Slow-twitch fiber glycogen depletion elevates moderate-exercise fast-twitch fiber activity and O2 uptake

Purpose: We testedthe hypotheses that previous glycogen depletion of slow-twitch (ST) fibers enhances recruitment of fast-twitch (FT) fibers, elevatesenergy requirement, and results in a slow component of V˙ O2 during moderate-intensity dynamic exercise in humans. Methods: Twelvehealthy, male subjects cycled for 20 min at 50% V˙ O2max with normal glycogen stores (CON) and with exercise-induced glycogendepleted ST fibers (CHO-DEP). Pulmonary V˙ O2 was measured continuously and single fiber, muscle homogenate, and bloodmetabolites were determined repeatedly during each trial. Results: ST fiber glycogen content decreased (P  0.05) during CON (293 24 to 204  17 mmol·kg1 d.w.), but not during CHO-DEP (92  22 and 84  13 mmol·kg1 d.w.). FT fiber CP and glycogenlevels were unaltered during CON, whereas FT fiber CP levels decreased (29  7%, P  0.05) during CHO-DEP and glycogen contenttended to decrease (32  14%, P 0.07). During CHO-DEP, V˙ O2 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 minof CHO-DEP, V˙ O2 increased (P  0.05) by 5  1% from 1.95  0.05 to 2.06  0.08 L·min1 but was unchanged during CON. Inthis exercise period, muscle pH and blood lactate were unaltered in both trials. Exponential modeling revealed a slow component ofV˙ O2 equivalent to 0.12  0.04 L·min1 during CHO-DEP. Conclusion: This study demonstrates that previous glycogen depletion ofST fibers enhances FT fiber recruitment, elevates O2 cost, and causes a slow component of V˙ O2 during dynamic exercise with no bloodlactate accumulation or muscular acidosis. These findings suggest that FT fiber recruitment elevates energy requirement of dynamicexercise in humans and support an important role of active FT fibers in producing the slow component of V˙ O2