Recovery from an anterior cruciate ligament (ACL) tear has traditionally been measured through strength, time, and biomechanics. However, a new variable is gaining prominence in return-to-play decisions: skin temperature.

Recently, two independent investigations have shed light on this phenomenon, demonstrating a remarkable alignment despite being conducted by different laboratories, using different cameras, and within different sporting contexts.

At ThermoHuman, we wanted to compare the evolution presented by Färber and his team at the University of Innsbruck (1) with the analysis performed by our researchers in the study by Escamilla-Galindo et al. 2025 (2). The objective was to evaluate whether ACL recovery in an individual case study (an elite skier) followed similar trends to the average observed in a cohort of 30 professional footballers.

The most fascinating aspect of this comparison is the number of similarities that emerge from independently collected data. Both studies confirm that the operated knee is not just an anatomical structure under repair, but a dynamic metabolic focus. In both the Austrian skier and the European footballers, thermal asymmetry manifests acutely following surgery, with differences exceeding 2°C or even 3°C compared to the healthy leg.

This initial "hyperthermia" is the result of post-operative inflammatory processes. The data show a progressive and parallel decline in both groups as the months progress. While the Escamilla-Galindo et al. 2025 study—processed entirely with ThermoHuman's computer vision technology—shows a steady normalization curve (typical of large cohort studies where the average is smoother), the Färber case allows us to see how that same trend is altered by training load peaks, proving that thermography is a faithful reflection of the physiological stress experienced by the tissue.

Despite differences in the mechanics of each sport, the findings converge on a critical point: the sensitivity of the knee to load. In the skier's study, spikes in thermal asymmetry aligned almost mathematically with increases in training volume. Meanwhile, the research by Escamilla-Galindo establishes that, although inflammation subsides, a persistent "thermal scar" remains. At the one-year mark, asymmetry in the footballers stabilizes at around 0.46°C, a figure that Färber’s skier also reached around the 12-month mark, following exhaustive load management and anti-inflammatory treatments.

ACL comparative

This parallelism between an individual skiing case and a professional football cohort highlights the robustness of infrared thermography as a tool for load control.

Key points of the comparison:

• Load-Asymmetry Relationship (Färber): It is visually observable how the asymmetry peaks in the skier tend to follow periods of higher training volume. For instance, the maximum initial asymmetry peak occurs after the first load phases, and the final spike between days 300 and 400 coincides with the return to skiing and increased training hours.
• Difference with the Cohort (Escamilla-Galindo): While the average in the Escamilla-Galindo study shows a progressive and "smoother" descent (typical of a 30-subject mean), the individual Färber case shows a much more sensitive and volatile response to the specific load variations of an elite athlete.
• Final Convergence: Despite load-induced fluctuations, both studies show that toward the end of the year (day 365+), asymmetry tends to stabilize near or below the clinical threshold of 0.5°C.

The main conclusion is clear: thermography is a tool that allows ACL recovery to be measured in a predictable manner. The integration of technologies like ThermoHuman into these processes transforms these trends into objective criteria for Return-to-Play. By eliminating the subjectivity of manual data collection, thermography establishes itself as the necessary bridge between graft biology and field performance, ensuring that the return to competition is not just a matter of time, but of thermal balance and joint health.

Interested in learning how to apply these metrics to your team? Explore more about our research and automatic analysis software in our science portal.

ACL injury evolution block

References

Färber, S., Mittermeier, R., Ebenbichler, C., & Raschner, C. (2025). Thermal evolution of skin temperature during rehabilitation from severe knee injury: A 12-month longitudinal case study of a female junior elite alpine ski racer. Infrared Physics & Technology, 106315.

Escamilla-Galindo, V. L., Felipe, J. L., Alonso-Callejo, A., Van-der-Horst, R., de la Torre-Combarros, A., Minafra, P., ... & Fernández-Cuevas, I. (2025). Return-to-play criteria based on infrared thermography during anterior cruciate ligament rehabilitation in football players. Biology of Sport, 42(2), 161-167.