Analysis of fluid movement in skeletal muscle using fluorescent microspheres

Introduction: Regional variability in interstitial fluid pressure confounds use of intramuscular pressure measurement to assess muscle force. It is hypothesized that interstitial flow is dependent on intramuscular pressure. The goal of this study was to assess the feasibility of using fluorescent microspheres to evaluate movement of interstitial fluid in skeletal muscle. Methods: Two diameters of fluorescent microspheres were injected into the rat tibialis anterior during both static (n = 6) and passively lengthened (10% strain) experimental conditions (n = 6). Microsphere dispersion was evaluated using confocal imaging of transverse muscle sections. Results: Fluorescent microspheres tracked interstitial fluid while not penetrating the muscle fiber. When compared with the static condition, significantly greater dispersion (P = 0.003) was seen with passively lengthened conditions (17 ± 9% vs. 31 ± 7%, respectively). Dispersion did not differ for the 2 microsphere sizes (P = 0.811). Conclusions: Fluorescent microspheres track movement of interstitial fluid, and dispersion is dependent on passive lengthening. Muscle Nerve 54: 444–450, 2016.