Free amino acid pool and muscle protein balance after resistance exercise

Med Sci Sports Exerc. 2003 May;35(5):784-92. doi: 10.1249/01.MSS.0000064934.51751.F9.


Purpose: The aim of this study was to assess the effects of a resistance exercise session (RES) on free amino acid concentrations and protein synthesis and breakdown of the vastus lateralis (VL) muscle during recovery in male subjects.

Methods: Both the exercise group (EG) and the control group (CG) consisted of six healthy physically active men. On the experiment day in fasting conditions, a stable isotopic tracer of L-[ring-2H(5)] phenylalanine was infused and EG started a heavy 50-min hypertrophic RES for lower extremities after 55 min of infusion. At the same time, CG was at rest. During recovery of 195 min after RES, several blood samples were drawn from the femoral artery (FA) and the femoral vein (FV) and muscle samples from the VL muscle. The enrichment was analyzed by GC/MS and leg muscle amino acid kinetics determined by three-pool compartment model between FA, FV, and VL.

Results: During recovery at 60 min after RES, there was no difference in muscle protein synthesis or muscle protein breakdown between the groups, but at 195 min, both muscle protein synthesis (P < 0.05) and muscle protein breakdown (P < 0.05) were increased in EG compared with CG. The protein net balance was negative and similar in both groups. Simultaneously in serum concentrations, there was a decrease in leucine (P < 0.05) associated with an increase in aspartate (P < 0.05). Furthermore, the exercise-induced increase in alanine concentration decreased both in serum and muscle.

Conclusion: In fasting conditions, protein net balance is negative and RES induces an increase in muscle protein synthesis and breakdown at 195 min but not yet at 60 min of recovery.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Amino Acids, Essential / blood
  • Amino Acids, Essential / metabolism*
  • Biological Transport
  • Case-Control Studies
  • Exercise / physiology*
  • Humans
  • Leg / blood supply*
  • Male
  • Multivariate Analysis
  • Muscle Proteins / blood
  • Muscle Proteins / metabolism*
  • Muscle, Skeletal / metabolism*
  • Muscle, Skeletal / physiology
  • Oxygen Consumption / physiology
  • Phenylalanine / blood
  • Phenylalanine / metabolism
  • Physical Exertion / physiology
  • Probability
  • Reference Values
  • Regional Blood Flow
  • Sensitivity and Specificity
  • Weight Lifting*


  • Amino Acids, Essential
  • Muscle Proteins
  • Phenylalanine