The effects of chronic undernutrition (UN) on respiratory muscle were investigated during UN producing a 50% decrease in body weight over a prolonged period (45 weeks) in Fischer 344 male rats. This model focused on progressive, aging-related changes in myosin heavy chain (MHC) profile over time, in which the confounding effects of early development and late senescence were avoided. With aging toward late adulthood (68 weeks), MHC composition of control diaphragms was shifted, with decreased type I (slow) and IIA MHC, and increased type IIB and IIX (fast) MHC. UN produced a divergence of this profile, with an increase in type I and IIA MHC, and decreased type IIX MHC. UN diaphragms in vitro were more resistant to loss of active force with fatigue, during repetitive contractions. However, passive tension rose disproportionately during fatigue, suggesting increased fatigability. We conclude that the observed changes in diaphragm mechanical function are consistent with the UN-induced shifts in MHC composition; however, the elevated passive tension with fatigue suggests additional UN-induced changes in mechanical properties that are possibly detrimental to respiratory muscle function. The UN-dependent divergence in phenotype and mechanical properties may be amplified by aging-related shifts in muscle MHC composition over time, in the control group.