RESISTANCE EXERCISE-INDUCED FLUID SHIFTS - CHANGE IN ACTIVE MUSCLE SIZE AND PLASMA-VOLUME

The purpose of this study was to test the hypothesis that the reductio n in plasma volume (PV) induced by resistance exercise reflects fluid loss to the extravascular space and subsequently selective increase in cross-sectional area (CSA) of active but not inactive skeletal muscle . We compared changes in active and inactive muscle CSA and PV after b arbell squat exercise. Magnetic resonance imaging (MRI) was used to qu antify muscle involvement in exercise and to determine CSA of muscle g roups or individual muscles [vasti (VS), adductor (Add), hamstring (Ha m), and rectus femoris (RF)]. Muscle involvement in exercise was deter mined using exercise-induced contrast shift in spin-spin relaxation ti me (T-2)-weighted MR images immediately postexercise. Alterations in m uscle size were based on the mean CSA of individual slices. Hematocrit , hemoglobin, and Evans blue dye were used to estimate changes in PV. Muscle CSA and PV data were obtained preexercise and immediately poste xercise and 15 and 45 min thereafter; A hierarchy of muscle involvemen t in exercise was found such that VS > Add > Ham > RF, with the Ham an d RF showing essentially no involvement. CSA of the VS and Add muscle groups were increased 10 and 5%, respectively, immediately after exerc ise in each thigh with no changes in Ham and RF CSA. PV was decreased 22% immediately following exercise. The absolute loss of PV was correl ated (r(2) = 0.75) with absolute increase in muscle CSA immediately po stexercise, supporting the notion that increased muscle size after res istance exercise reflects primarily fluid movement from the vascular s pace into active but not inactive muscle.