Graded reductions in preexercise muscle glycogen impair exercise capacity but do not augment skeletal muscle cell signaling: implications for CHO periodization

Hearris, MA ORCID: https://orcid.org/0000-0003-4909-6755, Hammond, KM, Seaborne, RA, Stocks, B, Shepherd, SO, Philp, A, Sharples, AP, Morton, JP and Louis, JB (2019) Graded reductions in preexercise muscle glycogen impair exercise capacity but do not augment skeletal muscle cell signaling: implications for CHO periodization. Journal of Applied Physiology, 126 (6). pp. 1587-1597. ISSN 8750-7587

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Abstract

We examined the effects of graded muscle glycogen on exercise capacity and modulation of skeletal muscle signaling pathways associated with the regulation of mitochondrial biogenesis. In a repeated-measures design, eight men completed a sleep-low, train-low model comprising an evening glycogen-depleting cycling protocol followed by an exhaustive exercise capacity test [8 3 min at 80% peak power output (PPO), followed by 1-min efforts at 80% PPO until exhaustion] the subsequent morning. After glycogen-depleting exercise, subjects ingested a total of 0 g/kg (L-CHO), 3.6 g/kg (M-CHO), or 7.6 g/kg (H-CHO) of carbohydrate (CHO) during a 6-h period before sleeping, such that exercise was commenced the next morning with graded (P 0.05) muscle glycogen concentrations (means SD: L-CHO: 88 43, M-CHO: 185 62, H-CHO: 278 47 mmol/kg dry wt). Despite differences (P 0.05) in exercise capacity at 80% PPO between trials (L-CHO: 18 7, M-CHO: 36 3, H-CHO: 44 9 min), exercise induced comparable AMPKThr172 phosphorylation (~4-fold) and PGC-1 mRNA expression (~5-fold) after exercise and 3 h after exercise, respectively. In contrast, neither exercise nor CHO availability affected the phosphorylation of p38MAPKThr180/Tyr182 or CaMKIIThr268 or mRNA expression of p53, Tfam, CPT-1, CD36, or PDK4. Data demonstrate that when exercise is commenced with muscle glycogen 300 mmol/kg dry wt, further graded reductions of 100 mmol/kg dry weight impair exercise capacity but do not augment skeletal muscle cell signaling. NEW & NOTEWORTHY We provide novel data demonstrating that when exercise is commenced with muscle glycogen below 300 mmol/kg dry wt (as achieved with the sleep-low, train-low model) further graded reductions in preexercise muscle glycogen of 100 mmol/kg dry wt reduce exercise capacity at 80% peak power output by 20 –50% but do not augment skeletal muscle cell signaling.