Adaptations in muscle metabolic regulation require only a small dose of aerobic-based exercise

This study investigated the hypothesis that the duration of aerobic-based cycle exercise would affect the adaptations in substrate and metabolic regulation that occur in vastus lateralis in response to a short-term (10 day) training program. Healthy active but untrained males (n = 7) with a peak aerobic power (\( \dot{V}{\text{O}}_{{ 2 {\text{ peak}}}} \)) of 44.4 ± 1.4 ml kg⁻¹ min⁻¹ participated in two different training programs with order randomly assigned (separated by ≥2 weeks). The training programs included exercising at a single intensity designated as light (L) corresponding to 60 % \( \dot{V}{\text{O}}_{{ 2 {\text{ peak}}}} \), for either 30 or 60 min. In response to a standardized task (60 % \( \dot{V}{\text{O}}_{{ 2 {\text{ peak}}}} \)), administered prior to and following each training program, L attenuated the decrease (P < 0.05) in phosphocreatine and the increase (P < 0.05) in free adenosine diphosphate and free adenosine monophosphate but not lactate. These effects were not altered by daily training duration. In the case of muscle glycogen, training for 60 versus 30 min exaggerated the increase (P < 0.05) that occurred, an effect that extended to both rest and exercise concentrations. No changes were observed in \( \dot{V}{\text{O}}_{{ 2 {\text{ peak}}}} \) measured during progressive exercise to fatigue or in \( \dot{V}{\text{O}}_{ 2} \) and RER during submaximal exercise with either training duration. These findings indicate that reductions in metabolic strain, as indicated by a more protected phosphorylation potential, and higher glycogen reserves, can be induced with a training stimulus of light intensity applied for as little as 30 min over 10 days. Our results also indicate that doubling the duration of daily exercise at L although inducing increased muscle glycogen reserves did not result in a greater metabolic adaptation.