higher effort following the high-intensity sets.
• This aligns with the changes observed in both temperature and muscle oxygenation.

What is the relationship between temperature and muscle oxygenation?

The study found consistent correlations between the increase in thigh temperature and the decrease in oxygen saturation during exertion. Additionally, the greater the temperature increase during recovery, the higher the oxygen recovery in the muscle.

This suggests that both variables reflect the level of muscle activation and fatigue, although they measure different physiological phenomena:

• Temperature represents a surface thermal response.
• Oxygenation reflects the availability of oxygen within the muscle.

Practical Applications

The findings of this study provide new tools for coaches, physiotherapists, and sports scientists:

• Thermography and NIRS can be used together to monitor internal load during strength training.
• They help identify areas with higher metabolic demand and adjust volume or intensity accordingly.
• They support decision-making on recovery and planning future training sessions.
• They may serve as early indicators of localized fatigue or overload..

Conclusion

• Thermography and NIRS are technologies that allow for non-invasive observation of what occurs in the muscle during and after strength training.
• Their combined use can improve load management, optimize recovery, and help prevent planning errors.
• This study reinforces the idea that beyond external load, understanding the internal physiological response is key to training more effectively.

References

• Vasquez-Bonilla, A. A., Boschetti, F., Sepulveda, R. Y., & Sillero-Quintana, M. (2025). Effects of Exercise Intensity on Skin Temperature and Muscle Oxygen Saturation in the Thigh During and After Strength Resistance Training. Journal of Thermal Biology, 104132.
• Santana, P. V. A., Alvarez, P. D., da Costa Sena, A. F., Serpa, T. K., de Assis, M. G., Pimenta, E. M., ... & Veneroso, C. E. (2022). Relationship between infrared thermography and muscle damage markers in physically active men after plyometric exercise. Journal of Thermal Biology, 104, 103187.
• Fernández-Cuevas, I., Marins, J. C. B., Lastras, J. A., Carmona, P. M. G., Cano, S. P., García-Concepción, M. Á., & Sillero-Quintana, M. (2015). Classification of factors influencing the use of infrared thermography in humans: A review. Infrared Physics & Technology, 71, 28-55.
• Requena-Bueno, L., Priego-Quesada, J. I., Jimenez-Perez, I., Gil-Calvo, M., & Pérez-Soriano, P. (2020). Validation of ThermoHuman automatic thermographic software for assessing foot temperature before and after running. Journal of Thermal Biology, 92, 102639.
• Vasquez-Bonilla, A. A., Tomas-Carus, P., Brazo-Sayavera, J., Malta, J., Folgado, H., & Olcina, G. (2023). Muscle oxygenation is associated with bilateral strength asymmetry during isokinetic testing in sport teams. Science & Sports, 38(4), 426-e1.