Inclusion criteria for participating clients in the healthy study group:  - Healthy individuals under the age of 25, with no history of SI dysfunction or lower back related illnesses  Inclusion criteria for participating clients in the healthy study group with the same demographic characteristics as the SI dysfunction patients:  - Healthy individuals between the age of 25 and 45, female sex and postpartum with no history of SI dysfunction or lower back related illnesses  Inclusion criteria for the MISJF group:  The patient is suffering from SIJ dysfunction (uni- or bilateral) because of postpartum pelvic instability or because of Ehlers Danlos syndrome and is enlisted for primary MISJF surgery. The patient is mobile enough to perform several tests at the motion lab, needed to obtain spatiotemporal parameters, pelvic obliquity, center of gravity and load capacity.  Exclusion criteria for all participating clients are:  The individual is not mobile and/or unable to perform tests at the motion lab, needed to obtain spatiotemporal parameters, pelvic obliquity, center of gravity and load capacity. Inadequate command of the Dutch language. Age under 18 years.