TCI showed meaningful responsiveness to internal load, particularly heart rate, highlighting its potential as a sensitive indicator of physiological load, while Tsk provides stable measurements but primarily reflects individual baseline characteristics rather than acute exercise responses.
Key Findings
Results
Skin temperature (Tsk) demonstrated good to excellent test-retest reliability across all regions of interest during incremental exercise.
ICC values (ICCA,1) ranged from 0.74 to 0.94 with confidence intervals of 0.44–0.98
Standard error of measurement (SEMA,1) ranged from 0.28 to 0.53
Nineteen participants completed two standardized incremental cycling tests
Reliability was assessed using coefficients of variation, intraclass correlation coefficients, Bland-Altman analysis, and mixed-effects models
Confidence intervals were generally wide, warranting cautious interpretation of the reliability estimates
Results
Thermal contrast index (TCI) reliability varied by region and exercise intensity, ranging from poor to good.
ICC values (ICCA,1) for TCI ranged from 0.23 to 0.80 with confidence intervals of -0.27 to 0.92
SEMA,1 for TCI ranged from 0.29 to 0.57
TCI showed a tendency toward higher reliability at greater exercise intensities
TCI showed higher reliability in the anterior thigh region compared to less active areas
Lower reliability was observed at low intensities and in less active regions
Results
TCI showed consistent and statistically significant associations with multiple physiological load indicators during incremental exercise.
TCI was associated with heart rate (β = 0.46, p < 0.001)
TCI was associated with blood lactate (β = 0.17, p < 0.001)
TCI was associated with energy expenditure (β = 0.14, p < 0.001)
These associations were described as 'consistent' across sessions, supporting TCI's responsiveness to internal load
Heart rate showed the strongest association with TCI among all physiological parameters measured
Results
Skin temperature was influenced more by individual traits than by acute physiological changes during exercise.
Mixed-effects models were used to distinguish individual baseline characteristics from exercise-induced physiological responses
Tsk provided stable measurements across sessions but did not track acute exercise responses as sensitively as TCI
In contrast to TCI, Tsk did not demonstrate meaningful responsiveness to cardiovascular, metabolic, or perceptual parameters
This pattern suggests Tsk primarily reflects interindividual differences rather than exercise-induced thermoregulatory changes
Results
TCI demonstrated a tendency toward good reliability specifically at higher exercise intensities and in performance-relevant muscle regions such as the anterior thigh.
Reliability of TCI was region-dependent, with the anterior thigh showing better reliability than other measured areas
Intensity-dependent reliability was observed, with higher intensities associated with better TCI reliability
Poor TCI reliability was observed at low exercise intensities
These findings highlight that measurement context (intensity and muscle region) substantially affects TCI reliability estimates
What This Means
This research suggests that two infrared thermography-based measures — skin temperature (Tsk) and the thermal contrast index (TCI) — behave quite differently during exercise and serve different purposes. The study had 19 participants complete the same incremental cycling test twice, allowing researchers to assess whether these measurements are consistent (reliable) and whether they track real physiological changes like heart rate, blood lactate, and energy expenditure. Skin temperature was found to be highly consistent between sessions (reliable), but it mainly reflected stable, individual characteristics of each person rather than changing meaningfully in response to exercise intensity. TCI, which compares temperature in active muscle areas to surrounding regions, was less consistent overall but showed a stronger connection to actual physiological stress — particularly heart rate — suggesting it responds more sensitively to how hard a person is working.
The reliability of TCI was notably better at higher exercise intensities and in muscle groups that are heavily used during cycling, such as the front of the thigh. At low intensities or in less active body areas, TCI reliability was poor. This means TCI may be most useful as a monitoring tool when exercise is vigorous and measurements focus on the primary working muscles. The statistical associations between TCI and internal load markers (heart rate β = 0.46, blood lactate β = 0.17, energy expenditure β = 0.14) were all significant, though the confidence intervals for reliability estimates were wide, meaning results should be interpreted with caution.
This research suggests that infrared thermography could have practical value in non-invasive exercise monitoring, but the choice of measure matters: Tsk may be useful for establishing stable baseline thermal profiles of individuals, while TCI appears better suited for tracking real-time physiological demands during moderate-to-high intensity exercise. Further research with larger samples is needed to narrow the confidence intervals and strengthen conclusions about when and where these measures can reliably be applied in sports science and performance monitoring contexts.
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Masur L, Willenbockel H, Cordes T, Sperlich B, Düking P. (2026). Reliability and physiological responsiveness of skin temperature and thermal contrast index during incremental exercise: a test-retest analysis.. Scientific reports. https://doi.org/10.1038/s41598-026-68741-x