Hypermobility of the first tarsometatarsal (TMT 1) joint is suggested to be an important factor in the cause and progression of hallux valgus deformity. Hypermobility of the TMT 1 joint is tested clinically in the sagittal plane, but an important deformation also exists in the transversal plane: metatarsus primus varus. This in vitro study was undertaken to investigate the relation between the mobility of the TMT 1 joint in these two planes and to investigate the correlation of the mobility with morphological variables. A second aim was to study the possible stabilizing effect of the tibialis anterior muscle, flexor hallucis longus muscle, and peroneus longus muscle on the TMT 1 joint.Nine embalmed human specimens were tested under standardized conditions. A 30-N force was applied to the head of the first metatarsal (MT 1) to pull in either the dorsal or medial direction. To simulate muscle force, 21 N was applied to the three tendons: all seven possible combinations of muscle action were tested in each plane of motion. Angular displacements were measured using 2-dimensional LED video registration.TMT 1 mobility is a relevant factor in MT 1 mobility in the sagittal and transversal planes, the peroneus longus has a stabilizing effect on this joint, and the effect of the flexor hallucis longus on this joint is different in both planes. When considering a Lapidus procedure for surgically correcting a hallux valgus, the mobility of MT 1 in the transversal plane should also be assessed, but so far no objective clinical test in this plane has been described.