AbstractThis study was designed to describe the development of posture control in sitting in response to a natural perturbation. Seven normal infants 2 to 5 months of age were tested at two stages of independent sitting development. Trunk support was removed from infants while sitting erect and the postural responses were videotaped and EMG recorded from the upper trunk extensors, lumbar paraspinals, gluteus maximus, rectus femoris, hamstrings, and abdominals. Kinematic variables (trunk displacement, trunk velocity, trunk curvature) and an EMG variable (pattern of muscle activation) were analyzed with computer programs. Between Stages 1 and 2 of sitting development, anterior trunk displacement and velocity decreased significantly, although the trunk extension curve did not change significantly. Infants had variable muscle responses during Stage 1; however, during Stage 2 EMG analysis revealed less variability and the emergence of postural synergies. Overall, lumbar paraspinals, hamstrings, and quadriceps were the muscles most frequently active during the postural response. Each subject had a preferred synergy, with the most common synergies being a lumbar paraspinalhamstring synergy and a lumbar paraspinal‐quadriceps synergy. These data provide evidence that trunk displacement and trunk velocity decrease in infants develop independent sitting posture, and these variables may be used to measure improvement in sitting control. We suggest that the control of sitting posture is related to the emergence and preferred use of the paraspinal‐hamstring and paraspinal‐quadriceps synergies. © 1993 Johan Wiley&Son