Abstract When a nonlinear biological system is under analysis, one may employ linear and nonlinear tools. Linear tools such as fractional order lumped impedance models have not been previously employed to characterize difference between healthy volunteers and patients diagnosed with kyphoscoliosis (KS). Nonlinear tools such as detection lines from nonlinear contributions in frequency domain have also not been employed previously on KS patient data. KS is an irreversible restrictive disease, of genetic origin, which manifests by deformation of the spine and thorax. The forced oscillation technique (FOT) is a non-invasive, simple lung function test suitable for this class of patients with breathing difficulties, since it does not require any special maneuvre. In this work we show that the FOT method combined with both linear and nonlinear tools reveals important information which may be used as complementary to the standardized lung function tests (i.e. spirometry).