Intramuscular lipids are stored as subsarcolemmal or intramyofibrillar droplets with potential diverse roles in energy metabolism. We examined intramuscular lipid utilization through transmission electron microscopy during repeated high-intensity intermittent exercise, an aspect that is hitherto unexplored. Seventeen moderately to well-trained males underwent three periods (EX1-EX3) of 10 × 45-s high-intensity cycling [∼100%-120% Watt(W)] combined with maximal repeated sprints (∼250%-300% W). M. vastus lateralis biopsies were obtained at baseline, after EX1, and EX3. During the complete exercise session, no net decline in either subsarcolemmal or intermyofibrillar lipid volume density occurred. However, a temporal relationship emerged for subsarcolemmal lipids with an ∼11% increase in droplet size after EX1 (= 0.024), which reverted to baseline levels after EX3 accompanied by an ∼30% reduction in the numerical density of subsarcolemmal lipid droplets compared with both baseline (= 0.019) and after EX1 (= 0.018). Baseline distinctions were demonstrated with an approximately twofold higher intermyofibrillar lipid volume in type 1 versus type 2 fibers (= 0.008), mediated solely by a higher number rather than the size of lipid droplets (< 0.001). No fiber-type-specific differences were observed in subsarcolemmal lipid volume although type 2 fibers exhibited ∼17% larger droplets (= 0.034) but a lower numerical density (main effect;= 0.010) including 3% less droplets at baseline. Collectively, these findings suggest that intramuscular lipids do not serve as an important substrate during high-intensity intermittent exercise; however, the repeated exercise pattern mediated a temporal remodeling of the subsarcolemmal lipid pool. Furthermore, fiber-type- and compartment-specific differences were found at baseline underscoring the heterogeneity in lipid droplet deposition.Undertaking a severe repeated high-intensity intermittent exercise protocol led to no net decline in neither subsarcolemmal nor intermyofibrillar lipid content in the thigh muscle of young moderately to well-trained participants. However, a temporal remodeling of the subsarcolemmal pool of lipid droplets did occur indicative of potential transient lipid accumulation. Moreover, baseline fiber-type distinctions in subcellular lipid droplet deposition were present underscoring the diversity in lipid droplet storage among fiber types and subcellular regions. max max max P P P P P P P NEW & NOTEWORTHY
