Slow-Twitch Fiber Glycogen Depletion Elevates Moderate-Exercise Fast-Twitch Fiber Activity and O2 Uptake

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Abstract
KRUSTRUP, P., K. SÖDERLUND, M. MOHR, and J. BANGSBO. Slow-Twitch Fiber Glycogen Depletion Elevates Moderate-Exercise Fast-Twitch Fiber Activity and O2 Uptake. Med. Sci. Sports Exerc., Vol. 36, No. 6, pp. 973–982, 2004. Purpose: We tested the hypotheses that previous glycogen depletion of slow-twitch (ST) fibers enhances recruitment of fast-twitch (FT) fibers, elevates energy requirement, and results in a slow component of V̇O2 during moderate-intensity dynamic exercise in humans. Methods: Twelve healthy, male subjects cycled for 20 min at ~50% Vdot;O2max with normal glycogen stores (CON) and with exercise-induced glycogen depleted ST fibers (CHO-DEP). Pulmonary Vdot;O2was measured continuously and single fiber, muscle homogenate, and blood metabolites were determined repeatedly during each trial. Results: ST fiber glycogen content decreased (P < 0.05) during CON (293 ± 24 to 204 ± 17 mmol·kg−1 d.w.), but not during CHO-DEP (92 ± 22 and 84 ± 13 mmol·kg−1 d.w.). FT fiber CP and glycogen levels were unaltered during CON, whereas FT fiber CP levels decreased (29 ± 7%, P < 0.05) during CHO-DEP and glycogen content tended to decrease (32 ± 14%, P = 0.07). During CHO-DEP, V̇O2 was higher (P < 0.05) from 2 to 20 min than in CON (0–20 min:7 ± 1%). Muscle lactate, pH and temperature, ventilation, and plasma epinephrine were not different between trials. From 3 to 20 min of CHO-DEP, V̇O2 increased (P −1 but was unchanged during CON. In this exercise period, muscle pH and blood lactate were unaltered in both trials. Exponential modeling revealed a slow component of V̇O2 equivalent to 0.12 ± 0.04 L·min−1 during CHO-DEP. Conclusion: This study demonstrates that previous glycogen depletion of ST fibers enhances FT fiber recruitment, elevates O2 cost, and causes a slow component of V̇O2 during dynamic exercise with no blood lactate accumulation or muscular acidosis. These findings suggest that FT fiber recruitment elevates energy requirement of dynamic exercise in humans and support an important role of active FT fibers in producing the slow component of V̇O2