Purpose: Preseason in football is crucial for optimizing physical fitness, team cohesion, and tactical readiness. This study investigated the effects of 2 distinct preseason training environments—mild altitude with cooler conditions and sea level with higher heat and humidity—on cardiovascular fitness, readiness, and match intensity in professional football players. Methods: Fifteen elite male players were monitored during 2 consecutive preseasons (2023-24 and 2024-25), with external- and internal-load parameters assessed via global positioning systems and heart-rate (HR) tracking systems. Cardiovascular fitness was evaluated using HR responses during submaximal running tests (HRex) and machine-learning models (delta HR), and readiness was assessed through vertical stiffness (Kvert) and locomotor efficiency (delta PL), also using machine-learning approaches. Results: Multilevel regression analysis indicated that general fitness, represented by HRex, improved independently of environmental conditions and was primarily influenced by cumulative training load (gamma = -0.045, P = .004). However, specific fitness (delta HR) improved more significantly under higher temperatures (gamma = 8.188, P = .009). Increased heat exposure reduced readiness levels, as reflected by declines in Kvert and delta PL by the end of the sea-level preseason. Match intensity showed no significant differences between environments. Conclusions: Sea-level preseason training environments appear to promote faster specific fitness gains, likely due to cardiovascular adaptations. We hypothesize that these adaptations could be related to the effects of heat exposure, such as plasma volume expansion. However, it also results in lower readiness levels, affecting vertical stiffness and locomotor efficiency. These findings provide valuable insights for designing preseason programs to balance cardiovascular fitness gains and fatigue mitigation.
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