Hypoxia-inducible factor-1a (HIF1a) as a context-dependent integrator of metabolic stress in skeletal muscle

Skeletal muscle experiences large fluctuations in ATP demand and redox state during contraction, ischaemia and chronic disease, requiring rapid adaptations in substrate selection, mitochondrial workload, vascular coupling, regeneration and protein homeostasis. Hypoxia-inducible factor-1a (HIF1a) is classically viewed as an oxygen-responsive transcription factor that mediates rapid adaptation to hypoxia. Accumulating evidence across tissues, including skeletal muscle, indicates that HIF1a is also responsive to physiological and pathological inputs, such as exercise, circadian timing, redox perturbations and endogenous/exogenous cytotoxins, even when tissue hypoxia is not detectable. During hypoxia, including transient mismatches between oxygen demand and supply during muscle contraction, HIF1a activation shifts metabolism from oxidative to non-oxidative energy production, suppresses non-essential energy-consuming processes, including protein homeostasis, and promotes vascular responses that improve oxygen delivery. Under normoxic conditions, persistent HIF1a activation promotes maladaptive responses, including impaired mitochondrial remodelling, reduced anabolic responsiveness, defective regeneration, fibrosis, and atrophy- and senescence-associated reprogramming. Current evidence shows that this shift from adaptive to maladaptive signalling is determined in part by post-translational mechanisms that regulate signalling duration and target gene engagement, as well as by fibre type, circadian state and the nature of the upstream stressor. Unlike the robust responses in muscle tissue observed in preclinical models, human muscle biopsies often show modest or transient HIF1a accumulation, yet transcriptional responses indicate meaningful pathway activation, suggesting that biologically relevant signalling occurs even when total protein levels appear low. Genetic models, multiomics, and human studies support HIF1a as a context-dependent regulator of metabolic reprogramming that balances short-term adaptation with long-term energetic cost.