A number of studies have explored the contribution of auditory information in speech production. On the other hand, little attention has been devoted to the possible role of somatosensory feedback in the achievement of speech goals. Nevertheless, the ability of individuals who become deaf as adults to produce intelligible speech could indeed be maintained by somatosensory information. This paper presents the use of a method which manipulates somatosensory feedback independent of speech acoustics, allowing direct assessment of the importance of somatosensation in speech. A robotic device applied mechanical loads to the jaw during speech production. The device significantly altered somatosensory feedback without perturbing the speech acoustics. In a previous study (Tremblay, Shiller & Ostry, 2003), we showed that sensorimotor adaptation to mechanical loads is observed during both vocalized and silent speech. That is, even in the absence of acoustic perturbation, subjects modified their motor commands in order to reach desired somatosensory targets. Thus, the Tremblay et al. study provided direct evidence that somatosensory input is central to the achievement of speech targets. However, in that experiment, the observed patterns of adaptation were specific to movements involving a vowel-to-vowel transition. To investigate this somewhat surprising outcome, the present study explores patterns of adaptation by manipulating the location of the vowel-to-vowel transition within the speech utterance. The goal was to identify the linguistic units for which the achievement of specific somatosensory targets might be important. The present results are consistent with the findings of the previous study: adaptation to a mechanical load is only achieved in portions of speech movements that are associated with a vowel-to-vowel transition. The results are discussed in terms of mechanical and acoustic properties of vowel production.