How temperature tunes muscle mechanics during eccentric contractions.

Eccentric muscle contractions occur when muscles actively lengthen, acting as brakes that dissipate energy and stabilize joints. When actively stretched, muscle force rises in two phases: an initial steep increase (phase-1), followed by a slower, sustained rise (phase-2). The temperature sensitivity of this response is poorly understood, despite its relevance for musculoskeletal models that often rely on data collected at nonphysiological temperatures. We studied active lengthening contractions in the mouse extensor digitorum longus muscle at 17°C, 27°C, and 37°C. Force development in both phases was temperature-sensitive. Phase-1 stiffness decreased at higher temperatures, consistent with faster ATP-dependent cross-bridge detachment, and contributions from mechanical strain-dependent detachment. In phase-2, stiffness increased with temperature, consistent with stronger and faster titin-actin interactions. The transition between phases (muscle "give") varied with temperature and may reflect lower temperatures delaying cross-bridge detachment and engagement of the parallel elastic elements. Together, these findings highlight the intrinsic tuning of muscle mechanics, with potential implications for susceptibility to muscle damage under different thermal conditions, and provide a foundation for the development for more accurate musculoskeletal models.