While playing expressively, cellists tend to produce postural movements, which seem to be part of their musical discourse. This article describes how their instrumental bowing gestures and timbral features of the produced sounds may be affected when constraining these postural (or ancillary) movements. We focus here on a specific acoustic timbre alteration qualified as harshness in the constrained condition. A method based on Canonical Correlation Analysis (CCA) is used to extract the correlations between the bowing displacement and the sound rendition with and without postural constraint among several cellists. Then a detailed investigation of the covariation between gestural and sound data for the duration of the note is carried out, using Functional Data Analysis (FDA) techniques. Results reveal interesting effects of the postural constraint on the coupling patterns between the bowing movement and the spectro-temporal acoustical features.