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Muscle Glycogen Content Modifies SR Ca2+ Release Rate in Elite Endurance Athletes

GEJL, KASPER DEGN1; HVID, LARS GRØNDAHL1; FRANDSEN, ULRIK1; JENSEN, KURT1,2; SAHLIN, KENT3; ØRTENBLAD, NIELS1,2

Medicine & Science in Sports & Exercise: March 2014 - Volume 46 - Issue 3 - p 496–505
doi: 10.1249/MSS.0000000000000132
Basic Sciences

Purpose The aim of the present study was to investigate the influence of muscle glycogen content on sarcoplasmic reticulum (SR) function and peak power output (W peak) in elite endurance athletes.

Methods Fourteen highly trained male triathletes (V˙O2max = 66.5 ± 1.3 mL O2·kg−1·min−1), performed 4 h of glycogen-depleting cycling exercise (HRmean = 73% ± 1% of maximum). During the first 4 h of recovery, athletes received either water (H2O) or carbohydrate (CHO), separating alterations in muscle glycogen content from acute changes affecting SR function and performance. Thereafter, all subjects received CHO-enriched food for the remaining 20-h recovery period.

Results Immediately after exercise, muscle glycogen content and SR Ca2+ release rate was reduced to 32% ± 4% (225 ± 28 mmol·kg−1 dw) and 86% ± 2% of initial levels, respectively (P < 0.01). Glycogen markedly recovered after 4 h of recovery with CHO (61% ± 2% of preexercise) and SR Ca2+ release rate returned to preexercise level. However, in the absence of CHO during the first 4 h of recovery, glycogen and SR Ca2+ release rate remained depressed, with the normalization of both parameters at the end of the 24 h of recovery after receiving a CHO-enriched diet. Linear regression demonstrated a significant correlation between SR Ca2+ release rate and muscle glycogen content (P < 0.01, r 2 = 0.30). The 4 h of cycling exercise reduced W peak by 5.5%–8.9% at different cadences (P < 0.05), and W peak was normalized after 4 h of recovery with CHO, whereas W peak remained depressed (P < 0.05) after water provision. W peak was fully recovered after 24 h in both the H2O and the CHO group.

Conclusion In conclusion, the present results suggest that low muscle glycogen depresses muscle SR Ca2+ release rate, which may contribute to fatigue and delayed recovery of W peak 4 h postexercise.

1Institute of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), University of Southern Denmark, Odense, DENMARK; 2Swedish Winter Sports Research Centre, Department of Health Sciences, Mid Sweden University, Östersund, SWEDEN; and 3The Åstrand Laboratory, Swedish School of Sport and Health Sciences, Stockholm, SWEDEN

Address for correspondence: Niels Ørtenblad, Ph.D., Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark; E-mail: nortenblad@health.sdu.dk.

Submitted for publication March 2013.

Accepted for publication July 2013.

© 2014 American College of Sports Medicine