Changes in tissue water content measured with multiple-frequency bioimpedance and metabolism measured with 31P-MRS during progressive forearm exercise.

Multiple-frequency bioimpedance analysis (MFBIA) has been used to determine the cellular water composition in the human body. It is noninvasive and has demonstrated good correlations with other invasive measures of tissue water. However, the ability of this method to study transient changes in tissue water in specific muscle groups has not been explored. In this study, MFBIA was used to assess changes in forearm intracellular water (ICW), extracellular water (ECW), and total water (TW) in seven healthy volunteers during and after a progressive wrist flexion exercise protocol. In an identical trial, (31)P magnetic resonance spectroscopy ((31)P-MRS) was used to assess changes in intracellular pH and phosphocreatine (PCr). At the completion of exercise, forearm ICW increased 12.6% (SD 0.07, P = 0.003), TW increased 10.1% (SD 0.06, P = 0.005), and no significant changes were recorded for ECW. A significant correlation was found between the changes in intracellular pH and changes in ICW during exercise (r = -0.84, P = 0.018). With the use of regression analysis, average changes in P(i), PCr, and pH were found to predict changes in ICW (R(2) = 0.98, P = 0.005). In conclusion, MFBIA was sensitive enough to measure transient changes in the exercising forearm muscle. The changes seen were consistent with the hypothesis that intracellular acidification and PCr hydrolysis are important mediators of cellular osmolality and therefore may be responsible for the increased volume of water in the intracellular space that is often recorded after short-term high-intensity exercise.