Foxo family transcription factors contribute to muscle atrophy by promoting transcription of the ubiquitin ligases MuRF1 and MAFbx/atrogin1. Foxo transcriptional effectiveness is largely determined by its nuclear cytoplasmic distribution, with unphosphorylated Foxo1 transported into nuclei and phosphorylated Foxo1 transported out. We expressed the fluorescent fusion protein Foxo1-GFP in cultured adult mouse flexor digitorum brevis muscle fibers, and tracked the time course of nuclear (N) to cytoplasmic (C) Foxo1-GFP mean pixel fluorescence in living fibers by confocal imaging. We previously showed that IGF1, which activates the Foxo kinase Akt/PKB, caused a rapid marked decline in N/C, whereas inhibition of Akt caused a modest increase in N/C [1]. Here we develop a 2 state mathematical model for Foxo1 nuclear cytoplasmic redistribution, where Foxo phosphorylation- dephosphorylation is assumed to be fast compared to nuclear influx and efflux. C is constant due to the much larger cytoplasmic than nuclear volume. Analysis of N/C time courses reveals that IGF1 strongly increased unidirectional nuclear efflux, indicating similarly increased fractional phosphorylation of Foxo1 within nuclei. Unidirectional nuclear influx was decreased in IGF1, indicating increased cytoplasmic fractional phosphorylation of Foxo1. Inhibition of Akt increased Foxo1 unidirectional nuclear influx, consistent with block of Foxo1 cytoplasmic phosphorylation, but did not decrease Foxo1 unidirectional nuclear efflux, indicating that Akt may not be involved in Foxo1 nuclear efflux under control conditions. New solution change experiments show that cultured fibers release factors into the medium that maintain low nuclear Foxo1. This study demonstrates the power of quantitative modelling of observed nuclear fluxes.