No net utilization of intramuscular lipid droplets during repeated high-intensity intermittent exercise

Christian B. Knudsen, Niels Ørtenblad, Magni Mohr, Kristian Overgaard, Jeppe F. Vigh‑Larsen

Research output: Contribution to journalArticlepeer-review


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% Wattmax (Wmax)] combined with maximal repeated sprints (250%–300% Wmax). 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 (P ¼ 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 (P ¼ 0.019) and after EX1 (P ¼ 0.018).
Baseline distinctions were demonstrated with an approximately twofold higher intermyofibrillar lipid volume in type 1 versus type 2 fibers (P ¼ 0.008), mediated solely by a higher number rather than the size of lipid droplets (P < 0.001). No fiber-type-specific differences were observed in subsarcolemmal lipid volume although type 2 fibers exhibited 17% larger droplets (P ¼ 0.034) but a lower numerical density (main effect; P ¼ 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.

NEW & NOTEWORTHY 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.
Original languageEnglish
Pages (from-to)700-710
Number of pages11
JournalAmerican Journal of Physiology. Endocrinology and Metabolism
Issue number6
Publication statusPublished - 17 Nov 2023


  • electron microscopy
  • fat
  • metabolism
  • subcellular
  • substrate


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