Advanced sarcopenia is prevalent among octogenarian women and yet little is known about myocellular quality and plasticity in this cohort. The aim of this investigation was to examine single muscle fiber contractile function and whole muscle characteristics before and after 12-wks of high-intensity progressive resistance training (PRT) in very old women (OW: 85±1 y, n=6). Young women (YW: 21±2 y, n=9) were included as a control group. Whole muscle strength (1-RM) and size (CT scans) were assessed before and after PRT. Functional experiments (size, peak force, velocity, power) were performed on vastus lateralis myosin heavy chain (MHC) I and IIa muscle fibers before and after PRT. With PRT, 1-RM strength increased (p<0.05) in YW (36%) and OW (26%). YW increased (p<0.05) thigh muscle CSA (5%), while OW did not hypertrophy. Before PRT there were no differences in SF parameters between YW and OW. With PRT, YW improved (p<0.05) MHC IIa fiber size (28%), peak force (31%), and power (28%), with no changes in MHC I fibers. There were no improvements in MHC I or IIa SF function in OW. These data show that OW have a similar myocellular functional profile to YW, but have a blunted hypertrophic response to PRT at the whole muscle and myocellular level. The limited myocellular plasticity in octogenarian women with PRT contrasts the YW and previous PRT studies in elderly women only a decade younger. These data suggest that attempts to greatly enhance skeletal muscle mass and function should begin before the age of eighty.