One size does not fit all: Methodological considerations and recommended solutions for intramuscular temperature assessment

Intramuscular temperature kinetics can provide insightful information for exercise and environmental physiology research. However, currently, there are no consistent method descriptions or guidelines for muscle temperature assessment in the literature. Studies have reported a great variation in muscle temperature assessment, from 1.5 cm under the skin to 4 cm under the muscle fascia. Moreover, a large variation in body composition components among participants exacerbates this issue, changing the depth and the muscle to be tested. For instance, in young adults (25 ± 5 yrs), the thigh subcutaneous fat thickness can vary from 0.11 to 1.69 cm, and vastus lateralis thickness from 1.62 to 3.38 cm; in older adults (68.5 ± 3 yrs), subcutaneous fat thickness plus gastrocnemius medialis thickness can vary from 1.03 to 3.22 cm. This variation results in inconsistent resting muscle temperature profiles and muscle temperature kinetics during and after an exercise or environmental thermal stress interventions (hot or cold). Hence, one fixed size does not fit all. Standardization and consistency in muscle temperature assessment procedures across studies are required to allow a better understanding and translation of the influence of a given stressor (exercise or thermal) on muscle temperature kinetics. This methodological manuscript i) summarizes the differences in muscle temperature assessment procedures and techniques used across different studies, ii) discusses current concerns related to variations in intramuscular needle depth, and subcutaneous fat and muscle thickness when assessing muscle temperature, and iii) suggests a systematic and more robust approach, based on individual body composition characteristics, to be considered when assessing intramuscular temperature.