This study explored the genetic and prenatal environmental influences on the development of reaching in twins. Detailed kinematic measures of arm movements and underlying muscle co-activation from two sets of monozygotic twins (MZ; one set was monochorionic; the other was dichorionic) and one set of dizygotic (DZ) were measured weekly from reach onset to 30 weeks, then bi-weekly to 1 year of age. Improvements in trajectories were not linear, but the MZ twins demonstrated higher correlations between kinematic variables compared to DZ twins and unrelated infants. Although all infants showed increase in muscle co-activation across the year, MZ twins showed no more correlations in muscle states than DZ twins or unrelated infants. The results suggest that reaches are not planned at the level of the muscles, and that anatomical, biomechanical, and energetic factors act as strong constraints on the inherent kinematic and muscle variability.